This definitely doesn't belong here; I'm just hoping that someone will
be able to point me in the right direction.

Many years ago, when using an angle grinder, I was careless and caught
my hand with the rotating disc. I expected blood and guts but merely
had a little roughed up skin.

This got me wondering why different tools cut different materials. A
diamond sharpener, for example, cuts metal and, as I discovered last
week, will file a broken tooth, but has almost no effect on
fingernails.

Obviously, size, hardness, edge shape, speed etc come into the mix, but
is there a branch of engineering that deals with this? My intermittent
googling efforts over the years have come up empty. All help/advice
appreciated.

Tony.

wrote in message 
news:1126807295.786557.170020@z14g2000cwz.googlegroups.com...

> This definitely doesn't belong here; I'm just hoping that someone will
> be able to point me in the right direction.
>
> Many years ago, when using an angle grinder, I was careless and caught
> my hand with the rotating disc. I expected blood and guts but merely
> had a little roughed up skin.
>
> This got me wondering why different tools cut different materials. A
> diamond sharpener, for example, cuts metal and, as I discovered last
> week, will file a broken tooth, but has almost no effect on
> fingernails.
>
> Obviously, size, hardness, edge shape, speed etc come into the mix, but
> is there a branch of engineering that deals with this? My intermittent
> googling efforts over the years have come up empty. All help/advice
> appreciated.


That's a very interesting question to which I knew the answer - fifty years 
ago.

The memory isn't what it used to be but there'll be some clever bugger along 
any minute who'll tell you.

Of course, there might be more than one - with different explanations :-)

Mary

>
> Tony.
> 

Of course, there might be more than one - with different explanations
:-)

I won my bet that the first answer, if any, would come from Mary
Fisher!  But I can't help wondering when she actually has time for any
DIY, when all her time seems to be spent here (:-

Now I'll have to wait for the clever buggers.

Tony.

wrote in message 
news:1126810864.649295.177030@g47g2000cwa.googlegroups.com...

> Of course, there might be more than one - with different explanations
> :-)
>
> I won my bet that the first answer, if any, would come from Mary
> Fisher!


I did my bestfor you :-)


> But I can't help wondering when she actually has time for any
> DIY, when all her time seems to be spent here (:-


Oh I do, you haven't been checking the times of my posts.They're fitted in 
between the diy.

>
> Now I'll have to wait for the clever buggers.


It will be interesting. I hope I remember this time!

Mary

>
> Tony.
> 

ton...@evemail.net wrote:


> Now I'll have to wait for the clever buggers.


I am neither thing.

 I noticed some years ago, when No. 1 son was getting a plaster cast
cut off his arm, that they used a disc cutter. The technician showed a
child that it was safe by sticking his hand into the spinning disc. I'd
always wondered how that worked.

Where are the clever persons when you need one?

Well, for both metals and ionic solids there's a good theoretical 
explanation why a given material can only be scratched by something about 
30% harder. This is the basis of the Mho's scale for identifying minerals 
which you can look up on Google. Mho was quite clever when picking his 10 
materials between Talc and Diamond so that the hardness difference between 
each successive one was close to the magic 1.3

Materials like skin behave quite differently. Those hospital tools for 
removing plaster casts have a vibrating abrasive cutter which whistles 
through plaster which is brittle (breaks when distorted a small amount) but 
when they touch skin they just drag it back and forward. Scary if you don't 
realise.

Aidan wrote:


> ton...@evemail.net wrote:
> 
> 
>>Now I'll have to wait for the clever buggers.
> 
> 
> I am neither thing.
> 
>  I noticed some years ago, when No. 1 son was getting a plaster cast
> cut off his arm, that they used a disc cutter. The technician showed a
> child that it was safe by sticking his hand into the spinning disc. I'd
> always wondered how that worked.


Because it probably wasn't spinning, just vibrating.

Badger wrote:

> Aidan wrote:
> Because it probably wasn't spinning, just vibrating.


NO, it was spinning, but I didn't get an opportunity to examine it.

Aidan wrote:

> ton...@evemail.net wrote:
> 
> 
>>Now I'll have to wait for the clever buggers.
> 
> 
> I am neither thing.
> 
>  I noticed some years ago, when No. 1 son was getting a plaster cast
> cut off his arm, that they used a disc cutter. The technician showed a
> child that it was safe by sticking his hand into the spinning disc. I'd
> always wondered how that worked.
> 
> Where are the clever persons when you need one?


It doesn't rotate, but vibrate backwards and forwards a few mm. If it 
contacts the skin, it just shakes it backwards and forwards a tiny amount.

Dave

In message , 
Aidan  writes

>
>ton...@evemail.net wrote:
>
>> Now I'll have to wait for the clever buggers.
>
>I am neither thing.
>
> I noticed some years ago, when No. 1 son was getting a plaster cast
>cut off his arm, that they used a disc cutter. The technician showed a
>child that it was safe by sticking his hand into the spinning disc. I'd
>always wondered how that worked.
>
>Where are the clever persons when you need one?
>

That's 'cause its not actually _spinning_ but resip, recep, er, moving 
back and forward about a quarter turn, which cuts hard plaster, but just 
waggles the bit of skin it touches
-- 
Jeanette

In message , jeanette 
 writes

>In message , 
>Aidan  writes
>>
>>ton...@evemail.net wrote:
>>
>>> Now I'll have to wait for the clever buggers.
>>
>>I am neither thing.
>>
>> I noticed some years ago, when No. 1 son was getting a plaster cast
>>cut off his arm, that they used a disc cutter. The technician showed a
>>child that it was safe by sticking his hand into the spinning disc. I'd
>>always wondered how that worked.
>>
>>Where are the clever persons when you need one?
>>
>That's 'cause its not actually _spinning_ but resip, recep, er, moving 
>back and forward about a quarter turn, which cuts hard plaster, but 
>just waggles the bit of skin it touches


Typical, wait all day for a clever person (tm), then three show up at 
once :-)
-- 
Keith

"jeanette"  wrote in message 
news:ip1w7nlmxdKDFwRW@blueyonder.co.uk...


>>
> That's 'cause its not actually _spinning_ but resip, recep, er, moving 
> back and forward about a quarter turn, which cuts hard plaster, but just 
> waggles the bit of skin it touches


Reciprocating?

Mary

> -- 
> Jeanette 

"Newshound"  wrote in message 
news:3ou403F7innjU1@individual.net...

> Well, for both metals and ionic solids there's a good theoretical 
> explanation why a given material can only be scratched by something about 
> 30% harder. This is the basis of the Mho's scale for identifying minerals 
> which you can look up on Google. Mho was quite clever when picking his 10 
> materials between Talc and Diamond so that the hardness difference between 
> each successive one was close to the magic 1.3


My Dad worked with Brinell (sp?) hardnesses for metals. But if you used the 
testing punch on your tongue you'd know about it!

>
> Materials like skin behave quite differently. Those hospital tools for 
> removing plaster casts have a vibrating abrasive cutter which whistles 
> through plaster which is brittle (breaks when distorted a small amount) 
> but when they touch skin they just drag it back and forward. Scary if you 
> don't realise.


Like the cutter for teeth developed a few years ago which would go through 
enamel, dentineand the like but leave gums undamaged.

I'm beginning to remember now ... all clever stuff, innit!

Now, when they invent a knife which will cut easily through swedes but leave 
fingers untouched so that red stuff doesn't leak on the vegetable and add to 
the gravy ...

Mary

>
> 

wrote in message 
news:1126887587.382406.109880@g14g2000cwa.googlegroups.com...

> >admission
>
> More male 'e's than 'ad's in the 1940's.


er ... ?

Mary

>
> Tony.
> 

tonybo@evemail.net wrote:

> I only belong to one. What these lucky sods with 'before and after'
> pills get up to, I don't know. :-)


Same as the rest of us get up to, just more often and for longer.

Owain

It's like stalks of wheat that bend with the wind. Rather than resist the 
wind and thus have to be massive and rigid, they
simply get out of the way. Same with your skin and the angle grinder I 
imagine. As to your fingernail, even that is flexible
enough to shrink out of the way of the bits of diamond as the come to bear 
on it.

Andy

>It's like stalks of wheat that bend with the wind. Rather than resist the
>wind and thus have to be massive and rigid, they
>simply get out of the way


Sorry Andy, I missed this somehow. Mary's fault - she leads me into
idle chatter (:-

Makes sense as far as the skin and grinder are concerned; and the fact
that different disks, rotating at the same speed, are needed to cut
stone and metal clearly implies that relative hardness is an important
element. It would be fascinating to know what others are involved
though.

I re-posted my original query on alt-mechanical engineering, but have
had no response, so far, so the search goes on.

Tony.

wrote in message 
news:1127057558.635856.291980@g47g2000cwa.googlegroups.com...

> >It's like stalks of wheat that bend with the wind. Rather than resist the
>>wind and thus have to be massive and rigid, they
>>simply get out of the way
>
> Sorry Andy, I missed this somehow. Mary's fault - she leads me into
> idle chatter (:-


Could be worse ... at least it's not the Primrose path.

>
> Makes sense as far as the skin and grinder are concerned; and the fact
> that different disks, rotating at the same speed, are needed to cut
> stone and metal clearly implies that relative hardness is an important
> element. It would be fascinating to know what others are involved
> though.
>
> I re-posted my original query on alt-mechanical engineering, but have
> had no response, so far, so the search goes on.


Yes, the more you know the more you know that you don't know as much as you 
thought you knew.

Or something ...

Mary

>
> Tony.
> 

>Yes, the more you know the more you know that you >don't know as much as you thought you knew.


Olde Worlde courtesy demands that I permit you the last word. So don't
read this, or you won't have it.

Tony.

wrote in message 
news:1127063883.945122.169560@f14g2000cwb.googlegroups.com...

> >Yes, the more you know the more you know that you >don't know as much as 
> >you thought you knew.
>
> Olde Worlde courtesy demands that I permit you the last word. So don't
> read this, or you won't have it.


Or, as I used toenda... discussion ... :

"If the last word is so important to you, you may have it."

It always worked :-)

Well, it has up to now. There's a first time for everything <G>

Mary

>
> Tony.
> 

Aidan  wrote:

>
> I noticed some years ago, when No. 1 son was getting a plaster cast
>cut off his arm, that they used a disc cutter. The technician showed a
>child that it was safe by sticking his hand into the spinning disc. I'd
>always wondered how that worked.


  In 1983, my brother broke his arm.  The hospital plastered it, but
  to allow for swelling cut a seam along the length.  When my mum
  spotted blood on his pillow, the nurse said it must have just been
  from the previous patient, but mum insisted and eventually they
  took the plaster off to discover they had cut through the skin on
  his elbow.  He still has the scar.
-- 
                                                         Selah

tonybo@evemail.net wrote:

>>Oh Tony, you just don't understand do you?  None of us really *do* any DIY!
> 
> 
> A bit like my generation's teenage boasts of sexual conquests then.
> 
> Tony.
> 

Is that an admission? :)

wrote in message 
news:1126885314.431089.51370@g44g2000cwa.googlegroups.com...

> >Oh Tony, you just don't understand do you?  None of us really *do* any 
> >DIY!
>
> A bit like my generation's teenage boasts of sexual conquests then.


Isn't every generation prone to that?

And not just teenagers!

Mary

>
> Tony.
> 

wrote in message 
news:1126898029.017643.217260@g44g2000cwa.googlegroups.com...

> >No, a casserole is easier :-) You don't need to have your five veg at one
>>meal. For lunch we had (homegrown) cucumber, tomatoes and apples.
>
>>That's real diy!
>
> Braggart. Pride cometh before the fall (or something like that) (:-


I know. I HAVE tripped in the garden but so far haven't done any real 
damage. However, not being all that good on my pins I'm very aware of what 
might happen one day.

I'm only human, despite all rumours to the contrary ...

Mary

>
> Tony.
> 

wrote in message 
news:1126888891.878231.220230@o13g2000cwo.googlegroups.com...

> >AND dinner's cooking in the oven! How's that for efficiency?
>
> Well, when "'Im Indoors" has finished his roasted rushnips,


No, we had lamb hot potwith celariac, Scots something potatoes, marmaladed 
carrots and butter-fried parsley. the rushnips are packed ready to be posted 
tomorrow.


> ask him
> which branch of engineering deals with cutting, which is what this
> thread is supposed to be about.

OK.
>
> Efficiency is chucking the whole lot in a frying pan (5 vegetables , of
> course). Less washing up.


No, a casserole is easier :-) You don't need to have your five veg at one 
meal. For lunch we had (homegrown) cucumber, tomatoes and apples.

That's real diy!

Mary

>
> Tony.
>