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  #31  
Old 09-03-2009, 09:56 AM
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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.
The tucks are cause by the excess being pushed out into the rest of the panel. That is really thinning, and not stretching per se. However, it is close enough for this example.

Now, if we narrow it down into terms of stretching is an increase of the surface area combined with thinning of the base thickness of the stock, then, yeah, I concede the point.

However, I tend not to narrow things down so much.

To my mind, stretching would be moving the metal's molecules further apart while drawing as little as possible into the area of the "stretch" and not pushing them out into the rest of the sheet either.

Maybe I'm just thinking too much?

Tim D.
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  #32  
Old 09-03-2009, 10:04 AM
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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.
Textbook definition or just a "sort of" kinda close enough definition? In most all of the books I've been reading, drawing is a deformation process - form, not shape. In terms of drawing, the area of the blank is at least functionally the same as the surface area of the resulting piece. Same surface area means same shape, right? Just a different form.

The formulas change depending on the ratio of height to diameter and whether or not you want to significantly change the stock thickness. Below a ratio of about 5:1 (height versus diameter), the blank should have the same area as the desired result. The wall thickness will also be at least VERY close to the original stock thickness.

Tim D.
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  #33  
Old 09-03-2009, 12:58 PM
David Gardiner David Gardiner is offline
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Tim, When a piece of metal is hit over a hollow or a bag of sand that has 'give' it stretches, I think we all agree with that. The edges also pucker so therefor these edges are being deformed and not drawn in the way you discribe.
In the photos you posted of the small cap parts the edges could not pucker because they were being constrained by the tooling. If you cut the blank so that it is larger than the final part requires and form that on the same tooling so that the edges are not constrained you will find that you get puckers. the tooling is basicly doing what I show on my DVD when I show how to shrink a flange without a machine.

I also have done a small amount of studying on drawing parts (when I did my diploma) I dont claim to be an expert but I learned the basics.

The three terms I mentioned earlier are the traditional terms used in coachbuilding (what you call metalshaping - for cars) each has a slightly different meaning, after all a hammer could mean anything but if I say to you a planishing hammer you have a much clearer idea what I am talking about. All the processes have different names, this is an ancient craft as you know and I would not want to lose the quirks and traditions that go with it. I dont see anyone on these sites using the accepted names for the common dollies that are used I wonder if you use these names? such as the heal dolly and toe dolly, or the universal dolly.

Most trades and crafts have weird and wonderfull names for tools and gadgets over here.

David
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  #34  
Old 09-03-2009, 01:29 PM
52pickup 52pickup is offline
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if it makes you feel better, David, I know and use those names for dollies... even when I'm talking to myself

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In terms of drawing, the area of the blank is at least functionally the same as the surface area of the resulting piece. Same surface area means same shape, right? Just a different form.
This is true, but you are comparing the die (which is the final shape) and the final sphape of the piece. To compare if there was any shrinking, you should be comparing the surface area of the blank you started with vs. the surface area of the finished part. I doubt you will find that they are the same. If they were, then on those little pucks you made, you should be able to cut out a piece of paper the same size as the blank you started with, and that piece should fit the puck without any folds, right? If not, there was either shrinking or stretching, or both, involved in making that piece.

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To my mind, stretching would be moving the metal's molecules further apart while drawing as little as possible into the area of the "stretch" and not pushing them out into the rest of the sheet either.

Maybe I'm just thinking too much?
I think you are looking to deep into it. As far as metalshaping is concerned, I would say any process that has an end result of reducing the thickness of the metal would be called "stretching", because as you are expanding the surface area of the material, not because of what is happening at a molecular level.

Think of it this way, if you put a panel in your wheel and went over the entire 6" x 6" panel, evenly, with enough pressure, you would end up with a slightly larger panel... granted, it may only be 6.05" x 6.05", but you would have a panel that is thinner, but has a larger footprint.
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  #35  
Old 09-03-2009, 02:25 PM
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Originally Posted by 52pickup
To compare if there was any shrinking, you should be comparing the surface area of the blank you started with vs. the surface area of the finished part.
blank diameter.jpg

Yup, that is the idea. Comparing the die and the final shape of the piece? No, comparing the blank and the final piece.

David, I know about the heel and toe dollies. The origin of their names lies with the cobbler. The universal dollie, the comma, etc.

Tim D.
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  #36  
Old 09-03-2009, 03:17 PM
David Gardiner David Gardiner is offline
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Tim, to draw a piece like you have shown, some parts would have to stretch and some shrink otherwise how does the metal take on the shape?.

You did not comment on what I said in my last post about, the edges puckering when a piece is hollowed out over a sandbag or stump. The fact that the edges pucker shows that the metal is deforming and not being drawn as you sugest it is. (if you take the whole hand forming process it can be compared to the drawing process in slow motion I think)

Glad to know that the names for the various dollies is the same there as they are here. Why do you not use the time honoured names for the various processes? I am sure you must have heard of bossing mallets and hollowing hammers, blocking hammers and dinging hammers ect.
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  #37  
Old 09-03-2009, 03:43 PM
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Originally Posted by David Gardiner View Post
Tim, to draw a piece like you have shown, some parts would have to stretch and some shrink otherwise how does the metal take on the shape?.
But the surface area stays the same. You brought up press drawing before I did. When I initially mentioned drawing it, it was, in essence, "borrowing" - the same way a debit card draws from your checking account. Two definitions of the same word as applied to sheetmetal work. Intermingle the two, and confusion is sure to result.

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Originally Posted by David Gardiner View Post
You did not comment on what I said in my last post about, the edges puckering when a piece is hollowed out over a sandbag or stump. The fact that the edges pucker shows that the metal is deforming and not being drawn as you sugest it is. (if you take the whole hand forming process it can be compared to the drawing process in slow motion I think)
Different type of drawing, actually. If the edges were constrained to limit the amount of material drawn (borrowed) from the rest of the sheet, there would be much more stretching and much less deformation for the same change in surface area. Since the tucks are formed from deformation, not stretch, they would be far fewer in the above example. Sure, holding the edge would prevent them from forming, hold with enough pressure inside the edges, and the results would be similar.

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Originally Posted by David Gardiner View Post
Glad to know that the names for the various dollies is the same there as they are here. Why do you not use the time honoured names for the various processes? I am sure you must have heard of bossing mallets and hollowing hammers, blocking hammers and dinging hammers ect.
Well, if I used my bossing mallet for blocking, would that be wrong?

Would it be wrong to use a dinging hammer for raising? What about using my raising hammer for dinging?

How much should I radius the edge of my planishing hammer, or should I in fact leave it square? Should I make a stiff frame for it or just keep the wooden handle?

Why don't I use more of the traditonal terms? Because the sources I have read from have often had differing or confusing names for the tools and no one has taken the time to explain the names and uses and even the processes they are used for in a way that I could comprehend and retain.

I can find all kinds of information regarding presswork and other types of sheetmetal forming infomation from even before the turn of the 20th century. The panel beater's trade was treated as a black art and deep secret, even then. While the language of some types of sheetmetal work has not changed and the math is the same as well, the language of the panelbeater, ne metalshaper, has evolved (or devolved, depending on your view) over the years. Witness the assertation of many in the past year or so that any machiine which moves metal by striking it with a die is a power hammer and the fact that a Pullmax is oft referred to as a power hammer by these same folks. In many cases, those folks are the ones disseminating the information to people formerly unaware of, or new to, "our" craft.

Only so much room in my little bitty brain to retain all this stuff. Eventually things will start draining out, leaving, perhaps a bit of room for the expansion of whatever is left.

Tim D.
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  #38  
Old 09-03-2009, 03:52 PM
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Oh, and I didn't write the example with the picture on how to determine the blanks size for drawing a shell - Franklin D. Jones did, in a book published in 1923 (first edition of the 5th printing, copyright 1915). Just one of the texts I've reread lately.

Tim D.
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  #39  
Old 09-03-2009, 04:12 PM
David Gardiner David Gardiner is offline
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Tim I explain and show many of the traditional tools and methods on my DVD. I show how and why many of the traditional tool are used. This is one of the things I wanted to put over and preserve.

I give up on the drawing/ shrinking/ hollowing thing my brain hurts! you seem to be saying two different things.

David
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  #40  
Old 09-03-2009, 04:31 PM
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Originally Posted by David Gardiner View Post
I give up on the drawing/ shrinking/ hollowing thing my brain hurts! you seem to be saying two different things.

David
Two different conversations about two different subjects. Drawing is drawing, but drawing isn't necessarily drawing. Isn't the English language so much fun?

Not to worry, David, I think I confused myself at one point, so I kinda tried to untangle things. Don't know if it worked or not.

Tim D.
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