Lathe spindle bearing selection

I always work on the basis of 1x bolt dia depth in steel and 2x in cast iron. These contact lengths are for full thread depth, so add a little to cover any chamfers. These contact lengths are in excess of the length at which the bolt will strip before breaking. So your bolt should be 25+16, 40 or 45 mm long.

I have always worked with 1.5 x D thread engagement as a "rule of thumb" for both steel & cast iron, so would be looking at around 12mm for an M8 bolt. That does assume, though, that you have at least 12mm of thread in the mating part - not always the case with "skinny" Chinese castings. Be aware that beyond a certain length SHCS are not threaded up to the head - make sure that any un-threaded portion of the shank does not protrude beyond the casting & prevent correct tightening.

Probably worthwhile changing the bolts (even if they had been long enough) for better quality items. I had the threads strip on a Chinese swivel bench vice base clamp bolts - the threads in the mating casting were good, but it turned out that sloppy 3/8 UNC bolts had been used in M10 tapped holes. New M10 bolts sorted that problem.

Nigel B.

Thank you all,

I have replaced the M8 bolts securing the headstock to the bed with longer high tensile ones and have 3Xbolt dia engaged in the bed. I am much happier now. Not able to use a torque wrench, I had to reduce the Short leg of an Allen key to get into the ones at the back but they are tight now.

The new FAG bearings are fitted and greased as per advice given.

The spindle is fully assembled in the machine and adjusted until no end float indicated just no pre load as such.

I have not run the motor yet, ran out of time.

Will work on it again tomorrow making adjustments as necessary to thr spindle but it dose feel very smooth when turned by hand.

When I fitted the drive belt I could feel either the belt flexing or perhaps it's the motor I don't know.

Thanks again for all your advice.

With a DTI on the toolpost and your solid carbide bar in the chuck, you can run along the front of the bar to get a good idea whether the headstock is still aligned with the bed. There are better alignment bars about, but what you have already will get you 99% perfect even before taking a test cut. A length of aluminium bar that will just pass through the chuck bore would be good for a test cut. It would be as stiff as you can get and also be clamped by the full length of jaws. With a projection of about 150mm and a very light cut you can compare the diameters at both machined ends of the bar for final alignment. Allowing for flexure, the diameter furthest from the chuck would be plus 0.02mm or better than the chuck end.

Edited By old mart on 15/12/2022 17:49:00

Thank you old mart I will work on that tomorrow.

I will post the findings hopefully it will be acceptable first time fingers crossed.

Or "Fischer-AG" as their local rep, years ago, pointedly insisted on calling them every time he said it.

+1 to everything else you said.

Edited By Peter Greene 🇨🇦 on 15/12/2022 18:15:31