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.

Before you cut off the gear, accurately measure the distance between the top of the metal shaft and the top of the cam stack, it should be around 4mm (see 3rd photo). The stack will undoubtedly move in the process of removing the gear and there is no way to lower the cam stack or raise the cam followers in the 760, so you should aim to reposition the cam stack perfectly.

Cut the gear off as cleanly as you can, without damaging the metal shaft (I used a small hobby lathe). Sand the bottom of the cam stack flat and square.

It is worth thinking about how you will join the gear to the stack. It will have to be glued but some interlock is beneficial. I cut 3 slots in the cam stack to receive 3 pegs in the gear, thus the transfer of rotational force is not reliant on the glue alone (visible in 4th photo).

Next determine the spacer height, this will depend on where you cut off the gear and how you intend to join the new gear to the stack. Enter this value in the SCAD file, render the gear and export it to STL.

Consider the type of filament. I am having sucess with annealed PLA, but perhaps PETG would be a better choice for standard temperature printers (although PETG is harder to glue).

If you choose to anneal PLA, try the following scaler settings in the slicer:

• X and Y : 101.7%
• Z: 96%

Set overall wall thickness to 1.2mm, this is where the strength needs to be.

Print the gear and continue in the section on post-processing...

Creality

Ender 3

Doesn't Matter

No

0.1mm

100%

Tempered PLA was used in this project, but alternatives should be considered.

If annealing PLA, put the gear onto the bare metal shaft (without the stack) and bake it at 90°C for 30 minutes or so. Watch the temperature, some ovens overshoot by quite a margin. The shaft will stop the gear from distorting out of true. The gear should adjust in dimension to the intended design and should come off the shaft easily.

Glue the gear to the shaft with a 2-part epoxy, the grooves help with the adhesion. Curing at 40-50°C will result in a stringer bond, the oven can be used again. Once cured, assemble the cam stack and check the vertical positioning, this is your last chance to get it right. If the stack ends up mounted too high on the shaft, you will need to start over because the cam followers will not engage correctly. Glue the cam stack to the gear using the bicarbonate of soda + superglue method.

The cam stack (with new gear) is now ready for re-assembly. Lubricate with a thick, quality silicon based grease that is compatible with plastics. Since everything is adjusted around the cam stack in the Touch and Sew, the rest of the machine will need to be re-tuned.

Since most 3D-printer filaments have a low glass transition temperature, it might be worth replacing the filament bulb in the machine with a LED version, this will stop the machine head from getting hot and possibly weakening the gear.

Happy sewing.