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Old 09-03-2009, 09:34 AM
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tdoty tdoty is offline
Join Date: May 2009
Location: Winchester, TN
Posts: 1,327

Originally Posted by Kerry Pinkerton View Post
If I changed this phrase:

to this:
the subsequent movement of the metal is what causes the tuck to form.
I'll agree that at a deeper 'physics' level, there is both stretching (thinnng) and drawing (pulling from surrounding metal) going on during the impact. Usually, I'll describe the process as 'stretching' to keep things simple and, at the end of the day, the result of a hammer blow over an unsupported area is a thinning of the metal and an increase in surface area which is generally called a 'stretch'. Right??
There we go! A lot of what is referred to as stretching involves very little stretch and a lot of drawing.

Marty's rubber die example where "the outer edges hold the material while the center stretches to the form of the die" is stretching. There is some drawing likely to occur to, since the rubber die can only hold the metal with so much resistance to movement - it will still slide.

A hammer with a high crown (small radius) will stretch more usually than a low crown (large radius) hammer. The higher psi = more movement at the point of impact and less movement of the surrounding metal.

What you have said, Kerry, more closely reflects my way of thinking. Tuck shrinking involves moving the metal to form the tuck. Forming the tuck moves the metal by drawing from other parts of the panel, though the surface area stays basically the same. The same thing can be done working in the hollow of a stump. Working the resulting tucks - formed by either method - changes the surface area and is indeed shrinking.

Does wheeling cause stretching? I would say yes, even though it is actually thinning the metal where the wheels meet and pushing the resulting excess metal out from the point of contact into the surrounding area. If you form a bowl purely by stretching in the wheel, how many sympathetic tucks or ruffles are formed along the edge? That is one example of why I say stretching does NOT cause tucks to form.

Another example would be to bolt a ring around a hollow (stump or whatever) to keep the metal from moving and push the center of the panel into the hoolow. Most of what is happening is stretching, though you will get some draw from below the surface. If you were to loosen the holding ring and perform the same operation, you would get a lot of drawing and not much shrinking.

I wish I could explain the rest of it. Everytime I try to put it into words without visual aids, it takes a few thousand words to explain what I could say in 3 or 4 pictures. Since I live in one place, do my sheetmetal work in another and my stump is currently in yet another place, it is surprisingly difficult to put everything together to do a photo session.

Maybe it is all semantics, but semantics are the road to universal understanding - or a universal misunderstanding It is not always the tools that you use, but the way that you use them that decides the results.

Tim D.
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