Jones' screw thread grinder, complete with wheel dressing attachment and counter-shaft, striking gear and swing frame

This machine was designed by J. H. Jones for grinding the threads on fuse-hole screw gauges, and was the first machine to grind successfully threads in hardened steel to the Ministry of Munition's limits of +0-0003 to —0-0006 inch in diameter.

The system comprises the use of a master screw mounted on the tail of the headstock spindle and working in a stationary nut. The spindle with the gauge is propelled past the grinding wheel as it rotates. This is an interesting reversion to the old system of cutting screws used in the early ornamental turning lathes.

The gauge blank is mounted between centres, the tailstock centre being spring loaded and giving to the travel of the gauge. In the headstock is a reversing mechanism operated by adjustable calliper-leg type of stops which swivel to and fro in sympathy with the movement of the spindle and engage with reversing pawls at the end of a pre-determined travel. The grinding wheel cuts the thread on both the outward and return travel, the feed of the wheel being applied by hand.

The wheel-trueing device consists of a carborundum dresser mounted on an arm at the back of the machine. This arm swivels through an angle of 27-5 0 on each side of the centre line, giving the angle of 55 0 for the standard Whitworth thread. The dresser is worked on both sides of the wheel by hand. forming a sharp pointed or a truncated V at the discretion of the operator. The desired radius on the top of the V is formed by using the newly dressed wheel for roughing out. The radius on the crests of the ground threads is formed by a separate wheel dressed with grooves at an angle of 90 0 and formed to shape on an old or rejected gauge.