The difference in runout may be due to the extra sideways forces on the revolving centre when actually taking a cut.

In my experience, Chinese bearings are too often very poorly made. Quite often replacing them with good quality brandname bearings such as FAG, Nachi, SKF etc will cure the problem.

If you are calling your axles 'scrap' , turn a test section down to the same diameter at the headstock end and measure that. This would check to see that there wasn't some sort of cyclic vibration coming from the headstock.
BobH

The solid centre should give better results, but you do not say what machine you are using, its condition or indeed what tolerances you are trying to achieve.

If you are expecting to get accuracy to a tenths of a thou then you have to establish whether your machine is capable of such tolerances. Indeed you may, if wanting very close tolerances, have just found out why commercial products are often ground to achieve the final size and tolerance. You state that your "worn" centre is showing a .0002/.0003 compared with .0001 for the fixed centre. This would be truly impressive if using a model makers lathe IMHO.

Backlash alone in your slides would normally prevent you achieving such accuracy.

There is a lot of good advice here, but first you must check what you are trying to achieve, and determine whether the lathe you are using is capable of doing this, and then look at the centres. HTH.

I am somewhat confused Norm. Exactly what sizes are you trying to produce by turning and what are your finished sizes. You see you have said that on your Harrison 140 you can achieve 1.1094 and 1,1092 over an 8 inch length with a chuck held piece of bar. Now you are trying to turn final diameters of 0.7500 inch over 6 inches! So why don't you simply use the Harrison?

I am somewhat sceptical that your lathe is that accurate.

A further confusion is caused by your comment of producing journal size 0.0008/0.0010 oversize. Eight tenths to one thou……and then using grit paper to finish to size???????????? Seriously??? What size and tolerance is the finished product, and what surface finish are you looking for?

Can I ask you check these sizes and tolerances, because they simply do not make sense to an old school chap like me. Sorry if that sounds rude, it is not meant to be but your descriptions of sizing capability on your lathe would suggest that you can turn to the finished size.

We ( precision engineers ) turned routinely to 5-7 micro meter. If it was needed 3 micro meter.

But normaly ( in 3 minute cycle time) 10 micron
In most cases ( spindles) these were then centerless ground..and delivered to +/- 2 micron.
Finish from grinding I don’t recall but oddly some parts rejected as too smooth..
That was with a ppm ( dimentional) figure of < 1 rejects. 1 micro meter is =0.0000393701 inch
Or =0.0393701 thou
..
And thats a production environment making parts on a 5 min cycle time.

Edited By jason udall on 30/12/2015 19:18:42

A tenth of a thou is a heck of a tolerance on an axle, and indeed a heck of a tolerance on a lathe. In all my time in engineering I only had one job where any tolerance was less than 0.001". I would think the level of accuracy you are trying to acheive will be difficult to get.

Edited By Tony Pratt 1 on 30/12/2015 20:32:55