I recently bought a blank chuck backing plate on Ebay, hoping that it would fit my Colchester lathe. It was $AUD110 plus postage, which, if suitable, would be an excellent price, but it was a gamble. It was old new stock.
When it arrived I cleaned off the old, hard grease, and nervously presented the backing blank to the lathe headstock. It fitted perfectly! The seller had another identical blank backing plate, so I bought that one too. Components for the Colchester are not readily available, so I was very happy with this find.
I had a use in mind for both of the backing plates, and a few days ago I machined up the first one as per the following photos.
The cast iron backing plate blank had a tough skin which a high speed steel cutter would not penetrate. So I use a carbide insert tool cutting 1mm deep to break through the skin. I finished the contact surface with a HSS tangential tool. (A diamond cutter from Eccentric Engineering)
Checking the runout off the newly installed collet chuck. With a piece of 10mm diameter silver steel, the total measured runout was about 0.005mm. Good enough. The backing plate is larger than required, but I will leave it as is in case I ever use it for another, larger chuck. C5 collets will hold round stock 2-26mm diameter, and some common square and hexagonal sizes. Very useful.