Comparing the drill bit diameter to the screw diameter

Clock Repair: My Meager Knowledge of The Hardness of Metals

It’s taken me a while to learn some basic metallurgy I need for clock repair. When I started I scratched up my brass clock plates by cleaning with SOS pads – steel wool – because I didn’t know that steel is harder than brass. In this post I collect what I’ve picked up in this metals game of Scissors, Paper, Rock.

Scratches from cleaning brass with steel wool
Scratches from cleaning brass with steel wool

If you look carefully at the above photo, you can see the lines scratched into the brass plate. That happened because I didn’t know that steel (SOS pads) is harder than brass. The Clock Repair Basics books I’ve seen don’t cover this very basic thing: that harder stuff scratches softer stuff, and what is harder than what.

Jewelry Notes has a nice chart of metal hardness. Here I go through what that means to clockmakers and clock repairers.

Here’s what I’ve learned so far:

One big point: The harder the metal, the more brittle it is. Carbide drill bits can easily snap off, but brass simply bends.

Brass is soft.

Work Hardened Brass is slightly harder than normal brass. Work hardening happens when brass is bent or hammered, such as when you use a smoothing broach to work harden a pivot hole so the pivot hole resists wear better.

Annealed Brass is softer than Work Hardened Brass. I only recently learned that you can anneal brass in the same way you anneal carbon steel: by heating to a particular color, then letting it cool slowly.

Carbon Steel is harder than brass. That’s why brass pivot holes wear more than steel pivots, and why steel wool scratches brass parts. It’s also why steel tools scratch brass parts and steel drills and steel saw blades cut brass.

The hardness of steel is complicated because carbon steel can be hardened by heating to a particular temperature and then quickly cooling by quenching in water or oil. Carbon steel can be softened (Annealed) by heating to a particular temperature and then letting it slowly cool in the air. Notice the big difference in hardness between Steel and Hardened Steel in the Jewelry Notes Chart.

There is an art to watching the color of the heated steel to know how hard it will be when it’s quenched – I don’t know much about this yet. Wikipedia has a good table of colors of steel. For the engineering-minded, I found this information about those steel colors.

For example, in his video on How to repair a broken mainspring end, Chris heats the end of the mainspring, then lets it cool slowly so he can drill and file a new end hole in the spring.

Carbide is harder than hardened steel. That’s why carbide drills are designed to drill steel and carbide tools are made to cut steel drill bits.

Stones and Minerals. I’m currently bewildered by this topic. There are various stones that can sharpen or grind carbide and other tools. Diamond-tipped or diamond-grit stones are also used.


Please add to my meager knowledge on the Twitter thread about this post.