On Thu, 26 Nov 2009 23:19:02 +0000, Mark Rand
wrote:

On Thu, 26 Nov 2009 16:06:51 -0500, "Ed Huntress"
wrote:




Ho-kay. Now, the reputation of Hardinge toolroom lathes is that the spindle
typically gets up to about body temperature or slightly above when they're
running and stabilized (this is second-hand info; I've never put my hand on
one). That's quite cool, but not out of reason for near-perfect bearings.

Apologies for doing this all in Fahrenheit and inch measurements, but if the
bearings are separated by, say, 8 inches, the spindle (and the spacer,
assuming it's steel) between them will expand by just under 0.002 in. with a
30 deg. F rise in temperature. (Steel expands at 7.3 x 10^-6 inch per inch,
per degree F, if anyone cares to check my calculations).

How do they cope with that? Does the headstock casting supposedly compensate
perfectly for that growth? Because 0.002 in. is enough to completely unload
any preloaded, high-quality bearings -- or to destroy them if they're each
facing the other way.


I had wondered about that.

Close inspection of the parts diagram (anyone want to give me an HLVH-H so I
know for sure, I missed out on two before I got the HLV), shows that the inner
races are separated by a spacer and clamped between the mandrel nose and the
pulley. The outer races are separated by a spacer closely fitted and keyed to
the headstock casting (can be assumed to be thermally part of it). The front
outer race is restrained on both sides, but the rear outer race is only
restrained on the "inside"


The spacer isnt keyed to anything. Its simply on the spindle tube.
Almost an interference fit.

So, assuming the mandrel heats up more rapidly than the headstock casting,
it'll get loose before regaining equilibrium. I don't know how good a fit the
rear outer race is in the headstock, but one must assume that it's free enough
to move before the bearing takes harm. There is no preload spring or similar
shown in the parts diagrams.

When the spindle gets hot..it expands and presses the bearings
harder..not softer.

This also tends to imply that you should run your Hardinge at your desired
speed for about an hour to heat soak it, before doing work of the highest
precision. But only if you're anal about it!

While they will indeed cut to a couple tenths..few people do.


We had an example where differential expansion ate bearings, a model air
turbine had an imperfect bearing retaining arrangement. Ate two sets of
bearings before they worked out the cause of the problem wasn't the mist
lubrication system. Matched pairs of 7" bore taper roller bearings at £6,000 a
set :-O.



Mark Rand
RTFM

"Aren't cats Libertarian? They just want to be left alone.
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