JEWEL HOLES FROM SYNTHETIC SAPPHIRE

Roy Harris FBHI

Single crystal synthetic sapphire is grown from chemically pure aluminium oxide, it will withstand high temperature, pressure and thermal shock.

Most important of all for the clockmaker and watchmaker it has a low coefficient of friction, excellent wear resistance and can be polished to a high degree of accuracy and smoothness.

It may not suit all applications if for instance you need to match the colour of the jewels in an antique watch movement.

George Daniels book Watchmaking has an excellent chapter on tools and techniques for jewelling and deals with the use of natural stones.

What follows is my description of the making of a jewel hole for a marine chronometer.

With the use of diamond compounds it is obviously not a good idea to employ your best lathe for the manufacture of jewel holes.

Something small that will take collets in the headstock and tailstock will suffice. The ability of the drill to be easily dabbed in and out of the forming hole is essential, if the tailstock has a lever quill that is good, with a screw type then that will have to be removed and an attachment made.

In fig. 1 is shown the set-up for parting off a section of 4 mm sapphire rod.

A square of brass reamed 4 mm is fitted in the toolpost a slot is cut with a slitting saw or diamond saw, this is then lined up with the diamond saw that will cut the rod.

I have sliced through the rod with it screw locked in position but found however sensitive you are the slice often breaks off when 75% through resulting in a jagged edge.

The best method is to set the thickness of slice by use of a collar attached to the rod or an extension of it and then rotate as it is being cut.

The result can be seen in fig. 2 eventually the rod will shear relatively clean as in fig. 3.

One side of the slice can now be flattened on a lap and shellacked into a carrier that has been reamed or slot drilled shallow enough to allow the dome of the jewel hole to be shaped (fig. 4).

This operation can be carried out with a diamond file or lap prior to polishing.

Diamond compound is readily available from lapidary and engineering material suppliers the coarse being 90 microns the fine 0.25 microns, what I have used here is 60, 30, 10 and 3 microns.

After the dome has been formed it is rough polished first with the 60 then the 30 micron compound.

The compound needs to be applied to a carrier which can be of different material depending on the work in hand, it ranges from paper to steel with wood, leather, copper all being employed.

It is worth experimenting to see which materials perform the best. I found that in this case the domed and flat surface could be polished down to 10 micron using a soft plastic material like a credit card or in my case the back of a diamond lap handle.

The work can be finished with pegwood and leather.

Next it is necessary to 'catch' the centre this can be done with a diamond burr in the tail stock collet fig. 5 this sometimes still leaves a pip in the centre which is removed by a hand held diamond point fig. 6.

The drilling is done by the use of a steel drill as shown in fig. 7 it is turned in the collet from pivot steel to the diameter required, in this case 0.3 mm.

The length of the drill should be sufficient to complete the work, as it will be reduced with the constant removal of the chamfered worn edges, but not long enough to flex and break.

The drill can be hardened and tempered to straw colour although I have not found that this reduces the ware rate to any great degree.

To drill the jewel it is the frequency at which the drill touches the face charged with compound that gives speedy results, great pressure is not required.

As the drill is withdrawn slightly the compound should run between the two faces but it does help to keep the drill charged by the use of pegwood to combat the centrifugal force (fig. 8).

The frequency of the dab I use is between 30-60 per minute and a rotation of 1000-1500 rpm and for some unknown reason a mixture of 10 and 30 micron compound I found gave the best results on the sizes in question.

A frequent check must be kept on the drill face and as it wears a few strokes of a stone will restore the corners.

The time to drill a 0.3mm hole to a depth of 1mm takes me about 1 hour.

An oil sink is produced by using a domed former (fig 9) and the visible faces polished with reducing grades of compound.

I do not drill completely through the slice but to a pre-determined depth that will enable the hole to be completed by the domed tool when the slice is reversed in the carrier (fig. 10).

The reason for this is that there is no chance of the drill bursting through and chipping and shattering around the hole exit.

The edges of the hole are finished with copper wire and compound to remove any sharpness.

Care must be taken when doing this as to force anything in or to get it jammed could result in the jewel shattering.

The jewel hole is now removed from the carrier and the shellac cleaned off (fig. 11).

Much information and the production techniques of synthetic sapphire can be found on the Internet.

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