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

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

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 .

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 .

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 .