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Spinning Lathe round dimensions

jfenwick

Plastic
Joined
May 4, 2015
Is there a good way to calculate how big your round of sheet metal should be to create a form on a spinning lathe?
I was hoping there would be a Solidworks tool that could unfold the correct shaped dimensions of material but so far I haven't found any sort of CAD way to do it. How did people figure it out back in the day?
 
Back in the day? Trial and error has solved problems for thousands of years. I only know the tiniest bit about spinning, but I seem to recall that forms were frequently trimmed after shaping. That would suggest to me that it may not be easy to set a precise blank diameter and spin it into a finished product right on the money.

I don't have Solidworks experience but I'd be surprised if there wasn't some way to develop a length of a contour for a first approximation. Can you put a spline on an irregular shape and then read out data on it?
 
I don't use Solidworks, but Rhino 6 and Rhino 7 have these menu choices:

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You can download Rhino 7 as a free trial and see if it works for you, or post some dimensions and I'll try it.
 
I found the flatten surface command in Solidworks. It's seems sort of cantankerous though, not sure if it's giving me the results I want.
It seems like guessing and trying it and then trimming is the only way so far.
 
Different operators will stretch the material more or less.

Yeah, depending on the shape you can get quite a variation in the amount of material left over after a part
has been spun. There's a metal spinning place near us that regularly uses simple cardboard templates to
start the process. Just make sure your first blank has excess material and trim to suit. After 3-4 parts
you'll have a pretty good idea of the optimum size for your blanks. It ain't rocket science. Too many
youngsters today figure that you've gotta do everything with a computer--sometimes the "old-fashioned"
way is the simplest...
 
Try this to see what you get:

Let Solidworks calculate the square inches of the part.

Divide that by pi.
Then find the square root.
The answer is the radius of the disc to start with (depending on the ratio of part stretch compared to compression and what you factor in for variance).

If you were deep drawing instead of spinning, I would use cubic inches.
 
Correct me if I'm wrong but isn't the important part of spinning to only stretch the metal and avoid trying to compress it or you will get wrinkles? From the few videos I've seen the blank diameter was only marginally larger than the final product diameter, and the depth of the bowl came entirely from the material stretch.
 
Correct me if I'm wrong but isn't the important part of spinning to only stretch the metal and avoid trying to compress it or you will get wrinkles? From the few videos I've seen the blank diameter was only marginally larger than the final product diameter, and the depth of the bowl came entirely from the material stretch.

OK, I'll correct you. If all you do is stretch the metal it will tear part. A good spinner compresses the metal to retain wall thickness and void tearing.
 
The extra material moves at right angles to the wrinkle. The direction that I would describe as "radially" where in not for the fact that it becomes axial as the work wraps around the chuck.

That's why it's diam+ depth not diamete+2depth
 
Stanley and Collvin in American machinists handbook calculate the size of the blank by the volume of the finished part. They also day it's a "try it and see" process which will change with the operator.
 
Stanley and Collvin in American machinists handbook calculate the size of the blank by the volume of the finished part. They also day it's a "try it and see" process which will change with the operator.

This is what makes the most sense to me. use a 2+ pound disc to make a 2 pound part. Or, a 30+ cubic inch disc to make a 30 cubic inch part.

It is hard to know exactly how many pounds or cubic inches are in a part that hasn't been made yet, so to me square inches is the way to go for the first off (If we are still talking about sheet metal).

Metal spinning while compressing (warning: loud, obnoxious music):
 
Cool video, clearly compressing the metal! Did anyone else get seasick watching it?

Is the flame for annealing or to keep the aluminum at an elevated temperature where it's easier to form?
 








 
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