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Old 09-03-2009, 06:47 AM
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Quote:
Originally Posted by David Gardiner View Post
Tim, You seem to be having an argument with yourself. When you hollow a piece of metal either into a sandbag or a 'stump' of course it is being 'drawn', it is also being stretched which is what 'drawing' does. To 'draw' a panel even in a huge press it is stretched, when this process is carried out the edges of the sheet are held and the sheet is forced into a female die by the male die, this requires the metal to be stretched to the required shape. The edges are held so that no puckers occur. When you form metal by hand you are doing the same thing but the edges are not held (most times) so they pucker, these puckers (tucks as you call them) have to be dressed out, because of the resistance in the panel caused by the amount of shape in it by this time the puckers 'shrink' into themselves. This is the process of 'hollowing' this is a time honoured method of shaping panels. (before there were any large presses) As I said earlier depending what tool you are using to do the hammering this process can be called 'hollowing' 'blocking' or 'bossing'.
David, you seem to have missed the post I was referring to, maybe I should have quoted it then:

Quote:
Originally Posted by Kerry Pinkerton View Post
the subsequent stretch is what causes the tuck to form, which can then be shrunk.
I disagree with that and have for a long time.

Funny you should mention press drawing, as I have been doing a lot of reading on that subject, primarily books written during the late 19th and early 20th century.

What you are referring to is stamping and or stretch forming. In drawing, the metal is allowed to draw into the die and the outer edge is not held fast. There is constraint above it to prevent buckling and wrinkling, but not a "pressure ring" per se that holds the metal fast. The area of a blank for a drawn shell is roughly the same as the area of the finished shell.

Here are a couple of experiments I did the other day with a very rudimentary punch and die set. Any significant stretching that occurred was due to a math error - the punch dimension I used should have been used for ironing after the pieces were formed with a slightly smaller punch. The wall thickness is surprisingly consistent, which seems to nullify the argument of stretching. The upper "blankholder" sits above a milled groove, the depth of which matches the material thickness. The material here is .026 steel.

100_1381.jpg

100_1384.jpg

Due to sloppy work and faulty math, the edges were a bit ragged and required trimming. Had I made and used the proper punch, the area of the finished part and the area of the blank would have been functionally the same. You can see a raw edge here and there because my blank wasn't perfectly centered. Luckily, my math allowed a blank that left me with a part that could be trimmed slightly to the desired dimension. Such is the nature of the process of drawing.

Anyway, how much do we want to narrow down the process names? It can be called any of 3 things and they are all interchangeable? And folks wonder why there is confusion about the terminology?

As I said, almost any shrinking method results in some stretch as well. Man, I wish I had more time for metalshaping, I would do a grid test to see exactly what the results are from the various methods - stump, tucking, thumbnail dies and doming dies. Heck, even Lancaster-style shrinkers cause a bit of stretch elsewhere in the panel, negligible though it may be.

Tim D.
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