Singer 760G cam stack drive gear

The "Touch and Sew" series of sewing machine are getting old. The cam stack drive gear is breaking on most 760Gs and people are binning them because they cannot find a readily available replacement. Shame, they don't make 'em like that any more. Fix it with this replacement gear.

There are two versions of the drive gear:
* Early production: A metal worm gear is fixed onto main arm shaft with 2 grub screws. The cam gear is just over 30mm in diameter.
* Later G3 models: Worm gear is cut into the arm shaft and the cam gear is a larger diameter with a bigger helix angle.

The gear in this project is the smaller one for earlier models. The larger G3 gear can still be purchased from various places.

**This fix is for the expert repair person**. The cam stack together with it's gear was assembled in the factory by glueing and pressing together two parts to form one continuous piece. Assuming the cams are still OK, the gear will need to be cut off and a new gear glued back on.

Befo