Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

Hi Jim,

Excellent video and photographs. I have used the system when ornamental turning, but never thought of using it on pulleys. I was taught to make a form tool with a radius of the core diameter of the thread and turn a groove to a little less than the radius of the core. Then use a similar system to yours to cut the thread? The only difference being the tap being supported by a male or female centre; the centre being a rod held by a Jacob chuck held in the tail stock. The work being done in brass and sometimes steel. I have used the system to make a worm and wheel gear arrangements.

Take care Tony

G'day Tony,

Yes, it's essentially a simplified version of true gear hobbing, using a tap to generate the grooves in our model pulleys.

My method is straightforward. I mark the pulley with a Sharpie so I can keep track of the rotations, then make 2–3 complete turns before slightly increasing the depth of cut. I repeat the process with another 2–3 turns, progressively feeding the tool in until I reach the groove depth I'm after. Throughout the process I use plenty of WD-40 as a cutting lubricant.

Over the years I've only broken two taps, and both were the result of trying to take too much depth in a single pass. Taking light, progressive cuts is the key.

I've also built up quite a collection of jigs to suit the various bore sizes of the pulleys I've hobbed. In addition, I've welded together a range of stepped fixtures that allow cone pulleys to sit at the correct height on the tool post. For jobs that require extra reach, I've drilled out steel rods and silver brazed taps into the ends, as there are times when a tap held in the three-jaw chuck simply won't reach far enough.