Tap and Die Puzzle

I have inherited a Sealey "Professional Tap & Die Set 40pc Metric, Model No. AK3012" which contains taps and dies for the common threads from M3 to M12. It also contains some less common metric fine threads from M6 to M12, and the ubiquitous 1/8" NPT.

However, I am completely stumped by three of the threads, viz:

• M3 x 0.6
• M4 x 0.75
• M5 x 0.9

My first thought was that they might be M3.5, M4.5 and M5.5 (if there is such a beast) mislabelled, but measuring them shows that the diameters given are correct. The pitches are all coarser than the standard threads

I have searched my technical reference books and the interwebs without avail. Has anybody come across these threads, or have any idea what they would be used for?

George B.

Edited By Georgineer on 29/11/2020 18:35:31

That triggered a vague memory. In a copy of Machinery's Handbook from August 1943 all the threads listed in the OP are in a table headed "French Standard for Metric Threads". What we now consider to be the standard pitchs for M3, M4 and M5 are not listed. The notes say that the French system followed the ISO system between 6mm and 90mm, but below 6mm they did their own thing. Even more bizarrely for threads less than 3mm the thread angle may be 60°, or 55°! There's also a table for a German Metric Thread that starts at 12mm, but with different thread depths and clearances from ISO, so not standard.

So much for standardisation.

Andrew

I have a load of M5 x 0.9 taps.

They came from a French workshop suppliers that our company took over a few years ago.

Phil

I dipped into my M4 box to tap a hole one day and found that the screw didn't fit the thread, on closer inspection the tap was an M4 x 0.75, quite irritating

M3 x 0.6 are used in French built Bulle clocks. Mike

Google is your friend, Metric Fine

MFine

Theses threads are from the SI standard used before the ISO one.
J M

Ahh! The beauty of metrication - it allows everone to have their own standard!

Check out P.A. Sidders "Guide to World Screw Threads" originally published as "Machinery Screw Thread Book" in the UK before being published by Industrial Press Inc. in the USA - it lists something in the order of 14 different variants of M6 for instance!

The great thing about standards is that there are so many to choose from.

Thank you all for your input. The likeliest answer seems to be that they are Systeme Française threads, though I am completely at a loss to explain why they would be included in a modern tap and die set aimed at the UK/US market.

Jacques, can you say how much the System Française is still used, say in French cars? I presume that is is obsolete in new designs like the British and Unified threads which have gradually been phased out in industry since about the 1970s, but are still widely used by restorers and old fogeys.

George B.