Hardening/tempering watch fusee ratchet pawls

[AlanB]

I find these very fiddly to make to replace fusee pawls which have worn out - they are not too bad to make from a turned blank with a spigot to go in the hole in the fusee plus some filing (usually aided by supergluing the part to a convenient steel handle), but then the question arises of hardening and tempering. I normally do this as per screws, heat to red heat in a little container tipping into water, polish and then blue in a pan over the spirit lamp, but then they need to be riveted in and at this point I find they break very easily, presumably because too hard around the rivet.

If they were bigger I'd just harden the ratchet tip (and use a shoulder screw rather than a rivet)

Does anyone have tips to make this easier?

Edited June 20, 2024 by AlanB
rivet not ratchet

[nickelsilver]

Depends a lot on the steel you're using. In the States O1 is a standard steel used for parts like this. It's an oil hardening steel (the O). There's also W1, and O2 (haven't been able to discern an actual difference between O1 and O2).

 

If you harden an oil hardening steel in water it gets even harder- to the point it may crack at stress points before you get to tempering. In the same vein, if you need extreme hardness you might harden an oil hardening steel in water for something like a cutting tool. In very extreme cases you may quench in salt water brine or even mercury.

 

For most watch parts it's good to harden, then temper to a dark blue. But depending on the steel a light blue may be more appropriate. Also, if not using an oven for the hardening stage, you can be off in temperature enough that you change the metal to a degree that even tempering past light blue results in brittle parts.

 

I also use a little container (ex- CO2 or NO2 cartridge, top cut off, welded to a steel rod), filled with finely ground wood charcoal. Parts at the bottom of that. The charcoal alights, burns out, looks like lava moving around for a while, then calms down. That's when I quench. The charcoal doesn't add any carbon, but it excludes oxygen enough that the parts come out grey. Tempering to color is easy then. The charcoal is ground in an espresso grinder at its finest setting- if you don't want to spring for a commercial level espresso grinder, you can buy it at health food shops and it's even more finely ground.

 

Steels I use are O1, Sandvik 20AP, and Klein Law 100x (same as 20AP minus the tiny bit of lead Sandvik used). O1 is clearly the best for cutting tools, the other two great for parts. Just made some 4th wheel pinions with extra long pivot in 20AP/Law 100x and no deformation in hardening.

Edited June 20, 2024 by nickelsilver

[nevenbekriev]

In the last 15 years I have worked mostly on fusee movements. Mostly verge, but english lever too. I have found that problems in the ratchet works in the fusee is common for such movements, so I have done many of them. I can not refer to something written in a book, but what I have found is that the ratchet pawls are not hardened. This is may be because the riveting in place, if they were hardened, then the degree of riveting needed would be not possible without breaking. I make this only parts in the watch movements from ordinary nails as material. The nail is harder than soft steel because of the plastic deformation in time of it's ptroduction and is just what is needed for the purpose.

Edited June 21, 2024 by nevenbekriev

[AlanB]

Actually thinking about it - the pawl to be replaced was clearly not hardenend, that's probably why it wore in the way it did, maybe the idea is for the pawl to wear rather than the ratchet wheel. 

As people like photos here is a (soft) one that I have made up

But I like the charcoal idea for hardening other parts without scale built up, will have to try that.  The surplus place often has old coffee grinders.

For steel we have the UK/US terminology differences, I've been using gauge plate and silver steel (maybe drill rod in us terms).  But I'll see if I can get some O1 round.  

[SwissSeiko]

On 6/21/2024 at 4:14 AM, AlanB said:

 

But I like the charcoal idea for hardening other parts without scale built up, will have to try that.  The surplus place often has old coffee grinders.

 

 

Another option is to use boric acid and mix it with denatured alcohol. Prevents scale buildup as well

[AlanB]

So following ideas above I repeated but only hardened the tip of the pawl then tempered the whole lot to pale blue. This was fine and the ratchet is now working. as I was able to rivet without breaking it. Scale build up control was very helpful as I find polishing steel slow work so the less I have to do the better!

[bramu]

Just for reference here are some equivalent standards of O1

ISO 95MnWCr5

DIN 1.2510 

B.S. BO 1 

ASTM A681

Here is a silver steel data sheet that includes a tempering curve.

https://www.coventry-grinders.co.uk/app/uploads/2014/03/Silver-Steel-Data-Sheet.pdf

At 300C (572F) tempering temp it's still around RC53 (Rockwell C scale hardness) which is still quite hard. According to steel tempering color charts I looked at deep blue is around 300C.

My guess is that the closest current alloys to what was used originally would be SAE 1075 or SAE 1095. These are plain carbon steels with a bit of manganese. For reference 1095 blue temper is RC50 and 1075 blue temper is around RC45.

SAE1095 & 1075 are similar to Swedish steel of the time which would have been available in England. England made blister steel and later crucible steel. There would have been quite a bit of variability in the the chemistry of the steels of the time. The old sources I have read recommend conducting heat treatment tests of each new batch of steel due to this.

That's a long winded way of saying that the hardness of "blued steel" has likely increased over time. To complicate things a bit more alloy steels may have less brittleness at higher hardnesses than plain carbon steels.