newbie seeks advice on what I can expect from a live centre

I have a virtually brand new "Royal" precision high speed centre (RLC-10212B) which is spec'd as :- accuracy +/- 0.0025mm,angular contact bearing plus needle bearing and thrust bearing, 3000rpm etc.. When i put it up and clock it there is virtualy no reading however light finger pressure will show a deflection of 0.15mm. When I put up a piece of stock and put some pressure on with the tailstock handwheel then the deflection is much reduced -- with a fair bit of pressure from the tailstock ram it now requires a firm pressure with the hand to give a reading of about 0.03mm to 0.04mm. When I have an ordinary dead centre, firm hand pressure shows no deflection.

This doesnt seem good enough for a £180 centre but then I have no experience. I would like to know what the experienced among you think i should expect.

Tahnks in advance Nigel

Well I'm no expert but if you want precision I think most people use a dead center. At least that was the what was done in the old days.

Hmmm, I hope this doesn't mean Royal's quality is on the skids. I bought one of their spring-loaded CNC-type centers a couple of years ago. I use it infrequently, and went to use it the other day. Under hand rotation of the lathe I couldn't get it to run out less than about 0.0005 TIR when it is spec'ed to be less than 0.0002. I had to switch to my Bison precision grade center, which did meet its 0.0001 TIR accuracy spec, but has a fat head that gets in the way. I don't think the Royal is bent or anything, just loose. Thinking aloud here, the looseness may be on purpose. I wonder if it does better at speed, when the lubricant takes up the clearances hydrodynamically. I know that needle bearings like those in the Royal center typically require clearance and can't tolerate preload or they will quickly destroy themselves. The Bison doesn't appear to use a needle bearing in the front at least, so maybe can tolerate some preload to eliminate looseness even when it isn't turning. The cutaway of the Bison center in the MSC catalog shows a ball thrust bearing near the front. Assuming that bearing is made correctly, the presence of some thrust will automatically eliminate radial slop.

The comparison is not valid. Dead centers cannot run with out clearance. Most of us know what happens if you screw them in solid and go for it. The Royal on the other hand will improve with loading until the angular contact bearing complains.

John

There's a correct loading for a live center, measured with a pressure gage. Since my centers have no gage built in, I turn the tailstock in till fairly snug, which is just below pretty snug.

The centers will stand a tremendous amount of thrust.

Barry Milton

Aha so increasing the pressure could well be the answer then. I note that the specs state an end load of 1071lb quite how that translates to how tight to wind the tailstock I aint sure; but it sounds like quite a lot. Anyone have any ideas how to judge this without danger of overloading the centre.

Nigel

Nigel:

The bearing will be happy to tell you if you use your hand to judge temperature until you become familiar with your new Royal. If it never even gets warm, you can probably use a little more pressure, etc. If it gets too hot to keep your hand on comfortably, you are being given the message to back off.

You probably know without coolant long parts get longer during machining, so act accordingly.

John