I built a CNC milling machine and due to the size of the machine (100x60cm) I chose to mount two stepping motors on the Y axis. The motors are connected by belt to an RM1605 lead screw. The belt transmission has a ratio of 2:1 with the idea of having more power. All this is driven by an Openbuild X32 controller. The motors are CONUCON type 2330. (Nema 23, torque 1.9 Nm, 3.0 amp) Now the problem is that very sometimes the motors do not run synchronously and whether this is because one of the motors skips steps or the controller does not control the motors properly I do not know. This only occurs when the machine wants to move from point A to B unloaded. So not during milling operations. The maximum travel speed in the software is set at 600 mm/minute. Because of the gear ratio of 2:1, this is actually 300 mm/minute. This seems pretty slow to me . If I turn the controller off I can then turn the screw leads around by hand without any problems. If I then adjust the X axis back to square I can then use the machine as normal. Now how can I find out what is causing the problem?
i think you are wrong there. Assuming you have correctly calibrated the steps/mm values, then that accounts for the belt drive AND the microstepping AND the leadscrew pitch, and the maxrate setting will be the actual maximum rate of movement. Easy to check, set zero somewhere convenient with space to the rear and issue: G1 Y60 F60 which will move 60mm at 60mm per minute, so it should take 1 minute within a few seconds to allow for acceleration. Given your slow movement (my machine rapids at 4500mm/min because I slowed it down from 7500mm/min because 500mm of travel disappears very quickly at 7500mm/min I would look at acceleration, motor limits due to high inductance, and dirt. So, we are left with a few things to check current settings on the two Y drivers must match each other and match the motors maxrate is only part of the story, maybe acceleration is too high for the mechanics? dirt on the rails/leadscrew/wheels, dirt will cause sudden and massive increase in rolling friction which is most likely to cause lost steps during rapid movements.
The torque that steppers provide falls off rapidly at high rotation speeds. As noted by David the Swarfer, trying to accelerate too fast is well worth checking. It sounds odd, but swapping the belt pulleys around (or at least going for a 1:1 ratio) may mean you can accelerate faster and get more reliable top speeds. That is, instead of two rotations of the motor giving one rotation of the shaft, having the pulleys do one rotation of the motor giving two rotations of the shaft. At very low speeds, then there will be 1/4 of the torque that you currently have, but given how torque curves behave, there will still be more than enough force available to do cuts. As a rough calculation, as for a lead screw, force = efficiency * 2* pi * gear_ratio * torque / pitch, currently you have a gear ratio of 2, a torque of 1.9Nm at stationary, and a screw pitch of 5mm. If we assume it is 90% efficient transmission, then that is a force at standstill of about 429kg (enough to do serious damage and turn a leadscrew into a corkscrew... ask me how I know!). Swapping the pulleys to drop the torque by a factor of 4 will still give a standstill force of 107kg. As the travel speed increases, the motor only has to rotate 1/4 of the rate it is doing at the moment, which will keep it in its high-torque region for longer. Evan.
Hi David and Evan, Thank you for your responses. I can do something with this and will make some changes to my machine. It will take some time to do this, but I will share the result with you.
