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

Empirical measurements from screwplates available to me suggests

Latard Plate Sizes

Size Diameter (mm)

17 0.48

18 0.42

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

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).

Randall C. Brooks. Standard screw threads for scientific instruments. Part II: The British Association Screw Gauge. History and Technology, volume 6, pages 45-59 (1988).

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