Lathe run out

I am turning 13.5mm steel by 50mm long and after several finish passes I have a run out of .02 mm.

Using my test bar which is 200mm long I have within .01 mm in all directions, so it is parallel.

I done some very light cuts with 38mm aluminium and found it within .01mm over 200mm.

Ive noted in the passed that near the chuck using steel that I have a deviation, could it be that the cross slide is moving when under cutting pressure.

Ive noted that on a mill the cross slides have a locking bolt but lathes don't, would a locking bolt fix this problem, if so Ill strip it down and put two on.

Bob.

0.02mm is less than 8/10 of a thou!

you could have some backlash in the leadscrew/nut.....

Hi Bob I have put locks on my M Type by drilling an extra hole in the compound and cross slide taping it for an extra screw which clamps the gib strip. I Also fitted a proper clamp for the main carriage for facing off. But before you go to all that bother try putting a dial test indicator on the slides in order with a magnetic base to try and get an idea where any play may be. If you find any play it may be a that some adjustment is need to be done.

David

Hi Bob,

By runout do you mean the part is coming out with .02 mm taper ? Runout is when you put a piece in the chuck then set up an indicator to check for runout as you spin the chuck by hand , if you are setting up an indicator on the cross slide or toolpost then indicating the part by moving the saddle along the bed you are reading taper .

If it is tapered it is because of deflection off the cutting tool , 1/2 inch bar deflects easily and .02 is less than a thousandth so over 50 mm not bad and the further you get away from the chuck the worse the deflectoin will be . What tooling are you using ? Carbide insert tooling will cause deflection also and no amount of light cuts will fix it because carbide insert tooling will not take very light cuts so switch over to a nice sharp honed HSS cutting tool and try a very light spring pass . I usually just use some emery cloth and give the piece a polish to get rid of any small amount of taper , .02 mm or a thou doesnt take much work to clean up .

If the cross slide gibs are really loose you will find it will cause the piece to come out with things like steps ,dig ins ,grooves and a poor finish so if the part looks ok for finish the gibs aare probably ok , i don’t have a cross slide lock either .

Thanks for the replies.

For XD 351, it a taper smaller chuck end, plus I was using a indexing tool. So points taken, and I think I shall put two locks on .

Bob.

If the peice measures smaller at the chuck end this definitely indicates deflection or “push off “ - try the HSS tool .

No amount of cross , compoud or saddle locks will fix it as it is caused by the cutting forces imparted onto the workpiece by the cutting tool and carbide inserts have tiny radii on the cutting edges to help stop chipping which makes matters worse . You could use a following steady or put a centre in the end of the workpiece and support it with the tailsock centre also .

Those carbide insert tools don't like fine cuts. HSS honed to a good sharp edge will perform better for this as XD351 says. Or you can do the final finish with a piece of emery cloth and take a bit extra metal off the large end of the job.

And if you really want slick set up you could use a toolpost grinder or external hone ! When i made the crank for the current engine I’m making i left it a thou or two oversize and finished with emery on the important surfaces like bearing journals , this gave a very smooth finish to within a couple of ten thousandths and the rest of the crank wasn’t critical so i just cleaned it up so it looks nice . All this took maybe 15 min or so and was much less stressful than trying to machine it dead nuts on .

Dont think I,d be too upset about that amount of run out !

But then,I suppose it,s a case of "Horses for course ". My errors tend to be a bit bigger than that !

I,d say,live with it.

I used a HSS tool for the final finish and it came out almost spot on over the 50mm, so as said the inserts did make that slight taper.

Thanks all Bob.