This knot of four mutually linked rings was CNC-milled from one piece of massive wood on the FabLab’s mill. It has the rotational symmetry of a cube and can therefore be obtained by applying the same three-axis roughing and finishing operations six times, once from every face of the cube. The challenge of precisely realigning the stock cube after every rotation, so that the milled surfaces would line up, was met successfully using the approach described here, with deviations on the order of 0.1 millimeters.
The rough 90 mm basswood cube was cut on a circular saw to a precision of about 0.5 mm, which is respectable, but not sufficient for the precision required here. Therefore a 86 mm cube embedded in it is taken as the stock geometry from the point of view of the milling and precisely shaped using the mill itself. Here its top four corners have been milled. Only the corners are required, not the whole edges and faces – for the side faces the cutter would be too short anyway. Alignment is not critical here, the inner cube just needs to fit completely into the outer one.
As a fixture for repeatable positioning of the cube, its four bottom corners are milled into MDF pieces fixed to the milling table. From here on, the work origin of the mill must not be modified anymore. Because I did the milling on two days, I had to do this procedure twice.
The cube fits into the fixture pretty tightly and is tied down using scrap metal brackets to prevent it from being tilted out.
Once that is done, the other four corners of the inner cube are milled.
Roughing of the first side done (6 mm end mill, 2 mm levels).
First side finished (4 mm ball mill, 1 mm spacing), second side roughed.
Milling some air on the fourth side.
Four sides finished.
Milling done and central cube removed by hand using a Dremel.
Separated using a fretsaw.
Once I had the process going, completing one side took 37 minutes, of which 16 minutes roughing, 9 minutes finishing. Total machine time 7 hours. Plus many hours of drilling, carving and sandpaper work afterwards.