Coventry 1/2″ Die Head Screws

[Peter_HParticipant @peter_h]

The four screws holding the faceplate to the body, which holds the die chasers in place, are held down by four unusual screws.. They are 14.3mm overall length and 6.05mm head  diameter, raised countersunk with a steep countersink angle. The thread has an outside diameter of 3.80mm and a thread rate of 42TPI. Length of thread is 10.7mm.

What is that thread please??

Yes, it’s close to M4 but it definitely isn’t. There are some UNC’s in the ballpark but they are 44TPI not 44TPI. I’ve hunted through the Machinery Handbook and several others but no luck. Any ideas please?

 

Peter

 

[Nick HughesParticipant @nickhughes97026]

I think this has been asked before on here and if I remember correctly, they are specials, with an oversize thread Dia for the pitch.

Could be an oversize 5BA thread.

Probably available from here:- https://threadtools.com/

 

Edit:- found the previous discussion after Nicholas posted his reply below. link:- https://www.model-engineer.co.uk/forums/topic/coventry-die-heads-2/

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Peter, maybe these two scans will help.

Regards Nick.

[Michael GilliganParticipant @michaelgilligan61133]

Following Nick’s excellent example …

Not sure, but I think this might be a good investment:

https://www.jurassictools.com/product/alfred-herbert-c-ch-style-coventry-diehead-spare-parts-list-photocopied-a4-6-pages-1-4-4-5-inch-dieheads-covered/

MichaelG.

[Peter_HParticipant @peter_h]

Thank you guys, what amazing help from you all, and to those in the previous thread which I failed to find first time.

The scans are invaluable. I don’t think I mentioned the head is a CH. The CHS seems to have had BA threads throughout. I was very familiar with BA which I worked with quite a bit in the 60’s/70’s and I am fairly sure these screws are not BA – they just feel too sharp. I could go with 55 degree thread though. It seems most folk have measured 40 TPI, rather than the 42 TPI I got. I’ve checked this with two different gauges now, and 42 does seem to just sit in there nicely.

Now, if I could just find a set of 42TPI die chasers I’d be away… :). Guess I’ll have to turn something up

Again, many thanks.

Peter

 

 

[HopperParticipant @hopper]

42TPI would be close as dammit to 0.6mm pitch.

But still looks like an inhouse special thread. Bizarre!

[Howard LewisParticipant @howardlewis46836]

Beiung a Coventry Die Head, possibly over 80 years old, the dimensions are most likely to be Imperial.

From memory, these screws are a special, not conforming to any of the usual standard thread sizes or pitches. (Possibly to make them exclusive)

It would be worth checking the thread form (55 degrees at a guess)

There have been earlier postings (Loathe to use the word “thread” in this context) on this subject, which would be worth finding. The answer might well be contained in one of those posts.

Howard

 

 

[bernard towersParticipant @bernardtowers37738]

Yes it does Howard, I posted it on the old forum but cannot find it. I think I have the info with a couple of spare screws stored with the die heads, will check today.

[bernard towersParticipant @bernardtowers37738]

The ones I have are 0.155″ x 42Wh!!!

[bernard towers]

On bernard towers Said:

The ones I have are 0.155″ x 42Wh!!!

I wonder if they used the odd pitch thread on their shouldered screws so they could not be replaced by a common fully-threaded 5/32″ BSF etc screw if the original got lost? Such a replacement would fit but probably not function correctly, leading to problems. Hence, make it unique so it must be replaced by the correct part.

Or was it simply a cunning plan to make you buy the genuine part from them? Seems a lot of faff to sell a few extra screws.

[Clive FosterParticipant @clivefoster55965]

The front plate screws have a non standard head form allowing the plate to be removed via the keyhole slots when the screws are merely loosened, not removed. A worthwhile refinement for production device.

My experience with such keyhole slots is that they don’t play well with standard form countersunk heads. (Allways assuming I didn’t cock the job up the one time I tried.)

Clive

[Howard LewisParticipant @howardlewis46836]

0.155″ is close to 5/32″ (0.15625) But the 42 tpi makes them a special

Have searched a lot of posts, but failed to find Bernard’s giving the exact vdetails of the thread and the screws.

Have looked at Tracy Tools catalogue, and nowhere do they list anything at 42 tpi.

So you may need to make them!

I can only think that scvrewcutting a 5/32 x 42 tpi thread will involve very gentle work!

Howard

[peak4Participant @peak4]

At first glance, an oversized 3.5mm might be close enough; 0.6mm is about 42.3tpi

 

Bill

[Howard Lewis]

On Howard Lewis Said:

[…]

Have searched a lot of posts, but failed to find Bernard’s giving the exact vdetails of the thread and the screws. […]

I think this must be Bernard’s earler post, Howard

https://www.model-engineer.co.uk/forums/topic/coventry-diehead/

.

.

MichaelG.

[bernard towersParticipant @bernardtowers37738]

Well done Michael I couldn’t find it for the life of me!

[DiogenesParticipant @diogenes]

Pardon me for the aside, but is anyone aware of a reason some old threads seem ‘findable’ and others just result in the ‘Lego’ page? ..seems very random..

 

[Howard LewisParticipant @howardlewis46836]

Thank You Michael

Was obviously calling for a wrong search.

So 0.155 x 42 tpi it is.

Any chance that Wisemans will have any at a reasonable proce, or will it be DIY job, I wonder?

Howard

[bernard towersParticipant @bernardtowers37738]

If you ring wisemans do it sitting down!!!!

[Howard Lewis]

On Howard Lewis Said:

Thank You Michael

Was obviously calling for a wrong search.

So 0.155 x 42 tpi it is.

Any chance that Wisemans will have any at a reasonable proce, or will it be DIY job, I wonder?

Howard

Rotagrip also sell spares, but they are price on application

Bill

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