Rather than clutter up another thread I am posting a picture of a Coventry diehead that is non-standard, for scale it is sitting on the Bridgeport table:

The question is what is it used for?

As it happens I do know its purpose, although I don't have any suitable chasers. No doubt JohnS knows too, so I hope he won't spoil the game.

Andrew

Superficially it looks a bit like a pipe threader, but it is a true Coventry diehead intended primarily for use on capstan lathes. However it is not a standard Coventry diehead. It is intended for cutting tapered threads. For short, or shallow, tapers the die chasers can just cut deeper as they move forward. Much like cutting an internal NPT thread with a tap.

However for longer, or steeper, tapered threads that doesn't work, so another method is needed. Here's the sequence of operation. First is the diehead in the fully closed position:

Note that the lug bottom right is at the front of the slot. Also note the gear teeth on the body of the diehead. Next is the diehead with the lug moved back a bit, as it would be if it was up against a stop while the capstan moved forward:

Looking at the gear teeth there are fewer visible indicating that the diehead body has rotated slightly clockwise opening the die chasers. Finally at the end of the slot the lug triggers the diehead to fully open, as per a normal diehead:

By this means a long tapered thread can be cut as the dies open in sync with the diehead moving forwards. I think there is a plate hidden behind the lug that controls the taper being cut.

I have no die chasers for this diehead; I'm not even sure if they are still available. They may well have been specials even when dieheads were universal. I don't know why I bought the diehead; one of my Ebay impulse buys! Still it wasn't expensive, £10 I think, and it's interesting to see how it works.

Andrew

…….and the 22nd edition, which is where I got my information.

Andrew