Small Shaft Straightening with Speed Lathe (?)

This guy Kev · Jul 31, 2019

Hey Gents,

I work in a shop that works with various forms of SS almost exclusively. We specialize in drills, small shaft, and cannulated devices. Much of what we do is below .125 OD.

We have recurring rework issues regarding runout. Right now, they use the old dial indicator and press method to rework, which takes up tons of man hours.

Anyone have experience with using a speed lathe to work out the runout? I remember seeing an old-timer years ago correcting runout by applying pressure perpendicular to the shaft as it spun in a chuck. Many times, they just used their thumb to apply the pressure, given the small OD.

Any feedback would be greatly appreciated!

sfriedberg · Jul 31, 2019

Did your old-timer have the tail end of the shaft captured by a tailstock center, or was the shaft free at the tail end?

This guy Kev · Jul 31, 2019

No sir, no tail stock used, just free. We're talking 3-5" lengths.

Milland · Jul 31, 2019

This guy Kev said:
No sir, no tail stock used, just free. We're talking 3-5" lengths.

I (being an old-timer now) used to and still do that, but generally not just using the thumb. The principal is taking the stock to yield with deflection, but supporting it such that as you reduce deflection you are releasing the deformation at the same time. It's easy to do if you know the tricks, and is a process best demonstrated.

You also don't do it when the stock already has features cut into it, straighten the clean/smooth stock first, then cut your features (unless you really don't like your fingers).

And yes, longer pieces must be supported, either by hand or fixture to prevent whipping and subsequent cursing. High speed is not needed, by memory I use around 200-400 RPM.

Where in MA are you? I could be cajoled into a demonstration with suitable inducements (food usually works).

FredC · Jul 31, 2019

Had no idea you could straighten under power. We straighten small assemblies to less than .001" TIR by indicating the high spot and applying pressure. The straightener in the photo is a Harig spin fixture modified with an off set crank handle and slide made from a Taig micro lathe. We use Hardinge accuracy collets to hold the work.

FredC · Jul 31, 2019

Milland said:
Where in MA are you? I could be cajoled into a demonstration with suitable inducements (food usually works).

Make a youtube video and paste a link here. I know I will try and send you something and maybe others also.
Maybe you could have a sign posted in the video "will teach for food"

Milland · Jul 31, 2019

FredC said:
Make a youtube video and paste a link here. I know I will try and send you something and maybe others also.
Maybe you could have a sign posted in the video "will teach for food"

I have a face made for radio, so a video would be traumatic for me and my audience. Besides, I'm fat enough, if this caught on I'd start looking like Mr. Creosote.

spaeth · Jul 31, 2019

Kev,
Worked in a shop years ago where they had a guy who took care of straightening small shafts. He used a beat to death SB 9" lathe and two sticks of hardwood. The wood was about 3/4" square and 10" long. He had a piece of leather attached to one end of both pieces to make like a hinge. It had worn different diameters into the wood from use. He had several of those and really didn't want anyone messing with them. He never shared his secret methods, it looked simple and his results were always good. I could never get it to work as well as he did. Spun the shaft slow, a drop of oil on the wood then he would squeeze the shaft between the sticks and pull the thing along the shaft and repeat as needed. Not sure how close they were when they were done but way straighter then when he started. Lost art I suppose.
spaeth

Nmbmxer · Aug 1, 2019

I keep a bearing on a QC toolpost for bumping stock in, mostly to get the far end running true -- not trying to bend it.

I've also seen a hardwood stick used like a spinning tool to apply side pressure to the stock as it spun fairly slow.

Same process to centering a wiggler, move it gently just past center and then back off. For the same reason I don't use my thumb nail to center a wiggler I wouldn't straighten stock by hand, too easy for it to jump off center and slice you up.

moonlight machine · Aug 1, 2019

spaeth said:
Kev,
Worked in a shop years ago where they had a guy who took care of straightening small shafts. He used a beat to death SB 9" lathe and two sticks of hardwood. The wood was about 3/4" square and 10" long. He had a piece of leather attached to one end of both pieces to make like a hinge. It had worn different diameters into the wood from use. He had several of those and really didn't want anyone messing with them. He never shared his secret methods, it looked simple and his results were always good. I could never get it to work as well as he did. Spun the shaft slow, a drop of oil on the wood then he would squeeze the shaft between the sticks and pull the thing along the shaft and repeat as needed. Not sure how close they were when they were done but way straighter then when he started. Lost art I suppose.
spaeth

That's total B.S. I will teach anyone what I know. Sure, I figured out a few things myself But a few guys taught me a lot, One in particular was just magic, that guy knew EVERYTHING! and he loved to pass it on, every day was another learning experience. Denny Hein where are you? He gave me a machinist handbook, it is one of my most cherished possessions.

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