Tips for turning to an accurate diameter

[waddycall]

I'm having another go at making a new steering stem for my vintage ktm. First time round I messed it up at the final stage of putting the finishing cuts on the diameters for the bearings and press fit into the bottom yoke. So before I mess it up again has anyone got any tips for turning aluminium to a precise diameter to within 10 microns or so on a worn out Victorian lathe??

 

[BrokenBiker]

are you able to tighten up any gibbs that have slop in them?

also when you take a cut to get close, for instance you have .2mm to go, so you take a .1mm cut, then measure again, sure enough you now have .1mm to go, instead of increasing the cut, run the tool up it again with the same rpm and feed rate, called a spring cut iirc, @doubleboost does it in a few of his vids, especially in the series about the dividing head.

once you have measure again and got down to .1mm to go (100 microns) take very small (0.02) cuts if possible, measure again, then another spring cut, and so on until you are bang on. I get a bit ocd about bearing fit and spend usually more time than it should take, but when you want perfection you have to work slowly and be meticulous about measuring and taking the smallest cut you can

 

[shenion]

Get it close then use a file or emery cloth to get to the final diameter. I usually "polish" to the final diameter.

 

[BrokenBiker]

Get it close then use a file or emery cloth to get to the final diameter. I usually "polish" to the final diameter.

I didn't want to give that advice, I usually lose concentricity doing that, I only ever use emery to get rid of any warks or polishing on stuff that isn't that critical, but each to their own, if it works use it

 

[CRB]

Get it close then use a file or emery cloth to get to the final diameter. I usually "polish" to the final diameter.

A bit difficult on internals though.

To take smaller cuts indirectly, set the tool post over to 60* and use the topslide.....if you have one. Make and model of lathe would help in the advice too.

 

[Pete.]

Make a sliding-fit plug from a scrap piece of the same alloy and measure it with a micrometer. Then make your stem and add whatever you want for your press fit to the micrometer reading taken off the plug. Don't press the stem into the yoke - shrink it in. Heat the yoke to about 80 degrees and the stem should pop straight in - but get it seated home first time as they will stick together in moments.

 

[shenion]

I don't see how you can lose concentricity on a spinning part. Never an issue for me.

For inside bores, I use a shaft with a slot cut to hold some emory cloth; works like a flapdisk.

 

[BrokenBiker]

sorry concentrcitiy wasn't the word I wanted looking at it, I mean parallel, I usually end up with one bit a smaller dia than another because ive either focused on one bit too long or pressed a bit hard.

I have however used your shaft with a slot method for cleaning up a bore, and ive also put the same shaft in a drill chuck and used it clean out exhaust pipes when I haven't had a wire brush that would do. works well

 

[garethp]

Make sure the tool is razor sharp for the final cuts, if it's rubbing at all it'll rub more as you put the cut on before digging in and taking too much off, also the tool needs to be on centre height or ever so slightly below for the same reason. If it's old lathe are you using hss or tipped tooling?

 

[waddycall]

Thanks for the replies chaps. Last time I messed this up I was using the compound set as suggested but still managed to overshoot. Before deciding to have another crack at it I turned the end of a piece of the bar down to within 5 microns. Being paranoid I'm convinced ill mess it up on the real thing. BBs suggestion of spring cuts is what I used to get the practice piece right. The lathe is an ancient Britannia that has had different headstock , tail stock and carriage fitted to it with a home made top slide!! The pic shows the lathe and profess so far.

 

[waddycall]

Sorry any chance the picture could be rotated please??

 

[waddycall]

Home ground hss tool. Good point on the rubbing.

 

[MattF]

With regards to finishing on the lathe, when you get to almost the final diameter, take several passes without altering the slide position. You'll likely find, especially with thin or long pieces, that you'll have quite a bit of flex, depending on how you have the piece set up. The second pass may still take off a surprising amount. Also remember that if you've got the piece hot during turning, you want to let it cool off before taking the finishing cuts. A minute amount of expansion could throw things off completely. Also, turn a slight portion of the piece down to size before doing the rest. You have a constant reference point then.

Get it close then use a file or emery cloth to get to the final diameter. I usually "polish" to the final diameter.

I tend to do the same a lot of the time, by filing to final finish. I think I just spent so many years without a lathe and using files instead, that I still find it hard to break away from my fondness of using them. Files are a vastly under-rated piece of equipment these days, IMHO.

 

[MattF]

sorry concentrcitiy wasn't the word I wanted looking at it, I mean parallel, I usually end up with one bit a smaller dia than another because ive either focused on one bit too long or pressed a bit hard.

You should stroke a file lightly across the surface at an angle to the piece, not try to dig it in, at a right angle, and take shedloads off. You're using the file incorrectly if you're achieving that result.

 

[Paul.]

Sorry any chance the picture could be rotated please??

Picture rotated

 

[BrokenBiker]

You should stroke a file lightly across the surface at an angle to the piece, not try to dig it in, at a right angle, and take shedloads off. You're using the file incorrectly if you're achieving that result.

don't use file just emery, only ever use a file for rounding an end off if I haven't got the right tool or time to make one

 

[waddycall]

Thank you

 

[MattF]

don't use file just emery, only ever use a file for rounding an end off if I haven't got the right tool or time to make one

The way you want to do that then is to get a thin strip of ply or similar and wrap the emery lengthways over that. Literally make a file from emery cloth and ply. Saves creating any inconstency from finger pressure and gives you the same type of control you'd have with a file. If you do just hold the cloth in your fingers/hand, keep it constantly moving along the piece.

 

[BrokenBiker]

keep it constantly moving along the piece.

this is what I try to do but get distracted when I notice a mark and just focus on that, although now ive altered a few things on the lathe im getting less and less tool marks when I make finishing cuts, and the addition of a stronger tool post have helped a lot

 

[Rig Pig]

Cover the bed if your using emery paper, any carborundum that drops off will make a loverly lapping compound!