Advice on turning a part

**charlieuk** · 29-08-2014

Dont know if any one can help but im looking for some advice on turning a part on a lathe

basically i need to bore the same size holes in the pinion on the top as the one on the bottom and then shorten the shoulder and then continue the thread.

the holes need to be accurate an a really good fit so there is no play but the hole sizes are not a standard drill bit. (12.68mm)

I have a lathe all be it not that great so a little concerned about getting it right however at least the pinions are relatively cheap but not to sure what sort of tool i would be best to do it with? drill a smaller hole first then enlarge it some how?

Im think the shaft will be a little harder though and assume that the shaft will be hardened in some way?

best regards

**EddyCurrent** · 29-08-2014

What about drilling it to the nearest size down, say 12.5mm then use an adjustable reamer to finish ?

**Neale** · 29-08-2014

For the pinion, the hole is probably a nominal 12.7mm - or half-inch as us old-timers know it! Easy enough (but not necessarily very cheap) to get a reamer for that. Biggest issue is maintaining concentricy. Personally, I would hold it by the teeth in a collet. You can make a one-off collet which will be very accurate even if you only have a tired 3-jaw chuck. A drill to open out the hole might work but a small boring tool would be better, whether or not you use a reamer for final sizing.

From what I can see, the shaft is more difficult. Even if it's not hardened, I'm not sure how you will hold it for turning which you will need to do before cutting the thread Possibly another home-made collet would do. Assuming it's just for a securing nut rather than an accurate location surface, a die would do the threading job but there will be a groove between existing thread and shoulder that might give a problem. If it is hardened - make another one from scratch?

**EddyCurrent** · 29-08-2014

Originally Posted by Neale

Easy enough (but not necessarily very cheap) to get a reamer for that. Biggest issue is maintaining concentricy.

You are correct, but I was thinking of a morse taper reamer held in the tailstock, same with the drill.
I can't believe I didn't realise it was 1/2", damn mm have warped my brain.

**Neale** · 29-08-2014

Problem is holding the pinion concentric. With a worn three-jaw, plus locating on teeth, the hole is unlikely to end up in the middle. A home-made split collet can fix that. And a worn lathe probably doesn't have an accurately set tailstock, hence suggestion of a boring tool. It's easy to start over-complicating, isn't it? It's what I would do, but I have the tools to hand. There are probably folks out there who would hand-hold the pinion and use a drill press (but please, don't try that one at home...)

**mekanik** · 29-08-2014

Hi Charlie
If you have a 4 jaw chuck you could get some silver steel rod or suitable drill shanks, place 4 in the spaces between the gear teeth and use a clock to set to the drill/rod outer dia, that will get the gear running true, bore it if you can, run @ a slow speed so you don't burn out your boring tool, if it wont cut you could anneal it and then machine it.
Mike

**charlieuk** · 29-08-2014

thanks for all the advice i only have the basic stuff and only a 3 jaw chuck, sounds like im best of trying to find some one to do it for me as its going to cost me a bunch just to get the mentioned tools and im not that sure my lathe is all that accurate any more.

**mekanik** · 30-08-2014

Hi Charlie
was just having another look @ your wee jobby, the dia of the boss looks smaller than the one shown with the key in it, what sort of wall thickness would you be left with if you opened it up to 1/2" ?
Mike

**charlieuk** · 30-08-2014

no they are the same size just one is a fraction shorter.

**mekanik** · 30-08-2014

Try to run a file in the bore of the gear to see if it is possible to machine it, i think your best option for the pin would be to remake from silver steel, harden/ temper and polish the shaft.
Mike