Would it not depend on the size of the fixing and the thickness? Also rather depends on what you call a ‘coarse thread’ in a 3.2(?)mm plate. If I were you I would go with what clockmakers have done in the past. No point in trying to re-invent the wheel.

BA was only formulated In 1893 (I think?) so relatively recently in horological terms. It was designed at the behest of the GPO by Mr Lehman who set up it's production by his company Lehman, Archer & lane….horological use came later

Before BA, and in the great era of English clockmaking the individual clock makers made up their own 'thread plates' and taps, as did other 'techy' trades such as gunsmiths and silversmiths etc. to make a thread plate you first need to make the taps..one method is to wind wire around a carbon steel pin and then file the outline to give the thread pitch…..then you only need to cut the flutes and harden and temper. This method served the industrial revolution and up to the victorian era.

When engineering 'by the seat of your pants' like this you need to be an intuitive worker….in brass one needs to be aware of the softness and keep thread pitch large enough to give strength…..

Edited By Bob Stevenson on 12/01/2020 19:00:42

My understanding is that fine threads are always stronger than coarse ones, and that the pitch should increase with diameter (or in imperial tpi should rise with reducing diameter.)

As BA was designed for instrumentation, quite a good guide for clock making. Or if you want to be more modern, the small fine metric threads that replace BA sizes.

Strength isn't the only important characteristic of a thread. In practice fine threads are a pain because they require careful alignment, are more likely to cross-thread, and take longer to tighten and undo. Generally industry prefer to use coarser threads unless there's good reason not too. Clocks are – I think – an example.

At least 3 full turns are needed for a screw to develop full holding power, so fine threads are preferred in plate. (Always exceptions in engineering – self-tapping screws are coarse for another reason – speed.)

Meccano threads are a good example of a very coarse thread (5/32" Whitworth) cleverly used to meet a particular need. Screws in a construction toy needn't be strong and it doesn't matter much if they tend to come loose. At the same time, screws quick and easy for children to assemble and disassemble are advantageous. Otherwise 5/32" BSW is a poor choice for real work, and downright inferior in a clock – the BSW thread is too coarse for that small a diameter.

Dave

You could use other thread forms, acme, square, buttress or truncated.

Vintage engineer On a stud in a 1/8 inch thick brass plate, I doubt it

Roy     

Edited By roy entwistle on 13/01/2020 09:20:18

Although this does not really answer the question there is a brass Whitworth thread – Whitworth form and 26 t.p.i. This must not be confused with the much loved British Cycle thread which has a very different thread form. There are also microscope threads (microscopes used to be made of brass) which have Whitworth form with very fine thread pitches.

JA

A coarse pitch thread in thin material will have minimal thread engagement, (A silly example, but think about 8 tpi in 1/8" plate to get the point )

The British Standard brass is Whit form, and 26 tpi. So this would give 3.25 threads engaged without any chamfers in the plate.

My temptation would be to use ME 40, to get 5 threads engaged, even if they are only 0.016" deep.

For your low stress application this should be adequate without risk of threads stripping bin either part..

Howard

Coarse threads for strength fine threads for grip. You have already said it's not under much stress you just dont want it come loose so a fine thread is required.

regards Martin

Edited By Martin Kyte on 13/01/2020 13:28:10