Flame straightening a shaft

[Pete O]

I tried something this afternoon that I have not done for about 25 years, using the oxyacetylene torch to straighten a shaft. The results were very confusing and I'd be interested in opinions as to why.
I set my mill up with the horizontal arbor for the first time yesterday and discovered that the arbor is bent- it's a 1" shaft about 20" long with a NT50 drive end. After discovering the shaft was bent, I put it between centres in the lathe and measured the bend with an indicator- the bend was right in the middle and there was about .037" bend, i.e. the indicator read .037" difference between two points 180 degrees apart around the centre of the shaft.

I put the shaft on some blocks, applied heat to the apex of the bend (i.e. the high spot) and then quenched it with water. Put it back in the lathe and the bend was now .010" less. Repeated the procedure and this time the bend was worse than when I started, had about .045" bend at the same point. Repeated the procedure, heating the high spot and quenching with water from a jug. this time the bend was .052".
Thought I might be doing it wrong after all these years, so I heated and quenched the inside of the bend. A slight improvement, back to about .040". Tried the same again, and was back to .050". I have now tried more heat, less heat, heat the inside of the bend, heat the outside, and the results have been random- no pattern to what seems to pull the bend in the direction that I want to. I'm now back to the starting point of about .035" bend in the same place as where it was originally. I think I'll be up for a new arbor but I'm puzzled as to why this procedure has produced such random effects.

[f350ca]

Have never tried flame straightening Pete, could the bend have been caused by torque. Might act differently then a side loaded bend.

[Vinny]

I haven't tried flame straightening either. When I have something that needs to be straightened I measure the error, add a few thou to it and put that down on one side with the high point up in the shop press.

[oldskoolron]

used to do lots of flame bending ect in my muffler shop .never quenched as if caused problems .if i had a driveshaft or fork leg to straighten id put it on the bench heat the opposite side to the bend then use either the press or clamp to bring it back an let it cool naturally .with the flame unless you get exactly the correct spot an area then quench it will pull at diff angles ect
anuther way we would have our dies and put the shaft tube or bar[outboard shafts n inboard boat shafts]in a jig and bring back to straight then heat the whole thing but never get it hot enuff to discolour the hard chrome[on forks]or tarnish the stainless [boat shafts] then once it was all even roughly with the torches we would lwt cool again by itself .if it was still out a tad normally do as vinny says an give it a yweek in the press .

[Dr Stan]

A bend is caused by internal stress and the application of heat removes and/or alters the stress.  As you have well documented all you have been doing is chasing the stress around.

For a 1" shaft heat will just give you problems.  In general using heat starts to straighten starts for me at about 2 1/2" diameter.  I do not remember the company name but while I was in Northern Italy a company was checking hardened shafts for bends.  If they were not within specs they used a CNC straightening machine which applied very localized heat and then pressure like in a press.  Afterwards they were checked again and if they still needed straightening back into the straightener they went up to 3 more times.  After a total of four they were scrapped.

After some reflection I think the company was building both machines and had sample shafts from the customer.  I have a vague memory of the shaft being used for a small CV joint assembly.  It was probably 25mm or so at the largest diameter and 250 to 300mm in length.

In general I like Vinny's approach better than applying heat, but in any case there will be a high likelihood the shaft will bend again in roughly the same spot.  Generally the bend will be worse the 2nd time around.

So if the shaft is 1" or less  I may put some time & effort into straightening it.  However not much time say 15 minutes to an hour depending on the complexity of the shaft and the cost/time issue of buying/making a new shaft.

[Pete O]

Sounds very feasible that the shaft is too thin for the technique to work, is probably getting hot enough right through the shaft to defeat the purpose. I have only ever done it before on larger shafts without the same precision required.
Unfortunately I don't own a press nor have access to one, it is on the project list, perhaps it needs to climb a few rungs on said list. I've actually used my petrol-powered hydraulic log splitter to press a few bearings but no way would that do this job.

[oldskoolron]

you got a press mate ute or tractor an bottle jack

[Mayhem]

Mount it between centres in your lathe and rotate until the high spot is facing directly towards you. Take a solid lump of wood and place one end between the ways. Now, use the wood to press against the high spot.

[Vinny]

Bring it over Pete, you can use mine.

[Dr Stan]

(11-11-2016, 02:41 PM)Vinny Wrote: Bring it over Pete, you can use mine.    

I'm making the same offer and will do the job for a case of Swan lager.