This picture should give you some guidance as to the size of the dies for a given size of head:

There are four dies per set, labelled 1 to 4. Not only do the thread type and die type have to match for all dies in a set there is also an arbitrary number on the dies which needs to be same for all dies in a set. The arbitrary number is related to the numbers on a tapered rule used when sharpening the dies, 51 in this case:

Andrew

Odd chaser dies can be used on their own with a suitable holder as chasers. They can easily be sharpened at home and provide excellent formed threads on their own (without the die box).

Bob

If the Whitworth dies are for free I'd definitely acquire them. Assuming they are in sets they have a value in the Uk.

Andrew

If you can clamp one chaser in your lathe's toolpost, it makes a nice screwcutting tool.

I'm unconvinced as to the general suitability of MT taper for mounting a Coventry Die Head. By their very nature there is significant lengthways pulling force on the head as it cuts the thread which will tend to pull it out of the MT socket.

When using one with an MT taper in a tailstock the tailstock has to be fed hard enough to keep it properly seated in the taper but not so hard as to interfere with the thread profile. When reaching the end of the thread the taper seating grip has to be sufficient to resist the spring forces as the head pulls forward to trigger the opening process. If the taper grip is inadequate the head will go round with the job rather than continue cutting the thread. A bit fraught methinks.

If the tapers are less than perfect it would seem that the only safe way is to eschew the self opening feature and use it like a simple fixed tailstock die carrier keeping continuous feed force applied and stopping the lathe when you get to the end of the thread. The head can then be opened manually. Perfectly practical unless the tapers are distressed almost to the point of being unusable but it does loose most of the benefits. Frankly an ordinary sliding die holder is may then be easier, although they too will spin if the tapers are poor.

My die head has a parallel shank so I made a simple holder to mount it in a Dickson QC tool holder by welding a split tube to a solid bar. I feed up to a bed stop using the saddle hand wheel whereupon the thread pulls the head forward to trigger the disengagement. The disengagement pull forward distance is significant. Power feed by setting up the appropriate pitch and driving as per screw cutting works well but the half nuts need to be dropped promptly and final feed to the stop done by hand.

I trimmed the bar to match the tube radius on the lathe with it mounted in the toolpost to ensure all was aligned. Did the same to finish the bore. The portion behind the die head shank is bored little larger so I can easily set it on centre using a short cylinder gauge mounted in the tailstock taper.

It lives permanently in that toolholder, carefully selected as being the grottiest one which I didn't mind loosing from general stock.

Clive

Posted by Clive foster

I'm unconvinced as to the general suitability of MT taper for mounting a Coventry Die Head. By their very nature there is significant lengthways pulling force on the head as it cuts the thread which will tend to pull it out of the MT socket.

There is no problem with using die heads with a morse taper, I have cut probably thousands of threads mainly with 1/2" or 1/4" units and never a problem.

Those with a parallel spigot are generally use on capstan lathes, the one I have and use is a 1/2" unit and to facilitate its use on my lathes made a No2 Morse taper holder with a 1" hole for the die box — never a problem.

John

If you want to see one in action have a look at Mr Crispin's channel on Youtube.

He does not use the MT in the tailstock barrel but has made a sleeve to clamp the diehead and tailstock barrel together thus eliminating the risk of the DH pulling out of the tailstock socket.

HTH

Dave