Four toothed-wheel circular steel cutters, or milling tools, patented by John George Bodmer (Patent No. 8070) in 1839. The cutters have teeth very similar to those of a saw or file, but shaped to the profile of the spaces between the teeth of the wheel to be cut. This made them very difficult to sharpen, and for gear-cutting purposes 'hobbing' became more widely established practice, patented by Joseph Whitworth in 1835.