I needed to make a form tool to make the base for the air pump on my triple expansion steam engine.
It required a 1/4″ radius section and a 15 degree straight section.
The dimensions for the cavity in the air pump, and the cutter to produce the cavity. And the piece of cold saw blade which I used to make the form tool.
I considered machining the arc and the straight sections separately, but I did not have suitable tools, so I made a form tool.
A friend had previously suggested using steel from a broken cold saw blade to make form tools, and on this occasion I used his suggestion. (Thanks Manuel!).
The blade was 1.6mm thick which was ideal. I had some trepidation about cutting it.
The broken cold saw blade. The steel is superb. Painted with layout dye. The air pump base is visible lower right of photo, bolted to the engine base.
Using an angle grinder with a 1mm cutting disk. It cuts through the cold saw blade easily. Like a hot knife through butter … almost.
Grinding the cutter to shape on an aluminium oxide wheel.
Grinding the 1/4 inch radius arc.
Marking the shape of the form tool cutter
Cutting a 1.6mm slit in 10mm mild steel rod.
Initially I fastened the cutter steel to the rod using 2 grub screws, then, after checking the dimensions and the 15 degree angle I cut it to size. In use, I found the grub screws would not hold the tool steel securely, and I eventually silver soldered the join.
The tool, prior to soldering. I ground the relief angles on my Quorn T&C grinder. (See old post). Except for silver soldering the tool steel into the rod, this is the finished tool.
Commencing the machining of the air pump base cavity. I had planned to do the machining using a boring head on my milling machine, but quickly realised that would not work. So I used the recommended method of a 4 jaw chuck on the lathe. The 45 year old 4 jaw is still in excellent condition.
The end result.
The complex cavity was initially centre drilled, drilled then bored to size. Then the home made tool was used to machine the undercut cavity. It worked perfectly!
I learned about using cold saw blade steel as a source of tool steel from Manuel. I am aware of a professional contract machinist who uses this method to turn complex shapes in brass and steel, in preference to using a CNC lathe.
The material can be heated to red heat, (during silver soldering) and it does not lose its superb ability to take and retain a sharp cutting edge. Very impressive.