Spherical Objects and dome shapes
The arc slide on the sliderest allows us to rotate the tool to keep it pointing perpendicularly to the work as we move along a 3D shape such as half an egg. We can see this best looking vertically down from above.
Cutting around a sphere or an egg, the cheapest option, is geometrically rather like cutting a curved cone... As the work progresses the arc is moved to maintain perpendicularity with the surface.
It is essential that the tool remains perpendicular to the work surface. The movement of the arc to achieve this is judged by eye. Considerable skill is required to keep the pattern even especially in a big egg as it is not uncommon to have to reset the sliderest halfway through the job. Here, the final cuts are being made.
A 100mm diameter hemisphere with a circular zig zag pattern. The need to keep the tool as near to properly centred as possible means that the sliderest often needs moving and resetting during the processing of objects like this. This example would need at least one movement of the sliderest to keep the tool centred accurately. The limit for cutting in one go is usually a little bigger than a chicken's egg. If this resetting is not done, the spacing of the cuts would noticeably change, at first close, then further apart, the closer again at the end. A truly spherical sliderest could resolve this issue, but engine turners usually rely on the method above because spherical sliderests were not generally available.
Non circular work on spherical objects
Some of our most spectacular work on eggs was actually done entirely on a Straight Line machine! Here we use a spherical Chuck to position the work and rotate it between cuts.
In the process of being cut on a purpose designed Spherical Chuck, an exact replica of the 150mm high 18ct Fabergé Coronation Egg. This design consisted of a spiral lattice giving over 100 lozenge shapes each of which has a sunray pattern of radiating lines from it's centre. There were over 14,000 cuts in the sunrays and the job took many days to complete. Perhaps surprisingly, this egg only saw a circular machine while the two halves were being centred on the wax. We did a series of exact reproductions of Fabergé eggs for the firm that made this, including the Chanticleer, Romanov and several other great eggs. The craftsmanship quality of the reproductions exceeded that of the Fabergé originals.
The Great Kutchinsky Egg. This piece, set with thousands of diamonds stood over two feet high and is believed to be the largest gold egg ever made. Much of our engine turning can only be seen on the inside! All the work was straight line.