source (metric) keyed shaft to replace bent one

[petertha]

I have determined that the power feed shaft on my Taiwan 14x40 lathe has developed a bow that is causing friction in the driveline to the extent the clutch is complaining. Mainly around the worm gear bracket I think. I'm in the process of getting it straightened, but it was maybe not great quality to begin with so thinking about a backup plan.

It is 19mm OD (0.748") x 1372mm length (54.0") with a 5mm key slot (0.197"). The tailstock end is reduced to 15mm for bearing & keyway terminates just short, but that can be worked around with continuous keyed stock. I considered a 3/4" nominal shaft which is more common, but I have to confirm what OD tolerances because it slides along a hardened worm gear with ID to accommodate the 19mm dimension (ie 0.750" +0.001 -0.000 shaft likely too tight for worm). Also they are typically 3/16" (0.188") slot so would have to be widened anyway.

I'm also chasing down possibility of retrofitting a new part off a similar lathe but that's still ongoing.

My measurements are maybe not the best but with ends constrained in the HS drive socket & TS bearing & just rotating by hand with a dial, I'm measuring max run-out from headstock to tailstock at roughly 6" increments like: 0.015", 0.025", 0.048", 0.050", 0.052", 0.050", 0.045", 0.027", 0.017".

Leads or advise welcome.

 

[rick9345]

Parker
http://www.metricmetal.com/
check with hydraulic cylinder repair shops (Motion) lots of cyl/rods are metric.

 

[Bob Korves]

If you try to make your own feed shaft from a piece of shafting, and have to cut a full length keyway in it, you will probably have more warpage in it than you have now after cutting the keyway. Look at some of Keith Fenner's (turnwright machine works) YouTube videos on "how to" shaft straightening. Or send the shaft to him for straightening. A single long bow in a shaft is pretty easy to straighten if you take your time, be careful about considering each straightening push, and then don't overdo the straightening pushes. It will take some time, but you should be able to get it within a few thou fairly easily. It will have to come off the machine to do it.

Edit: I am confused as to how a feed shaft "develops" a bow in it...

 

[petertha]

Thanks Bob. Yep, watched Keith's videos both on his lathe rebuild (which looks to be a distant, older Taiwan cousin to mine) and also his more general shaft straightening episodes using his heat/cool technique. That's what makes me think there is technique & experience involved. Your point about post keyway cutting distortion is exactly what I was concerned with. Thanks for the heads up.

I think a replacement (lathe) shaft solves 3 out of the 4 problems but so far limited hits. Some of the newer lathes have gone to 20mm. But I will keep trying.

 

[RJSakowski]

McMaster sells keyed metric shafting.
https://www.mcmaster.com/#precision-shafts/=17jypiz

edit: unfortunately, not 19mm

 

[strantor]

19mm is weird, not a standard metric trapezoidal. it's not a size I can find for sale anywhere, nor can I even find a spec on it.
I don't aim to make either of us look dumb by asking, but are you sure it's metric?
I know nothing about your Taiwan lathe, but if the machine cuts imperial threads, maybe it has imperial lead screw. It's only .002" off a 3/4"

 

[Bob Korves]

19mm is weird, not a standard metric trapezoidal. it's not a size I can find for sale anywhere, nor can I even find a spec on it.
I don't aim to make either of us look dumb by asking, but are you sure it's metric?
I know nothing about your Taiwan lathe, but if the machine cuts imperial threads, maybe it has imperial lead screw. It's only .002" off a 3/4"

It is not a lead screw. It is a feed rod, a shaft with a keyway the full length to drive the feed gear behind the carriage.

 

[strantor]

It is not a lead screw. It is a feed rod, a shaft with a keyway the full length to drive the feed gear behind the carriage.

Thanks for the correction. Still any chance it's imperial?

 

[Bob Korves]

Thanks for the correction. Still any chance it's imperial?

I suppose that is a possibility, but the OP said the keyway in the shaft is 5mm, which does not have a close imperial nominal size equivalent, and the lathe is made in Taiwan, so I think it unlikely.

 

[petertha]

It's hard to know with 100% certainty if its nominal 19mm OD, because lathes like these can be a mixed bag of IMP/MET. But that's my guess because its 'mostly' MET. The worm gear is for sure has 5mm key, that shows up in the parts manual. My manual does not state shaft OD but, again, sometimes they just don't bother with certain dimensions. I could measure the worm pitch to satisfy myself its a metric gear, that would probably settle it.

Anyway, thanks for suggesting the McMaster link nonetheless. They don't stock 19mm OD keyed shaft as you also determined, but I finally see a stated dimensional tolerance to this kind of shaft. The 3/4" size says -0.0025, -0.0010" which works out to 0.7475" (18.99mm) to 0.7490" (19.02mm). That might work with the ID of the worm gear which is a pretty loose sliding fit right now. Or at least within emery paper rectification range? But the standard 3/16" keyway would have to be re-milled to 5mm key to match the one in the worm, unless I got creative there. Its a hardened gear so that leaves some Franken-key modification.