How quickly things can change

[JGrainger]

Decided to have a go at a creative project today, was really pleased and frustrated

Centered (in a wax chuck) and drilled a .3mm through hole through a Peseux 330 center wheel (from a parts watch) with a PBC drill.. felt it clearly go through, did a cheer, then decided to give it a bit more just in case.. at this point my drill jammed and snapped. Upon removal the drill was 1.5mm through and really didn't need all that extra - I've tried to remove it and broken a bit of the drill off. It's now in meths in case there's still some shellac in the hole.

I'm not really concerned and it's not a repair, there are also a few spares if it doesn't work.. but it goes to show how quickly things can change (lacking sufficient emojis ).

 

[JGrainger]

To conclude this adventure, it took about 1hr 15 to remove the drill with some help from a soldering iron, oil, jewelling press, staking block, tiny pin punch.

[nickelsilver]

Broken drills in arbors is a real problem. When repivoting, which generally requires a much shallower depth/diameter ratio (general rule is at least 3 but better 4 or 5 diameters depth), I almost always use a spade drill. These I make up as needed freehand out of tungsten carbide, and make them with noticeable back taper. The advantage is twofold; being hand ground they will almost certainly drill slightly oversize, and being "waisted" they don't stick in the hole in the event of breakage. I just did a LeCoultre escape wheel with one last week- pivot was 0.08mm, was able to do a hole of 0.14mm and have decent wall thickness. Then plug and turn down and burnish- I always figure the ultimate test of the new pivot is if it stands up to the Jacot tool.

 

With handmade drills they are better held in a pinvice and guided by hand as they are rarely perfectly centered on their shank. As long as the center is caught correctly before drilling it's no problem.

[JGrainger]

I'd bought some PCB drills a bit back for general use which were to hand so thought I'd try them. The PCB drills fit in a pin vice with the tailstock centre as a guide..

..I am hoping to try carbide spade drills once my normal work has resumed but hadn't realised there would be such advantages to using them.. it sounds like they would do a nice job on this and reduce the risk of binding too.

This evening I've removed the 4th wheel pivot and drilled it to accept an extended pivot onto which I'll fit a friction fitted wheel, that is not a through-hole but of reasonable depth for repivoting. Is there a standard length to pivot ratio for the new extended part of the pivot which protrudes through the bridge plate? - I'm thinking of making it stand proud of the bridge by 1.5 to 2mm so that a friction fitted wheel boss will fit.

Is it worth adding a bit of glue to the new pivot?

[nickelsilver]

The length of an extended pivot whether to take a hand or a driving wheel really depends on the design, but 1.5-2mm is reasonable. I sometimes add a little Loctite 648 or 638 when friction fitting the new pivot, but I think it's mostly psychological, as there really isn't any room for it if the pivot is correctly sized for a friction fit. If the pivot is loose enough in the drilled hole for the Loctite to have room, then it won't hold anyway, there just isn't enough surface area for it to do its magic.

 

You really have to watch the size of the new pivot- often the wall thickness of the drilled arbor is quite thin, and the interference fit is on the order of a few microns. Too much and you can split the arbor... and of course too little and it doesn't hold.

[JGrainger]

Thank you for your advice,

Today (between other stuff) I fitted a new longer pivot, it was actually over 2mm with the intention to reduce as necessary but part way through burnishing the pivot part it snapped. I've re-drilled it and started to drill a spare - hoping to turn and fit a new extended pivot tomorrow.. I might increase the hole a little too

[JGrainger]

Figured I'd post an update..

I've got a couple of 4th wheels to extend - decided to try fitting a longer pivot on one, and turning the other down to fit an extension onto another, but that would require me to remove a 4th wheel jewel and cut through the plate, then use an additional bridge to support a short pivot above the new 4th wheel extension.. it's only a small movement with pivot diameters optimised for the relatively light usual loads.

I'm not sure how many extra bridges this will all add, so am focusing on making enough parts to get the height of a new bridge over the 1st wheel with a Daniels/ Rolex inspired clutch above it.

The clutch pinion is presently too long but is helpful for figuring out how short/ low it can be remade.. the sketch was "off the top of my head" and I allowed too much height for components - even my thinner (than sketched 1 to 0.55mm) clutch piece will be remade thinner.. total clutch height may be nearly halved.

 

[JGrainger]

A little bit more progress.. I've done some more machining and drawings - bring it down to size, and have started one of the two bridges.. awaiting taps, will probably utilize existing movement screws and/or some Russian watch spares if larger screwheads are required. This bridge is in two pieces and will total 3.9mm in height, which should allow room for a clutch.

There isn't a lot of meat on the movement (the small size is one reason I like it but a mixed blessing), so I'm going to fasten the bottom part of this bridge to the existing screw hole on the movement and rivet an internally threaded post onto the existing part (to hold the top part of this bridge). There will possibly be another locating pin fastened to the lower part of the bridge to assist in lining up the existing plate with the two parts of the bridge.

Edited July 20, 2020 by JGrainger
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