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External Chuck Bearing

An 8MM collet, when all is said and done, is a delicate thing with little mass, that is intended to hold small pieces of brass and soft steel for high precision machining.

Lately, I've been working on some pieces of larger diameter chucked up in two Sherline Chucks (a 3 jaw and 4 jaw) using a threaded collet adaptor. I quickly found out that anything with a great diameter than a half an inch would try and "climb" over the bit even with light cuts. I didn't want to damage the beautiful antique lathe headstock hard bearing surface and the climbing made cutoff and even moderate depth cuts nerve racking and even impossible. Since I'm an inventor, I decided to solve it by defeating the deflection forces at the chuck itself thereby protecting the cone bearings on the antique headstock while achieving the higher performance and rigidity I wanted.
The result is an external chuck bearing and custom collet adaptor with bearing surface you see below.
Watchmakers lathe external chuck bearing

I adapted a Sherline chuck plate that threads into the back of the chuck so that it mates up with a collet I machined from some mild steel.
Watchmakers lathe external chuck bearing with roller fingers

I decided to design it like it was a large steady rest with extendable fingers with bearings on them so that it would adjust in size for different chuck diameters and run smoothly without adding resistance. This meant having the hoop of the bearing frame extend below the bed of the lathe which of course meant making a drop saddle that would fit over the bed and pick up the hoop at the sides.
The whole assembly uses the same kind of attachment bolt through the split bed that all the accessories for the little lathe use.
watchmakers lathe external chuck bearing and chuck runout adaptor

The hoop is a cutoff piece of 8 inch diameter steel tubing. The capture housings, the fingers and the bed attachment frame are all "castings" from shapeways metal rapid prototyping process.
Watchmakers lathe external chuck bearing made from rapid prototype printed steel fittings.

It runs very quietly and I can now take chatterless parting cuts and moderate cuts off of much larger diameter pieces of metal without any stress being transmitted to the headstock.

A secondary advantage is that the adaptor for the chuck allowed me to really tune in the runout on the jaws down to less than 1/2 thousands of an inch!


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