This American machine is designed to cut the teeth of gear wheels.
The cutting tool that you can see is called a ‘hob’. The gear wheel into which the teeth are to be cut is fitted onto the spindle in the middle of the turntable in front of the hob.
The machine was used in the Coatbridge works of Anderson Brothers from the 1930s. One of the engineers there, Jimmy Houston, used this machine to build the miniature traction engine ‘Tigger’ which is now at Summerlee
This illustration from an engineering manual shows a similar machine cutting the teeth of a bevelled gear wheel. You can see that the hob of the machine is angled to match the angle of the teeth it is cutting.
For a gear hobbing machine which is a theoretical involute, the fundamental rack is straight-sided, with sides inclined at the pressure angle of the tooth form, with flat top and bottom. The necessary addendum correction to allow the use of small-numbered pinions can either be obtained by suitable modification of this rack
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