New day. New challenge. Hopefully one of the last ones. During the verification of the travel I realized when the Z axis moves up it’ll accumulate about 0.15 mm deviation in the first move, then this will be a constant deviation. So I’m making it move 10 mm but it only moves 9.85 mm. Then 19.85, 29.85 and so on. Here is a video where it clearly shows the deviation:
http://youtu.be/wbXK3GG57qII started to investigate what might have caused it. I focused my attention on the couplers that join the stepper motor axis and the threaded rod. I did not like the idea of putting the entire gantry weight on it. Initially both the motor and threaded rods were inserted into the coupler ~6.5 mm, leaving space between them. Here is an analytical calculation for the gantry mass. Since my machine is assembled I did not want to take it apart just to weigh the gantry assembly. I only included the main components.
As you can see the entire X gantry weighs 1.67 kg or 3.67 lbs. This will translate to 16.4 N force. This force acts on two couplers, so one coupler will see approximately half of this only - depending on the extruder position.
From engineering perspective the coupler is a helical spring with rectangular cross section. It’s probably made out of 6061 aluminum, and has very low stiffness. Also this coupler is not designed for axial loads.
I’ve made some calculation for a helical spring with rectangular cross section with the force shown above applied to it. As you can see the deflection is turned out to be 0.126 mm. This is not far from the indicated 0.15 mm.
- deflection.jpg (30.21 KiB) Viewed 315226 times
I started to look for solutions to prevent the compression of the coupler. Initially I wanted to have brackets machined/printed and bolted to the makerslides. These would house bearings and provide support for the lead screws.
Then I had a better idea, I simply dropped a ¼” ball into the coupler to act as a spacer between the motor shaft and lead screw.
Here is a video after its fixed:
http://youtu.be/jDOvTZG_zrE]
I'm pretty sure some people just shoved the axes together in the coupler until they're solidly stacked, I beleive this will make the coupler more rigid. The ball I used is 1/4" and I beleive it was in a bicycle front wheel cup and cone bearing.