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  #11  
Old 04-11-2020, 09:26 AM
Marc Bourget Marc Bourget is offline
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The Tinman, aka "The Tinmeister" has earned the right to be aggressive in his teaching methods.


Despite that, he shows much grace by his reply.



Truth be told, I was probably being a little selfish/lazy in posing my question.



Forgot to consider - what I feel is one of his best attributes, the visibly "fierce" concentration he brings to his tasks.


Properly chastened and motivated, I increased my concentration and, best lesson ever, developed the answer by observation. "Meister instructor" is he!


Onward and upward
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  #12  
Old 02-12-2022, 02:42 PM
Denny Graham Denny Graham is offline
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Kent, I've been beating my self to death, or should I more correctly say, beating a $200 sheet of .063" 3003 to death for the last two months in a futile attempt to form an extreme reverse curve in the upper cowl of a 3/4 scale Miller 91 Indy car that I'm building the body for. I have an air hammer, I have an e-wheel, I have many hand tools, that is, slappers, hammers, mallets, stumps, bags. I do 'not' have a Pullmax or any sort of expensive power hammer, etc. with linier stretching or shrinking dies.
The discussion in this thread seemed to be about as close to the shape that I'm looking for as it is very similar to a Hyperbolic paraboloid, which was mentioned in one of the earlier replies.
This saddle or potato chip or reverse curve shape obviously is one of the hardest to master as there is NO, NOTA, ZIP video taking you thru the process. Lots of metal shapers get you started on the basics for a 'shallow' reverse curve, but I have yet to see anyone complete the shape for an extreme reverse curve. Having been an EAA member for many decades, I've seen this shape applied to many wing farings where the wing meets the fuselage. Many of the 20's GP cars used flared cockpit cowl's around the steering as did most of the 30's thru the 60's midget, sprint and Indy dirt cars. Any advice would be greatly appreciated.
dg
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  #13  
Old 02-12-2022, 03:51 PM
Jaroslav Jaroslav is offline
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Denny, this exercise is good for understanding the basics. Make this thing without a hammer - only with EW and jaws for shortening and stretching the material. Caution, you must not use a hammer.
DSC03134.JPG

DSC03128.JPG

DSC03127.JPG
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  #14  
Old 02-12-2022, 07:46 PM
Denny Graham Denny Graham is offline
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Riiiiight Jaro!!!! When I can't even get the top curve to a 15" wide cowl that's
12" deep with a 14" radius after literally two months worth of trying, you
honestly think I would try something like that???
Thanks but I'm still at square one.
I think the main problem I'm having is the material needs to be shrunk a LOT in the
middle of the panel where it kicks up. I don't have any fancy Pullmax or tools like that
which would let me shrink the center. I've tried bending the panel, clamping it to the
table and with torch heating the center hump and hammering it down, but that doesn't
seem to be enough.
Upper Cowl Panel on Wire buck.jpg

Upper Cowl Panel on Sub-Frame.jpg

My E-Wheel.jpg
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Last edited by Steve Hamilton; 02-12-2022 at 11:26 PM.
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  #15  
Old 02-12-2022, 08:48 PM
blue62 blue62 is offline
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Quote:
Originally Posted by Denny Graham View Post
Riiiiight Jaro!!!! When I can't even get the top curve to a 15" wide cowl that's
12" deep with a 14" radius after literally two months worth of trying, you
honestly think I would try something like that???
Thanks but I'm still at square one.
I think the main problem I'm having is the material needs to be shrunk a LOT in the
middle of the panel where it kicks up. I don't have any fancy Pullmax or tools like that
which would let me shrink the center. I've tried bending the panel, clamping it to the
table and with torch heating the center hump and hammering it down, but that doesn't
seem to be enough.
dg

I looked at some pictures of 1929-1930 Miller 91 Indy cars.
If your trying to create the reverse that is at the steering wheel end of the panel then you need to stretch that end.
It is just like the reverse on some old motor cycle rear fenders.
Stretching the end and down the sides a short distance will cause the metal in the middle at that end to go hollow that is the beginning of the reverse. Keep slowly stretching don't touch the middle where you want the metal to go hollow (reverse). There is no shrinking involved in creating that reverse.

No need for for a pullmax or a torch or any fancy tools.
All you need is a blocking hammer a stump and perhaps a sand bag. An English Wheel would be a plus for smoothing it up.
Get some masking paper or brown kraft paper and use it to make a paper pattern of the area in question.
When a paper pattern is made of the area "correctly" it will tell you exactly where you need to stretch and how much.
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Last edited by blue62; 02-12-2022 at 10:41 PM.
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  #16  
Old 02-13-2022, 02:28 AM
Jaroslav Jaroslav is offline
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Yes, as Dave said. Make it out of paper, then cut it so that it is straight. You'll see what to do.
In your case, you're stretching the edges. You must not touch Wednesday.
Make a narrower attempt to see it better.
You need EW and maybe jaws. A hammer? Careful .... Skip the center. Don't touch him at all. He bends himself.
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  #17  
Old 02-13-2022, 02:25 PM
crystallographic crystallographic is offline
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Hi Mr. Graham,
...... okie dokie, catching up with the missed bus here ....


As Jaro sez: " paper pattern...."
The patterns come in varieties in my shop, and here is a "metal explanation" - (but this is only what I do and teach, up in the mountain woods ...)



W_R finger pattern c.jpg
One finger fit, wingroot fairing copy.jpg

"work edges...."
Wednesday is day off..."
P1120365 copy.jpg
P1120369 copy.jpg
P1120370 copy.jpg
P1170018 c.jpg


or
.....


AH fairing dies +saddle2.jpg
P1010186.jpg
P1020859 copy.jpg
P1050830 c.jpg
C swoop D copy.jpg


Might notice that I am not using Eckold, Pullmax, Wheel for these easy shapes ....
But .... but I do teach other methods, when it is correct/appropriate to do so.
H1 over mountains.jpg


This is just a brief sample, to get your mojo going.
Success and Joy!
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"All it takes is a little practical experience to blow the he!! out of a perfectly good theory." --- Lloyd Rosenquist, charter member AWS, 1919.

Last edited by crystallographic; 02-13-2022 at 02:48 PM.
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  #18  
Old 02-13-2022, 03:05 PM
blue62 blue62 is offline
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Quote:
Originally Posted by crystallographic View Post
Hi Mr. Graham,
...... okie dokie, catching up with the missed bus here ....


As Jaro sez: " paper pattern...."
The patterns come in varieties in my shop, and here is a "metal explanation" - (but this is only what I do and teach, up in the mountain woods ...)



Attachment 62797
Attachment 62801

"work edges...."
Wednesday is day off..."
Attachment 62788
Attachment 62789
Attachment 62790
Attachment 62791


or
.....


Attachment 62792
Attachment 62794
Attachment 62795
Attachment 62796
Attachment 62793


Might notice that I am not using Eckold, Pullmax, Wheel for these easy shapes ....
But .... but I do teach other methods, when it is correct/appropriate to do so.
Attachment 62802


This is just a brief sample, to get your mojo going.
Success and Joy!

Kent,
In the series of four pictures of the panel with the writing on it.
Is there any "shrinking involved in that panel?
By the way You always have the greatest pictures
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  #19  
Old 02-14-2022, 04:47 AM
Blackfinger Blackfinger is offline
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Think its opposite to crowning a panel where you stretch the middle and shrink the edges.I think Lazze has a video of doing a basic reverse curved panel.
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Last edited by Blackfinger; 02-14-2022 at 05:03 AM. Reason: spelling
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  #20  
Old 02-14-2022, 11:25 AM
BTromblay BTromblay is offline
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Hi,

I have struggled with reverses as they are very common on airplanes, here is several things to consider.

When developing a reverse, the edge must become thinner that the middle of the panel. We can do this by shrinking the middle and stretching the edge. Shrinking of the middle is the most challenging of metal shaping processes, for some reverse's, you can get a way with only stretching the edge. What is critical, is not stretching at all in the middle or what we call the "No Blow" zone. Only extremally light planishing can occur in the middle or you will take the shape back out of the panel, if you can avoid planishing this area, the better off you are. The "No Blow" zone can be found with a paper pattern, where the cut in the paper ends, is the start of the no blow area, in basic terms. For the work that I do on aircraft, material thickness can be critical, so I can't always stretch the edge into oblivion to develop the reverse. I use a Anticlastic stake and shrink the middle first, then go onto the edge stretching. By doing this, I can create a tight reverse and only loose several thousand of an inch in material thickness. I can post pictures, but if you look up "anticlastic forming" on Youtube you will get the idea.

All of this can be done with basic tools and it appears to me that you would not need to shrink the center as your reverse is not that tight.

I'm in Mukwonago Wi, if you need help.

B
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