are bigger steppers the answer?

[paul_m_hutton]

I have a cnc router that came with nema 23 18 lb/in steppers.(I bought it as a kit) I find that if I try to run it faster than 750mm/min (30in/min) it will lose steps. I think this thing should be able to be run far faster. Is it as simple as getting more powerful steppers or is something else limiting my speed? the machine itself is perfect no binding ,smooth as glass ,the accuracy is all I could hope for, I just wish it was faster!

[CJL5585]

Originally Posted by paul_m_hutton I have a cnc router that came with nema 23 18 lb/in steppers.(I bought it as a kit) I find that if I try to run it faster than 750mm/min (30in/min) it will lose steps. I think this thing should be able to be run far faster. Is it as simple as getting more powerful steppers or is something else limiting my speed? the machine itself is perfect no binding ,smooth as glass ,the accuracy is all I could hope for, I just wish it was faster!

Are you sure that the steppers are 18 lb/in steppers. Could they be 18 in/oz instead? A Nema 23 stepper is a small stepper motor.

There are several factors that determine the max speed of a router.

1 -- Power supply voltage
2 -- leadscrew turns per inch
3 -- Drive - Max voltage and current the drive can handle. Namebrand
4 -- Stepper motor steps per revolution, voltage & current requirements
5 -- Is stepper driver a microstep drive?
6 -- Is there a website for this kit?


In order to give you any assistance at all, one would need to know the specifications of the unit listed as #1 thru #5 above.

Jerry

[paul_m_hutton]

thanks for the reply jerry. here are the specs for my motors http://web4.automationdirect.com/sta...stepmotors.pdf they are model number 23079 ,it is a 30vdc power supply. the lead screw pitch is 1/4" per turn. as for the driver specs I will have to get back to you on that. the web site for the kit is here www.durhamrobotics.com although as of right now it isn't working for me. maybe you will have better luck. I will try and get in touch with gary(the owner) he would know the specs of his driver. (i'm sure it is on his site so hopefully I can get to it soon) what a great machine though.

thanks again- paul

[CJL5585]

Paul,
The website is online. The company is moving to a new location. I did read the specs for the components.

Drive voltage is 30 VDC.
Drives are rated at 6 Amps Max.
Max drive frequency is 16,000 Hz. Drives are 1/2 step. Motors are 200 steps per revolution.

So, if one takes the 16,000 Hz, divides it by 200 (steps per revolution) and divides that by 2 (half stepping), the max theoretical movement is 40 revolutions per second, or 2400 revolutions per minute.

I would check with the manufacturer and get some insight into the performance of the machine from his perspective. It could be the power supply voltage or the software settings.

Jerry

[CJL5585]

Paul,

The steppers are 276 oz/in and have a fairly high inductance. It could be a combination of the power supply voltage and the inductance which causes the system to miss steps at higher speeds. The acceleration factor could be set too high in software causing this. Some of the larger steppers cannot achieve acceleration speeds comparable with some smaller ones.

Jerry

[--colin--]

My setup:

200 oz-in NEMA 23 Steppers
Gecko microstepping drives: 2000 steps/rev
24VDC, 6A motor power
Direct drive leadscrews: 8 pitch (1/8 inch/rev)
Max speed: about 160 inch/min (IPM)
Practical max: about 120 IPM

I have finally settled on 90 IPM rapid speeds to not .loose steps. I'm using a plastic anti-backlash leadscrew nut and sustained operation at 120IPM results in the screws getting hot and tightening on the leadscrew. Otherwise I could use faster rapids for short periods.

Since you are using motors with higher torque and faster screws, you should be able to get better speeds. However, since you are running half-step with a fast screw, the torque required to get the mass moving is much higher and that may be the problem. Perhaps look at changing the acceleration curve?

It is my understanding that the practical limit for stepper motors is about 1000 rpm. I don't know how this relates to full step/half step/micro step modes. At 1000 rpm x .25IPR = 250IPM. You should be able to get better speeds but may be torque limited as mentioned above.

--Colin--

[CJL5585]

Paul,

Go to the stepper motor curves for your steppers. You will find the curves show that at a step speed of 4500 Hz that the stepper motor torque has dropped from 276 oz/in of torque down to 50 oz/in.

In essence, your system appears to work as designed, but is limited in speed by the torque available at the higher speeds. As speeds increase, it loses the ability to have enough torque to turn the ballscrew.

This goes back to my prior post about the motor inductance.

Jerry

[paul_m_hutton]

thanks for the replies- i will try messing with the acceleration of the motors first. I'm pretty ignorant when it comes to electronics. could you explain inductance briefly?

Paul

[paul_m_hutton]

disregard my request for an explaination of inductance guys! i just looked it up on wikipedia. I won't ask anyone to explain that to me. I will take your word for it. i do thank you all for taking the time to help me.

paul