Developed by G P Potts in the 1920's, this jig is widely respected and a coveted addition to the workshop. Based on the works of Van Rouen in 1913 (details included), the cone geometry generated by the jig provides near perfect back-off, providing an easy cutting drill in the important range of Ø1/8" to Ø1/2". The geometry is adjusted according to the drill diameter by the simple use of a caliper. The jig can be mounted either at the side of your bench grinder, on the base of your Worden Mk3 or on a bar bed for the Quorn Mk3.
The current design has been kindly vetted by Professor Dr.-Ing. Joerg Hugel of the Swiss Federal Institute of Technology at Zurich, who calculates that it is capable of grinding the "perfect drill".
The kit includes 6 castings together with all necessary bar materials and fasteners. The drawings which now accompany the kit are of a much higher standard than previously available and have taken many, many hours to prepare. Included in the design are a couple of interesting modifications proposed by Duplex in 1951 to improve repeatability.
Construction notes unfortunately are not available and, owing to the complexity of the compound angles, this may not be a project for the beginner to tackle.