On Sun, 7 Jan 2018 17:11:03 -0700, Bob La Londe
wrote:

On 1/7/2018 12:25 PM, 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.

I have two thoughts:

I can offset the tail stock and use a ball bearing center.

I can machine off the melted center, center drill it, and insert a ball
bearing between it and the center drill on that end of the stock.* The
ball bearing certainly won't melt, and it will more evenly distribute
the load from the stock to the center.* It would also have more contact
with the drilled out center.* It would also allow me to have a small
reservoir of grease behind the bearing.

I really don't have a strong feeling for which approach will work out
better other than I dislike the time spent adjusting my tailstock back
in.* I can leave the offset center setup for this taper indefinitely.
Ok, maybe I hope the ball bearing method will work out atleast as well.

The taper is pretty shallow.* .157 over 10 inches.




So how much RPM do you think a ball bearing can take?

I thought I was turning at 300 RPM the first time, but it turns out I
was actually turning at 460. Oops. The only ball bearing I have that I
know is hard is one of the little bearings out of the ACBs I pulled out
of the Hurco mill. Its only about .20 so I know it doesn't have enough
mass to sink much heat.

~~~ time lapsed ~~~

I finished it at 115 RPM. Wow. Talk about slow. On the other hand it
looks really nice after three passes with the vertical shear and then
hand finishing with fine grit.

I did have the grease around the bearing start to smoke once so I let it
cool for a while and then used another bearing with fresh grease. In my
mind a larger bearing would have worked better, and taken longer to
overheat.
I'm glad you got it done Bob. I used to do lots of between centers
work. Dead centers for some of it. Ball centers too. When I used a
ball center I would usually first use a same size ball and a hammer to
form a depression in the drilled center to fit the ball. This results
in more surface area for the lube and lowers the pressure per area. So
less heat is generated. When needing to turn the work at relatively
high rpm with dead centers I would lube the center and leave it a
little loose. Then spin up the lathe and let the part warm up. And
check the tightness of the center in the part during the warm up. And
once machining commences the center tightness needs to be checked on a
regular basis. This all done by feel but it doesn't take too long to
learn. I tried all sorts of high pressure lubes and mixed some of my
own.
Eric