We have published a number of guides to imperial, metric and BA screwcutting relatively recently. However, Brian's examples are unusual and merit publication, not least as the Cowell is a rather small lathe, with a metric leadscrew of just 1mm pitch. Before starting I must say, some years ago I designed a gear hobbing arrangement so I am able to cut any gear wheels needed. At the present time I am building a 5 inch gauge steam loco and have reached the point of boiler fittings, which require: 26-32 and 40 TPI threads as well as 2 BA and 6 BA. My Cowell lathe has 1mm pitch lead screw and gear wheels of 32dp. Not having a 32dp hob, the nearest being 0.8 module, and calculating the size of wheels required for imperial threads, the 127 tooth wheel was just too big to fit. The next size down I have is a 0.7 module hob. After calculating sizes for 0.7 module, I found these would fit the lathe.
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