Another challenge was put forward – to make a leather cup washer to replace a worn one from a bicycle track pump. These cup washers were often used in pump applications, both air and water, although I’d never really thought much about how they were made.

After a bit of research the method of construction was to press the leather into a form to shape the leather. Typically, in making leather items, the leather would be soaked in water before being pressed into a form. Once dried, the leather would retain the shape of the form.


However the general advice was to soak the leather in hot paraffin wax rather than water. I was quite intrigued by the process so the challenge was accepted!

The project dimensions were:

The only issue I could see was my hole punches came in 1mm increments. Although the bore diameter is not critical so the options are to ‘over-punch’ an 8mm puce or use a 9mm one.
A basic CNC model was knocked up. I opted for a hole equal to the bore diameter simply to give definition to where to punch the hole.
Similarly a raised section equal to the bore diameter was put on the male form to aid aligning the punch later on.

The choice of polyethylene for the form was not the best. The heat generated by the end mill would melt the swarf back to the base material but it would do.
The only tooling leather I had was 3.6mm thick – the target ranger was 1.5mm to 2mm.
The tooling leather therefore needed to be split down to size. My generous 55mm wide strap of leather was at the limit of what my 6” splitter could realistically handled due to the effort to pull the strap through.
The paraffin wax comes in bead form.
Melting the paraffin wax without direct hear
After 15 minutes soaking in paraffin wax, the leather was quickly transferred to be pressed between the two halves of the form. A 20 ton hydraulic press was used to press the leather tightly into the form.
The indentation worked well to provide a guide for centring the holes punch
Not so successful was that I hadn’t allowed a sufficient gap between the two halves to allow a flat bottomed skiving knife to remove the excess leather.
Cup Washer Prototype 1
The absorbed paraffin wax make a the once pliable leather quite stiff which should enable a tight seal. fingers crossed!