#1
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The effect of shrinking across the width of a gouge
In Robert Sargent's 2nd edition of Automobile Sheet Metal Repair , the third paragraph on page 111 states:
"The effect of shrinking across the width of a gouge is greater than it is lengthwise. Unless the shrinking operation is controlled closely, it will tend to affect length and width uniformly. The result is that the length is overshrunk as width is reduced to the proper dimension" Doesn't the first sentence contradict the third? If shrinking affects width more than length, won't the width be reduced to the proper dimension before the length?
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Mike Schweitzer Erie, Colorado "You can never stop a body man" - Roy Winterhalder |
#2
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Quote:
His premise is correct, that the width shrinks more readily than length, but the subsequent statement reverses "width" and "length" to successfully oppose his premise. Good catch.
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Kent http://www.tinmantech.com "All it takes is a little practical experience to blow the he!! out of a perfectly good theory." --- Lloyd Rosenquist, charter member AWS, 1919. |
#3
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Thank you Kent for your reply!
Just as the most pious pilgrim daily reads their Good Book, I study Mr. Sargent's text. I also study your posts to this board as well So this paragraph should read: "The result is that the WIDTH is overshrunk as LENGTH is reduced to the proper dimension. Overshrinking the WIDTH of a gouge will have a drawing effect on the metal on both sides of it." Okay, now this makes sense! Putting this advice into practice, it might read "shrink to properly restore the contour length-wise, then stretch to restore the contour width-wise" This seems to be a corollary to, or simply another example of, the general statement that metal should be stretched immediately after it has been shrunk.
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Mike Schweitzer Erie, Colorado "You can never stop a body man" - Roy Winterhalder |
#4
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I admire the study of theory in complex details…
But in practice it does horrible things to me. A small note, do you understand paper templates? It is absolutely essential knowledge.
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Jaroslav |
#5
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Quote:
As for my "rule" on this, I let the hot metal cool to "uniform temperature over the whole panel." Not slightly warmer in one spot. The senior craftsmen I trained with - who did "roll-jobs" in the 1930's and 1940's - would use sunlight heat to remove hail dents. This was instructive to me. Measuring the residual shrinkage during the last 150deg F fall is non-trivial. ---Especially on flat panel work. When all the shrinkage is accounted for, then I know whether to shrink more, leave it be, or stretch it back a little. The most difficult jobs show how careful your practice has to be..... and "difficult" seems to keep right on a-goin" .... along with our experience.
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Kent http://www.tinmantech.com "All it takes is a little practical experience to blow the he!! out of a perfectly good theory." --- Lloyd Rosenquist, charter member AWS, 1919. |
#6
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Quote:
I guess it's time to take them off of the peg board and put them to work. I suspect routinely checking the contour that I'm busy restoring, against an unmolested original, will show me right away where the problems are. This I can easily do, as I have a perfect original to check against. I am working on a very flat hood that appears to have had something fall on it, bending the top skin against the unyielding, rigid hood frame, and giving me a nice bend to remove. I have successfully removed this bend and am now shrinking the general area, which remains slightly higher than the correct contour. My shop has great lighting (thanks to the Colorado sun and all the windows), plus I've oriented the hood so that the reflections show the issues to be addressed. As I shrink and planish (on-dolly, light upward pressure, light weight slapper, shrinking disk), working the metal hot as Ken often advises, these issues become less and less, thankfully, so I know I'm going in the right direction. It's the last 2% of the job that has caused me to slow down and spend more time thinking about my next move. As I sneak up on making this hood perfect, I'm going through the beginner's problem of whether to shrink or stretch. I can see now how profile gauges would help answer these sorts of questions, beyond simply sight and feel.
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Mike Schweitzer Erie, Colorado "You can never stop a body man" - Roy Winterhalder Last edited by schwemf; 04-30-2023 at 12:28 PM. |
#7
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Jaroslav, you were so right about regularly checking the profile. This weakness in my technique almost got me into more trouble, as the area that I thought needed shrinking was actually too low. It feels high, it looks high, but the profile gauge tells a different story.
Kent's comment about proceeding carefully, especially on difficult jobs, was timely! I should probably start a new thread so that this good advice doesn't get missed by those not interested in typos in old text books!!
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Mike Schweitzer Erie, Colorado "You can never stop a body man" - Roy Winterhalder |
#8
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You should post a few pictures of what your working on.
Visual enhances written.
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Dave Bradbury |
#9
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Hi Dave,
I'm starting a new post under the title "Metal finishing a low crown hood", as I've wandered a bit off of the original topic. Thank you for your interest!
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Mike Schweitzer Erie, Colorado "You can never stop a body man" - Roy Winterhalder |
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