View Full Version : Advice on turning a part
29-08-2014, 05:18 PM
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?
29-08-2014, 05:55 PM
What about drilling it to the nearest size down, say 12.5mm then use an adjustable reamer to finish ?
29-08-2014, 06:04 PM
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?
29-08-2014, 06:13 PM
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.
29-08-2014, 06:44 PM
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...)
29-08-2014, 09:07 PM
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.
29-08-2014, 09:31 PM
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.
30-08-2014, 09:45 AM
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" ?
30-08-2014, 11:52 AM
no they are the same size just one is a fraction shorter.
30-08-2014, 01:06 PM
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.
30-08-2014, 01:07 PM
is there any one that may be interested in doing this for me before i go and find some company to do it? Its literally the last part to get my machine up and running!
30-08-2014, 01:26 PM
Need to know if the gear can be bored, do the file check and if it's possible and you supply a piece or two of silver steel(extra for F**K up) for the shaft, and 4 pieces of smaller dia silver steel that will fit into the space between the gear teeth i will give it a try. Can't promise it will be a success but will give it my best shot before you have to spend silly money on the professionals.
Get the file out
The chances of the gear being hardened are somewhere between slim and nil.
If all you have is a three jaw, then make a soft collet.
Easiest way is get a bit aluminium wide enough to cover the gear width, and about a couple mm larger diameter than the gear. Clamp it fairly lightish, then bore it out so the gear will slide in, then tighten the chuck and the aluminium will deform onto the gear.
Alternatively, get a thicker and bigger diameter bit, bore it out deep enough for the gear but leave it a couple mm undersize, remove and cut three slots in, place suitable bits of material to hold the slots open, reclamp, bore to size so the gear slides in, unclamp slightly, remove spacer bits, and clamp onto the gear. Think http://www.arceurotrade.co.uk/Catalogue/Collets/5C-Collets/5C-Step-Collets but held by jaws and not a collet holder.
The bolt may be hardened, but probably not hard enough that a decent carbide insert won't handle it. If it is hardened, it's not likely to be through hardened, so once you're throught the skin it shouldn't be a problem. However picking up the thread accurately to extend it using a single pointed tool will be a bigger problem, so it would be easier to do with a die.
30-08-2014, 08:33 PM
Would it not be easier to just make a new bolt to fit the bore size of the new gear, it would obviously have a smaller thread but I would of thought it would still be strong enough and a lot easier than boring out the gear
31-08-2014, 09:40 AM
Can i assume the purpose of the exercise is to install the larger dia gear into your belt drive pulley ?. If this is the case you are still going to need the drive key.
As m-c has mentioned the chances are you should be able to machine the gears OK with a machined sleeve, an option you could try is to use the original keyed pinion and remove the geared portion, bore the new gear to suit but leave a slight clearance between OD & ID and silver solder the new gear onto the original sleeve, so no need to machine the pin. If you fancy this option, to bore the new gear make the sleeve with a spigot ( ie like a top hat) this will be pressed hard up to the chuck jaws, size the OD of the sleeve 4mm greater than the gear OD, machine the bore of the sleeve but leave a step in the back of the bore to locate the gear on do not remove the sleeve from the chuck or you will loose concentricity.push the gear into the sleeve using tailstock and then tighten the chuck to deform the sleeve and grip the gear.
Hope that's of some help.
31-08-2014, 10:38 AM
I too am the owner of an old Lathe (WW2) similar to a Bantam and as my knowledge and experience grew over the years I had to come to the conclusion I would never get Precision work from it. The cross slide nuts are worn, I don't think the tail-stock has ever truly lined up with the chuck.. Most Lathes have a hollow shaft with a Morse taper (or other) behind the Chuck. This should be more accurate than the chuck if you could get to it and may be able to rig something up if you have access to bits and bobs..
Probably the easiest option is to draw up in 2d on a piece of paper exactly what you want, look under 'Precision Engineers' in Yellow pages and find a smaller firm and just go down there with the Gear and Drawing and do 'puppy dog eyes at them..'
(If you phone them they will put you off.) Say you are a 'CNC Hobbyist' and this may well endear you to them.
This only has a 30% chance of working as they may not have any machines free, but what may take you hours will take them minutes and once you have posed the problem, his engineering brain has already started working on 'How'..
Alternatively, spend 2 grand on a better lathe..
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