Hardening clock pinions in EN8 steel

I am about to start making some pinions for my current clock project and have both silver steel and EN8 steel available. I have previously always used silver steel but note that Malcolm Wild, in his book 'Wheel and Pinion Cutting in Horology' advocates EN8 steel for its free cutting properties. He states that hardening can only be achieved by 'flame or induction hardening methods' which are outside the scope of a home workshop. Malcolm also implies that hardening is not necessary and EN8 pinions can be left unhardened. Elsewhere I have read that you can harden and temper EN8 like silver steel but I'm not sure what can be reliably achieved in an amateur workshop. There is also debate about whether hardening a clock pinion actually makes it last longer. I have now cut some trial pinions in EN8 and the finish is excellent with almost no polishing necessary so am tempted to go down this route.

Any advice on the practicality or value of hardning pinions in EN8? I could make the pinions seperately and assemble them to the arbor with anaerobic adhesive, a technique which does simplify the hardening process.

Edited By lfoggy on 04/06/2022 20:39:57

Edited By lfoggy on 04/06/2022 20:40:42

I know there'll be others with a more thorough explanation. Meanwhile, might I suggest Ifoggy, that you take a piece of EN8, heat it to bright (cherry) red, and quench it in clean cold water.

A rub with a file should show that it has remained soft.

A quick scan for the carbon content of EN8 describes it as Mild Steel with between 0.35% and 0.45% carbon. Silver steel contains in excess of 1.0% and given the same heat treatment (as above) will be pot hard.

For work pieces, tempering is highly recommended.

Regards,

Sam

Is this any help ?

**LINK** https://www.smithmetal.com/pdf/engineering/en8.pdf

MichaelG.

Ifiggy,

Due in part to declining years, I failed to realise, or even to check that you have been building clocks for many years.

As a one-clock ‘expert’, I relied almost entirely on my engineering background, missing things like ‘shake’ etc. as I progressed.

As revealed via Michael’s (Smith Metal) link, EN8’s key features are …

• Unalloyed medium carbon steel

• Reasonable tensile strength

• Can be flame or induction hardened (well I never)

• Readily machinable

• Moderate wear resistance if heat treated

Sam

Ive just tried that and a file skates over the EN8 just like it would over similarly treated silver steel suggesting it has hardened. Will it soften by tempering according to colour, like silver steel?

Hi,

I haven't played around with it but have seen Clickspring on Youtube make cutting tools by hardening and tempering it,

David.

Hello ifoggy, yes you can use an anaerobic adhesive for the pinions. John Wilding even recommends it.

David

Yes.

I used EN8 when I wanted some 'hard' washers in an application..

FWIW high-tensile carbon steel screw shanks (Allen screws) will also harden and temper.. I've hacked & filed small boring tools out of them in times of need..

Edited By DiogenesII on 05/06/2022 09:31:45

Joining the dots:

All steels can be hardened more or less, but not necessarily easily. It's important to understand the time and temperature characteristic of the particular steel and what needs to be done to stop it warping and cracking etc. Much depends on the size and shape of the object; generally small objects are easier to harden than big ones. (Very handy in the workshop!)

• Low carbon steels like EN1A are so difficult to harden it's considered impractical. (They can be case hardened though.)
• From the spec, EN8 is a medium carbon steel that's moderately difficult to harden, requiring the temperature to be held in within a 30C range and then oil quenched. Certainly not impossible, but easier to do with a thermostatically controlled oven and all the data than with a casually waved blowlamp. EN8 can be case and induction hardened.
• Silver Steel is specifically formulated to be easy to harden in an ordinary workshop. It's forgiving: neither timing or temperature are awkwardly critical. Not the most machinable, or strongest, or toughest, or hardest or cheapest steel possible, but highly convenient and more than good enough for most needs.  Tends not to be used for large objects and production runs because of the cost: when a lot of steel is to be hardened, it's cheaper to invest in the equipment needed to reliably harden cheaper steels.

The value of hardening a clock pinion is reduced rate of wear: clocks with hardened pinions run for much longer than clocks with unhardened pinions. Hard brass wears better than soft steel, and hard steel is better hard brass. Jewels are harder than the best hardened steel.

Is it worth putting the effort in? I think so, Having gone to the trouble of making an attractive clock that keeps reasonable time, it seems a shame to spoil a potential heirloom due to a load of soft pinions grinding to a halt after several years. Though we might all be dead before then!

Dave

Edited By SillyOldDuffer on 05/06/2022 10:36:32

This may be of use.

https://bortec.de/en/blog/what-is-flame-hardening/

**LINK**

Suggests that it is a surface hardening process. Temperature seems to be critical - heat to austenization temperature and quench immediately.

Being a case hardening method, maybe subsequent tempering would be inadvisable and unnecessary?

I profile milled these pinions in the end of a FC steel bar, probably EN8. I didn't bother to harden as they are for motion work not a train, but maybe I should have tried this technique?

Hi,

Is that a confidently known fact ? Seems to be some debate about this in the horology world but it does seem likley.

Have just been playing around hardening EN8 pinions by heating to red heat, holding there for 60s then quenching in water. They are definitely hard according to the file test. Who knows how hard though, and the effect of tempering is even harder to measure in a home shop...

Very useful, thank you. I agree this implies that tempering is not going to have any effect. I am thinking to either use EN8 and leave unhardened or go back to silver steel.

Amazing work on that pinion. Presumably you can programme your milling machine to generate any tooth form you like?

Apart from buying a hardness tester you can use these to get an indication of the hardness**LINK** Niko.

Edited By speelwerk on 05/06/2022 13:03:08

Update. The hardened EN8 pinions are so brittle you can snap the pinion leaves with your fingernail. After tempering they are still too brittle. This is not going to be viable in a home shop I think.

Searching the horology forums as well as this forum, there are quite a few threads on whether to bother hardening clock pinions, which include contributions from some eminent horologists. There is no real consensus and clearly there are plenty of constructors and restorers who do not harden.

Executive decision: EN8 left untreated. I will report back in 100 years or so whether this was the right choice.

EN24T is the other suggestion. Supplied hardened and tempered it will machine well and needs no further heat treatment. Incidentally silver steel is I think generally supplied half hard and annealing before pinion cutting will help both with the ease of machining and with surface finish.

As to the question harden or not, my understanding is that French clocks had hardened pinions whilst englissh clocks did not. The pertinant question to ask is why? At first glance you would assume that the reason was to make them wear resistant. However clock pinions do not run at high speeds and are generally lightly loaded but are however not lubricated. If you examine a clock that has run for a considerable time you will find that the brass wheels have cut into the steel pinions. The mechanism of this is that the brass acquires embedded dust in the surface of the wheel leaves and the brass leaves then act as a lap against the harder steel and gradually erode the pinion leaves. As this is a grinding process I don't think that hardened pinions would help much. Far better to ensure a closed dust free case or a protective glass dome.

My opionion of the reason for hardening and then "polishing" is to reduce friction, something of the greatest desire in clockmaking. Although hardening does not change the co-efficient of friction much in steel generally it is assumed that harder matherial have less friction than softer, I think the hardened pinion will take and hold a polish better than the pinion in the soft state and will be much less likely to pick up any of the lapping material used in the polishing.

The above is to an extent speculation on my part as I have no experimental evidence for all this so I would be interested in the comments of fellow and perhapse more knowledgable clockmakers.

regards Martin

I also guess that is right decision for EN8.

Some things to consider: being plain carbon I suggest you just need to get to 'at least' the hardening temp, but exceeding it can still work. The lower carbon means it is farter way from the 'eutectic' so will need a higher heat to get the max out of it. So getting it beyond 'cherry red' is probably a good idea. For small items in low alloy steel, you do not need to hold the high temp for long at all - just get to heat then quench immediately. If hotter and held longer then you will get grain growth which will make it brittle. (For some higher alloy steel, an exact temp is much more important to get the desired composition of things before quenching)

The spec sheet listed by MichaelG suggest a max hardness of less than 26HRC which is way below 'skating file' hardness (which for decent files is above 60HRC). So I guess that means it is not intended for applications above that and should be tempered to get that. The tempering temp is therefore quite high at about 600C... too high for the kitchen oven where I usually bake my stuff. That might be beyond purple and then colours start to get very inaccurate.

You can probably buy it in the 26HRC 'tempered' condition and machine at that hardness.

The 'brittle' behaviour could be due to longer heat and/or lack of high temp tempering.

Wear will be increased if the pinions are not polished as well. It will be much easier to polish a soft EN8 than a hard silver steel. Lots of other things contribute as well to wear - like dust, or the other material, or the gap, the maintaining power etc.

So if you are of the opinion that hard pinions are good... go with silver steel. It is specifically designed for machining and heat treat. But I think it has a characteristic of being quite large-grained (might be to help machining). It also has extra carbon to create gritty carbides which also resist wear but will cause the wheel to wear faster.

If you are not convinced of the need for hard pinions... EN8 without hardening is going to be much easier to machine and polish.

All theory, no practical experience in EN8. I read a lot of books though.

Edit - Martin posted while I wrote. I am in complete agreement with everything he writes. Some might find contradictions with what  I wrote, but I don't think there are.

Edited By jaCK Hobson on 05/06/2022 13:54:39

Would I be throwing a spanner in the works by mentioning blue steel?

**LINK**

As for which will wear more, here's an old chestnut. Back in the 50’s I removed the substantially worn and scored gudgeon pin from a small 2-stroke petrol engine.

The replacement pin was a near perfect fit in the small end (bronze?) bearing.

In a similar vein, tests on a lawn mower spur gear made from HDPE (high density polyethylene) meshing with a similar steel gear resulted in the steel gear wearing out. Lapping comes to mind.

Sam

Edited By Sam Stones on 05/06/2022 21:09:23

Please forgive the digression, but I just remembered this interesting discussion about ‘pinion wire’

**LINK**

https://www.model-engineer.co.uk/forums/postings.asp?th=91503

MichaelG.

.

I think that sums it up nicely

Martin makes a good point about polishing, but ultimately what you need in a pinion is toughness.

MichaelG.

It is obvious but to reduce wear it also important that pinion and wheel are made as accurate matching (diameter/pitch/modul) as possible. Niko.