This is an alternative whacker hub for the Tertill by Franklin Robotics.
My garden soil is clay with many small rocks. This chewed through the original stock of replacement whacker lines in a few weeks. This custom part allows standard weed whacker line to be used, secured with a M3x0.5 grub screw.
Determining the length relative to the flexibility of the whacker line is important, as the Tertill whacker motor will stall if the line encounters much resistance.
Since the hub is friction fit to the Tertill, tight tolerances are required. The guide holes in this part should be bored with a 2mm bit after printing. Print this part on the flat face with at least 4 shell walls (1.6mm shell) and 25% infill. The additional walls provide needed margin for drilling the holes, and support for the grub screw threads. The extra infill provides a similar weight compared to the stock part.
If you don't have metric bits, use a 5/64 in place of the 2mm bit. Using a 3/32 to countersink the grub screw hole makes it much easier to start the screw. The fit on the motor shaft will be very tight - be careful not to damage it.
Metric Grub Screw Assortment
Arnold .065-Inch Maxi-Edge Commercial Trimmer Line