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

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
SizeDiameter (mm)
61.05
170.48
180.42

## 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
SizeDiameter (mm)Pitch (mm)
02.100.3703
11.950.3703
21.800.3125
31.650.2941
41.500.2941
51.350.2439
61.300.2777
71.200.2500
81.100.2272
91.000.2272
100.900.1923
110.800.1724
120.750.1724
130.700.1562
140.650.1470
150.550.1388
160.500.1086
170.450.1136

## Thury System

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

$P= 0.9nmm$

The diameter is given by

$D= 6P6/5mm$

Size (n)Diameter (mm)Pitch
06.001.00
15.290.90
24.660.810
34.110.729
43.620.656
53.190.590
62.810.531
72.480.478
82.180.430
91.920.387
101.640.349
111.490.314
121.320.282
131.160.254
141.020.229
150.9010.206
160.7940.185
170.6990.167
180.6160.150
190.5430.135
200.4790.122
210.4260.109
220.3720.0985
230.3280.0886
240.2890.0798
250.2540.0718

## 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)