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Threading with Coventry Die Heads

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  Threading Bolts Using Coventry Die Heads     Published : 12/08/11

Off at a Tangent Again – Coventry Die Head’s and Thread Cutting
For anyone reading who is not familiar with Coventry Die Heads - which are a commercially used device for cutting threads, these are fascinating tools, and although they originate back to a different engineering age (at least 50 years I would imagine), they are still widely used in the industry. They come in a variety of head sizes, and are cylindrical in shape, with an opening on one end to feed onto the blank rod to be threaded, and a hollow shaft on the other end, that can be held in a suitable toolstock. They are manufactured by a company called Alfred Herbert Ltd. and I believe they were originally used primarily on machines such as the Ward Capstan lathe – which was the closest thing to a modern CNC machine, in that it allowed for a modicum of automated repetitive machining. These machines – not unlike a normal centre lathe, but with a collet quick release chuck, and a feed for a long length of steel to be continually fed through the chuck from the back, then parted off, also had a large rotating tailstock that allowed for a number of different tool attachments to be fitted. The idea was that a machinist could keep feeding bar through the chuck, then quickly rotate tools, allowing for each operation, including threading the bar, to be carried out, to ensure that there was as little time as possible wasted between each operation. Not much good for one-off jobs out of the ordinary, but great for large production jobs, where every extra second of a job is multiplied by the number you need to make.

Coventyr Dike Head with lathe
This photo shows one of my Coventry Die Heads, which is mounted in the jig that allows it to be mounted to the toolpost of my Smart and Brown lathe. The 'trigger' with the black handle, which allows the diehead to be 'cocked' can be seen in the foreground
Coventyr Dike Head with lathe
And here, with the face of the Die Head removed it is possible to see how the four chasers are loaded. These chasers are each marked with a number from 1 to 4 and are loaded sequentially

Anyway, the Coventry Die Head, uses a set of 4 hardened teeth, each numbered sequentially, that are inserted radially around the head then a cover plate retains them. Every set of cutters is unique for a particular teeth form – i.e. BS Cycle, BSW, BSF, Metric etc, and a particular diameter, i.e, 5/16”, 7/16”, 1/2” etc, which is why there are different size die, heads, as diameters get large.
The clever principle of a Coventry Die Head is that it has a small lever (trigger) coming out of the side of the head cylinder which allows the device to be ‘cocked’. Once cocked, the cutting chasers close down to the correct cutting diameter and then the cutting can begin. The teeth engage with the bar to be cut, and to an extent the die head is pulled along by the pull of cutting the thread. You then place a fixed stop at a suitable point where the die head is stopped from progressing any further, at which point the head stops, the inner body continues to travel forward for another fraction of an inch which pulls the ‘trigger’ and the teeth open up again and release from the bar being threaded. By doing this it allows for a stop to be placed to exactly correspond where you want the thread to end, then the work can be threaded very quickly, knowing it will automatically stop at the correct point.
Because these die heads are meant for high quantity production work, the quality of the threads is excellent, and the teeth (providing they are not abused) last a very long time without losing sharpness – something that cannot be said for many inferior quality tap and die sets.
A complete set of chasers, ready to be inserted. In this case 1/2" by 20 TPI, one of the more unusual BS Cycle threads used by Nortons
Coventyr Dike Head with lathe
The chasers are expensive to buy new, but can often be picked up very reasonably second hand, if you dont mind hunting for your specific size or can get to Engineering auctions. Many of those I have acquired have been through this route. Beware broken teeth when bought second hand
Die Heads in  3 sizes
The 3 most common and useful size of Die Heads for the home workshop - 1/4" on left, 5/16" in centre and 3/8" on right. I find 5/16" the most useful for our average diameters of bolts
            The downside of these tools is that they are extremely expensive, I seem to remember the last time I checked, about 5 years ago, a standard 5/16” Coventry Die Head cost about £600 each, and the cutting teeth cost about £35 for a single set of 4. If the teeth get damaged or worn they can be reclaimed by the use of a very special jig that allows for all the teeth to be held together at a particular angle and be cut back on a horizontal grinder      
Die Head in box
I was amazed when walking round a Car Boot Sale in Luton many years ago, to see this distinctive box on a stall, which I think I paid £5 for!
    Coventry Diehead Grinding Fixture
Amazingly, a few years ago I picked up a beautiful boxed example of one of these lovely jigs (again – made by Alfred Herbert Ltd, specifically for the Coventry Die Head), complete with fittings – and what a wonderful precision instrument they are! The purpose of this device is to allow for the chaser sets to be re-sharpened in the event they get damaged or blunt over time. To do this, the jigs allow for a full set of four chasers to be mounted in the jig together, then mounted on a milling or grinding table, while a grind stone is traversed over them. The jig allows for both rake and the throat of the chaser to be re-cut. The throat of the chaser is the entry point for the bar being fed into the device, so obviously, it is this part of the chaser that takes the brunt of the initial cut. With this in mind, it is not unusual when buying these sets of chasers second hand to find that almost half ot the number of teeth on the chaser are ground back - indicating that the engineering company have made very good use of the teeth over the years and have constantly re-sharpened them, as they get worn. This does not detract from the quality of the finish - as I would imagine it is only the first few teeth that do the actual cutting, but it does have the slight disadvantage that it is more difficult to gauge when you should stop cutting, if you are cutting threads to a precise point on the bar shaft, as you lose that visual aid, the further the teeth are cut back
Die Head in box 2
Contents revealed this elaborate and beautifully manufacturered jig, that allows for the re-grinding of the chaser teeth of the two smaller sizes of chasers, it they get damaged or blunt with age
            However, the jig is even more clever than that! As well as allowing for recutting of throat and rake for 2 different chaser sizes, it also allows for both right and left hand threads and for different angles to be selected, dependent on the chaser type or the metal type to be cut. The facility for this last variable is made possible by the design of the top holder being made cylindrical, so it can be rotated in the base of the jig to a number of preset angles - how clever is that!
I am a self confessed engineering nurd (albeit a very amateur one), and this is one of my very favourite engineering tools, simply because of the sheer cleverness of the design and economy of its execution - it performs so many different tasks from one single jig. There are some tools in the garage that you may only use once or very rarely, but just owning them and being able to look at them and marvel at how they are made is enough, and this jig falls very much into that category for me.
Having said all that, how I came by my example in a pretty unusual and fortunate way - and not because I went out looking for one! On a visit to the inl-laws in Luton some years ago, we took a walk round a local car boot sale on the Sunday morning. Amidst all the normal stalls selling old dross and pirate CD's I spotted a bloke with a small toolbox of engineering bits - old lathe tools and drill bits mostly, but on the small trestle table he also had, there were a couple of larger machine tools and this lovely looking wooden box.
Die Head Grinding jig
Here you can clearly see what a wonderful piece of engineering this jig is. It allows for both rake and throats (i.e. where the teeth open to first cut the bar) to be re-ground.
Not surprisingly, although attracting a lot of curiosty interest, he had not had many takers for it - in fact, I got the prize for being the only punter who had been able to tell what it was, so after a little bit of friendly bartering I got it knocked down to me for the princely sum of £8 - bargains do not come much better than that! Not sure where it might have started life, but I suspect it is of approximately 1960's vintage, and as Vauxhall Motors was just round the corner, maybe it started life making cylinder studs for Vauxhall Vivas.
After saying all that, I confess I have not had to use it in anger yet, simply because I do not use the Coventry Die Heads and chasers enough to need to have to resharpen any. I look forward to the day I do though, though I think it will mean having to setup a grinding stone on my vertical miller and use maximum revs, which is not ideal, but will allow me to bolt the jig to the milling table
Die Head  cutting instructions
In this close up of the instructions on the inner box lid you can clearly see how the chasers are intended to be re-ground. Normally the base of the jig would be bolted to a milling table which could be traversed under the grinding stone
Die Head  in Myford
Not my lathe this one, but a nice picture of a Myford Super 7 using a Coventry Die Head in the tailstock. On my own Smart and Brown I have a jig for holding the Die Head in the toolpost, so I can engage screwcutting feed. By the way, I have a very nice Myford Super 7 with quickchange gearbox for sale if anyone is interested. Not cheap, but a really nice machine and fully tooled with Myford stand\chest - contact me on the normal email if interested
      Buying Coventry Die Heads
I picked up my Coventry Die Heads (the three most common sizes for our size of threads) many years ago at an engineering closing down auction, as well as two drawers full of cutting chasers, in various sizes, which included many of the rare to find BS Cycle sizes. I love using them for their accuracy, and have made a special holding jig that replaces the toolstock holder on my lathe, so that it can travel with the main lathe saddle, when I engage screwcutting. The problem I have with them, is that there is a ‘fine adjustment’ setting on these die heads, which needs to be setup exactly right first, or the thread will be under/oversize and this normally takes a couple of attempts to get right. Also, they are quite fiddly and time consuming to change over from one thread size to another, as all 4 chasers need to be removed and replaced in the correct order. Because of this, I only tend to use them when I have a large batch of bolts in the same thread size to make.
Die Head  in Myford
And another reference shot - this one shows a Capstan style production machine, where the rod blank is mounted in a quick release collet type chuck. The Coventry Die Head is held in a revolving toolpost. These have the facility to hold multiple tools, normally arranged in sequence and by going backwards and forwards with the turret feed handle it rotates round to the next tool. This style of lathe is great for high quantity repetitive production work

Actually, since my father died, I have meant to collect the extra couple of heads he also has (he was with me when I bought mine!), so I can at least set up a couple of the most common sizes I use, which would save time.

For me though the best part about using a Coventry Die Head (particularly the way I have them set up to work on my lathe), is to watch it automatically chunder down a blank bolt, until it hits the stop and the trigger pops it open, and when you pull the head back, you are left with a perfectly cut thread - which providing it is setup correctly, has the perfect tolerance to the nut.

As a final snippet of useless information – to all but us Norton buff’s, I am informed that the bevel gears fitted in Norton cammy engines were manufactured by the same company that make Coventry Die Heads - Alfred Herbert Ltd. This is indicated by the little oval 'AH' trademark stamped onto all original Norton gears, which is also stamped on Coventry Dieheads– that is something I bet you wont get asked in your normal Pub Quiz!

Die Head  in Myford
Ahhh . . . and now the final connection - notice how similar the logo stamped on an original Norton International top bevel gear looks when compared to . . .

Die Head  in Myford
. . . the logo on the front of the Coventry Die Head Grinding Attachment box - that because they are both the same Alfred Herbert company logo!

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