Brown & Sharpe screw missing

[David BroadbentParticipant @davidbroadbent33226]

I have just reassembled an old Brown &Sharpe dividing head. A fascinating project which I have enjoyed – but the problem I have is that one of the screws is missing! The particular screw is that which holds the detent pin in place which bears on the direct indexing pin. See pictures. Elsewhere on the unit I have found the threads to be unlike anything I have previously met – so I'm struggling to know how to replace this one? I have measured the hole at 0.19", which would be the thread root diameter. The screw should ideally have a countersunk head, the maximum diameter of which should be about 0.32". Length of the screw would be about a half inch I would guess.

Does anybody out there have any brilliant idea of how this screw might be replaced – or ideally have just such a screw kicking around in some old oddments tin?

 

Edited By David Broadbent on 25/01/2020 19:36:34

[David BroadbentParticipant @davidbroadbent33226]

[old martParticipant @oldmart]

You need to get an idea of the thread pitch. I would stick something like a match stick down the hole, and then the right diameter small screwdriver down the hole alongside it to squeeze it against the thread. Remove the screwdriver and then the match stick, and with luck, there will be an impression of the thread for you to measure the pitch.

[Jeff DaymanParticipant @jeffdayman43397]

Could be 1/4"-28 UNF thread, if you can find one of those near you. The div head probably had a round drum-shaped slotted head screw. A modern socket head screw may fit.

[Clive FosterParticipant @clivefoster55965]

David

As Brown and Sharpe is an American company I'd expect American threads. 1/4" ANC / UNC has a core diameter of 0.1887" which is in reasonable agreement with your 0.19" measurement and is 20 TPI.

Standard US countersunk machine screw head for 1/4" is 0.507" diameter but, as I'm sure you have already found elsewhere normal practice for this sort of equipment was to use smaller diameter heads with a parallel portion above the countersink cone. Tricky bit is whether they are 80° or 100° nominal countersink angle.

Yup les crazy yanks used both!

I'd be unsurprised to discover that the basic dimensions were basically as per the Slotted Undercut Oval Countersunk Head Machine Screw but of reduced diameter with a flat, rather than oval top. So the countersink angle would be 80° nominal and overall head height about 0.18" and diameter 0.30" or a shade under. If the dividing head is was old as I think probably 19/64" diameter.

Machinery's Handbook is your Frenemy for this sort of thing. Nearly everything is in there but finding it is different matter! As is making sense of what it all means when you have found it!

Clive

[old martParticipant @oldmart]

Looking at the photograph, I have my doubts about the thread being 1/4 20 UNC, it looks too fine to me. I would remove some of the other screws and measure them to get an idea of what threads B & S were using. It is common practice to use coarse threads in cast iron, but not universal, Smart & Brown lathes use BSF rather than BSW in their castings.

I think it might be 1/4 28 UNF.

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