We plan on using a lead screw for an off-season project. We plan on 3d printing an adapter from the lead screw to a hex shaft, and then affix the adapter to the lead screw by boring a hole through it and using a cotter pin to secure it. The current plan is a Neo 550 running on a 10:1 Planetary gearbox. The weight of the device attached to the lead screw will be no more than 10lbs.
What size of cotter pin should we use, and is there any other obvious issues with this? I don’t think the torque should be too much if I print at 100% infill in CFPLA, but I could be wrong.
You are going to want to reduce point loads, cotter keys are relatively soft by design and generally not used to transmit torque of any substantial amount. That being said they can totally work and don’t require much in terms of tools.
I would look for something in the neighborhood of 1/8" cotter for a half inch hex shaft, make things beefy on the 3dp part. So you are probably fine with whatever you choose.
A more conventional option would to be to use a roll pin of a similar size. Just need to pay attention to the tolerances (friction fit) . These will be more tolerant to torque transfer.
With all that being said, the weak link in the system is probably the 3dp part.
Be careful crossdrilling the lead screw if it isn’t already.
We used lead screws on 708 in 2016, 2018, and 2019. My recommendation, if you have access to a mill, would be to machine some sort of feature into the screws. We machined hex ends into our screws, I think it was 3/8 hex machined into a 1/2 screw. But you could add flats or a square or a keyway and have the matching geometry in the 3d print.
Personally in not the biggest fan of pinning but that should work too, especially for low load applications.
Sadly, the speed regulator on our mill halfway through the last season, and our school took it and never returned/replaced it. So we don’t have access to one. We have a drill press which is how we plan to drill the hole for the cotter pin.