stepped collets

[ozzy]

I just bought 4 stepped collets with the intention of using them to hold the pocket watch wheels while I burnish them. My problem is that I can't get the burnisher into the collet, (because it is stepped)!!!
Have I wasted my money?
What is the real function of a stepped collet?

OZZY

 

[Ansomnia]

Ozzy, I'm not sure I understand your message. Why are you trying to burnish your pocketwatch wheels?

AFAIK, the function of a step collet is to provide a more convenient alternative method to wax chucks when you need to safely grasp a (trued) wheel (by the edges) so as to perform turning operations on other parts of the wheel.

Do you or have you taken courses on pocketwatch repairing? I believe the NAWCC offers them. Failing that, you should consult relevant books written by experts before trying any important work or investing too much on tools. A while back, some people also posted links to PDF copies of the "Master Watchmaking" course from the Chicago School of Watchmaking. You can probably find them by searching past postings.


Michael

 

[Dushan Grujich]

...
What is the real function of a stepped collet?

Good Day Ozzy,

Just as one would re-bush a plate or a bridge of a watch, when the pivot hole has moved due to the wear, wheels can also be ex-centric, loosing the concentricity out of various reasons.

Concentricity of wheels is very important, because if the wheel is even slightly ex-centre then depthing of the teeth will be impaired and the power off the mainspring will not be transferred equally and watch will exhibit periodical loss of balance amplitude and will eventually stop with mainspring still unwound.

Stepped collets are used to hold wheel by its rim and allow boring the centre in order to restore the concentricity. Normally one would bore the new wheel centre and then either use a new pinion with a larger diameter or place a blind brass bushing in the opening, rivet the bushing in place then drill and bore to size the new centre, thereby restoring the concentricity of the wheel.

The stepped collets are used most often to correct concentricity of mainspring barrels, and barrel covers, which happens due to excessive wear after lubricant is spent, the larger the acting force the bigger is the wear.

Cheers

Dushan

 

[Dave B]

I remember Mr. Goodman once repairing an antique surveyor's compass jewel using a stepped collet. The jewel was a large one, in a brass mount, but had some nasty scratches in it. He could not get a replacement, so he chucked the mount in the collet, and polished them out with diamantine powder. I was impressed, and tucked that trick away in my memory banks against the day I might need it.

 

[ozzy]

Thanks everyone for your help.
I figured that if polishing pivots of the balances desired, then polishing ALL the pivots is also desired. I now see how that can't work.
I guess that I haven't wasted my money as I now see a different purpose for the collets.

Thanks, OZZY

 

[Ansomnia]

In case this is of help to some people, I've made a few photos of a Jacot pivot lathe setup. This type of lathe is ideal for working on pocketwatches and should work on wristwatch movements as well. You should be able to polish any watch pivot on this sort of lathe.

Hopefully, my photos are self-explantory. BTW, I was using a fine sapphire burnisher because I couldn't find my steel ones. The pivot on this demo wheel is obviously far too rough at this point to require a sapphire burnisher!

The wheel is held in place by the application of the burnisher on the pivot. If you spin the wheel on the Jacot drum without the burnisher, it can fly off the lathe. So be forewarned! The nice thing about hand-powered lathes is that you are far less likely to overdo something before you realize it. You have much better control.


Michael

 

[Dave B]

I have been thinking - off and on for several years now - and one of the things that I've been thinking is I really should break down and get myself one of those Jacot pivot lathes. I have been using my 8mm Peerless lathe for polishing pivots, but it seems I rarely have the right sized collet for the arbor at hand. A Jacot-type polisher would eliminate that hassle. I have gotten pretty good at guessing the length of shim stock for wrapping the arbor, though.

Here's a runner I made for the business end. A piece of 1 1/2 diameter brass, drilled with a series of holes of varying diameters, then turned down to the center of the hole circle, and a couple of pieces from the junk bin. By mounting it in the tailstock runner, I can drop it down on top of the pivot and tighten the thumbnut, and away I go. If I want to see what I'm doing, or polish from above, I can rotate the push spindle to put the runner anywhere that is convenient.

 

[Ansomnia]

Dave, nice job on making the tool! I assume you drilled the holes using the headstock and runner of the same lathe. The Jacot drum runner you've made should work very well for clock pivots. However, I would advise against trying to making such a tool for watch work. You will not be able to make them remotely small enough or accurate enough for watch work (the pivot I used was 0.29 mm and already on the large side - they go down to 0.02 mm).

Based on my personal understanding of the Jacot lathe, I have 2 suggestions to improve your Jacot drum design.

A Jacot drum should not be round. Instead it should be multi-sided with each bed or groove sited in the middle of a flattened face along the perimeter of the drum. This makes located the pivot with the burnisher much easier. When burnishing a pivot, the procedure involves sliding the burnisher back and forth as the (oiled) pivot is turned. This alleviates the effect of creating a worn spot on the burnisher and also provides a more even burnishing effect. It's very hard to locate the pivot if it is just a small bump on a round drum. Most of the time you will miss it as you slide the burnisher back and forth.

The drum would also be better if you make it out of tool steel which can be hardened. If the drum is made of soft metal, the burnishing action also has an effect of rounding off the lips of the adjacent beds as they present themselves "high spots" to the burnisher. This will eventually compromise those beds.


Michael

 

[Dave B]

Actually, I made the runner and the tailstock taper on a Unimat 3. The brass flat has a slot in it, that allows the drum to move up and down, as well as to rotate. That way, I can chuck up the arbor in the head stock, loosen the thumb nut, and drop the runner down onto the workpiece. That solves all the measurig and centering problems I would have had to deal with had I made it rotate on a fixed offset from center. (The notch in the flat serves no purpose - it was just a piece of stock I had lying around in the junk bin, and the notch was already in it when I started with it.)

It's true - I should have made it from tool steel - or at least cold rolled stock instead of brass. In just six months or so of use, I am already beginning to notice wear in the pivot rests.

I could probably make one for watches - I have pivot drills - but I suspect turning the runner down after drilling would tend to throw a burr across the smaller holes. Besides - with watch parts, there is still the problem of how to chuck the short end - often there is little or no arbor extending past the wheel or pinon. Chucking things up by the pivots is not recommended, and with the cone taper that watch pivots typically have, not practical, anyway. So I really need to invvest in a Jacot, with a driving dog. I am curious, though - Why doesn't the wheel tend to push away from the center on the driving end? - I realize it is held captive when the burnisher is on the opposite pivot, but what keeps the whole shebang from falling off of there when the burnisher is lifted from the workpiece for inspection?

 

[Ansomnia]

Dave, I think your polisher is well made and should be within tolerance for clock pivots. But for watch pivots, I suspect the entire lathe needs to be extremely rigid and integral. And this probably led to the way Jacot lathes are made; essentially as 1-piece tools with very rigid fittings and close tolerances. From what I can tell, the Jacot lathe relies on very accurate alignment between its runners and the female centres that pick up the pivot on the opposite end of the staff.

You bring up a very good question about whether the staff or arbor would tend to push away from the female centre unless something is pressing the pivot against the centre.

As far as I know, square-shouldered pivots like the one in my photos (with the pinion) should be carefully fitted on the Jacot lathe so that the flat surface of the pivot shoulder rests against the face of the Jacot runner. You have to check with a loupe and your sensitive fingertips. There should not be noticeable endshake or the pivot may wobble on the bed although the downward pressure on the burnisher should keep the wheel in place. I use my fingernail to hold down the pivot while I test spin the assembly. I can feel for any endshake at the tips of my fingernail.

With a conical pivot, only the cylindrical portion of the pivot can rest on the beds so I believe the staff is kept in the female centre partially by how well the round-shouldered burnisher is held. I think the concept is similar to square-shouldered pivots but I've never worked on a conical pivot. BTW, there are different radii to the conical shapes of various pivots and you must use a burnisher that has the same radius on its curved side as the radius on the conical portion of the pivot. You can see how this can get expensive if you only do one of these pivots... ever!

If I am wrong I hope the experts will jump in to correct me.


Michael

 

[Dave B]

Having looked at the Jacot tool once again, I have another question. When the direction of rotation is reversed, does the dog slam against the opposite spoke? I would have expected to see the spoke trapped between two pins, since driving in a Jacot is in two directions.

 

[Ansomnia]

Dave, AFAIK, all the Jacot lathe pulleys use a single arm to push the spokes so there is a slight bump on the reverse stroke of the bow.

OTOH, some of the pulley arms like the one in my lathe are made of brass and can easily be bent to fit closer to the bottom of the spokes, closer to the hub. This reduces the force of the "bumps" and should not affect how well the wheels are spun or pose any possibility of damage.

You can also replace the arms with one of your own designs. I think the single arm is more universal because it works with all widths of spokes and allows the watchmaker to "fit" a wheel to the lathe much easier. Trying to "skewer" a spoke might get tricky. I think it's safer to minimize fussing with the watch wheel while it is on the lathe.


Michael