On 1/7/2018 5:11 PM, 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.Â* No I can't just shorten it up.Â* So
I still have just one to do.Â* LOL.

I picked up one of those little offsettable centers and it worked well
enough, except the tip melted.Â* I kept it lubricated, but it might
have just been to high of an rpm for the method.Â* 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.Â* The ball bearing certainly won't melt, and it will more evenly
distribute the load from the stock to the center.Â* It would also have
more contact with the drilled out center.Â* 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.Â* 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.Â* .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.Â* Oops.Â* 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.Â* 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.Â* Wow.Â* Talk about slow.Â* 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.Â* In my
mind a larger bearing would have worked better, and taken longer to
overheat.

I used plane old waterproof wheel bearing grease, and it worked fine.
Not great, but fine for high pressure low RPM. I was going to try some
MolyKote BG-20, but I couldn't find my tube of the stuff. Of course
this morning when I walked into the shop... there it was on my desk.
Right where I thought it as. LOL.

I may buy a few carbide balls to keep in my lathe tool cart for future
projects, but like I said before. If find myself needing to do a bunch
of these I'm going with to make a linear rod taper attachment.

I've got some other tapers I want to make for myself down the road anyway.