Although not strictly speaking specifically gear cutting per, se I had another ‘first’ yesterday. This ‘first’ was single point thread cutting which I have never attempted before. For those that don’t know what single point thread cutting is, it’s the cutting of a thread using a cutting tool in the lathe rather than a traditional threading die (which I used to cut the 0BA worm a few days ago).

The pitch of the thread is determined either by altering gearing via several levers (if your lathe is so equipped) or in the case of many smaller hobby lathes it’s achieved by the use of change gears. Mine is the latter. What the change gears do is alter the rotation of the lead screw so that when the automatic feed is engaged, it moves the carriage a certain distance for each revolution. In my case I needed a 1.5mm pitch so I needed to swap out my standard gear set up for a 30 tooth (A), a 50 tooth(B), a 55 tooth(C), and a 35 tooth(D)

This was the standard gear layout

This is the gear layout for a 1.5mm pitch

Once I had them setup, I put some layout fluid on a piece of nickel bar and did a scratch pass to make sure that I hadn’t made any mistakes.

Then I had a go at cutting a thread using a proprietary standard* 60 degrees thread cutting tool.

*Threads come in many different flavours depending on whether you are using metric, imperial, BA or many of the other thread types. Below is a thread tool gauge designed to assist when grinding your own cutting tools. As you can see US Imperial and Metric use the same standard which is 60 degrees.

I used this mainly because I had never done it before and I wanted to use a known good set up rather than a tool I had ground myself (which proved a wise choice).

I am happy to report that all went as planned, although I did over shoot the thread gutter a couple times because I hadn’t really made it wide enough to allow the machine to stop at this pitch and I successfully cut a screw thread.

Another good session in the workshop. On the back of this I had another go later in the day, using a piece of steel and the cutter that I had ground. Sadly, as I half suspected, the tip broke off the cutter after about 5 passes. This was due I think, to being ground on a taper, which meant there was nothing under the point to offer any support. Plus being carbide, it was quite brittle.

This actually turned out to be a blessing in disguise as when I started to grind a proper piece of tool steel, I realised that my 20 degrees angle, actually gave me 40 degrees included angle. A mistake that I had also made on the first one.

Thankfully it was a case of taking more off rather than starting again so I just need to finish off grinding it to shape after posting this.