Cutting (OT)

[tonybo]

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.

[Mary Fisher]

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

[tonybo]

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.

[Mary Fisher]

I did my bestfor you :-)

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

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

Mary

[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. 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

[Aidan]

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?

[Newshound]

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.

[Badger]

Because it probably wasn't spinning, just vibrating.

[Aidan]

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

[Dave]

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

[jeanette]

In message , Aidan writes

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

[Keith]

In message , jeanette writes

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

[Mary Fisher]

Reciprocating?

Mary

[Mary Fisher]

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

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

[Aidan]

Ah, explains that then.

So why didn't Ton get his hand ripped up? I will not try this at home.

[Chris Bacon]

As does wear rate, lubrication, and flushing.

Um. Thinking cap on.

[Chris Bacon]

"Moh's", dear boy. Goes back to the early C19th.

[OG]

Seems a bit harsh; is it IKEA, the Volvo, or the general Social Democrat smugness you object to?

[Rob Morley]

It may spin if it's not in contact with anything because the disk is free to rotate, when it's pressed against something it just oscillates.

[tonybo]

That's what the etc was in my post - and , no doubt, a lot of other variables - but it brings me no nearer the branch of engineering which deals with the subject. There have been such enormous advances in cutting technology in recent years, that a specialist branch seems highly probable.

I can't believe that Mary doesn't have a mate there (:-

Tony.