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  #21  
Old 09-02-2009, 11:19 AM
David Gardiner David Gardiner is offline
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Quote:
Originally Posted by tdoty View Post
Blocking and hollowing are not the only operations that can be carried out on the stump.

I've been told that working on the stump is more of a drawing operation than stretching. For me, it depends on how I put the material on the stump and where I hit it. If you leave the edge of the panel hanging over the hollow of the stump and start beating, you can encourage the sympathetic ruffles to form along the edge, and then shrink them into themselves using the hollow of the stump to help trap them.

I can work over the hollow of the stump and have little to no formation of sympathetic ruffles, or a I can move the edge over the hollow and get a lot. When I work on the bag, I tend to get more sympathetic ruffles, which shows me the metal is being drawn toward the center and not really stretching.

It's more technique than tools or "operations". It's not blocking or hollowing, it is shrinking using the stump to help form the tucks instead of using a tucking fork or the like.

I'll also admit to having no clue what the "Kirk-style" shrinking hammer or technique are. I didn't have much luck the the modified door skin hammer either. As I said earlier, what I know about tuck shrinking came from Wray, Randy, Kerry and John Kelly. If they haven't advised it, I likely haven't tried it. That said, stump shrinking is still tuck shrinking - you just don't need an extra tool to form the tucks.

That's my story, and I'm sticking to it!

Tim D.
This is what I show on my DVD.

I am not saying that they are not both methods of shrinking but one is better for some jobs and the other is better for others.
If you want a panel with a flat surface that then curves down say, using a stump would not be the best way to shrink. I use buth methods on the same panel sometimes. But I dont use a fork to do the tuck most times.
Tuck shrinking is basicaly raising while working on the stump is hollowing, blocking or bossing depending what tool you are using.
These are the traditional terms for the techniques.
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  #22  
Old 09-02-2009, 12:14 PM
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Johnny i use the kirk style hammer on the little tuck's i get on the edge of a pannel that seem to fold over on themselves when i hit them into a stump not the nice big ones that smash down dead easy with mallet.
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  #23  
Old 09-02-2009, 02:10 PM
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I still contend that stretching does NOT cause tucks or ruffles to form - drawing causes that. You mught get some stretch during the drawing, but the stretching doesn't cause the tucks to form.

Almost any form of shrinking will have some stretch to go along with it. Planishing out those shrinks will definitely have a bit of stretch to go with it.

Tim D.

Last edited by tdoty; 09-02-2009 at 02:10 PM. Reason: Been awake and running for 22+ hours so far........I'm tarred!
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  #24  
Old 09-03-2009, 01:49 AM
David Gardiner David Gardiner is offline
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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'.

A pucker should never fold over itself, if this happens it is because you are not doing the process correctly. Several small 'tucks' one after the other will net you the same result as one large one and you will not get the problem of the metal folding over on itself.

David
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  #25  
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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  #26  
Old 09-03-2009, 06:53 AM
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Marty Comstock Marty Comstock is offline
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I have used a doorskin hammer with good sucess. Works wekllif you dont mind the marks, very agressive. I have tried but not totally mastered Kirk's method, it is very controllable and systematic. A light hammer and precise blows work well with this one. Kirk has given me his permission to make and sell his style hammer, but other projects are in front of that.

as for one vs the other? Like has been said before, if you need shrink close to a flat panel area stump shrinking isnt the way to go, and generally I use whatever is either nearest or whatever will accomplish the task at hand in a reasonable amount of time. I used to throw dies at the pullmax, now I use the recip machine Kerry made for us. If that fails me for whatever reason, I go to the tucking forks.

I bet there are folk out there that can and do use the stump on everything, and may even do it faster than my machines. Matter of fact, I can use the doorskin hammer and hand tuck faster than a recip machine, but the marks it leaves are of a very agressive nature. for some parts, this isnt a problem, but for most, it very well would be too much.

I agree too, that most anytime you stretch, waves or ripples are started that would turn into tucks if kept at it, except for one process, a rubber / steel die in my pullmax or on a helve can stretch without making waves, its like the outer edges hold the material while the center stretches to the form of the die.

Marty
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  #27  
Old 09-03-2009, 08:15 AM
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OK, I'm confused Tim...I wrote:

Quote:
Originally Posted by Kerry Pinkerton View Post
I agree. Anytime you hit a panel, you're pushing the metal in the path of least resistance. If you're over a hole in a stump, the subsequent stretch is what causes the tuck to form, which can then be shrunk. Since most panels are made by both stretching and shrinking it works fine.
Then you said...(Pulled from a couple posts here)

Quote:
Originally Posted by tdoty View Post
...I disagree with that and have for a long time.
......

I still contend that stretching does NOT cause tucks or ruffles to form - drawing causes that. You mught get some stretch during the drawing, but the stretching doesn't cause the tucks to form.

Almost any form of shrinking will have some stretch to go along with it. Planishing out those shrinks will definitely have a bit of stretch to go with it.

Tim D.
Are we debating semantics here?

If I changed this phrase:
Quote:
the subsequent stretch is what causes the tuck to form
to this:
Quote:
the subsequent movement of the metal is what causes the tuck to form.
would you still disagree?

Or perhaps this:
Quote:
the subsequent increase in surface area in the impact zone 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??
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  #28  
Old 09-03-2009, 09:34 AM
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Quote:
Originally Posted by Kerry Pinkerton View Post
If I changed this phrase:

to this:
Quote:
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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  #29  
Old 09-03-2009, 09:46 AM
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Quote:
Originally Posted by tdoty View Post
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.
If you had a circle of 12"dia and only wheeled inside a 6"dia, then tucks would form.

Most of the time, when a bowl is made by e-wheel use only, the operator would wheel to the edge, creating a even dispersment of stretch, little at the edge, gradually increasing to the center.

Stretching can cause tucks to form, depending on the method used.

Marty
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  #30  
Old 09-03-2009, 09:53 AM
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Originally Posted by tdoty View Post
There we go! A lot of what is referred to as stretching involves very little stretch and a lot of drawing....
But when you get right down to it, isn't drawing in fact stretching? Just not stretching IN the impact area but rather around the impact area.
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