On Sun, 09 Jan 2005 18:27:48 -0600, Ken Grunke
wrote:

Just added to the dropbox:

http://www.metalworking.com/DropBox/...ET_CENTERS.jpg
http://www.metalworking.com/DropBox/...ET_CENTERS.txt

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BEARING BALLS FOR OFFSET CENTERS


This is an idea that sprung to my mind a while ago, although I'm
sure I'm not the first one to think of it.

When offsetting the tailstock for taper turning, or using a special
tailstock fixture for the same purpose, the 60 degree center points
don't fit well in the centerholes of the work being taper turned.

This method needs custom-made lathe centers for both headstock and
tailstock.
The sharp point is turned off for a short distance, and centerdrilled
just as is done for the work being turned.

Hardened steel balls are captured in the centerholes between the lathe
centers and the work, at each end.

The correct centerhole size is important in relation to the bearing ball
diameter.

For a standard 60 degree centerdrill, the opening of the hole at the
ends should ideally be between 88% and 90% of the diameter of the ball.

If larger, there may not be enough clearance between the lathe center
and work to allow any offset.
If the hole's opening is smaller than 87% of the ball's diameter, only
the corner of the hole's opening will contact the ball and the whole
thing may come loose under heavy cutting pressure.

In practical experience, I've had very good results with this technique
while turning morse taper shanks.

For the purpose of accurately setting the tailstock setover, the
effective length of the workpiece is measured between the centers of
each ball.

Just mike the workpiece with the balls in place, and subtract the total
of one half the diameter of each ball.

Be sure to use your favorite tailstock center lube on that end
(I use white lithium grease).

Hope this is useful,

Ken Grunke
West Lima, WI
Jan. 09, 2005


This is a pretty good technique. It's perhaps
worth emphasising that it removes the length uncertainty
that's always present when turning between centre points.
With centre points the bar pivots about a point a bit inside
the pivot hole so the effective length is always an
uncertain bit less than the overall bar length.

With balls this is not a problem. The measurement
of overall length with both balls in place less 1/2d +1/2d
(d=ball dia) precisely defines the effective length.

With this checked with a decent vernier and the
offset set by gauge blocks pretty precise tapers are
possible.

Jim