I built this concave faceting machine in the winter of 2000/01 but at that time I did not have access to a digital camera. I have always intended to put a description and some photos of this machine on the web and I am now able to do so.

There are some design decisions which have to be made up front. How fast should the machine run. I worked back from the use of standard flat laps and via surface speed in metres per minute calculated the rotation and the reciprocal speeds. The lap should be able to pivot around the long axis of the facet head. There should be enough room to move the facet head so all angle can be ‘dialed in’.

Figure 1. – The Concave Machine.

To the left of the machine is an small box which contains the Beale Woolley Depth of Cut meter fited to my Ultra Tec faceting head. If you want more information on this modification you might like to read the page which describes my Experiments to Understand the Beale Wooley Depth of Cut Indicator/.

The machine is built on a 3/8″ aluminium plate and both the faceting head and the cutting head are mounted on 1/2″ aluminium. Under the base plate is a 12v torodal transformer with 2 12v 15 amp windings. Alongside the transformer are two PWM ( Pulse Width Modulated ) Speed Controllers; one controls the RPM of the rotating mandrel the other its speed of reciprocation. You will notice in Figure 2 that the motor terminals are un-insulated on my machine. My next job is to fit heat shrink boots onto these terminals. The motors, pulleys and speed controllers have been chosen to give mandrel rotation in the range 3,000RPM to 50RPM with reciprocation speeds of 30 to 150 per minute.

Notice how heavy the side stop is. There is a 15mm micrometer head fixed to the stop. After discussion with other faceters I realised that some machines had been built with side stops which were too light and did not resist a side movement of the head. I built my stop from 1/2″ steel and 3/8″ aluminium plate.

Figure 2. – The machine end on.

Figure 3 shows a close up of the reciprocating mechanism – basically the machine was built according to KISS principles so the reciprocating mechanism was the simplest I could design to work. I would suggest that the slot in the cam follower in future be machined in a piece of Delrin however I didn’t bother for this version. I do use a silicone grease to lubricate this slot.o

Figure 3. – The reciprocating mechanism.

The photo shown as Figure 4 shows a view of the machine from the pulley end. The pulleys are fabricated from 1/2″ Aluminium mounted on stubs of 1″ diameter rod. The belting used is 5mm PolyBelt.

Figure 4. – The machine showing the pulleys.

Figure 5 shows the cutting head rotated about its pivot which is located immediately below the centre of the stone. The photo shows the cutting head rotated about 45 degrees anticlockwise and on the right fully clockwise.

Figure 5. – The machine with the cutting head rotated either side of the main axis of the machine.

I reckon that with this machine there are 5 degrees of freedom for the stone/lap relationship. On a conventional machine there are only 3. I found that this increase in degrees of freedom increased the difficulty in cutting stones using this machine.

Figure 6. – Three cutting mandrels.

Figure 6 shows three of my cutting mandrels or laps. From the left the mandrels are:

• a polishing mandrel using diamond on Phenolic.
• one with cerium oxide on perspex.
• and finally a cutting mandrel using diamond on Copper.

You will notice that the spindles on my mandrels are 12mm diameter. The copper mandrels are made from domestic water pipe – approx 15mm external diameter soldered to the spindles. They are then turned down to 14.5mm. The Perspex or lucite mandrel was drilled and tapped M8 then screwed onto its shaft, held with “Loctite” before being turned down to 14.5mm. The phenolic mandrels are made in a similar fashon. It should be noted that there are various types of Phenolic. In the UK it is called Paxolin and is manufactured with both paper and cloth as the base into which the resin is impregnated. When I purchased my off-cuts the vendor said I would have problems with the paper based material and I needed the cloth based. I have heard anecdotal evidence which confirms this view.

Figure 7 shows a further view of the machine. I should mention that I have a left handed UltraTec Machine and the photos are not mirrored on this page. It was suggested that a left handed machine was better for a right handed person as the stone was manipulated with the left hand – the right attending to all the other jobs. I previously had right handed machines before buying this UT in the mid 70′s. Now I woulds not like to go back to a right handed machine. But it is a matter of individual preference and familiarity.

I hope this page is of interest or use to you – maybe it will encourage you to build your own machine and join the ranks of those of us who try to cut concave faceted stones.

To see some excelent concave stones you might like to visit Chris Algar’s Concave Faceting Page. He is much more able than I to take good photos of his stones.

Figure 7. – A further view of the machine.