Straightening spindle via peening

lrak · Apr 26, 2020

Not too long ago, I had to straighten a motor shaft - bent where the shaft left the motor - Shaft stuck out about 100mm[4"] and it it was out 0.5mm[.020"] at the end. I thought of doing flame straightening - tried it on some scrap material - it sort of works but easy to over shoot and hard to fine tune. Then I remembered an exceptional tool maker I knew back in the '70s - one day he was tweaking a shaft via peening - it stuck in my mind.

It was hard to find the info on that process - for a shaft you want to form a tool with a grove that fits the shaft - rounded polished edge. I set the bottom of the shaft on some copper - and peened - took a while. If I did it again I would use a press and get close - then peen the rest of the way. The key is I could dial in about .002mm[tenths] - much more accuracy than I needed, I was surprised what was possible. Once I finished I tapped the end for a while to see if it would move - applied a bit of heat - it stayed put.

I have a spindle that is just a tiny bit out - so in the back of my mind, I've wondered if I could set it up on rollers - indicate - map out where the ding is and peen it straight? Different metal than a motor shaft - but it might work? so I'm wondering if anyone here has wondered down this road? (There was an old thread here about straightening a spindle - mentioned flame straightening, press straightening - but nothing this delicate.)

michiganbuck · Apr 26, 2020

We, I straightened hard spline broaches with a small air hammer and round nose chiseis. we would map the error with tenths indicators and hammed small dents into hard tool steel and return them to straight.
the dents would cause the material to expand so they were applied to the hollow or concave side of the error.

Ken hosford · Apr 26, 2020

Motor shaft will typically have a larger bearing than the out put shaft leaving a shoulder you peen this in the corner on the smaller dia section using a rounded cold chisel
you straighten crankshafts this way
I straightened d2 punches for my nibbler this way no shoulder on them though

steve45 · Apr 26, 2020

It's amazing, but they use the same technique for straightening crankshafts on large industrial engines, like Coopers and Clarks.

johansen · Apr 26, 2020

Ive heard that bent shafts want to be straight and i never believed it until i had a bent motor shaft. 5/8 diameter sticking out 2 inches, but the motor bearing was a 6205 and the shaft reduced.. So i figured it was safe to give a strong blow from a hammer and bloxk of wood.

Increased the blow until it yeilded and.. Zero runout in one blow.

Motor shafts are uaually soft. If it got bent and didnt damage the bearing, you can bend it back the other way. Also bearings are cheap. Just replace them after straightening the shaft.

wesg · Apr 27, 2020

I have a piece of hollow Thompson shaft that was a bit crooked. Put it high side up and gave it a good whack with a dead blow mallet. Dead straight. May have just got lucky.

I've heard of bent shotgun barrels being smacked into a telephone pole.

lrak · Jun 3, 2020

Thanks for verifying my hunch.

I'm puzzled that I've not seen videos etc where someone straightened a spindle via peening - lots via grinding, but there are problems with grinding ( some spindles - like kwik-switch are not really fixable with grinding ) . I think peening spindles straight might be sort of a secret sauce?

kustomizingkid · Jun 3, 2020

I remember years back Archie Cheda talked about how peening to a mill table could warp it, something I found beyond fascinating.Not totally relevant but shows the power peening can have.

Starts at post 200.

Lucas Horizontal Boring Mill going to Tuckahoe . . .

michiganbuck · Jun 3, 2020

You can sometimes straighten with putting a part/shaft in an arbor press over two risers and push it opposite the error a controlled amount .and add to that amount only .015 or so with each push. And test it after each push.
I have straightened hard parts the way. But care taken because pushing a little too far and the part/shaft may break.
Learned that from a journeyman black smith.

PackardV8 · Jun 4, 2020

Cast iron has the incorrect reputation of being brittle and impossible to straighten. Tell that to the camshaft grinders who remove the finished shaft from the grinder, put it between centers and bring it true with a couple of expert taps with a square nose chisel and a 1# drilling hammer. Crankshafts with a very slight bend can be straightened the same way.

And yes, really bent cranks are straightened on a crank press frame.

jack vines

MCritchley · Jun 4, 2020

PackardV8 said:
Cast iron has the incorrect reputation of being brittle and impossible to straighten. Tell that to the camshaft grinders who remove the finished shaft from the grinder, put it between centers and bring it true with a couple of expert taps with a square nose chisel and a 1# drilling hammer. Crankshafts with a very slight bend can be straightened the same way.

And yes, really bent cranks are straightened on a crank press frame.

jack vines

You would be surprised at how easy it is to straighten brittle cast iron. I didn't believe it until Rich king showed me how to bend a gib back before scraping.

Done it a few times,haven't broken one yet.

MCritchley · Jun 4, 2020

michiganbuck said:
You can sometimes straighten with putting a part/shaft in an arbor press over two risers and push it opposite the error a controlled amount .and add to that amount only .015 or so with each push. And test it after each push.
I have straightened hard parts the way. But care taken because pushing a little too far and the part/shaft may break.
Learned that from a journeyman black smith.

I have done quite a few precision shafts with this method,with patience you can get a shaft back to original within a few tenths. Motor shafts bend back in a predictable fashion, the only down side is that you must take the motor apart.

I think shaft peening while assembled might be somewhat bad on the bearings, specifically if they are of the rolling element type. You may end up brinneling the bearing races.

ratbldr427 · Jun 4, 2020

About automotive crankshafts I would not put any Tuftrided-Nitrided shafts in a press.Peening is much safer and not likely to crack the shaft.

pops2004 · Jun 5, 2020

For twenty years I worked in a electric motor repair shop. At least once a week we would have a motor come in with a bent shaft. I have straightened them with heat, peaning or what ever method would work. I do remember we had a 10,000 hp motor with a 12” diameter shaft that was bent .012”. I have no idea how it got bent but we did get it straight again with a little well placed heat.

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dian · Jun 5, 2020

ratbldr427 said:
About automotive crankshafts I would not put any Tuftrided-Nitrided shafts in a press.Peening is much safer and not likely to crack the shaft.

what do you peen that crank with so not to damage the hard layer?

Straightedge · Jun 6, 2020

If you tape off the critical surfaces, you can make things straight to tenths with a sandblaster.

pops2004 · Jul 10, 2020

Years ago when I was working I used to work at a motorcycle shop in Southern California that repaired wrecked bikes and also made custom choppers. We made “radius blocks”, as we used to call them that were two pieces of about 3” x 3” metal that were bored out to the diameter of the front fork tubes plus .005”. These were placed on two thick blocks on our press and an indicator was mounted under the tube to be straightened. That was easy to tell where the high spot was. Just keep pressing and rotating until it’s straight. I worked there for about five years back in the 60’s.

We had a lot of business from Evel Knievel. He was a regular at our shop.

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eKretz · Jul 10, 2020

dian said:
what do you peen that crank with so not to damage the hard layer?

You don't peen the working surfaces. Normally peening on a crank is done in the fillet radius area.

hobbyman · Jul 10, 2020

When I was a kid my neighbour told me of him being called out to straighten a shaft on a plate shear. It was WW2 and you could not get parts for anything so he went over with a ball peen and a hydraulic jack. He jacked on the shaft to give it tension and then used his hammer. The next day he had it running true again.
To this day I wonder if he peened on the short side to stretch it or peened on the long side to upset and shrink. Old Cliff is long gone.

lrak · Jul 10, 2020

Reading this thread got me digging into peening even more - a bunch of reading. it looks like they do CNC peening - impressive:
YouTube

They also do LASER peening - short laser pulse makes sort of an explosion in some water on the surface - compressive shock wave does the rest - some millimeters deep. Sets up compressive residual stress that makes a very tough part.

I had always thought of peening as a way to relieve welding stress (contraction force in tension) - hadn't realized it was used for so much more. Putting the surface in compression improves crack resistance - and fatigue life. Reminds me of the holes they drill in the ceilings of mines - then put in an expanding plug to put the surface into compression so crack won't form.

Anyway - this thread turned up some interesting tips and hints.

Straightening spindle via peening

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