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I have a Boxford STS 10-20 lathe. This is one of what they call the X10 series. I find these a good machine for model engineering apart from a few minor niggles.
Link to X10's **LINK**
One of the problems with them is the availability of change gears for screw cutting. There was just not the vast numbers of them made like the more 'traditional' Boxfords so 2nd hand spares are limited. Such things as the change gears, steady's etc have either been lost or they stay with the machine and don't get sold on their own.
These are available still directly from Boxford but are very expensive. (tripple the price that is on their website as it's been so long since that was updated.)
I recently came into contact with a chap who offered to make me some out of Delrin. With them having a spline in the hub against the traditional keyway I would not imagine an easy thing to do. He did do so and upon arrival they fit perfectly.
This is the first time he has made them for an X10. So I would imagine they could be made for other lathes if sizes etc were available.

Now I know they will not be a durable as steel ones. (but Delrin is pretty tuff stuff) But they will only ever be used for screw cutting so will be only subjected to minimal loads. – He did also say. "Break one I will replace it for free."
I should add that I have no financial connection with this chap. In fact I have never even met him and only spoken on the phone. But he did say he makes this sort of thing as a bit of a hobby and not large profits. …….. They must be quite time consuming to produce in small numbers I would think.
So if anyone wants such send me a mail and I will provide his contact details. – I cannot give you a cost as we did not actually swop any money as we did a deal with something else. But they will not be expensive and a fraction of the cost of the OEM ones.
Nick