Am I getting all I can from my stepper motors?

[farmertom]

Hi,
My DIY CNC machine seems kind of mediocre or lethargic, I am hoping someone can look at my specs and tell me if I am getting all I can out of my system.
So the details:
1) Gecko 540 motor controller running four motors ( two are run together. Two motors on x axis)
2) 3500K resistor on each motor connector for 3.5 amp current
3) 48 Volt power supply ( I think 10 amps)
4) Nema 23 570 oz\in stepper motors wired to unipolar (motor specs in attachments)
5) Mach3 Controller software.
6) I use Acme ½ inch one start lead screws (.1 inch per rotation)
7) The most I get out of my system is 60 IPM reliability before the motors tend to stall.
Am I expecting too much or am I missing something?
Suggests on upgrades to system are also appreciated!
Any advice would be appreciated!

Thanks
farmertom

[harryn]

I am not an expert, but I think you should wire the motors in bipolar series, not unipolar for better performance with that driver.

It might be good to post this question in the wood router build section. The reason I mention it, is that your screw drives are not optimal for speed either, and the gantry uprights might need beefing up.

60 ipm x 10 rotations per inch = 600 rpm - that is a lot for a 1/2 in dia. screw. For some reason, I am thinking that 50 - 100 rpm is closer to where it needs to be for better speed.

Edit - I looked at your weebly web site - very nice carving work. The fact that the lines are coming out so clean means that you certainly are doing something right.

[ger21]

You really have 2 issues here.

1) you're choice of screws. A good rule of thumb, is that 1/2-10 single start acme is only good for 60-100ipm. Usually, the smaller the motor, the faster it will spin.
Changing to 5 start scres would give you a roughly 5x speed increase. Possibly more, possibly less. Issue #2 factors into this.

2) You're motors are not a good match for the G540.
You can try wiring them bipolar series, but I think the unipolar (half coil?) might be the better choice.

You have 3 options to improve performance.
1) change to 5 start screws. This should really be your first choice, as without changing the screws, options 2 and 3 won't make much difference
2) Change to smaller, 380 oz motors from Keling. You're motors usable rpm will go up substantially, and you'll still have plenty of power.
3) Change the Gecko G201 or G203V drives. This will allow you're current motors to spin much faster, but you'll also need a new power supply that can supply at least 15 amps.

#1 is the easiest way to increase performance. After you change the screws, you can decide whether you need to do #2 or #3.

#2 is probably a better choice than #3, due to price. 4 new motors = $200. 4 new drives and a new power supply = $600-$700

A fourth option would be to convert the machine to rack and pinion. I'm not sure how easy this would be, or if it's even feasible at all.

[farmertom]

Hi,
Thanks for all responses!
A correction first. The motors are wired unipolar and not bipolar, sorry about that. So that mis-type fixes some suggestions

If i change to the 1/2 inch 5 start acme screw will the accuracy be reduced?
And will I notice the reduced accuracy?

What can I do about the motor stalling?

Thanks again
farmertom

p.s. harryn thanks for the comment on the carvings! made my day!

[KOC62]

Another aspect I found in my situation with single start 10TPI leadscrew was kernel speed. I was aiming for 120 IPM jog speed. This meant a leadscrew RPM of 1200. My stepper and G540 drive required 2000 step pulses per revolution. I couldn't get a 40KHZ kernel speed with EMC2, due to the high latency of my PC. Maybe Mach3 is better in this regard.

Then again, as has been mentioned, even if I could get the electronics to whip the motor to 1200 RPM, the whipping action of the 1/2" leadscrew would most likely prevent it.

So it seems that the "voice of experience" ends up where ger21 has pointed to.

[ger21]

Motor stalling is caused by trying to go faster than the motors are capable of. Steppers lose torque rapidly as rpm's increase. Changing to 5 start will allow the motor to spin 5 times slower at any given speed, where the motor will have much more torque.
Also be aware that 5 start is typically about twice as efficient as single start. With single start, about 60% of the torque is lost to friction. With 5 start, it's only about 20-25%. So it's possible that at the same rpm your motors are currently stalling at, the 5 start screws will have more torque available due to lower efficiency losses. Which may allow even higher rpm's than the single start, with 5 times the speed.

It's very important to make sure that you don't have any binding in your system, either from misaligned screws or misaligned bearings. Even slight misalignment can drastically reduce performance and cause stalling at much lower speeds.

[farmertom]

Hi again,
ger21 thanks for all the advice. I am upgrading up to 1/2 -10 five start lead screws.
Thanks
farmertom.

[H500]

60 ipm isn't bad for lead screws. But the biggest upgrade you can make is to use ball screws. My motors have half the holding torque as yours but I get 250 ipm reliably on x and y with a 34v power supply. You should be able to get that easily since your machine has less friction.

You should also wire the coils as bipolar parallel even if the driver cannot supply 5 amps. Your performance won't change, but the motors will run cooler.