Tooth Repair on a Fine Clock Wheel

I thought it might be useful, and enjoyable, to show a few techniques I use when repairing/dovetailing a tooth on a French wheel. These teeth are very small and can be challenging to replace. I hope it offers something to someone.

I have used a French wheel from my parts bin.

The tooth in question can clearly seen to be bent in Pic 1.  Before the tooth is  removed, using a sharp scribe, score a line from the centre of the base of the old tooth on the radial line to centre of the wheel, no deeper than half the depth of the rim, pic 2. Once this is scribed the tooth can be carefully taken off using a very fine blade on a jewelers scroll saw; fine enough to fit between the teeth and cut along the line of the wheel circumference.

Using the scroll jaw, cut into the wheel along the scribed radial line to the desired depth, pic 3. Avoid cutting deeper than half of the depth of the wheel rim. When using a scroll saw for these micro jobs I have found the scroll saw easier to use and control if only approx a quarter of a blade is used.

Use the scroll saw to carefully cut the dovetail sides, to the same depth as the centre line already cut, pic 4. The scroll saw can be carefully used to remove any brass in the dovetail hole. Using a very thin cut of 600 grit wet and dry cloth in a ‘to and fro’ motion with both hands will effectively smooth the base of the dovetail, pic 5. Wet and dry paper will break doing this, the fiber backed type is ideal for this.

Finishing or widening the dovetail side is tricky in such a tiny space. My preference here is too use the cheap Indian suspension springs with wet and dry paper glued to one side, pic 6. This technique creates a tool that can work effectively in this small space and also will not abrade other areas of the working space, pic 7.

               

 Once you’re happy with your dovetail hole, pic 8, cut a piece of donor brass, place it under the dovetail hole, mask the wheel up with tape; holding the donor piece tight to the wheel with the tape, pic 9. Spray the dovetail very lightly with  an aerosol paint and leave to dry. Any paint on the wheel will dissolve in some acetone, and you will have the impression of the dovetail left on the donor brass, pic 10, which will be a great help when cutting the new tooth/dovetail insert with the scroll saw. Leave plenty  of brass at the large end of the donor piece for handling and holding in a vice, pic 11. It will also be a lot easier to find if you drop it, or subconsciously put it down on the bench whilst looking for a different tool.

  

The donor piece needs to be a tight fit. It is a good idea, with a very fine paint brush , to apply a little solder flux in the joint before fitting. I use Tix flux and solder.

When you are happy that it will go in with a little gentle persuasion, I have found the best way is using a staking tool and a small piece of ‘Bluetac’, pic 12, a little tap with a brass ended flat punch will fit the piece squarely. Ensure there is excess donor brass either side of the wheel, and using a steel flat punch in the staking tool, ‘peen’ the donor brass to fit/fill the dovetail hole well, pic 13. This is why the flux has already been applied in the joint.

Only very tiny chips of solder are needed for the joint, It is helpful to squash the solder in smooth nose pliers, cut teeth longways and then trim tiny pieces off and place them on the seams on the joint, pic 14. This needs doing under mag, I use 10x for this as I like to have the solder pieces ‘standing up’ as in the picture. Placing the solder in this way I find that the solder, melts, drops and flows straight into the joint and much less on the wheel rim, pic 15/16.

A mini butane soldering torch with a fine hot air nozzle attachment, to ensure there is no flame, and at a low/med heat from underneath does a great job. Some prefer other heat sources, including alcohol lamps. After it has cooled clean off any flux thoroughly.

I prefer to leave a ‘long tooth’ throughout, up until the next stage, as it is easier to see that the new tooth is still on the radial line. When cutting off the excess tooth, and filing to length it is a good idea to protect the adjacent teeth tips with tape, and keep checking it has not been breached, pic 17.

If the gaps between the new tooth and adjacent teeth has been compromised, by solder for example, it can be very tricky to ‘make good’ in such a small gap. Get another of those cheap Indian suspension springs, put it in the vice, and ‘skip’ a needle file across its edge to give it very fine teeth, pic 18. Using this will make a good job of re-defining the 90deg root angle of the tooth in the gap, where most files will not fit, pic 19.

If the new tooth is too wide, use the suspension spring with the wet and dry paper stuck to it to reduce its width. Again, using this will ensure only the width is abraded, not the gaps base. These can also be used to finely finish the tooth’s profile. Carefully remove excess brass from the tooth to match the wheel rim width, with damaging/marking the rim. The dovetail can be carefully finished/polished with very fine wet and dry paper, to a point where it is virtually invisible to the naked eye.

When using files to shape the tooth, it is best to use as fine a file as you can, cut grade 4 and above. ‘Escapement’ files are smaller than other needle files, for shaping the top of the tooth a Barette/safety escapement file is ideal. Good ones are not cheap, and should be saved for delicate jobs.