Procedure for unbending an acme lead screw

Reddinr

Reddinr

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So I bought a cheap 3/4" acme screw for a project. Six foot long. $28. Of course, it came in bent, probably shipped bent. I can either hassle with returning it or I can try to unbend it. It is way beyond torch unbending method. About 1/3 has a longish bend in it and 2/3 is straight enough. The application will be a low RPM, low precision one. I just need it straight enough. Having tried unbending stuff before, I usually made matters worse, not better.

Is there a step by step process for clamping, pressing etc. For example, if the bend is an arc that is about 18" long, where would I want to put the fulcrums to press against? Or, am I just overthinking it (again). I'm thinking V-blocks with clamps would be good to use.
 
Find the center of your bend using straight edge and mark center and outer limits with a sharpie.

Put over V blocks or similar and gently press in center with even pressure. You will need to over bend as it will spring back, but go slow and don't deform it worse than it is. A block of wood or two of different lengths may help.

Take measurements via the old MK1 eyeball as you go keeping track of any smaller bends you may be inducing and act accordingly.

This process is slow and tedious, but your results will only be limited to the time you are willing to spend.

You can look up gun barrel straightening for a more complete overview.
 
I straightened my acme spindle mounted in the 3 jaw chuck and supported by a live center. I searched for the highest spot, heated that spot red hot using a propane torch and straightened the spindle by pushing a bit on that spot using a small wooden block. I repeated this for 2 hours and got a reasonable result.

I also tried once by heating the high spot red hot and cooling it on the opposite side using water. This is a very slow process and suitable for light bends. I didn't get the spindle straight enough because it looked like I was getting some cracks due to the fast cooling of the red hot steel.

I expect that all rolled spindles aren't straight and need some work.
 
gun barrel straightening! Sometimes, just knowing what to search on is key. Will look into that.
Good excuse to finally buy a couple of larger v-blocks that I've been wanting anyway. The price of the acme screw just went up...
 
Believe it or not I have used a tree to straighten rods and bend conduit. You need a good eye and a tree with a narrow Y at the base
Doesn't hurt the tree too much, but you could put some cardboard to cushion it. Sharpie also of course, and patience.
 
Yes, mark everything as described above. Then, in a repeatable setup, use some kind of press or lever to put force on the bent object. Use an indicator to judge how far to bend it. Guessing how much force isn't as easy to judge as watching the displacement. I saw a clip of Richard King helping a student straighten a gib this way and I've found that it works well. Also, there's a video of a guy who built a really nice brake. He eventually shows how to adjust the angle of a bend using the displacement of the brake. Again, it's measured with an indicator. Thousandths seems to be enough resolution for most work. (You'll find that archers are also quite adept at unbending metal tubes.)

 
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A suitable press, 2 V blocks and a bent shaft is all that’s needed, a dial indicator is a plus but not absolutely necessary if one is not readily available. small scraps of soft metal may help to protect threads being acted upon by hydraulic force
 
Ok. I've got an idea what to do now. I'm off to unbend something.

In an unrelated note, in my search for a quality spindle to replace my router I happened on these guys:
https://www.artcotools.com/ If I had a much larger wallet, I'd be shopping there quite a lot. They have good candy.
 
Well, all right then. It took about an hour or so and got it as straight as needed. I snuck up on the main over-bend, increasing the number of pumps of the press handle over a few iterations and got it 95% there. It took much more overbend than I thought it would. Then a tweak here and there and done!

Thanks for the advice. It was easier than I thought.
 
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