Evolution of screw threads

[rotrax]

An interesting thread is Brass Thread, (sometimes improperly called a standard). A fine thread, 26tpi through the range, stemming, I believe, from when brassware was hand-turned, and the turner could traverse the screwcutting tool at a constant rate irrespective of the diameter.
Re. metrication, I once owned a pre-war Morris car which had metric fine threaded bolts with imperial-sized hexagons.

IIRC Brass is the same as Cycle Threads-26TPI-and buying Brass taps and dies was always much cheaper than buying Cycle ones once restoring old motorbikes became popular.

The suppliers cottoned on to charging more for Cycle pretty quickly, but missed that Brass Threads were completly interchangeable in most sizes.

As an aside, my 1930 Rudge motorbike-the one I won my first race on-had 9mm across flats nuts with a 5/16 cycle thread bore holding the cylinder to the crankcase. Good thing there were plenty of them, as the walls were pretty thin!

 

[earlybird]

Cycle thread is 60 deg. so not quite the same as brass thread, which is 55 deg. There's a standard, BSC, quite a number of different pitches over the range of diameters, apart from 26 tpi.
Rudges on the race-track! That's going back a bit.

 

[davidej]

Re. metrication, I once owned a pre-war Morris car which had metric fine threaded bolts with imperial-sized hexagons.

This is because William Morris bought a job lot of machine tools from a French motor company that was going out of business. These tools could only cut metric threads but he got the heads made to fit Imperial spanners as the garage mechanics would not have metric ones.

This carried on, to my certain knowledge, at least until the passing of the MG T-Types (1955)

 

[JumbleDuck]

And I seem to remember that 2BA and 6 mm are the same.

To the horror of imperial nostalgists, BA threads are metric! For nBA, the pitch p_n = 0.9ⁿ mm and the outside diameter D_n = 6 p_n^1.2 mm, starting with 0BA at 6mm diameter, 1mm pitch.

Do any of the electrical engineers here know why odd numbered BA threads are more-or-less ubiquitously and more-or-less only used in electrical devices? Everywhere else uses the even numbers.

 

[JumbleDuck]

This is because William Morris bought a job lot of machine tools from a French motor company that was going out of business. These tools could only cut metric threads but he got the heads made to fit Imperial spanners as the garage mechanics would not have metric ones.

There were all sorts of metric threads floating around before things were standardised in (I think) the 60s. For example, M5 is now 0.8mm (coarse) or 0.5mm (fine) pitch, but in the 50's Citroën committed their systems to M5 with a pitch of 0.75mm, and went on using them till 1970.

 

[halcyon]

To the horror of imperial nostalgists, BA threads are metric! For nBA, the pitch p_n = 0.9ⁿ mm and the outside diameter D_n = 6 p_n^1.2 mm, starting with 0BA at 6mm diameter, 1mm pitch.

Do any of the electrical engineers here know why odd numbered BA threads are more-or-less ubiquitously and more-or-less only used in electrical devices? Everywhere else uses the even numbers.

Have a read https://en.wikipedia.org/wiki/British_Association_screw_threads

Brian

 

[JumbleDuck]

Have a read https://en.wikipedia.org/wiki/British_Association_screw_threads

Still can't see why so much electrical stuff uses odd numbered BAs.

 

[penberth3]

Still can't see why so much electrical stuff uses odd numbered BAs.

Not something I've ever noticed. Have you got any examples?

Because the sizes are so close together, I can see the sense in going for the evens, 0 2 4 6, which does seem to have happened.

 

[penberth3]

There were all sorts of metric threads floating around before things were standardised in (I think) the 60s. For example, M5 is now 0.8mm (coarse) or 0.5mm (fine) pitch, but in the 50's Citroën committed their systems to M5 with a pitch of 0.75mm, and went on using them till 1970.

ISTR finding an "in between" pitch on a Fiat truck, many years ago, so possibly the Italians had their own system as well.