Siff' Build Log

[Jonathan]

If I understand correctly, even though the motors require 4.2A and the gecko only provides 3.5A, this would mean that the motors just run at reduced torque, and wouldn't damage them, or the controller. Please correct me if i'm wrong.

That is correct. You could always run them in bipolar series ... then you only need 2.1A. That will reduce the torque slightly at higher speed than with parallel, but I'm pretty sure it will be less of a loss than using only 83% of the rated current. The power dissipation of the motors will be about 70% of rated running on 3.5A. Why gecko? Something like PM752 or PM542 on from eBay or Zapp should be cheaper and will let you run at the full current and (with PM752) higher voltage.

I agree unsupported rail is a bad plan, especially on the X-axis as the end fixity is poor. Even 30mm unsupported rail will flex more than 12mm supported. Granted the load ratings of the bearings on the 30mm rail would be higher but they're plenty more than required anyway. I would use 16mm, or more if you can afford it. 25mm supported rails are good as they have an additional row of balls ... though on this size of machine you don't need that size. Have you looked on eBay for them? They're cheap from linearmotionbearings2008.

You might want to bolt the Y axis plate to the back of the gantry, not the front, as that will get the centre of mass closer to being half way between the pairs of X-axis bearings.

There is a lot of overhang, as Robin said do you really need that much Z travel? If so you're best to eliminate the gantry sides altogether, like I did here:



Plate is also the worst profile for the gantry sides since it bends easily in the direction parallel to the Y-axis, however you've compensated for that by having a plate parallel to the Y axis, so the only thing it will lack is torsional stiffness. You could use something lighter if you changed the cross section - maybe extrusion? That's expensive though. Steel box section is cheap and the weight isn't really an issue since the gantry is fixed. Just make it as strong as you can.

Also if you want the machine to be fast use timing belts on the stepper motors/screws to get the motor operating in the right region. Since you're machine is quite light you will almost certainly want the pulley on the stepper motor to be bigger than on the screw. What pitch ballscrews are you using, probably 5mm? I have not noticed increased backlash from using timing belts/pulleys. They do stretch very slightly, but I don't think that's a big issue.

Nice drawing ... if you do decide to use supported rails I can send you the drawings I did of them to save you a bit of time if you like...