Thread: Getting maximum speed from Mach3

1. Getting maximum speed from Mach3

Evening all

I am building my second machine, a medium size router and upgrading from ACME to ball screw, adding bigger motors and so on.

I have this kit (430 oz-in 3-axis CNC Pack with Sinusoidal Drivers :: Large Stepper Kits :: CNC :: Ocean Controls) all hooked up and running fine. My ball screws are metric 5mm pitch, i.e. 1 rev = 5mm travel.

I have configured the drivers for 8 microsteps so I get the maths of:

200 steps native on the motor
8 microsteps = 1600 steps per rev
5mm per rev = 320 steps per mm.

Mach3 is configured accordingly and running at 25kHz on a quad core 2.4Ghz CPU under Windows 7.

My issue is that anything over 3500mm per minute, 140 inches, under load and the motors stall out.

Am I expecting too much from this configuration, I would have thought something like 7000mm per minute, near the 275 inch mark was more likely ?

Would playing with kernel speed and microstep combinations improve the speed while maintaining torque ?

Since the torque/speed curve on this configuration is quite steep and non-linear, would I be better off running the motor slower then using a gearbox/pulley system to speed up while still maintaining torque. Let's say that doubling motor speed cuts torque by 70%. If I keep the base motor speed but increase shaft speed by a 2:1 pulley then I only get a 50% loss of torque for the same speed. Does that sound right?

2. 3500mm/min = 700 rpm, which about what you can expect.

According to Gecko, for maximum motor performance, the voltage should be 32 x sq root of the inductance. This is about 80V. Since you're running at 40V, you're only getting about 1/2 the maximum speed those motors are capable of.

So, you have 3 option.
1. Change the motors to lower inductance, higher current motors, that will have more torque at higher speeds.
2. Change the drives and power supply to run your motors at 80V.
3. Use a gear reduction to spin the screw faster.

I'd go with #3, as you suggested.

As you're currently set up, 25Khz kernel speed is good for about 4700mm/min
25000 steps/sec x 60 sec divided by 320steps/mm

If I keep the base motor speed but increase shaft speed by a 2:1 pulley then I only get a 50% loss of torque for the same speed. Does that sound right?
Yes, but keep in mind that moving the machine twice as fast at any given motor speed requires twice as much torque. You'll definitely get more speed, but it may not be double.

Here's a good explanation of how spinning a stepper slower can give you higher speeds.
Mechanical Power - PMinMO.com

3. Hi

Thanks for the quick reply, much appreciated.
I'll see what I can do on the gearing option for now.

Cheers