Thread Types

[Bruce_in_Oz]

As someone who has cut a few threads in his time, I find the thread data for small arms quite fascinating.

The trick with a lot of the British threads is that they were developed at Enfield and standardised for small arms production because even in the mid-19th century, "engineering standards" were still a "personal" thing.

Martinis and Lee-Enfields used "Enfield specials". Some of them are downright wierd in comparison to modern practice. In those early series threads, about the only one that is close to a modern thread is the stock bolt, which is Whitworth in pitch/diameter, but a little different in form.

No4s are full of BA threads, with the exception of the stock bolt and the front trigger-guard screw, which is BSF. BA (British Association) threads are themselves derived from a Swiss clockmakers series (Thury), hence the gently rounded form that was designed to prevent tearing in softer materials like brass. "0" BA is has essentially the same diameter and pitch as M6 (coarse) but with the special 47.5 deg form. Each BA "increment" is 0.9 times the size of the previous one; i.e., to get "1 BA" dimensions, multiply "0 BA" by .9. This keeps the overall proportions of the screw series identical: most "modern" thread series use the more practical but less elegant "nearest to theoretically perfect" ratios found on common gear-boxes of various machines. Further trivia: "14 BA" has the same major diameter as M1 coarse; 1mm / 0.0394".

It got weird in Australia when Lithgow was set up with a plant build by Pratt and Whitney in the US. P&W used the "P&W inch", Enfield used the "Enfield inch" They are NOT the same. There used to be a chart in the old Lithgow archives that plotted the difference. Apparently, there was a certain amount of eyebrow raising when the first Lithgow samples arrived at Enfield.

The FORM of early Enfield threads is almost as odd as a lot of the pitches (try cutting 33 or 37TPI on a basic lathe): radiused roots and crests like a BA but with strange flank angles. The "Peddled Scheme" of sub-contracting in WW1 must have been a tool-makers nightmare.

It is not surprising that No4s went to general "industry standards" for mass production.

Then again, Arisaka Type 38s are full of "Imperial" spec. threads, as are Mausers. Why? Because the machinery used to build them was made in The US or Britain and, at the time, those two countries were the only ones that had formally set out to "standardise" standards AND were making the machines to make other machines to make the components. Note that the later Type 99 Arisaka went "METRIC" The entire Japanese ship-building industry was built around British practice and still retains quite a bit of "Imperial" metrology.

Then again, until recently, the global bicycle industry was base on CEI (Cycle Engineers Institute) threads, which are generally 26 TPI, regardless of diameter. That certainly simplified the gearbox in your thread-cutting machinery!

/Anorak / Beany-copter Off