There are several notes by various authors on the best types of Brass and Steel for use in watchmaking. Here is a summary of those consolidated notes from various sources.
Brass Alloys for Watchmaking
TL;DR: Use Engraving brass e.g. CZ120, CZ121 (CZ118 and CZ119 are also acceptable). Brass is used for wheels, but can also be used for many other elements in the movement, ranging from plates to escape wheels (as seen in [Daniels]). Brass is available in different alloys to make it more machinable.
[Wild] Notes on Brass:
“For modern clocks the material most suitable is CZ120 compo engraving brass, with the addition of lead to aid free machining. Slices of normal extruded brass bar CZ121 can also be used; this can be parted off at the approximate thickness. It is also useful if a small wheel or brass pinion is to be cut, as it can be machined straight from the bar.
“For modern clocks the material most suitable is CZ120 compo engraving brass, with the addition of lead to aid free machining. Slices of normal extruded brass bar CZ121 can also be used; this can be parted off at the approximate thickness. It is also useful if a small wheel or brass pinion is to be cut, as it can be machined straight from the bar.
Normal rolled brass sheet CZI08 is not really suited for either wheels or plates, as it tends to be 'tough' and sticky when machining due to the work-hardening process in manufacture; it is therefore difficult to maintain a sharp edge after cutting. Problems are also experienced when crossing out the wheel (cutting out the spokes) with the piercing saw, as the blades snap like carrots!”
“The most common type of brass used for watchmaking is an alloy of 60 per cent copper and 40 per cent zinc, which is yellow in colour and can be cut, drilled and threaded, etc., with ease. It can be bought in hard-rolled sheet or strip form. In these conditions the stresses introduced by rolling will cause bowing or warping if the surface is machined, as for example when preparing the surfaces of a watch plate. However, the stresses can be relieved by heating to 400°C for thirty minutes and allowing to cool slowly. The heat will reduce the hardness induced by the rolling but the metal will remain hard enough for the watchmaker. The slow rate of cooling will prevent further softening, which will occur if the hot metal is quenched, and, by precipitation, will reintroduce some of the lost hardness.
Where possible the material for wheels should be obtained in the correct thickness. These are best made from the hard-rolled metal, which will warp to cause unequal thickness of the blank if the surface is machined to reduce the thickness. The addition of up to 1.6 per cent of lead will help reduce burrs and produce a better finish from the cutter. The alloy most commonly recommended is in the order of 60 per cent copper, 38.4 per cent zinc and 1.6 per cent lead.
It is sometimes suggested that brass for making escape wheels should be extra hardened by hammering. In my own experience this is not necessary. It is only necessary that the material is hard enough to withstand deformation during the wheel's manufacture and the hard-rolled condition would be adequate for this. If the impact of locking has any effect it will be to work-harden the contact points. If the friction of unlocking is high enough to cause wear then the escapement could not function. Escape-wheel teeth will show signs of wear only if the jewels are incorrectly polished. It is especially important to ensure that the corners of chronometer locking stones and lever pallet stones are not finished to a sharp edge that can cause wear to the tip of the tooth by scraping.
As with steel, the addition of 3 per cent to 3.5 per cent lead will produce free-cutting brass, with swarf in the form of powder or small chips. However, this brass is too brittle for fine components and will offer lower resistance to deformation. It is particularly unsuitable for wheels with delicate teeth.”
“Brass for wheels should be hard or half-hard leaded engraving brass such as CZ 118, CZ 119 or CZ 120, with a hardness of 120-130 VNP. Do not use soft brass which will clog the cutter and spoil the work.”
[Adventures] says he uses Leaded type 360 round and square rod and for sheet he uses Leaded type 353 engravers brass.
Steel Alloys for Watchmaking
TL:DR: Pinions and small parts which need to either be either robust or hardwearing need to be made out of a high-carbon steel alloy which is either water hardened or oil hardened (the method varies so check for your alloy) such as “O-1” “tool steel” (aka “Drill Rod” or “Silver Steel”). Such steel is supplied in an annealed state, you must then both harden and temper the steel. Low carbon steel (aka mild steel, cold roll steel/CSR, free machining steel) can be used for larger or non-critical parts, this is more free machining, but cannot be hardened or tempered.
“Until recently, the most commonly used steel for pinions was a free-cutting silver steel; its reference was KEAI08, and it was manufactured by Sanderson Kayser in Sheffield. As anyone who has machined normal silver steel will know, it is tough, and it is not easy to obtain a good finish. KEI08 had an addition of 0.2 per cent selenium,which gave it its free machining properties. This is now considered to be a dangerous substance,however, and can no longer be used in steel making. Alternative steels for pinions are EN24t, EN8DM and silver steel.
EN24tEN24t CBS Spec. 817 M40) is a steel supplied already hardened and tempered. Whilst it is tough, it will machine quite well, and a good finish can be obtained. It is a nickel chromium molybdenum steel with a carbon content of 0.36- 0.44 per cent. The advantage is that once machined and polished, no further heat treatment is required. Sizes available are from 6mm CY<in) dia. upwards, and equivalent metric sizes, and the condition of finish is bright drawn.EN8DMA steel that is more easily machined than EN24t is EN8DM CBS Spec. 212 A42), a medium carbon steel. Whilst it can only be hardened by flame or induction-hardening methods, it will give reasonable results for long-term wear. The steel has a carbon content of the range 0.40- 0.45 per cent, but with a higher sulphur content than normal EN8, which gives improved machinability. Sizes available are basically as the EN24t range.Silver SteelFinally there is silver steel, a high-carbon tool steel. This varies considerably in machinability, but once finished and polished, will give excellent long-term wear results. Carbon content 'C' is 1.10- 1.20. It is supplied in the annealed condition, and is available in centreless ground lengths in sizes from 3mm to 40mm CYsin- l Y<in) dia. Smaller sizes than 3mm are also available in the drawn and sometimes ground finish; this makes the material extremely suitable for arbors as well as pinions. Hardening and tempering can quite easily be carried out in the home workshop without any specialist furnace or heattreatment facilities, although more controlled results can be achieved by the use of a small muffle or pot furnace, which can be either made or purchased. With some form of temperature control, improved heat-treatment results can be Achieved.”
“A carbon content of between 1 per cent and 1.5 per cent will harden if quenched in oil at red heat. A lower carbon content may need a higher temperature and quenching in water. However, this treatment is likely to cause the component to warp if it is an irregular shape. Oil-hardening steels are easier to control but may not polish or blue so well. A steel which contains additives to enhance its hardening properties should be tested first by completing the heat treatment and finishing stages so that their effects may be fully understood. Steel hardened and tempered to a blue colour will turn smoothly and cleanly with a hand graver.”
“Watch pinions are usually made of oil (0-1) or water hardening drill rod containing enough carbon to assure hardenability. For example, one American watch company used SAE 1095 high carbon steel for watch winding wheels, and SAE 10120 for balance staffs and pinions. Because of the small quantities we need, it is often difficult to obtain specific steels for our use. Realistically, nearly any good oil or water hardening drill rod will contain enough carbon for our purpose.”
- Uses O-1 tool steel (aka Drill Rod or Silver Steel) which is machined and then heat treated. He uses this for all watch parts which require hardening. He uses L.S.Starrett brand which is available in metric sizes. Alloys include ASTM-A681-94 and AISI/SAE O1.
- He also uses Low Carbon Steel (aka mild steel, cold roll steel/CSR, free machining steel). A common alloy is 1018 but he finds that the leaded 12L14 is more machinable.
- He uses M-42 HSS (8% cobalt) high speed steel for making hand gravers.
Steel Clock pinions may be cut from silver steel which has been fully annealed (it is supplied in normalised condition), but steel with a lower carbon content such as EN8 or EN9 are to be preferred.
I also note here that a popular low carbon steel which is free machining is 12L14, which in the UK is also known as “Bright Mild Steel” or “EN1A”.
