The rear wheel treads on these 97mm diameter wheels are just 0.75mm thick and so after some consideration were designed as a machined feature rather than a riveted tread.
The rim itself is just 2.75mm thick, designed to give a 2mm thick rim and 0.75mm thick tread.
The two wheel rims were machined from stock 100mm mild steel round bar.
There are 35 treads and so the rim blanks needed setting up on the rotary table.
This is the Warco HV4 rotary table with a 4-jaw chuck mounted to it, or more accurately about to be mounted.
Rotary table was set upright on the milling machine table and at an angle of 10 degrees.
The rear wheels are handed and so I will need to rotate the indexing table 10 degrees in the opposite direction.
The blue marker is there on opposite sides so that I could mark the diameter, a line across the outer tread surface and a centre to the tread.
The diameter marks allowed me to set the wheel upright with an engineers square. This then allowed me to align the milling table with the centre of the rim.
I counted the treads on a number of wheel rims that I could find on drawings and images and got to 35 treads.
I ummd and arrgghd over 35 or 36 and decided that the odd number was preferrable as a machined 4mm gap gave me the right ratio of gap to tread.
Is there anything in the fact that there are an even number of spokes and an odd number of treads? I’m not sure.
This next image gives you a bit more of a feel for the setup on the Sieg SX2.7 milling machine, along with the dividing plates on the rotary table.
The rims are mild steel and the slot drill is 4mm diameter and tungsten carbide.
I used a cutting speed of 1800rpm with a generous amount of cutting fluid applied with a brush.
I’m thoroughly enjoying building all of the tricky parts for this miniature traction engine.