What is a “Coventry die head”?

Came across a cideo of a machinist using a Coventry die head in the tailstock of their lathe and after a bit of searching I can’t find explanation of this tool.

How is it different to the normal split die I’m used to? Note that I have no formal metalwork training.

A thorough explanation would be appreciated.

Is this book of any use?

Thor

Some years ago they were commonly used on capstan lathes in a production environment for “fast” threading of components – screws, threaded rod, that sort of thing. It is probably done by CNC these days and I would guess that the die head is nearly an antique!

Rik

I n a nutshell, without having to read a whole book (which is very good BTW thanks Thor) the Coventry die head is like your ordinary die and holder, but the die is in effect split into four pieces, each with a set of teeth on it to cut the thread. The holder holds all four in teh correct position. This means that once you have cut your thread, instead of having to reverse the lathe and wind your one-piece die and holder back off again, you simply yank that handle on the Coventry head and the four toothed cutting sections snap outwards, clearing the job and allowing the carriage to be wound back toward the tailstock with the lathe still running. Saves heaps of time on production work but not that essential for home use.

There is the added advantage that as the cutting teeth wear and the thread they cut becomes a bit oversized, the Coventry head allows you to minutely adjust their depth of cut to bring the thread back into spec, like squeezing down a split die with the clamp screws but much quicker and easier.

On the ones I've seen and used, 'twas t'other way about. You 'cocked' the diehead by rotating the handle till the 4 chasers clicked into closed position, ran the diehead up the workpiece to a stop, when the chasers sprang open so you could whip back the turret to index to the next tool.

Setting up and using the Coventry diehead was on of the first things taught on the Capstan Setter/Operator course I started at a Government Training Centre in 1974. They're magnificently robust, and still used, at least in heritage railway workshops.

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+1 on the thanks to Thor

Nice summary, Hopper.

Further to Rik's observation ... The individual inserts can be useful to the amateur, as thread chasers.

MichaelG.

One size of insert(11 tpi I think), can be used as a form tool for a Poly V belt pulley.

Ian S C

Hi all, the days of cutting threads with Coventry Die heads are far from over, for example the Simba Plough Company of Sleaford Lincolnshire had a requirement for some rather large "studs" threaded 2" Whitworth, the longest of which was 12 feet long with 2 feet of said thread at one end, we had to cut a hole in the wall of the wooden shed that was our workplace, feed the bar through, and the turret of the Herbert No 7B, headstock & finally a well greased wooden plummer block! Then one may take 1 rough & 1 finish pass, job done, proven technology etc etc. Compare this with Cnc, right hand end of machine often blocked with swarf conveyer, left hand end of spindle blocked by obscenely expensive magazine bar feed for delivering bars little over 3 feet long, oops run out of space. Rolled thread? I don't fink so, Thread milling? your having a larff, have you any idea how much such technologies cost? for a thread to be fitted with 2 (very large) nuts & washers, go with the tried & tested.

Bob H

"Further to Rik's observation ... The individual inserts can be useful to the amateur, as thread chasers."

Indeed they can Michael - here is a holder to do that very job which I made a while ago:

I have a set of single inserts and a holder from Tracy Tool for use as a hand chasers.

In practice I hold one in a QC tool holder.

Neil

We used them on centre lathes as well as capstans, make for rapid threading and you can run at a higher speed plus they cut a much more accurate thread than a split die. Another feature is that you can do a roughing cut and finishing cut on most of the models, if memory is good its the smaller sizes where this feature is omitted. I have and use a 1/4 & 1/2 sizes.

John

Yes, it was a sort of dance. You might have another operation step between the diehead and part-off, like a drill and ream or tap, or just possibly a roller-box - though that would be more likely before the diehead.

"it was a sort of dance"

​I totally agree. That was the sweet spot, get it right and you could (and did) do it all day long, with good production rates and accuracy.