Strange thread on an Archimedean drill

This topic has 10 replies, 7 voices, and was last updated 25 April 2026 at 07:27 by rjenkinsgb.

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10 April 2026 at 12:09 #844025
Andrew Tinsley
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@andrewtinsley63637
I have a very old Archimedean drill with a Brass type pin chuck to hold the drill. The nose of the pin chuck has been lost and I need to make another. The thread has me stumped. It is 5/16″ diameter and 25 tpi . I have checked the thread angle and it is 55 degrees and The 25tpi is correct (checked on a projection screen).

I cannot find such a thread, is it a special? The details of the thread are correct. There are a couple of threads that are almost the same, but they are not 25 tpi.
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10 April 2026 at 13:04 #844035
noel shelley
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@noelshelley55608
5/16″ BSB is 26 TPI so could be a candidate ? The thread your checking is short ? Or an 8mmX 1mm pitch gives 25.4 TPI Just some thoughts. Noel.
10 April 2026 at 13:07 #844037
ChrisLH
Participant
@chrislh
Could it be 1 mm pitch ? I’ve searched my copy of Robertson’s “Guide to World Screw Thread Series” and found 24 tpi, 26 tpi and 27 tpi but no 25 tpi. There are some weird threads though e.g. the Australian “7/8 Skein” with a pitch of 2 and 2/7 tpi !
10 April 2026 at 14:33 #844051
Andrew Crow
Participant
@andrewcrow91475
Tooling manufacturers have regularly used their own unusual threads, usually so you had to buy their spare parts, Alfred Herbert were very good at it.
Back to the problem in hand, is it possible to take the drill apart and reduce the thread to 9/32 × 26 or 32 t.p.i you should then be able to make a new nose piece to suit.
Andy.
10 April 2026 at 15:06 #844059
Andrew Tinsley
Participant
@andrewtinsley63637
Thanks everyone,

I am just intrigued as to the origin of the thread. I can easily screwcut the required thread, that isn’t a problem.

I have a volume that covers just about every thread you can think of (including jam jar threads etc!!!) The mystery thread isn’t there.

It is certainly 25tpi, there is 1/2″ of thread so it isn’t a measuring error. I have not considered an 8nmm x 1mm thread. The drill belonged to my grandfather and I suspect it is WW1 vintage, maybe it is of continental origin, so I will check to see if the 8mm x 1mm thread is possible.

Thanks,

Andrew.
10 April 2026 at 20:50 #844107
Andrew Tinsley
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@andrewtinsley63637
Just carefully checked the 8mm x 1 mm thread suggestion. It is very close but not quite the same as the 25,tpi mystery thread. The differences can be clearly seen on the projector. An 8mm x 1mm metric fine nut screws onto the thread and just starts to bind when fully screwed on.

Conclusion must be that it is a manufacturers special! Thanks everyone for your input, mystery seems to be solved.

Andrew.
11 April 2026 at 01:01 #844129
Pete
Participant
@pete41194
Fwiw Andrew,

5/16″ X 25 tpi was also used on British or older machinery, and for specialized automotive applications. Google gives an example of it being used on Morse Minor vacuum pipes. Since I’ve never owned one, then I can’t say for sure that’s true or not.
11 April 2026 at 02:16 #844130
Idler
Participant
@idle1
<p style=”text-align: left;”>Standard threads are only for fasteners, so a nut made anywhere in the world will fit a bolt made anywhere, to the same standard.</p>
Machine parts don’t have to fit anything except the matching machine part from the same manufacturers. So they very commonly don’t use standard threads. Cars and motorcycles through to kitchen appliances etc are full of whatever combination of diameter and tpi the maker chose to use.
<p style=”text-align: center;”></p>
11 April 2026 at 08:47 #844150
Andrew Tinsley
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@andrewtinsley63637
Agreed Idle One, If you read my original post you will see that I surmised that the thread was a special. as indeed it turns out to be, so your explanation is somewhat wasted.

Andrew.
11 April 2026 at 12:22 #844188
Idler
Participant
@idle1
On 11 April 2026 at 08:47 Andrew Tinsley Said:

Agreed Idle One, If you read my original post you will see that I surmised that the thread was a special. as indeed it turns out to be, so your explanation is somewhat wasted.

Andrew.

You specifically asked “is it a special?” And said “It’s got me stumped.”

Sorry if I wasted my time answering your specific question. I won’t make the same mistake again.
25 April 2026 at 07:27 #845758
rjenkinsgb
Participant
@rjenkinsgb
Your odd thread matches a 5/16 “Douglas” thread, which was an odd standard used in the early 1900s, up to around the WWI era.

(Douglas motorcycles used both that and a 5/16″ x 26tpi BSC (British Standard Cycle) thread in different parts of the machines, which sound like a recipe for confusion!)
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