PDA

View Full Version : What is optimal motor speed ??

flyinwilly
04-03-2010, 09:46 AM
I've reached the point where I am ready to purchase my drive components. I am using a Gecko 540 with 425oz steppers on the Y and Z axis, and 2 slaved 283 oz on the x. It will be a belt drive on the x and y. I am ready to purchase the timing pulleys and have a question on the optimal motor speed.

I calculated the steps/in at 4800 for a 3:1 reduction and 8533 at 4:1 reduction. With a cutting speed (estimated) of 50 IPM this gives a rpm of 120 at 3:1 and 213 at 4:1.

The motors sound fine at either speed although they seem (purely subjective) smoother at the 4:1 reduction. However, they don't have any load on them at this point.

Is there a optimal speed that stepper like to run at ?? I have room in my design to go up to a 6:1 reduction if necessary although I really don't need that kind of accuracy.

Thanks,

Willy

Pandinus
04-03-2010, 12:30 PM
I think your calcs are off somewhere...

4800 / 3 = 1600
1600 * 4 = 6400

Final gearing will depend on weight ... final speed / acceleration you want ... motor torque... torque dropping off at higher rpms etc.

flyinwilly
04-03-2010, 01:59 PM
You are correct. My spreadsheet used the reduction twice in its calcs. I guess you can't believe computers, especially when I program them.

The correct numbers are 1600 steps/in at 3:1 with 40 RPM and 2132 steps/in at 53.3 RPM at 4:1.

I changed Mach and tested both configurations and neither one sounded as good as the previous settings (which were 3 or 4 times faster).

I calculated the weight of the gantry at around 31 - 33 lbs and it's riding on Pacific Bearing V rails so it should move pretty easily with 2 drive motors.

What is a normal cutting speed (IPM) for 11 - 16GA steel? My only experience is freehand and it seems pretty quick. I've used 50 IPM in all my calculations but this is just a number I've picked up in the forums.

Any help appreciated.

Thanks,

Willy

Pandinus
04-03-2010, 08:46 PM
The cutting speed depends on your plasma cutter.. make/amperage... my Powermax45 would cut
11 gauge @170-200+ IPM
16 gauge @350-400+ IPM

My machine is rack and pinion at about one revolution of the motor per inch G540 and 387Oz motors and about 36V on the 40V linear power supply under load... it gets about 800 IPM at Mach3s 25Khz base speed.. I tested with EMC and pumped it up to about 1800 IPM... the acceleration can be set high enough to make it look like I'm hitting the machine with a sledge hammer... lol ... I have no idea what the weight of the gantry is, but its relatively light and made from aluminum. Motors and gears/rack probably weigh as much as the structure :p

I just realized that means I was getting up to around 1800 RPM out of the steppers before I lost the torque to drive the weight... I love my Gecko :D

flyinwilly
04-03-2010, 09:49 PM
Wow, thats pretty fast cutting.Hope I can get near those numbers.

One of the things that is lurking in the back of my mind is stopping the gantry with a belt drive when it really gets moving. I am going to use 3/4" drive belts to help eliminate stretching or skips. It might be little overkill but the cost difference is minimal.

I have a 40A inverter that cuts pretty good for 3/8" or less.

From the numbers you gave, I should be able to safely increase the design cut speed to 150 IPM for 11 to 16 ga crs. 99% of all my work is 1/8 or less.

This should increase the motor speed to 120 RPM with a 3:1 reduction or 160 RPM for the 4:1. The motors run pretty smooth at this level although they still have no load yet.

I believe that I will design it for the 4:1 since the motors and the 540 have plenty of upside left at these numbers.

Thanks for the info.

Willy