Given that a surface gauge (and indeed height gauges)
need to sit securely on a surface plate,
why don't they have a tripod support
(which sits secure regardless),
instead of having a flat base, which
is only stable if it's carefully made truly flat?

Surely a 3-point base gives equivalent functionality
in perpetuity at a lower build difficulty.

    BugBear (slightly confused)

In article <YrSdnXN1jdg61fbVnZ2dnUVZ8s7inZ2d@posted.plusnet>, bugbear 
<bugbear@trim_papermule.co.uk_trim> writes
>Given that a surface gauge (and indeed height gauges)
>need to sit securely on a surface plate,
>why don't they have a tripod support
>(which sits secure regardless),
>instead of having a flat base, which
>is only stable if it's carefully made truly flat?
>
>Surely a 3-point base gives equivalent functionality
>in perpetuity at a lower build difficulty.
>
Interesting question. Acting as counsel for the defence, as it were, I 
suggest the following:

(1) A 3-point surface is more likely to damage a cast iron surface plate 
than a flat base, if set down rather heavily, and raise slight bumps 
which would be most detrimental. If slid about (as one has to do to 
score reference lines) it would be more likely to cause scratches.

(2) A 3-point base is stable only if there is room for all 3 points to 
rest on the reference surface. If you were short of space, a flat base 
can hang over the edge; a 3-pointer can't.

(3) Making flat surfaces to within a tenth or two by surface grinding is 
relatively easy and cheap.

David
-- 
David Littlewood

"bugbear" <bugbear@trim_papermule.co.uk_trim> wrote in message 
news:YrSdnXN1jdg61fbVnZ2dnUVZ8s7inZ2d@posted.plusnet...
> Given that a surface gauge (and indeed height gauges)
> need to sit securely on a surface plate,
> why don't they have a tripod support
> (which sits secure regardless),
> instead of having a flat base, which
> is only stable if it's carefully made truly flat?
>
> Surely a 3-point base gives equivalent functionality
> in perpetuity at a lower build difficulty.
>
>    BugBear (slightly confused)

Wouldn't the points be likely to be damaged or quickly lose accuracy due to 
wear?

AC

On Jul 2, 5:21 am, bugbear <bugbear@trim_papermule.co.uk_trim> wrote:
>
> Surely a 3-point base gives equivalent functionality
> in perpetuity at a lower build difficulty.
>
>     BugBear

A kinematically correct support may be easier to make in low volume by
hand; optical surface-plate instruments like spherometers are made
with 3 legs, with hardened feet added. I was warned not to treat them
like machinists' measuring instruments because they aren't as
durable.

Large flat surfaces are easy to make in high volume if you have
invested in a surface grinder. The cast iron of the base is a very
good wear surface because of the hard iron carbide.

In article 
, Jim 
Wilkins  writes
>On Jul 2, 5:21 am, bugbear <bugbear@trim_papermule.co.uk_trim> wrote:
>>
>> Surely a 3-point base gives equivalent functionality
>> in perpetuity at a lower build difficulty.
>>
>>     BugBear
>
>A kinematically correct support may be easier to make in low volume by
>hand; optical surface-plate instruments like spherometers are made
>with 3 legs, with hardened feet added. I was warned not to treat them
>like machinists' measuring instruments because they aren't as
>durable.
>
Surely - if I have understood what you mean by spherometer* - it is 
essential to its function to have 3 legs along with a central 
measurement point.

*an instrument for measuring the radius of curvature of a surface.

David
-- 
David Littlewood

AC wrote:
> "bugbear" <bugbear@trim_papermule.co.uk_trim> wrote in message 
> news:YrSdnXN1jdg61fbVnZ2dnUVZ8s7inZ2d@posted.plusnet...
>> Given that a surface gauge (and indeed height gauges)
>> need to sit securely on a surface plate,
>> why don't they have a tripod support
>> (which sits secure regardless),
>> instead of having a flat base, which
>> is only stable if it's carefully made truly flat?
>>
>> Surely a 3-point base gives equivalent functionality
>> in perpetuity at a lower build difficulty.
>>
>>    BugBear (slightly confused)
> 
> Wouldn't the points be likely to be damaged or quickly lose accuracy due to 
> wear?

no - the whole point it they don't matter.

Hmm. That applies to a surface gauge, but not a height gauge.

   BugBear

On 2 Jul, 11:49, "AC"  wrote:
> "bugbear" <bugbear@trim_papermule.co.uk_trim> wrote in message
>
> news:YrSdnXN1jdg61fbVnZ2dnUVZ8s7inZ2d@posted.plusnet...
>
> > Given that a surface gauge (and indeed height gauges)
> > need to sit securely on a surface plate,
> > why don't they have a tripod support
> > (which sits secure regardless),
> > instead of having a flat base, which
> > is only stable if it's carefully made truly flat?
>
> > Surely a 3-point base gives equivalent functionality
> > in perpetuity at a lower build difficulty.
>
> >    BugBear (slightly confused)
>
> Wouldn't the points be likely to be damaged or quickly lose accuracy due to
> wear?
>
> AC

if the three points were hardened ball bearings they wouldn't ...


the  iron surface plate is many peaks and valleys from scrapping and
meant to add up to flat when something flat is placed on it

granite surface plates ...do they have peaks and valleys ? ...i don't
know, as i don't have one ....if not a 3 ball bearings would work.

all the best.markj

Visualize a 3-point base on a flat surface.  If the lengths of the 3 points (legs) were not all 
exactly the same the base would not sit in a plane parallel to the surface plate.  This would cause 
the scribe ( or meter ) to swing in an arc away from perpendicular.

Bob Swinney
"bugbear" <bugbear@trim_papermule.co.uk_trim> wrote in message 
news:YrSdnXN1jdg61fbVnZ2dnUVZ8s7inZ2d@posted.plusnet...
Given that a surface gauge (and indeed height gauges)
need to sit securely on a surface plate,
why don't they have a tripod support
(which sits secure regardless),
instead of having a flat base, which
is only stable if it's carefully made truly flat?

Surely a 3-point base gives equivalent functionality
in perpetuity at a lower build difficulty.

    BugBear (slightly confused) 

** Posted from http://www.teranews.com **

On Wed, 02 Jul 2008 10:21:43 +0100, bugbear
<bugbear@trim_papermule.co.uk_trim> wrote:

>Given that a surface gauge (and indeed height gauges)
>need to sit securely on a surface plate,
>why don't they have a tripod support
>(which sits secure regardless),
>instead of having a flat base, which
>is only stable if it's carefully made truly flat?
>
>Surely a 3-point base gives equivalent functionality
>in perpetuity at a lower build difficulty.
>
>    BugBear (slightly confused)
On a granite surface plate trying to rotate a surface gauge or height
gauge is likely to cause chatter. The gauge will tend to skitter when
not moved in a straight line. I worked at a place that had a height
gauge with three flat feet about 3/4" diameter. It was a pain to use
because of this vibration when roatating the gauge. You had to be real
careful and move it slowly.
Eric

Robert Swinney wrote:
> Visualize a 3-point base on a flat surface.  If the lengths of the 3 points (legs) were not all 
> exactly the same the base would not sit in a plane parallel to the surface plate.  This would cause 
> the scribe ( or meter ) to swing in an arc away from perpendicular.

The tip of the scribe (the only part that matters)
is at "some height", and I can't see why this height would change,
under translation (sliding) or rotation.

It is perfectly normal for the *beam* of a surface gauge
to not be vertical, even with a flat base.

    BugBear

etpm@whidbey.com wrote:
> On a granite surface plate trying to rotate a surface gauge or height
> gauge is likely to cause chatter.

More so than a cast iron plate? How curious (and interesting)

 > The gauge will tend to skitter when
> not moved in a straight line. I worked at a place that had a height
> gauge with three flat feet about 3/4" diameter. It was a pain to use
> because of this vibration when roatating the gauge. You had to be real
> careful and move it slowly.

Did "flat bottomed" gauges show the same behaviour (assumiung
you had some) ?

   BugBear

bugbear, waxing trollish, sez:  "Did "flat bottomed" gauges show the same behaviour (assumiung you 
had some) ?

Fuggegitabbit already!  Go back and visualize the arc the "vertical" member would swing in if the 3 
points weren't all equal.

Bob Swinney


"bugbear" <bugbear@trim_papermule.co.uk_trim> wrote in message 
news:0LOdnWIUpatfPfbVnZ2dneKdnZydnZ2d@posted.plusnet...
etpm@whidbey.com wrote:
> On a granite surface plate trying to rotate a surface gauge or height
> gauge is likely to cause chatter.

More so than a cast iron plate? How curious (and interesting)

 > The gauge will tend to skitter when
> not moved in a straight line. I worked at a place that had a height
> gauge with three flat feet about 3/4" diameter. It was a pain to use
> because of this vibration when roatating the gauge. You had to be real
> careful and move it slowly.

   BugBear 

** Posted from http://www.teranews.com **

"bugbear" <bugbear@trim_papermule.co.uk_trim> wrote in message 
news:YrSdnXN1jdg61fbVnZ2dnUVZ8s7inZ2d@posted.plusnet...
> Given that a surface gauge (and indeed height gauges)
> need to sit securely on a surface plate,
> why don't they have a tripod support
> (which sits secure regardless),
> instead of having a flat base, which
> is only stable if it's carefully made truly flat?
>
> Surely a 3-point base gives equivalent functionality
> in perpetuity at a lower build difficulty.

1) If both surfaces are flat then depressions or damage to either doesn't 
matter. A tripod would follow such depressions.

2) For a surface gauge perpendicularity doesn't really matter but for a 
height gauge it does. How do you know if a tripod is set up perpendicular? 
The answer is you don't.

3) Wear. Three points of contact would wear much faster than a flat base.
-- 
Dave Baker

"Dave Baker"  wrote in message 
news:g4gds5$trq$1@news.datemas.de...
>
> "bugbear" <bugbear@trim_papermule.co.uk_trim> wrote in message 
> news:YrSdnXN1jdg61fbVnZ2dnUVZ8s7inZ2d@posted.plusnet...
>> Given that a surface gauge (and indeed height gauges)
>> need to sit securely on a surface plate,
>> why don't they have a tripod support
>> (which sits secure regardless),
>> instead of having a flat base, which
>> is only stable if it's carefully made truly flat?
>>
>> Surely a 3-point base gives equivalent functionality
>> in perpetuity at a lower build difficulty.
>
> 1) If both surfaces are flat then depressions or damage to either doesn't 
> matter. A tripod would follow such depressions.
>
> 2) For a surface gauge perpendicularity doesn't really matter but for a 
> height gauge it does. How do you know if a tripod is set up perpendicular? 
> The answer is you don't.
>
> 3) Wear. Three points of contact would wear much faster than a flat base.
> -- 
> Dave Baker

Surface plates aren't necessarily *locally* flat. A scraped cast-iron plate 
is only flat at the peaks of the scraping marks. Depending on the size of 
the tripod feet and the finish on the plate, you might not get it right.

Granite surface plates, too, have a tolerance for waviness. I forget what 
the tolerance is, but they're not locally flat.

--
Ed Huntress

"Ed Huntress"  wrote in message 
news:486bbc52$0$5004$607ed4bc@cv.net...
>
> "Dave Baker"  wrote in message 
> news:g4gds5$trq$1@news.datemas.de...
>>
>> "bugbear" <bugbear@trim_papermule.co.uk_trim> wrote in message 
>> news:YrSdnXN1jdg61fbVnZ2dnUVZ8s7inZ2d@posted.plusnet...
>>> Given that a surface gauge (and indeed height gauges)
>>> need to sit securely on a surface plate,
>>> why don't they have a tripod support
>>> (which sits secure regardless),
>>> instead of having a flat base, which
>>> is only stable if it's carefully made truly flat?
>>>
>>> Surely a 3-point base gives equivalent functionality
>>> in perpetuity at a lower build difficulty.
>>
>> 1) If both surfaces are flat then depressions or damage to either doesn't 
>> matter. A tripod would follow such depressions.
>>
>> 2) For a surface gauge perpendicularity doesn't really matter but for a 
>> height gauge it does. How do you know if a tripod is set up 
>> perpendicular? The answer is you don't.
>>
>> 3) Wear. Three points of contact would wear much faster than a flat base.
>> -- 
>> Dave Baker
>
> Surface plates aren't necessarily *locally* flat. A scraped cast-iron 
> plate is only flat at the peaks of the scraping marks. Depending on the 
> size of the tripod feet and the finish on the plate, you might not get it 
> right.
>
> Granite surface plates, too, have a tolerance for waviness. I forget what 
> the tolerance is, but they're not locally flat.

I thought that was the point I was making.
-- 
Dave Baker

On Wed, 2 Jul 2008 18:52:31 +0100, "Dave Baker"  wrote:

>
>"Ed Huntress"  wrote in message 
>news:486bbc52$0$5004$607ed4bc@cv.net...
>>
>> "Dave Baker"  wrote in message 
>> news:g4gds5$trq$1@news.datemas.de...
>>>
>>> "bugbear" <bugbear@trim_papermule.co.uk_trim> wrote in message 
>>> news:YrSdnXN1jdg61fbVnZ2dnUVZ8s7inZ2d@posted.plusnet...
>>>> Given that a surface gauge (and indeed height gauges)
>>>> need to sit securely on a surface plate,
>>>> why don't they have a tripod support
>>>> (which sits secure regardless),
>>>> instead of having a flat base, which
>>>> is only stable if it's carefully made truly flat?
>>>>
>>>> Surely a 3-point base gives equivalent functionality
>>>> in perpetuity at a lower build difficulty.
>>>
>>> 1) If both surfaces are flat then depressions or damage to either doesn't 
>>> matter. A tripod would follow such depressions.
>>>
>>> 2) For a surface gauge perpendicularity doesn't really matter but for a 
>>> height gauge it does. How do you know if a tripod is set up 
>>> perpendicular? The answer is you don't.
>>>
>>> 3) Wear. Three points of contact would wear much faster than a flat base.
>>> -- 
>>> Dave Baker
>>
>> Surface plates aren't necessarily *locally* flat. A scraped cast-iron 
>> plate is only flat at the peaks of the scraping marks. Depending on the 
>> size of the tripod feet and the finish on the plate, you might not get it 
>> right.
>>
>> Granite surface plates, too, have a tolerance for waviness. I forget what 
>> the tolerance is, but they're not locally flat.
>
>I thought that was the point I was making.

Curiously enough, my final year City & Guilds project at college was
designed because of error observed when measuring on a surface plate.

I'd noticed that when sliding a surface gauge base with a clock on it
along the plate to check a reference part surface that I knew was
flat, I could get small variations on the clock reading depending on
how much pressure was used to push down on the surface gauge base as
it was slid along the surface plate.

I designed and made a low friction slide block running on bearing
balls in vees and a flat. It had a small handwheel on the end of the
base with dual feedscrews for coarse and fine movement of the top
slide, which carried a frame for mounting a clock in various
positions. You mounted the clock, zeroed the dial, and used the
feedscrew handwheel to feed along the length of the part.

The base was heavy enough not to move anywhere, and the clock could
index along the part, with the only deflection coming from changes in
the part, and not the plate or the pressure applied to move along it.

Milling, turning, grinding, hardening, lapping, screwcutting and
metrology all in one project, and it earned me a Distinction at the
end of the year.
Then I didn't cut another piece of metal for 25 years<G>.

Peter

"Dave Baker"  wrote in message 
news:g4gf8q$11r$1@news.datemas.de...
>
> "Ed Huntress"  wrote in message 
> news:486bbc52$0$5004$607ed4bc@cv.net...
>>
>> "Dave Baker"  wrote in message 
>> news:g4gds5$trq$1@news.datemas.de...
>>>
>>> "bugbear" <bugbear@trim_papermule.co.uk_trim> wrote in message 
>>> news:YrSdnXN1jdg61fbVnZ2dnUVZ8s7inZ2d@posted.plusnet...
>>>> Given that a surface gauge (and indeed height gauges)
>>>> need to sit securely on a surface plate,
>>>> why don't they have a tripod support
>>>> (which sits secure regardless),
>>>> instead of having a flat base, which
>>>> is only stable if it's carefully made truly flat?
>>>>
>>>> Surely a 3-point base gives equivalent functionality
>>>> in perpetuity at a lower build difficulty.
>>>
>>> 1) If both surfaces are flat then depressions or damage to either 
>>> doesn't matter. A tripod would follow such depressions.
>>>
>>> 2) For a surface gauge perpendicularity doesn't really matter but for a 
>>> height gauge it does. How do you know if a tripod is set up 
>>> perpendicular? The answer is you don't.
>>>
>>> 3) Wear. Three points of contact would wear much faster than a flat 
>>> base.
>>> -- 
>>> Dave Baker
>>
>> Surface plates aren't necessarily *locally* flat. A scraped cast-iron 
>> plate is only flat at the peaks of the scraping marks. Depending on the 
>> size of the tripod feet and the finish on the plate, you might not get it 
>> right.
>>
>> Granite surface plates, too, have a tolerance for waviness. I forget what 
>> the tolerance is, but they're not locally flat.
>
> I thought that was the point I was making.
> -- 
> Dave Baker

?

--
Ed Huntress

"Dave Baker"  wrote in message 
news:g4gf8q$11r$1@news.datemas.de...
>
> "Ed Huntress"  wrote in message 
> news:486bbc52$0$5004$607ed4bc@cv.net...
>>
>> "Dave Baker"  wrote in message 
>> news:g4gds5$trq$1@news.datemas.de...
>>>
>>> "bugbear" <bugbear@trim_papermule.co.uk_trim> wrote in message 
>>> news:YrSdnXN1jdg61fbVnZ2dnUVZ8s7inZ2d@posted.plusnet...
>>>> Given that a surface gauge (and indeed height gauges)
>>>> need to sit securely on a surface plate,
>>>> why don't they have a tripod support
>>>> (which sits secure regardless),
>>>> instead of having a flat base, which
>>>> is only stable if it's carefully made truly flat?
>>>>
>>>> Surely a 3-point base gives equivalent functionality
>>>> in perpetuity at a lower build difficulty.
>>>
>>> 1) If both surfaces are flat then depressions or damage to either 
>>> doesn't matter. A tripod would follow such depressions.
>>>
>>> 2) For a surface gauge perpendicularity doesn't really matter but for a 
>>> height gauge it does. How do you know if a tripod is set up 
>>> perpendicular? The answer is you don't.
>>>
>>> 3) Wear. Three points of contact would wear much faster than a flat 
>>> base.
>>> -- 
>>> Dave Baker
>>
>> Surface plates aren't necessarily *locally* flat. A scraped cast-iron 
>> plate is only flat at the peaks of the scraping marks. Depending on the 
>> size of the tripod feet and the finish on the plate, you might not get it 
>> right.
>>
>> Granite surface plates, too, have a tolerance for waviness. I forget what 
>> the tolerance is, but they're not locally flat.
>
> I thought that was the point I was making.
> -- 
> Dave Baker
>
He's a yank from RCM - you can't expect him to speak the same language :-)

Sorry Ed that time of day

"Ed Huntress"  wrote in message 
news:486bcc91$0$5022$607ed4bc@cv.net...
>
> "Dave Baker"  wrote in message 
> news:g4gf8q$11r$1@news.datemas.de...
>> I thought that was the point I was making.
>> -- 
>> Dave Baker
>
> ?

That a tripod will follow the surface waviness.
-- 
Dave Baker

"Dave Baker"  wrote in message 
news:g4giv8$8c6$1@news.datemas.de...
>
> "Ed Huntress"  wrote in message 
> news:486bcc91$0$5022$607ed4bc@cv.net...
>>
>> "Dave Baker"  wrote in message 
>> news:g4gf8q$11r$1@news.datemas.de...
>>> I thought that was the point I was making.
>>> -- 
>>> Dave Baker
>>
>> ?
>
> That a tripod will follow the surface waviness.
> -- 
> Dave Baker

Ok.

--
Ed Huntress

bugbear wrote:
> Given that a surface gauge (and indeed height gauges)
> need to sit securely on a surface plate,
> why don't they have a tripod support
> (which sits secure regardless),
> instead of having a flat base, which
> is only stable if it's carefully made truly flat?
> 
Because they ARE carefully made truly flat, often by hand scraping.
If yours isn't, you can scrape it in quite easily once you have a 
surface plate.

> Surely a 3-point base gives equivalent functionality
> in perpetuity at a lower build difficulty.
If you have a big square base, you can carve away some of the middle
and 2 corners to achieve a 3-point base.  But, make each spot at least
a cm square so it won't wear too quickly.

Jon

mark wrote:
> On 2 Jul, 11:49, "AC"  wrote:
> 
>>"bugbear" <bugbear@trim_papermule.co.uk_trim> wrote in message
>>
>>news:YrSdnXN1jdg61fbVnZ2dnUVZ8s7inZ2d@posted.plusnet...
>>
>>
>>>Given that a surface gauge (and indeed height gauges)
>>>need to sit securely on a surface plate,
>>>why don't they have a tripod support
>>>(which sits secure regardless),
>>>instead of having a flat base, which
>>>is only stable if it's carefully made truly flat?
>>
>>>Surely a 3-point base gives equivalent functionality
>>>in perpetuity at a lower build difficulty.
>>
>>>   BugBear (slightly confused)
>>
>>Wouldn't the points be likely to be damaged or quickly lose accuracy due to
>>wear?
>>
>>AC
> 
> 
> if the three points were hardened ball bearings they wouldn't ...
> 
> 
> the  iron surface plate is many peaks and valleys from scrapping and
> meant to add up to flat when something flat is placed on it

A properly scraped iron plate is also pretty flat, after precision 
scraping.  There are roughed plates that look like they were hacked with 
a dull axe, but that is nothing like precision scraping.  I have made 
some precision straight edges and angles by hand scraping, and it 
produces a very flat surface when you are completely done.

> 
> granite surface plates ...do they have peaks and valleys ? ...i don't
> know, as i don't have one ....if not a 3 ball bearings would work.

Granite plates have waves, but they are VERY small, if the plate was 
made properly.  There are tiny valleys between crystals, but any object 
placed on it will sit across the high spots, which are the true surface.

Jon

On Wed, 02 Jul 2008 16:37:38 +0100, bugbear
<bugbear@trim_papermule.co.uk_trim> wrote:

>etpm@whidbey.com wrote:
>> On a granite surface plate trying to rotate a surface gauge or height
>> gauge is likely to cause chatter.
>
>More so than a cast iron plate? How curious (and interesting)
>
> > The gauge will tend to skitter when
>> not moved in a straight line. I worked at a place that had a height
>> gauge with three flat feet about 3/4" diameter. It was a pain to use
>> because of this vibration when roatating the gauge. You had to be real
>> careful and move it slowly.
>
>Did "flat bottomed" gauges show the same behaviour (assumiung
>you had some) ?
>
>   BugBear
I don't know about cast iron plates. We did also have a round based
height gauge with the vertical beam mounted close to one edge and it
chattered the worst. Rectangular based height gauges did not have this
behaviour.
Eric

He's a yank from RCM - you can't expect him to speak the same language :-)

Yeah but:  Ed has already forgot more of that language than you 2 clowns will ever know.

Bob (never built racing engines) Swinney

** Posted from http://www.teranews.com **

"Robert Swinney"  wrote in message 
news:a655a$486bd968$21434@news.teranews.com...
> He's a yank from RCM - you can't expect him to speak the same language :-)
>
> Yeah but:  Ed has already forgot more of that language than you 2 clowns 
> will ever know.
>
> Bob (never built racing engines) Swinney

Charmed I'm sure.
-- 
Dave Baker

"Steve"  wrote in message 
news:xs-dned-IIHtkvDVnZ2dneKdnZydnZ2d@bt.com...
>
> "Dave Baker"  wrote in message 
> news:g4gf8q$11r$1@news.datemas.de...
>>
>> "Ed Huntress"  wrote in message 
>> news:486bbc52$0$5004$607ed4bc@cv.net...
>>>
>>> "Dave Baker"  wrote in message 
>>> news:g4gds5$trq$1@news.datemas.de...
>>>>
>>>> "bugbear" <bugbear@trim_papermule.co.uk_trim> wrote in message 
>>>> news:YrSdnXN1jdg61fbVnZ2dnUVZ8s7inZ2d@posted.plusnet...
>>>>> Given that a surface gauge (and indeed height gauges)
>>>>> need to sit securely on a surface plate,
>>>>> why don't they have a tripod support
>>>>> (which sits secure regardless),
>>>>> instead of having a flat base, which
>>>>> is only stable if it's carefully made truly flat?
>>>>>
>>>>> Surely a 3-point base gives equivalent functionality
>>>>> in perpetuity at a lower build difficulty.
>>>>
>>>> 1) If both surfaces are flat then depressions or damage to either 
>>>> doesn't matter. A tripod would follow such depressions.
>>>>
>>>> 2) For a surface gauge perpendicularity doesn't really matter but for a 
>>>> height gauge it does. How do you know if a tripod is set up 
>>>> perpendicular? The answer is you don't.
>>>>
>>>> 3) Wear. Three points of contact would wear much faster than a flat 
>>>> base.
>>>> -- 
>>>> Dave Baker
>>>
>>> Surface plates aren't necessarily *locally* flat. A scraped cast-iron 
>>> plate is only flat at the peaks of the scraping marks. Depending on the 
>>> size of the tripod feet and the finish on the plate, you might not get 
>>> it right.
>>>
>>> Granite surface plates, too, have a tolerance for waviness. I forget 
>>> what the tolerance is, but they're not locally flat.
>>
>> I thought that was the point I was making.
>> -- 
>> Dave Baker
>>
> He's a yank from RCM - you can't expect him to speak the same language :-)
>
> Sorry Ed that time of day

I think I was reading Dave's post upside-down. d8-)

Anyway, I see that Jon and Mark said exactly the same thing, so I shoulda 
just stayed quiet.

--
Ed Huntress
>
>

Ed Huntress wrote:
> "Dave Baker"  wrote in message 
> news:g4gds5$trq$1@news.datemas.de...
> 
>>"bugbear" <bugbear@trim_papermule.co.uk_trim> wrote in message 
>>news:YrSdnXN1jdg61fbVnZ2dnUVZ8s7inZ2d@posted.plusnet...
>>
>>>Given that a surface gauge (and indeed height gauges)
>>>need to sit securely on a surface plate,
>>>why don't they have a tripod support
>>>(which sits secure regardless),
>>>instead of having a flat base, which
>>>is only stable if it's carefully made truly flat?
>>>
>>>Surely a 3-point base gives equivalent functionality
>>>in perpetuity at a lower build difficulty.
>>
>>1) If both surfaces are flat then depressions or damage to either doesn't 
>>matter. A tripod would follow such depressions.
>>
>>2) For a surface gauge perpendicularity doesn't really matter but for a 
>>height gauge it does. How do you know if a tripod is set up perpendicular? 
>>The answer is you don't.
>>
>>3) Wear. Three points of contact would wear much faster than a flat base.
>>-- 
>>Dave Baker
> 
> 
> Surface plates aren't necessarily *locally* flat. A scraped cast-iron plate 
> is only flat at the peaks of the scraping marks. Depending on the size of 
> the tripod feet and the finish on the plate, you might not get it right.
> 
> Granite surface plates, too, have a tolerance for waviness. I forget what 
> the tolerance is, but they're not locally flat.
> 
> --
> Ed Huntress 
> 
> 

This thread just reminded me of something I hadn't thought about in years.

Assume you have a four legged stool with equal length legs standing on a 
wavy floor and it is "rockable" because only three of its four legs 
touch the floor at the same time.

Didja know that just by rotating the stool less than a quarter of a turn 
around its central axis you'll find a position where all four legs are 
in contact with the floor and the "rockability" stops?

Understanding why that is so makes for an interesting thought exercise.

To be a bit more specific, the floor can't have any step discontinuities 
and the stool legs should best terminate in sharp points, But, the 
rotating technique works pretty well with any average stool.

Jeff

-- 
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
The speed of light is 1.8*10^12 furlongs per fortnight.

On 2 Jul, 09:21, bugbear <bugbear@trim_papermule.co.uk_trim> wrote:
> Given that a surface gauge (and indeed height gauges)
> need to sit securely on a surface plate,
> why don't they have a tripod support
> (which sits secure regardless),
> instead of having a flat base, which
> is only stable if it's carefully made truly flat?
>
> Surely a 3-point base gives equivalent functionality
> in perpetuity at a lower build difficulty.
>
>     BugBear (slightly confused)

I think you`re overlooking the fact that height gauges have other uses
other than sitting on a surface plate and if it was three legged it
would not be as useful.
Mark.

mark@ems-fife.co.uk wrote:
> On 2 Jul, 09:21, bugbear <bugbear@trim_papermule.co.uk_trim> wrote:
> 
>>Given that a surface gauge (and indeed height gauges)
>>need to sit securely on a surface plate,
>>why don't they have a tripod support
>>(which sits secure regardless),
>>instead of having a flat base, which
>>is only stable if it's carefully made truly flat?
>>
>>Surely a 3-point base gives equivalent functionality
>>in perpetuity at a lower build difficulty.
>>
>>    BugBear (slightly confused)
> 
> 
> I think you`re overlooking the fact that height gauges have other uses
> other than sitting on a surface plate and if it was three legged it
> would not be as useful.
> Mark.

I use my height gauge to scribe my workpiece.
Most of the time this means sliding the height gauge on
the surface plate.  Much less damage to the surface plate
using a full periphery contact surface than if that pressure
were concentrated on three points, yes?

--Winston

On 2008-07-02, bugbear <bugbear@trim_papermule.co.uk_trim> wrote:
> Given that a surface gauge (and indeed height gauges)
> need to sit securely on a surface plate,
> why don't they have a tripod support
> (which sits secure regardless),
> instead of having a flat base, which
> is only stable if it's carefully made truly flat?
>
> Surely a 3-point base gives equivalent functionality
> in perpetuity at a lower build difficulty.

	Well ... a 3-point base has less surface area, so it is more
likely to wear out of spec more quickly for a given amount of use.
Remember that it is important that the height gauge's beam be truly
vertical.  I guess that you *could* fit the base with a high precision
bullseye level and make two of the three contact points adjustable so
you could set it truly horizontal (and the beam truly vertical) before
each use.  But, of course, this requires the surface plate to be truly
level before you start. :-)

	Enjoy,
		DoN.

-- 
 Email:      | Voice (all times): (703) 938-4564
	(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
           --- Black Holes are where God is dividing by zero ---

"Jeff Wisnia"  wrote in message 
news:MfCdnVeNkrfrYPbVnZ2dnUVZ_qXinZ2d@posted.choiceonecommunications...
> Ed Huntress wrote:
>> "Dave Baker"  wrote in message 
>> news:g4gds5$trq$1@news.datemas.de...
>>
>>>"bugbear" <bugbear@trim_papermule.co.uk_trim> wrote in message 
>>>news:YrSdnXN1jdg61fbVnZ2dnUVZ8s7inZ2d@posted.plusnet...
>>>
>>>>Given that a surface gauge (and indeed height gauges)
>>>>need to sit securely on a surface plate,
>>>>why don't they have a tripod support
>>>>(which sits secure regardless),
>>>>instead of having a flat base, which
>>>>is only stable if it's carefully made truly flat?
>>>>
>>>>Surely a 3-point base gives equivalent functionality
>>>>in perpetuity at a lower build difficulty.
>>>
>>>1) If both surfaces are flat then depressions or damage to either doesn't 
>>>matter. A tripod would follow such depressions.
>>>
>>>2) For a surface gauge perpendicularity doesn't really matter but for a 
>>>height gauge it does. How do you know if a tripod is set up 
>>>perpendicular? The answer is you don't.
>>>
>>>3) Wear. Three points of contact would wear much faster than a flat base.
>>>-- 
>>>Dave Baker
>>
>>
>> Surface plates aren't necessarily *locally* flat. A scraped cast-iron 
>> plate is only flat at the peaks of the scraping marks. Depending on the 
>> size of the tripod feet and the finish on the plate, you might not get it 
>> right.
>>
>> Granite surface plates, too, have a tolerance for waviness. I forget what 
>> the tolerance is, but they're not locally flat.
>>
>> --
>> Ed Huntress
>
> This thread just reminded me of something I hadn't thought about in years.
>
> Assume you have a four legged stool with equal length legs standing on a 
> wavy floor and it is "rockable" because only three of its four legs touch 
> the floor at the same time.
>
> Didja know that just by rotating the stool less than a quarter of a turn 
> around its central axis you'll find a position where all four legs are in 
> contact with the floor and the "rockability" stops?
>
> Understanding why that is so makes for an interesting thought exercise.
>
> To be a bit more specific, the floor can't have any step discontinuities 
> and the stool legs should best terminate in sharp points, But, the 
> rotating technique works pretty well with any average stool.
>
> Jeff

I can't fold my feet behind my head anymore and my brain doesn't twist that 
easily, either. But it does sound interesting.

--
Ed Huntress

"DoN. Nichols"  wrote in message 
news:slrng6oc8p.48f.dnichols@Katana.d-and-d.com...
> On 2008-07-02, bugbear <bugbear@trim_papermule.co.uk_trim> wrote:
>> Given that a surface gauge (and indeed height gauges)
>> need to sit securely on a surface plate,
>> why don't they have a tripod support
>> (which sits secure regardless),
>> instead of having a flat base, which
>> is only stable if it's carefully made truly flat?
>>
>> Surely a 3-point base gives equivalent functionality
>> in perpetuity at a lower build difficulty.
>
> Well ... a 3-point base has less surface area, so it is more
> likely to wear out of spec more quickly for a given amount of use.
> Remember that it is important that the height gauge's beam be truly
> vertical.  I guess that you *could* fit the base with a high precision
> bullseye level and make two of the three contact points adjustable so
> you could set it truly horizontal (and the beam truly vertical) before
> each use.  But, of course, this requires the surface plate to be truly
> level before you start. :-)
>
> Enjoy,
> DoN.



The height-gage base is flat, and covers some area, considerably more than 
three small "feet", (think *pounds-per-square-inch* of contact area, here) 
so it can be moved around on the surface plate and still remain true, plumb, 
lever - whatever - CONSISTENT. To minimize wear (which translates to 
*inaccuracy* ).  Dimensional stability at temperature-change is better on a 
flat base, too.

Good granite surface plates can easily be lapped to .00001, if the operator 
is savvy, and the big metrology companies have some people who travel around 
and do just that - by hand!  Small dings in the surface will not affect the 
result of a good height gage with a big base, whereas a three-legged thing 
could easily be rendered utterly useless if one leg got into a small ding.. 
That is another reason that granite plates are preferred for inspection-lab 
and calibration-lab purposes.

 Iron, no matter how good a job of scraping is done - and I have seen a 
number of really good ones -  get dinged and they raise a spot. Maybe good 
enough for work-cell spot-checking, but not the most durable nor precise.

Flash

David Littlewood writes:

> Surely - if I have understood what you mean by spherometer* - it is 
> essential to its function to have 3 legs along with a central 
> measurement point.

There are two-legged versions.  A ring will also do.  But not a plate, I 
agree.

http://www.truetex.com/spherometers.htm

Ed Huntress writes:

> Granite surface plates, too, have a tolerance for waviness. I forget
> what the tolerance is, but they're not locally flat.

In other words, high-frequency spatial noise.  The average height is what 
is accurately constant.  Thus the need for an integrator (flat bearing 
surface) on the sampling device (height gage, etc), instead of a point 
detector (tripod tip).

My Mitutoyo height gage base is largely hollow, only about 1/2 inch 
perimeter is flat bearing surface.

Jeff Wisnia  wrote:

> This thread just reminded me of something I hadn't thought about in
> years.
>
> Assume you have a four legged stool with equal length legs standing on a
> wavy floor and it is "rockable" because only three of its four legs
> touch the floor at the same time.
>
> Didja know that just by rotating the stool less than a quarter of a turn
> around its central axis you'll find a position where all four legs are
> in contact with the floor and the "rockability" stops?
>
> Understanding why that is so makes for an interesting thought exercise.
>
> To be a bit more specific, the floor can't have any step discontinuities
> and the stool legs should best terminate in sharp points, But, the
> rotating technique works pretty well with any average stool.

Interesting puzzle.

Thinking a bit, I see it's not true in general for any surface.  The
surface could be like a random pile of rocks (only rolling hill-like to
meet your "no step discontinuities").  Every little motion will move at
least 3 feet to a new location on the surface and yet another potential
imbalance.  Real floors aren't commonly like that however (unless you are
trying to make it balance on a rock floor :)).

However, floors made out of large flat 4x8 sheets tend to distort along a
single axis forming a ridge or cylinder distortion.  If you are on a ridge
(aka cylinder), you can always twist the chair to straddle the ridge and
keep it from rocking.  Even if the 4 feet are not flat, I think you can
still fix it by twisting the chair as long as the floor's ridge distortion
is larger than the leg distortion (distance the two diagonals connecting
the corner feet fail to meet by).

If you are located on the seam of two sub-floor sheets, and they are both
twisted, but in different ways, and bowed at the seam to make a third
distortion to add to the mix, I doubt there can be any gurantee that
twisting the chair will fix the rock.

> Jeff

-- 
Curt Welch                                            http://CurtWelch.Com/
curt@kcwc.com                                        http://NewsReader.Com/

In article , Richard J 
Kinch  writes
>David Littlewood writes:
>
>> Surely - if I have understood what you mean by spherometer* - it is
>> essential to its function to have 3 legs along with a central
>> measurement point.
>
>There are two-legged versions.  A ring will also do.  But not a plate, I
>agree.
>
>http://www.truetex.com/spherometers.htm

Interesting, thank you.

David
-- 
David Littlewood

bugbear wrote:
> 
> The tip of the scribe (the only part that matters)
> is at "some height", and I can't see why this height would change,
> under translation (sliding) or rotation.
> 
> It is perfectly normal for the *beam* of a surface gauge
> to not be vertical, even with a flat base.

Further reading of this thread has pointed
out that in the particular case of a height gauge, the beam
is (and must be) vertical for the calibrations to have any meaning.

   BugBear

Jon Elson wrote:
> 
> 
> bugbear wrote:
>> Given that a surface gauge (and indeed height gauges)
>> need to sit securely on a surface plate,
>> why don't they have a tripod support
>> (which sits secure regardless),
>> instead of having a flat base, which
>> is only stable if it's carefully made truly flat?
>>
> Because they ARE carefully made truly flat, often by hand scraping.

Indeed.

I'm not questioning wether a (truly)flat base works,
or can be made to work.

I was trying to explore
whether an alternative design, with lower production
costs, can also be made to work

   BugBear

In article <ApudnS_pcIi7K_HVnZ2dnUVZ8u-dnZ2d@posted.plusnet>, bugbear 
<bugbear@trim_papermule.co.uk_trim> writes
>bugbear wrote:
>>  The tip of the scribe (the only part that matters)
>> is at "some height", and I can't see why this height would change,
>> under translation (sliding) or rotation.
>>  It is perfectly normal for the *beam* of a surface gauge
>> to not be vertical, even with a flat base.
>
>Further reading of this thread has pointed
>out that in the particular case of a height gauge, the beam
>is (and must be) vertical for the calibrations to have any meaning.
>
Well, easy to make these assumptions without too much thought, but check 
the numbers.

Let's take a case where the vertical post of a height gauge is 0.5 
degrees off vertical - that's quite a whopping deviation. Then measured 
height would be true height times cos 0.5 degrees, or true height x 
0.999962. Less than half a tenth out per inch. I doubt anyone outside a 
metrology lab would even be able to detect such a difference, let alone 
measure it reliably. For example, a 2 degree temperature fluctuation in 
your workshop would give a change in your stainless steel height gauge 
of about the same amount.

To put that in context, a 27 thou pip under one end of a height gauge 
with a 3" base would be required to give that 0.5 degrees tilt. That's 
about 3/4 mm, nearly enough to trip over, never mind to feel.

Even at 1 degree of tilt, it's still only 1.5 tenths per inch. Above 1 
degree, it would get rapidly worse.

You would do better to check the temperature at which your height gauge 
is calibrated, and seeing how your workshop temperature compares.

David
-- 
David Littlewood

"bugbear" <bugbear@trim_papermule.co.uk_trim> wrote in message 
news:ApudnS_pcIi7K_HVnZ2dnUVZ8u-dnZ2d@posted.plusnet...
> bugbear wrote:
>>
>> The tip of the scribe (the only part that matters)
>> is at "some height", and I can't see why this height would change,
>> under translation (sliding) or rotation.
>>
>> It is perfectly normal for the *beam* of a surface gauge
>> to not be vertical, even with a flat base.
>
> Further reading of this thread has pointed
> out that in the particular case of a height gauge, the beam
> is (and must be) vertical for the calibrations to have any meaning.
>
>   BugBear

I may not be following you, but the cosine error that results from a slight 
departure from vertical is well within the tolerance of the gage. It has to 
be off-vertical by quite a bit before the error becomes significant.

--
Ed Huntress

Ed Huntress wrote:
> "bugbear" <bugbear@trim_papermule.co.uk_trim> wrote in message 
> news:ApudnS_pcIi7K_HVnZ2dnUVZ8u-dnZ2d@posted.plusnet...
>> bugbear wrote:
>>> The tip of the scribe (the only part that matters)
>>> is at "some height", and I can't see why this height would change,
>>> under translation (sliding) or rotation.
>>>
>>> It is perfectly normal for the *beam* of a surface gauge
>>> to not be vertical, even with a flat base.
>> Further reading of this thread has pointed
>> out that in the particular case of a height gauge, the beam
>> is (and must be) vertical for the calibrations to have any meaning.
>>
>>   BugBear
> 
> I may not be following you, but the cosine error that results from a slight 
> departure from vertical is well within the tolerance of the gage. It has to 
> be off-vertical by quite a bit before the error becomes significant.

Mea culpa; while a non vertical beam does introduce
an error, I didn't go as far as you and David Littlewood
(correctly) did, and calculate the *magnitude* of the error.

Speaking as someone who often castigates woodworkers
for talking about "dead square", as opposed to specifying
a squareness *tolerance*, I am duly ashamed.

   BugBear

"mark"  wrote in message 
news:6af7a29f-d91b-4dd4-bbc1-dd596057866b@p25g2000hsf.googlegroups.com...
On 2 Jul, 11:49, "AC"  wrote:
> "bugbear" <bugbear@trim_papermule.co.uk_trim> wrote in message
>
> news:YrSdnXN1jdg61fbVnZ2dnUVZ8s7inZ2d@posted.plusnet...
>
> > Given that a surface gauge (and indeed height gauges)
> > need to sit securely on a surface plate,
> > why don't they have a tripod support
> > (which sits secure regardless),
> > instead of having a flat base, which
> > is only stable if it's carefully made truly flat?
>
> > Surely a 3-point base gives equivalent functionality
> > in perpetuity at a lower build difficulty.
>
> > BugBear (slightly confused)
>
> Wouldn't the points be likely to be damaged or quickly lose accuracy due 
> to
> wear?
>
> AC

>if the three points were hardened ball bearings they wouldn't ...
>
>
>the  iron surface plate is many peaks and valleys from scrapping and
>meant to add up to flat when something flat is placed on it
>
>granite surface plates ...do they have peaks and valleys ? ...i don't
>know, as i don't have one ....if not a 3 ball bearings would work.
>
>all the best.markj

Ah, OK, I was visualising points.

AC

Curt Welch wrote:

<snipped>


> 
> If you are located on the seam of two sub-floor sheets, and they are both
> twisted, but in different ways, and bowed at the seam to make a third
> distortion to add to the mix, I doubt there can be any gurantee that
> twisting the chair will fix the rock.
> 
> 
>>Jeff
> 
> 


Well, the way it was explained to me is:

The stool's four legs are located on the corners of a square.

Label the legs A, B, C, and D.

With A, B and C touching the wavy floor and D not, start rotating the 
stool about its central axis, constraining it so that legs A, B and C 
remain in contact with the floor.

If you were to rotate the stool through a full 90 degrees leg A could 
replace leg D as being the one not touching the floor and leg D would be 
in contact with the floor.

Ergo, at some point in less than 90 degrees of rotation leg D must have 
made contact with the floor, and if you'd stopped rotating the stool at 
that point all four legs would be contacting the floor.

QED

Try it yourself....

Jeff



-- 
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
The speed of light is 1.8*10^12 furlongs per fortnight.

On Jul 3, 7:22 am, bugbear <bugbear@trim_papermule.co.uk_trim>
wrote:>
> > bugbear wrote:
> >> Given that a surface gauge (and indeed height gauges)
> >> need to sit securely on a surface plate,
> >> why don't they have a tripod support
...
> whether an alternative design, with lower production
> costs, can also be made to work
>
>    BugBear

Surface gauges also have to work on milling machine tables with tee
slots.

On Wed, 2 Jul 2008 14:43:33 -0500, "Robert Swinney" 
wrote:

>He's a yank from RCM - you can't expect him to speak the same language :-)
>
>Yeah but:  Ed has already forgot more of that language than you 2 clowns will ever know.
>
>Bob (never built racing engines) Swinney
>
>** Posted from http://www.teranews.com **

	...sigh.....I just remembered why I left RCM behind all those years
ago.
--

Chris Edwards (in deepest Dorset)      "....there *must* be an easier way!"

"Chris Edwards"  wrote in message 
news:imtp64tm85p86e9inps8oaojigsq35cpkm@4ax.com...
> On Wed, 2 Jul 2008 14:43:33 -0500, "Robert Swinney" 
> wrote:
>
>>He's a yank from RCM - you can't expect him to speak the same language :-)
>>
>>Yeah but:  Ed has already forgot more of that language than you 2 clowns 
>>will ever know.
>>
>>Bob (never built racing engines) Swinney
>>
>>** Posted from http://www.teranews.com **
>
> ...sigh.....I just remembered why I left RCM behind all those years
> ago.

Same here. I left a few days after 9/11 when some of the people on there 
started calling anyone from the Middle East ragheads and camel jockeys and 
wanting the entire region to be bombed into dust regardless of who had 
actually brought the towers down.

They got their wish I suppose and look what that achieved. I hope they're 
all happy now as their dollar and their economy sink steadily into the 
toilet.
-- 
Dave Baker

On Thu, 3 Jul 2008 08:45:00 -0400, "Ed Huntress" 
wrote:

>
>"bugbear" <bugbear@trim_papermule.co.uk_trim> wrote in message 
>news:ApudnS_pcIi7K_HVnZ2dnUVZ8u-dnZ2d@posted.plusnet...
>> bugbear wrote:
>>>
>>> The tip of the scribe (the only part that matters)
>>> is at "some height", and I can't see why this height would change,
>>> under translation (sliding) or rotation.
>>>
>>> It is perfectly normal for the *beam* of a surface gauge
>>> to not be vertical, even with a flat base.
>>
>> Further reading of this thread has pointed
>> out that in the particular case of a height gauge, the beam
>> is (and must be) vertical for the calibrations to have any meaning.
>>
>>   BugBear
>
>I may not be following you, but the cosine error that results from a slight 
>departure from vertical is well within the tolerance of the gage. It has to 
>be off-vertical by quite a bit before the error becomes significant.


Actually, since the original question was about surface gauges rather than
height gauges... If the scriber of the surface gauge were to overhang the
centre of the base by 3", then the half degree tilt would produce a 26 thou
error due to being a sine error. So, in the case of a surface gauge, a tilt is
not a good thing :-)


regards
Mark Rand

David Littlewood wrote:
> In article <ApudnS_pcIi7K_HVnZ2dnUVZ8u-dnZ2d@posted.plusnet>, bugbear 
> <bugbear@trim_papermule.co.uk_trim> writes
>> bugbear wrote:
>>>  The tip of the scribe (the only part that matters)
>>> is at "some height", and I can't see why this height would change,
>>> under translation (sliding) or rotation.
>>>  It is perfectly normal for the *beam* of a surface gauge
>>> to not be vertical, even with a flat base.
>>
>> Further reading of this thread has pointed
>> out that in the particular case of a height gauge, the beam
>> is (and must be) vertical for the calibrations to have any meaning.
>>
> Well, easy to make these assumptions without too much thought, but check 
> the numbers.
> 
> Let's take a case where the vertical post of a height gauge is 0.5 
> degrees off vertical - that's quite a whopping deviation. Then measured 
> height would be true height times cos 0.5 degrees, or true height x 
> 0.999962. Less than half a tenth out per inch. I doubt anyone outside a 
> metrology lab would even be able to detect such a difference, let alone 
> measure it reliably. For example, a 2 degree temperature fluctuation in 
> your workshop would give a change in your stainless steel height gauge 
> of about the same amount.
> 
> To put that in context, a 27 thou pip under one end of a height gauge 
> with a 3" base would be required to give that 0.5 degrees tilt. That's 
> about 3/4 mm, nearly enough to trip over, never mind to feel.

A 27 thou pip under one end of an equilateral tripod base would raise 
the zero point by 9 thou (if my trig is correct).

The center of the base would be raised. This effect would be far more 
important than the tilt.

-- Peter Fairbrother





> 
> Even at 1 degree of tilt, it's still only 1.5 tenths per inch. Above 1 
> degree, it would get rapidly worse.
> 
> You would do better to check the temperature at which your height gauge 
> is calibrated, and seeing how your workshop temperature compares.
> 
> David

bugbear wrote:
> I'm not questioning wether a (truly)flat base works,
> or can be made to work.
> 
> I was trying to explore
> whether an alternative design, with lower production
> costs, can also be made to work
Sure, a casting with 3 bumps that are then scraped (or for cheap
ones, ground).

Jon

"bugbear" <bugbear@trim_papermule.co.uk_trim> wrote in message 
news:YrSdnXN1jdg61fbVnZ2dnUVZ8s7inZ2d@posted.plusnet...
> Given that a surface gauge (and indeed height gauges)
> need to sit securely on a surface plate,
> why don't they have a tripod support
> (which sits secure regardless),
> instead of having a flat base, which
> is only stable if it's carefully made truly flat?
>
> Surely a 3-point base gives equivalent functionality
> in perpetuity at a lower build difficulty.
>
>    BugBear (slightly confused)

JS's favourite vehicle has three wheels and tips over easily by all 
accounts!
Would a three legged surface gauge not suffer the same fate and thus be 
awkward to use?

Henry

On Jul 3, 10:53 am, Jeff Wisnia  wrote:
> Well, the way it was explained to me is:
>
> The stool's four legs are located on the corners of a square.
>
> Label the legs A, B, C, and D.
>
> With A, B and C touching the wavy floor and D not, start rotating the
> stool about its central axis, constraining it so that legs A, B and C
> remain in contact with the floor.
>
> If you were to rotate the stool through a full 90 degrees leg A could
> replace leg D as being the one not touching the floor and leg D would be
> in contact with the floor.
>
> Ergo, at some point in less than 90 degrees of rotation leg D must have
> made contact with the floor, and if you'd stopped rotating the stool at
> that point all four legs would be contacting the floor.
>
> QED
>
> Try it yourself....
>
> Jeff
>

I don't think it's quite that simple, Jeff - you've got an
inconsistency there.

On the one hand, you say to constrain it while rotating, so that A, B
and C remain in contact with the floor.  You can do that.  But you
then say that after 90 degrees D will be contacting the floor and A
will be off it.  A can't be off it if the chair is constrained so that
A remains in contact.

John Martin

bugbear <bugbear@trim_papermule.co.uk_trim> wrote:
> Jon Elson wrote:
> >
> >
> > bugbear wrote:
> >> Given that a surface gauge (and indeed height gauges)
> >> need to sit securely on a surface plate,
> >> why don't they have a tripod support
> >> (which sits secure regardless),
> >> instead of having a flat base, which
> >> is only stable if it's carefully made truly flat?
> >>
> > Because they ARE carefully made truly flat, often by hand scraping.
>
> Indeed.
>
> I'm not questioning wether a (truly)flat base works,
> or can be made to work.
>
> I was trying to explore
> whether an alternative design, with lower production
> costs, can also be made to work

I would guess that making a block with a single flat side is in fact far
easier and cheaper to mass produce than any type of tripod.  The cheapest
way to make a tripod with precision flat feet might even be to make a block
with a flat side and them remove the extra material to turn it into a
tripod.

Making a tripod with pointed feet so that you don't have to deal with the
flatness issue ends up with something that will damage your surface plate
so that's not much of an option.  Making hard rounded feet on the tripod is
again, only harder than just making a block with a flat surface and also
risks damage or excess wear to the surface plate.  So though it might seem
like an interesting approach, I think the answer is that in practice, there
is no type of tripod that makes more sense than a block with a flat side
for use with a surface plate.

BTW, my new surface plate just showed up today and I just spent the last
hour making a flat piece of aluminum for the fun of it.

-- 
Curt Welch                                            http://CurtWelch.Com/
curt@kcwc.com                                        http://NewsReader.Com/

Jeff Wisnia  wrote:
> Curt Welch wrote:
>
> <snipped>
>
> >
> > If you are located on the seam of two sub-floor sheets, and they are
> > both twisted, but in different ways, and bowed at the seam to make a
> > third distortion to add to the mix, I doubt there can be any gurantee
> > that twisting the chair will fix the rock.
> >
> >
> >>Jeff
> >
> >
>
> Well, the way it was explained to me is:
>
> The stool's four legs are located on the corners of a square.
>
> Label the legs A, B, C, and D.
>
> With A, B and C touching the wavy floor and D not, start rotating the
> stool about its central axis, constraining it so that legs A, B and C
> remain in contact with the floor.
>
> If you were to rotate the stool through a full 90 degrees leg A could
> replace leg D as being the one not touching the floor and leg D would be
> in contact with the floor.
>
> Ergo, at some point in less than 90 degrees of rotation leg D must have
> made contact with the floor, and if you'd stopped rotating the stool at
> that point all four legs would be contacting the floor.
>
> QED
>
> Try it yourself....
>
> Jeff

Very cool.  I like your logic much better than mine. :)

Even without the constraint of the rotation, it's (now) easy to see that if
you keep three legs on a wavy floor and move it around randomly, you will
always be able to find a spot where the the forth leg would hit the floor
making all 4 touch at that point.  The rotation idea is simple proof that
there there must always be at least one location where that was true
(assuming the chair legs were correctly aligned on a plane).

-- 
Curt Welch                                            http://CurtWelch.Com/
curt@kcwc.com                                        http://NewsReader.Com/

John Martin wrote:
> On Jul 3, 10:53 am, Jeff Wisnia  wrote:
> 
>>Well, the way it was explained to me is:
>>
>>The stool's four legs are located on the corners of a square.
>>
>>Label the legs A, B, C, and D.
>>
>>With A, B and C touching the wavy floor and D not, start rotating the
>>stool about its central axis, constraining it so that legs A, B and C
>>remain in contact with the floor.
>>
>>If you were to rotate the stool through a full 90 degrees leg A could
>>replace leg D as being the one not touching the floor and leg D would be
>>in contact with the floor.
>>
>>Ergo, at some point in less than 90 degrees of rotation leg D must have
>>made contact with the floor, and if you'd stopped rotating the stool at
>>that point all four legs would be contacting the floor.
>>
>>QED
>>
>>Try it yourself....
>>
>>Jeff
>>
> 
> 
> I don't think it's quite that simple, Jeff - you've got an
> inconsistency there.
> 
> On the one hand, you say to constrain it while rotating, so that A, B
> and C remain in contact with the floor.  You can do that.  But you
> then say that after 90 degrees D will be contacting the floor and A
> will be off it.  A can't be off it if the chair is constrained so that
> A remains in contact.
> 
> John Martin

You're correct, John...My language was sloppy. I should have said that D 
could then be made to touch the floor and A would then be off it.

But, the real point is that before you get to 90 degrees of rotation 
you'll reach a point where all four legs will be touching the wavy floor.

It's worked OK for me every time I've tried it.

Jeff

-- 
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
The speed of light is 1.8*10^12 furlongs per fortnight.

John Martin wrote:

> On Jul 3, 10:53 am, Jeff Wisnia  wrote:
> 
>>Well, the way it was explained to me is:
>>
>>The stool's four legs are located on the corners of a square.
>>
>>Label the legs A, B, C, and D.
>>
>>With A, B and C touching the wavy floor and D not, start rotating the
>>stool about its central axis, constraining it so that legs A, B and C
>>remain in contact with the floor.
>>
>>If you were to rotate the stool through a full 90 degrees leg A could
>>replace leg D as being the one not touching the floor and leg D would be
>>in contact with the floor.
>>
>>Ergo, at some point in less than 90 degrees of rotation leg D must have
>>made contact with the floor, and if you'd stopped rotating the stool at
>>that point all four legs would be contacting the floor.
>>
>>QED
>>
>>Try it yourself....
>>
>>Jeff
>>
> 
> 
> I don't think it's quite that simple, Jeff - you've got an
> inconsistency there.
> 
> On the one hand, you say to constrain it while rotating, so that A, B
> and C remain in contact with the floor.  You can do that.  But you
> then say that after 90 degrees D will be contacting the floor and A
> will be off it.  A can't be off it if the chair is constrained so that
> A remains in contact.
> 
> John Martin


My curious mind got to wondering when I first got involved with this 
whole silly musing about four legged stools and wavy floors.

Looks like it was over 12 years ago, and this link provides a grandios 
explanation of why what I said really does work:

http://mathforum.org/kb/message.jspa?messageID=1083862&tstart=0

Happy Independance Day to all.

Jeff (Who can't recall any other US holidays referred to by their dates 
instead of their given names. He doesn't think that the shipment of 
mayonaise on the Titantic <Cinco de Mayo> is a US holiday yet.)

-- 
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
The speed of light is 1.8*10^12 furlongs per fortnight.

On 03 Jul 2008 20:19:38 GMT, curt@kcwc.com (Curt Welch) wrote:

>> in contact with the floor.
>>
>> Ergo, at some point in less than 90 degrees of rotation leg D must have
>> made contact with the floor, and if you'd stopped rotating the stool at
>> that point all four legs would be contacting the floor.
>>
>> QED
>>
>> Try it yourself....
>>
>> Jeff
>
>Very cool.  I like your logic much better than mine. :)
>
>Even without the constraint of the rotation, it's (now) easy to see that if
>you keep three legs on a wavy floor and move it around randomly, you will
>always be able to find a spot where the the forth leg would hit the floor
>making all 4 touch at that point.  The rotation idea is simple proof that
>there there must always be at least one location where that was true
>(assuming the chair legs were correctly aligned on a plane).

There is of course a "flaw" in this proof (pun intended)...it assumes
that the available floor space is big enough for all 4 legs of a stool
to be on it at the same time. You'd have a hard time proving it in my
workshop <G>

Regards,
Tony

"bugbear" <bugbear@trim_papermule.co.uk_trim> wrote in message 
news:0cWdnTIGTrlCT_HVnZ2dnUVZ8hmdnZ2d@posted.plusnet...
> Ed Huntress wrote:
>> "bugbear" <bugbear@trim_papermule.co.uk_trim> wrote in message 
>> news:ApudnS_pcIi7K_HVnZ2dnUVZ8u-dnZ2d@posted.plusnet...
>>> bugbear wrote:
>>>> The tip of the scribe (the only part that matters)
>>>> is at "some height", and I can't see why this height would change,
>>>> under translation (sliding) or rotation.
>>>>
>>>> It is perfectly normal for the *beam* of a surface gauge
>>>> to not be vertical, even with a flat base.
>>> Further reading of this thread has pointed
>>> out that in the particular case of a height gauge, the beam
>>> is (and must be) vertical for the calibrations to have any meaning.
>>>
>>>   BugBear
>>
>> I may not be following you, but the cosine error that results from a 
>> slight departure from vertical is well within the tolerance of the gage. 
>> It has to be off-vertical by quite a bit before the error becomes 
>> significant.
>
> Mea culpa; while a non vertical beam does introduce
> an error, I didn't go as far as you and David Littlewood
> (correctly) did, and calculate the *magnitude* of the error.
>
> Speaking as someone who often castigates woodworkers
> for talking about "dead square", as opposed to specifying
> a squareness *tolerance*, I am duly ashamed.
>
>   BugBear
As a retired metrology specialist I share your concern about unspecified 
tolerances. However, I have what I believe is an interesting concept. "Make 
it square enough that I can not visually determine the direction of any 
out-of square." Also, "Make it so that I am not aware of any inaccuracies". 
And, "Make it so that my measuring instruments do not detect any errors". 
This is not very practical to communicate requirements to others but it 
works for some occasions. It is interesting to me that a machine inaccuracy 
is probably only important if it produces unsuitable parts. That being said, 
I still think accuracy is important.

Don Young

Curt Welch wrote:
> BTW, my new surface plate just showed up today and I just spent the last
> hour making a flat piece of aluminum for the fun of it.

I did that, although I was making a small
aluminium sanding block (I'm now sure my sanding
block needed to quite THAT flat, but it was,
as you say, fun).

   BugBear

Mark Rand wrote:


> 
> Actually, since the original question was about surface gauges rather than
> height gauges... If the scriber of the surface gauge were to overhang the
> centre of the base by 3", then the half degree tilt would produce a 26 thou
> error due to being a sine error. So, in the case of a surface gauge, a tilt is
> not a good thing :-)
> 
>
I don't quite follow this. I thought the important feature was that the
scriber should be at a consistent height - and it will be even if you
rotate it.

Russell

On Jul 3, 4:44 pm, Jeff Wisnia  wrote:
> John Martin wrote:
> > On Jul 3, 10:53 am, Jeff Wisnia  wrote:
>
> >>Well, the way it was explained to me is:
>
> >>The stool's four legs are located on the corners of a square.
>
> >>Label the legs A, B, C, and D.
>
> >>With A, B and C touching the wavy floor and D not, start rotating the
> >>stool about its central axis, constraining it so that legs A, B and C
> >>remain in contact with the floor.
>
> >>If you were to rotate the stool through a full 90 degrees leg A could
> >>replace leg D as being the one not touching the floor and leg D would be
> >>in contact with the floor.
>
> >>Ergo, at some point in less than 90 degrees of rotation leg D must have
> >>made contact with the floor, and if you'd stopped rotating the stool at
> >>that point all four legs would be contacting the floor.
>
> >>QED
>
> >>Try it yourself....
>
> >>Jeff
>
> > I don't think it's quite that simple, Jeff - you've got an
> > inconsistency there.
>
> > On the one hand, you say to constrain it while rotating, so that A, B
> > and C remain in contact with the floor.  You can do that.  But you
> > then say that after 90 degrees D will be contacting the floor and A
> > will be off it.  A can't be off it if the chair is constrained so that
> > A remains in contact.
>
> > John Martin
>
> You're correct, John...My language was sloppy. I should have said that D
> could then be made to touch the floor and A would then be off it.
>
> But, the real point is that before you get to 90 degrees of rotation
> you'll reach a point where all four legs will be touching the wavy floor.
>
> It's worked OK for me every time I've tried it.
>
> Jeff
>
> --
> Jeffry Wisnia
> (W1BSV  Brass Rat '57 EE)
> The speed of light is 1.8*10^12 furlongs per fortnight.- Hide quoted text> - Show quoted text -

Here's an easier way to visualize it.

Stool with legs A, B, C, D all the same length, sitting on a wavy
surface.  Surface is only mildly wavy with no steps or
discontinuities.

Stool is unstable - it rocks on legs diagonally opposite.  Let's say
they are A and C.  It appears that legs A and C are too long.  Rotate
the stool 90 degrees, so that legs B and D are where A and C were, and
since they are all the same length B and D will now be solid and A and
C will appear too short.  They went in that 90 degree rotation from
too long to too short.  At one place at least in that rotation they
were just right.

John Martin

>Here's an easier way to visualize it.

>Stool with legs A, B, C, D all the same length, sitting on a wavy
>surface.  Surface is only mildly wavy with no steps or
>discontinuities.

>Stool is unstable - it rocks on legs diagonally opposite.  Let's say
>they are A and C.  It appears that legs A and C are too long.  Rotate
>the stool 90 degrees, so that legs B and D are where A and C were, and
>since they are all the same length B and D will now be solid and A and
>C will appear too short.  They went in that 90 degree rotation from
>too long to too short.  At one place at least in that rotation they
>were just right.

But not necessarily at the same time.

Henry

"Dragon"  wrote:
> >Here's an easier way to visualize it.
>
> >Stool with legs A, B, C, D all the same length, sitting on a wavy
> >surface.  Surface is only mildly wavy with no steps or
> >discontinuities.
>
> >Stool is unstable - it rocks on legs diagonally opposite.  Let's say
> >they are A and C.  It appears that legs A and C are too long.  Rotate
> >the stool 90 degrees, so that legs B and D are where A and C were, and
> >since they are all the same length B and D will now be solid and A and
> >C will appear too short.  They went in that 90 degree rotation from
> >too long to too short.  At one place at least in that rotation they
> >were just right.
>
> But not necessarily at the same time.

If it's a continuous floor surface with no steps, there's guaranteed to be
at least one spot where they will all touch the floor at the same time.

Before the A or C leg can lift off the floor in the rotation, the B and D
legs must first both touch the floor.  It's at that point in the rotation
where the B and D legs first touch the floor that they will all be touching
the floor at the same time.

It's the same reason you can't draw a graph of two lines - one tiled up,
and the other tilted down, that you can't get from the point on the x axis
where the line A is above B, to the point where line A is below B, without
going through the point where the lines intersect.  It's at that point
where both lines are the "same height" on the graph.

> Henry

-- 
Curt Welch                                            http://CurtWelch.Com/
curt@kcwc.com                                        http://NewsReader.Com/

>> But not necessarily at the same time.
>
> If it's a continuous floor surface with no steps, there's guaranteed to be
> at least one spot where they will all touch the floor at the same time.

That's what is to be proved.

> Before the A or C leg can lift off the floor in the rotation, the B and D
> legs must first both touch the floor.  It's at that point in the rotation
> where the B and D legs first touch the floor that they will all be 
> touching
> the floor at the same time.

And that does it nicely!
So I was wrong.

Henry

"Robert Swinney"  writes:

> Fuggegitabbit already!  Go back and visualize the arc the "vertical" member would swing in if the 3 
> points weren't all equal.


Think of it this way:

      A three footed gauge will generate error if ANY of the feet are
      imperfect.

      A flat-bottomed gauge has more that three points. It has
      hundreds. If one of those has an error, it will not affect the
      gauge.