On 2018-01-07, David Billington wrote:
On 07/01/18 19:25, Bob La Londe wrote:
After much adieu over nothing I suppose (since at the moment I only
have one to do) I decided to turn a tapered mandrel between centers.
The first one turned out pretty good except I screwed up the finish
pass(es) and came in under sized.* No I can't just shorten it up.* So
I still have just one to do.* LOL.

I picked up one of those little offsettable centers and it worked well
enough, except the tip melted.* I kept it lubricated, but it might
have just been to high of an rpm for the method.* I can turn it
slower, but if I get to do more of these I sure don't want to go slower.

Bob,

I think it was me that suggested those offsettable centres so shame the
centre melted. I think if it was me I would try and modify it to have a
rotating centre. Maybe you can fit a housing to the movable part to
support the outer races of a couple of ball races back to back and fit a
centre into those so you'll have a well supported rotating centre. I've
done similar before but not for a centre application but the application
is still going well, it was actually the rotating mass bearings for a
vibratory table, the housing was fixed to the vibrating table and the
centre shaft connected to the motor via a bit of flexible hose.

If you make a set of ball races in a cup for the offsetable
center, I would suggest that you also make it tiltable towards the
center line too. You want the axis of rotation for the workpiece to be
parallel to the axis of rotation of the center. Otherwise, you get lots
of friction as the angle of the center changes in the drilled center in
the workpiece. And if the workpiece is a soft alumin(i)um, you will
wallow the center hole out as you work unless the axis of rotation of
the tailstock center matches that of the workpiece.

I think that the amount of angle change at the headstock center
would still be a problem, so inlay a ball in that center so it has
maximum contact with the drilled center in the workpiece. (The same
problem exists with the offset tailstock, too.) Even worse was the way
the Unimat SL-1000 (and DB-200) did it, where the headstock was pivoted,
so the angle of contact of the headstock center with the drilled center
hole was worse, since the headstock and the workpiece axes were in
opposite directions.

Good Luck,
DoN.

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