Martin Screw Threads

Watchmaker's screwplates are often found bearing makes names such as Martin, Martin et fils or Perrelet and Martin. There does not seem to be much information around on what threads these plates actually cut. Hence I have collected as much as I can find.

Series

This is denoted by a letter at the top of the plate

L Latard (also called Perrelet and Martin)
B Bourgeaux
G Gauche (Left Handed)

Number

The number at the top of the plate is the number of holes in the plate (typically 36, 30 or 24). Conventionally there are two holes for each thread size in the plate (in case one becomes blocked, by a workpiece breaking off when in the process of being tapped). Each hole is numbered in the series which runs from 0 to 21. Typical examples

24 plate 5 through 16
30 plate 4 through 18
36 plate sizes 4 through 21

Sizes

Saunier contains the following advice.

For a long time we have made use of two Latard screw plates with holes of sizes such that a spindle which just passes into a hole of a given number will be of size most convenient for forming a screw in the next smaller hole.

The BA standardization committee report mentions the following ranges of screw plates sizes and their equivalent in the BA system

Plate BA

21 sizes Latard 15

23 sizes Bourgeaux 17

36 sizes Latard 23

Latard

Empirical measurements from screwplates available to me suggests

Latard Plate Sizes
Size	Diameter (mm)
6	1.05
17	0.48
18	0.42

Bourgeaux

Taken from A. B. Perkins, The Watchmaker's lathe and how to use it, the author measured them from a screw plate.

Bourgeaux Plate Sizes
Size	Diameter (mm)	Pitch (mm)
0	2.10	0.3703
1	1.95	0.3703
2	1.80	0.3125
3	1.65	0.2941
4	1.50	0.2941
5	1.35	0.2439
6	1.30	0.2777
7	1.20	0.2500
8	1.10	0.2272
9	1.00	0.2272
10	0.90	0.1923
11	0.80	0.1724
12	0.75	0.1724
13	0.70	0.1562
14	0.65	0.1470
15	0.55	0.1388
16	0.50	0.1086
17	0.45	0.1136

Thury System

The Thury system (which is a precursor to the BA system is defined through the pitch equation

P= 0.9

ⁿ mm

The diameter is given by

D= 6P

^6/5mm

Thury Thread Sizes
Size (n)	Diameter (mm)	Pitch
0	6.00	1.00
1	5.29	0.90
2	4.66	0.810
3	4.11	0.729
4	3.62	0.656
5	3.19	0.590
6	2.81	0.531
7	2.48	0.478
8	2.18	0.430
9	1.92	0.387
10	1.64	0.349
11	1.49	0.314
12	1.32	0.282
13	1.16	0.254
14	1.02	0.229
15	0.901	0.206
16	0.794	0.185
17	0.699	0.167
18	0.616	0.150
19	0.543	0.135
20	0.479	0.122
21	0.426	0.109
22	0.372	0.0985
23	0.328	0.0886
24	0.289	0.0798
25	0.254	0.0718

Further reference

Randall C. Brooks. Standard screw threads for scientific instruments.
Part I: Production techniques and the Filiere Suisse History and Technology, volume 5, pages 59-76 (1988).

Archie. B. Perkins. The Modern Watchmakers Lathe and How to Use It. AWCI (2003)

Plate	BA
21 sizes Latard	15
23 sizes Bourgeaux	17
36 sizes Latard	23