Broaching a watch key

Dr. Jon

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I bought a small and nice watch with front key setting. The problem is that it did not come with a key to set it and the minute hand arbor is a 0.6mm square. No key fit from my stock of watch keys and a check of catalogs showed the smallest standard size is 0.9mm.

I decided to make a rotary broaching tool to run in my Boley F1. Available tools are very expensive and I do not believe they will work for a hole this small, as I learned.

Here is the tool as set up.

in lathe.png

The tali stock is on the left and the tool mount in it using a shaft of 8mm diameter 3D printer rod stock. This steel is very precise and hard. I used an abrasive saw to cut it to length. The larger cylinder is the tool holder. It has a 10mm hole bored in at about 1.6 degrees off axis. This holds the inner tool holder a thrust bearing mounted ER-8 collet holder.

Here is a side view
side.png

The collet in the head stock is empty. I used this set up the check that the broach tool runs on center.

tailstock holder.jpg
Here is the outer tool holder. I faced it at 1.6 degrees on my Sherkine mill. It is 1144 steel and machines readily.

The inner tool holder tales ER8 collets and runs in a thrust bearing.

collet-insert.jpg

The broach us at teh right end. It is square and although it does not look it in the photos, it tapers back at about 5 degree. The brass ring holds the ball bearings of the thrust mount with a steel washer at each end. The second washer is there because the ER 8 collet does not seat all the way in because the broach shaft diameter is near the max bore of the collet.

The length of the broach cutter is important because it has to touch the work in center.

To see how these work it is better to look at a few YouTube videos on rotary braoching than for me to try to explain it here. The essential trick is that you rotate the work piece and the broach rotates with it off axis a bit to drive each corner as the work rotates.

For this project the problem is the the square is very small and both the tool and the work have a lot of inertia. It is verye asy ti break the corners either on the tool or the work. If the work and broach rotate with respect to each other, either the broach corners will fail or the hole will be round. I needed several attempts to get it all to work.

The YouTube videos show the technique working at high speed cutting a large square. These have enough on the corners that the work and tool lock up and spin together under power. For very small cuts there is not not enough "meat' to prevent the hole from rounding.

I made the broach roughing it using a universal grinder and touching it up by hand with a coarse and then a fine stone. I dished in the ends with a carbide center drill I had made.

The critical step is hardening the broach. I ground it from 1/16" music wire and thought it would be hard enough. It was not and its corners failed. I tried hardening and torch tempering but it came out too soft and failed. I finally got it right by hardening it in a water quench and baking it at 400F in a toaster oven.

I made the cut by applying pressure using a light hammer to drove it a tap or going a quarter turn so at a time and rotating the work and toot by hand, together. After a few passes, the tool locked in place and I made the cut.

The hole looks a bit ratty, but under a stereo microscope it looks better and more, important it really does work to set the hands. This square is about 0.65mm on a side.

end.png

Here is the final ensemble. ensemble.png

The watch is a ruby cylinder by Moulanie. The male key winds the watch. It comprises 5 parts. teh ring at thetop turns.

If anyone is interested, I can supply the drawings in Powerpoint.
I think I spent about 30 hours on this and about $75 on stock tools and parts.
 
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Dr. Jon

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More on broaching:

The thrust pressures are very high and I had to cut down the work piece a bit to make a lip to prevent the work from driven into the collet.

After doing all of this, I wonder whether it would have been simpler and better to make broach from a stake and simply hammer the broach using the staking frame.
 

Dr. Jon

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Broaching a hand hole is a different problem. It is a thru hole and on a thin plate. The work is easily secured by resting on a stump with a hole large enough to clear the broach.

Holding shaft vertical is a different matter. but now that I know what I know I probably could have taken the shaft down leaving lip that would hold in hole plate of the frame.

The other thing I learned is that rotary broaching and tool making is more difficult than I had thought.
 

gmorse

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Hi Jon,

You could make a through hole in the business end of the key and fabricate the rest of it afterwards by silver soldering. The OD of the key could be greater than needed to begin with, to make it easier to stand upright, perhaps with a step as you suggest, and turned down to its final size once the hole has been broached. Having said that I've made tangent screw setup keys with blind holes this way, the hole just has to be deep enough.

Regards,

Graham
 

Dr. Jon

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After I made the business end of the key I hardened it and tempered it to blue. I have had too many cheap keys wear out. Silver solder leaves the business end very soft.

This does not rule out using the staking frame method but it does make a through hole difficult.

In a blind hole, the chip stays in front of the cutting edge and accumulates until it blocks further penetration.

I suspect commercial key making uses a blind broach but heats the work piece.
 

measuretwice

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The other thing I learned is that rotary broaching and tool making is more difficult than I had thought.
I'm impressed with your efforts and it seems like you got a good result. Tool making holds its challenges, all the more so for doing it in minature.
 

SKennedy

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I've had to do a few one off keys, just used a static punch into a drilled hole, removing it several times to clear the metal. If its just for setting then it doesn't need to be very deep but I think this little one was for winding and needed to fit through a small hole in the cuvette (or dial - can't remember) so I turned it down afterwards. Always feels a bit touch-and-go as you need the punch hard enough but if it breaks off while trying to pull it out then you're back to square one with making punch and key blank!

_MG_6509.jpg

_MG_6510.jpg

_MG_6516.jpg

I've also made one to replace one missing from the end of a Breguet style key that was just used for hand setting and used gold, same process though.

IMG_7046.jpg
 

Betzel

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if it breaks off while trying to pull it out then you're back to square one with making punch and key blank!
Ouch. Would thread-cutting oil help at all with broaching?

Also not sure if it would work as a recovery option, but I was wondering if using heat on the outside and cold on the inside (sort of a heat sink into some ice water) might work to expand the outer work enough to free the cooler broken-tool in the center. If you were going to harden/temper the key, wouldn't it be after brooching the hole, so nothing is lost by trying?

Differential temperature works well to bench fit tapers into drill chucks. I put the chuck in the sun for half an hour and the taper in the freezer with a thin film of oil and then press them together. Faster than epoxy, and comes back apart! I also know old timers used alchemy (kidding, it was toxic chemicals) to rust out old steel screws from their brass plates, though this example is steel stuck in steel. I'm not a chemist, but hate when things break off during an operation, usually due to an excessive DOC, off angles and "grab" and/or bad work-holding.

Just thinking out loud...
 

praezis

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I wonder if it would be sufficient to use a shallow starting hole for centering the square punch (diameter = square diameter)?
This can avoid the guiding cylinder and give room for chips.

Frank
 

Dr. Jon

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I suggest back relief on the cutter. I put it on mine s required for rotary broaching but it would also be useful for this approach too. Less area in contact and yes oil is always a good thing for this application
 
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SKennedy

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Yes, the square is not parallel sided on my punch. I've not had one break, it just always feels a moment away from doing so!
 
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Dr. Jon

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Thanks for the reference. YouTube also has several videos on this, It is not workable for blind holes and these broaches are very easily broken, at least in my experience.
 

measuretwice

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This article may be of interest
I've made hex and square broaches similar to what is shown there. the multi staged cutters get challenging on a comparatively large scale, say 3/16....it would be a real challenge to make something like that for small hand
 
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