Coventry die head

[lug lordParticipant @luglord]

recently boight a myford ml7r and used to have a suoer 7. I am about to thread sone m8 bars using the coventry dir head fixed to a collet and mt2. I am worried about the die pulling out as my prrvious lathe had a clutch. The myford ml7r has no cluttch any ideas

[HuubParticipant @huub]

I have made a floating tailstock threading die holder that spins freely and has 2 handles. I set the lathe in its lowest speed and hold the holder by hands. When the die hits the shoulder, is lose my grip and that’s it.

You could thread M8 manually by rotating the chuck by hand, at least the last few rotations.

[bernard towersParticipant @bernardtowers37738]

If you are using a coventry die head the dies cannot pull out, if you stop the head moving towards the lathe chuck it will auto disengage.

[Andrew CrowParticipant @andrewcrow91475]

<p style=”text-align: left;”>If the taper in the tailstock quill is in good condition there shouldn’t be a problem, if there is problem I would suggest making a holder for your diehead that is located and locked to the external diameter of the  quill. In use let the whole tailstock/diehead slide along the bed, then either use the tailstock lock or a fixed stop so that the head automatically opens at the desired length of thread.</p>
It is not good practice to reverse a diehead off the just cut thread.</p>
Andy

[JohnFParticipant @johnf59703]

There should be no problem with the die head releasing from the Morse taper providing the taper is in good condition.

You can release the dies at the appropriate moment, some heads have a release lever intended for capstan use but you can use it manually.  If yours does not then just lock the tailstock a little before you reach the end of the thread and it will auto release.

Or depending on the model you will find you can release the dies manually by pulling the small lever that changes the cut from rougher to finisher cut.

John

[Clive FosterParticipant @clivefoster55965]

I’m not a lover of running Coventry die heads off the tailstock quill. Its hard to get a nice constant feed and, if the die doesn’t want start cleanly there is a risk of it twisting out of the taper.

I run mine from a tool post mount. Its easy to keep a nice constant feed using the longitudinal feed hand wheel whilst watching the die head bob back and forth on its positioning spring. A bed stop reliably sets the threading length. The pull forward distance to release the die once feeding has stopped is very repeatable.

My carrier was a simple weld up job. Fix a tube to bar to fit the QC post, weld bolting stubs on the side drilled to clamp bolt threading size, split the tube, tap the bottom parts of the stubs, drill the top plan generous clearance and clean up the bore in site with a between centres bar. Job done. I made an MT taper setting stub to fit the bore for rapid alignment using the stub in the tailstock.

If you don’t have a QC system a suitably bored block with similar clamping arrangements will work just fine.

Concerning clutches a simple spring loaded jockey pulley type works fine on smaller lathes such as the ML7. Simples way is to “obtain” a car auxiliary drive belt tensioner discarded after servicing “to the book” once the nominal service life mileage has been reached. Many makes use a complete assembly on a backplate so re-purposing is easy. Just a matter of arranging a suitable bracket and connecting a control lever. The spring will be too strong so a stop will be needed to regulate the pressure.

Clive.

[bernard towersParticipant @bernardtowers37738]

you can also fit an internal stop which takes away all the hassle of will it won’t it stop and if doing multiple items they are all exactly the same (handy for studs etc.).

[John MCParticipant @johnmc39344]

I’m with Clive F. on this one.  I think it a lot to ask of a small Morse taper.  This will, no doubt, be dependant of the size of die head and MT.  Better to mount on the saddle in the toolpost.

I have found occaisional difficulty in starting a thread with the die head so I set the gearbox to the correct pitch/TPI, let the leadscrew drive the head onto the workpiece, when a few turns have been established, release the leadscrew, the die head will then carry on until the saddle stop, the die head opens and is withdrawn.

I’m intrigued by Bernard T’s mention of an “internal stop”.  Is that the way the die head opens or something else?

The photo shows my toolpost mounting arrangement, 1″CHS die head on a Colchester Triumph.

 

[bernard towersParticipant @bernardtowers37738]

Hi John ,It’s the method of the die head opening at a precise length EVERY time, necessary when making batches of screws/studs. Basically an adjustable internal stop so that when the stock being threaded reaches the stop the die head commences to disengage itself. In the case of a 5/16 head it’s a further 3/32” which you allow for when setting the stop. Will try to post images tomorrow as only just walked in the door from visiting the Midlands Show.

[Julie AnnParticipant @julieann]

I’ve made batches of identical studs using a Coventry diehead on a repetition lathe without needing to use an internal stop:

The important point of an internal stop is that it stops on a feature of the work, not on an external stop that is located on the machine. Internal stops are normally used were the precise location of the work relative to the machine is not consistent from one piece to the next.

Julie

[bernard towersParticipant @bernardtowers37738]

which is why an internal stop is used on a standard centre lathe set up

[Clive FosterParticipant @clivefoster55965]

Redneck alternative to eternal stops on an ordinary centre lathe lacking inherently relatable setting of workpiece stick out is to use a second bed stop at the tailstock side of the carriage. Set carriage to second stop pull workpiece out until it touches the closed die. Tighten collet or chuck and have at it.

I find thread length is repeatable to within any precision I care to measure.

Fine with pull back collets like the 5C system spindle native to my S&B 1024. Variation in retraction distance is very small. One day I might measure it.

Fundamental issue with centre lathe when doing repetition work with a Coventry die head is lack of cut-off slide so the threaded job cannot be instantly removed from the stock without re-setting tooling.

Another place where a multi position cross slide stop would be really handy. I’m always surprised how rare such stop sets are as a factory accessory. So far as I’m aware neither aftermarket accessory or designs for shop made systems have been produced Which surprises me as I find my 6 position bed stop very useful if doing a number of parts and a cross slide system would be at least as useful for rapid replication of accurate diameters and bores.

Clive

[Julie AnnParticipant @julieann]

I have used a Coventry diehead on my centre lathe before I got the repetition lathe. But I was using a capstan unit, with a work setting stop, rather than the tailstock to hold the diehead, so external pull off worked fine.

Julie

[bernard towers]

On bernard towers Said:

Hi John ,It’s the method of the die head opening at a precise length EVERY time, necessary when making batches of screws/studs. Basically an adjustable internal stop so that when the stock being threaded reaches the stop the die head commences to disengage itself. In the case of a 5/16 head it’s a further 3/32” which you allow for when setting the stop. Will try to post images tomorrow as only just walked in the door from visiting the Midlands Show.

The reason I asked about the “internal stop” is that I thought it could be a feature I was unaware of.

These dieheads do not have a built in “stop”.  What they have is a an automatic trip/opener.  This is activated by the stops on the machine its mounted on, typically a bed or capstan slide stop.

Semantics?  Possibly, no harm in trying to get the meanings/use correct.

Incidentally, I’ve just realised my photo of the head mounted in a centre lathe is a DX with the external trip, although I rarely use that feature, relying on the internal trip.

This head was initially used on a “Herbert Auto Robot”, a plug board controlled capstan lathe.  I was tasked with getting this (new) machine up and running.  A waste of time, soon superseded by NC the CNC, that and a change in the work that the workshop undertook.

[bernard towersParticipant @bernardtowers37738]

The stops were available from Herbert’s but not many in circulation as when used like Julie on a capstan with stops they weren’t necessary. There just happened to be a 5/16 head on a stall at the Midlands yesterday with a genuine one fitted. The only awkward part about making one is the fixing of a fitting at the rear, you just need a few stops of different dia to miss the closed die if your screw thread is very short. The screws in the tool holder are just an example of its use.

[John MCParticipant @johnmc39344]

Bernard, I understand now.  I have a couple of what I now know, are called internal stops, a plain diameter type and a “pronged” type.  I’ve not used them.  These screw into the adaptor (die head to machine as shown in my photo), to be adjusted by accessing through the lathes capstan (turret), if memory serves.  Still not part of the head though, but I can see it could be?

I’m struggling to see any great purpose for the so-called internal stop

While browsing my Herbert, (and Tooling Investment), literature I noticed something of interest to the OP.  Herbert do not recommend tail stock mounting, difficult to control thread length, apparently.

 

[bernard towersParticipant @bernardtowers37738]

The internal stop was used for the screws in the photo, the threaded stock hits the stop and the head commences to disengage itself simples. The 4 pronged version is gone into more detail in the CDH manual, worth reading

[John MC]

On John MC Said:

…..I’m struggling to see any great purpose for the so-called internal stop

My copy of the manual implies that internal stops are not commonly used, at least in a commercial environment using capstan lathes.

Julie

[bernard towersParticipant @bernardtowers37738]

I did say in a previous post that they were used on centre lathes.

[John MCParticipant @johnmc39344]

I’m not sure the type of lathe determines the type of stop used.

One of the regular jobs for my die heads was threading components made elsewhere.  Therefore, holding the work piece in the correct axial position might be thought to be difficult.  Thats what soft jaws are for…

One final comment, apologise to the OP for taking this thread for a walk!

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