Use A axis for dual Y motors, or just connect both to Y

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Joined: Tue Mar 04, 2014 5:21 pm

Use A axis for dual Y motors, or just connect both to Y

Post by jbernardis » Wed Mar 05, 2014 9:30 pm

The grbl shield I am currently using - Reactive Substance - as well and the protoneer board - indicate that if you have dual Y motors, then you should hook one up to the A axis and one to the Y axis and have the A axis duplicate the Y. The shapeoko assembly wiki, as well as every 3D printer I've seen that has 2 Z motors, just connect both motors to the same axis.

Is there an advantage of doing one over the other? I have been planning on connecting both Y motors to the Y axis, but I have no planned use for the A axis, and so could adjust if needed. I realize that the motors need to turn in opposite directions, but my wiring already takes care of that. I am running NEMA17 motors, and am using the pololu A4988 Black drivers

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Re: Use A axis for dual Y motors, or just connect both to Y

Post by cvoinescu » Wed Mar 05, 2014 11:35 pm

Absolutely connect the second Y motor to the A driver.

The drivers can barely supply the rated current for one motor; you don't want to connect two motors to one driver if you can help it. The fact that is has four drivers instead of three is a major advantage of the and similar shields over the gShield/grblShield.

Even though you get about the same total torque if you split the current from one driver between the two Y motors, the Y axis needs more total torque than the X axis, for two reasons. When milling near one end of the gantry, the near Y motor handles more of the load than the far one; to withstand the same force as the X axis when milling at the extreme X positions, the Y axis needs about 80% of the X current in each of its two motors (more than that, if the gantry is longer). Secondly, the X needs to move just the carriage, but the Y needs to move the gantry (including the Y motors) in addition to that. To get the same acceleration on the Y, you need more (total) current than on the X. Given small drivers with limited thermal dissipation, it's much easier to get the extra current for the Y axis by using two drivers, one for each motor.
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