4th axis on the RC1530RH — where do you take Z zero, and how much chuck runout is normal?

[pete]

We have just put a 4th axis on our RC1530RH, mostly for tapered table legs out of 90 mm square blanks, around 900 mm long.

The rotary is mounted, it turns, no alarms. What I cannot settle is where to take Z zero. If I touch off on the top of the blank at the chuck end, the far end reads about 1.8 mm higher, because the blank is not perfectly straight and the tailstock lifts it a little. If I zero in the middle of the blank instead, the cut is even but the finished diameter comes out roughly 0.9 mm under at both ends.

Things I have already been through: indicated the chuck face and the jaws (0.04 mm TIR on the body, 0.12 mm on the jaws), shimmed the tailstock, re-centred the blank between centres with a dial indicator before tightening, and checked the rotary is square to the gantry — within 0.1 mm over the length.

So do you zero at the chuck and accept the taper, probe the stock and let the post work the offset out per pass, or put a steady in the middle? And what runout do you call acceptable on these chucks before you start chasing it?

[walt1962]

Zeroing at the chuck end and accepting the taper is what I gave up on — the taper moves with every blank, so you never converge. On my RC2400, turning similar long blanks, I stopped chasing stock straightness and instead touched off at three positions along the blank, split the difference, and left a 0.15 mm allowance for a spring pass so the finish cut evens the profile out. That took the finished diameter variation from roughly 0.8 mm down to 0.1–0.15 mm over an 850 mm blank.

On runout: I consider 0.05 mm TIR at the chuck body fine once things are indicated in, but your 0.12 mm on the jaws is where I would dig first. Boring the jaws in place — or turning a sacrificial soft jaw set — usually pulls that under 0.05, and on my machine that change alone removed most of the lift at the tailstock end.