Thread identification

[david gregg 1Participant @davidgregg1]

What is is this unusual thread

[david gregg 1Participant @davidgregg1]

I have a Coventry die head and it is missing one of the machine screws which hold on the front cover plate ,the screw measures .172 o/d and is 40 tpi ,I have tried all the thread charts and cannot find what the thread is , was this a special thread used by the manufacturer.
Hopefully somebody has come across the thread ,but I will probably end up having to make one

[peak4Participant @peak4]

From what I've read elsewhere, some of the threads used were specials.
It was argued that it was so that you had to buy spares from the manufacturer.

It seems to fall in between two model engineering 40tpi threads, so if you have Herbert chasers for one or the other, you could probably use the head itself to make them.
There's a table HERE

Bill

[bernard towersParticipant @bernardtowers37738]

Yes it is a special but they are available from the firm that now owns them (the name escapes me at the moment) it is a company that makes thread rolling heads. From memory the are about a fiver each!. I am surprised that you have only one missing most of the ones I’ve seen only ever have two screws left in them!

[bernard towersParticipant @bernardtowers37738]

Try wiseman threading tools

[Howard LewisParticipant @howardlewis46836]

Very slightly rounded, 11/64" is 0.1719" and is listed in Zeus Charts as 0.1719.

No 40 tpi ME threads are listed as 11/64

None of the figures quoted match BA or BSP.

So it may well be a "Special" 11/64 x 40 tpi., exclusive to the Coventry Die Head

What is the thread form? 55 degrees is Whit, which is what I would feel inclined to expect, but if it is a special it could be different.

Howard

[martin haysomParticipant @martinhaysom48469]

nothing in my caxton engineering workshop data [1947] but if its helps all mention of 40 tpi is in relation to whitworth thread form

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