Stepper motor speeds

[nikolatesla20]

Hi, Cnc newbie here

I posted in another forum about some steppers I have and I remarked about the fact that I didn't think I would be able to go very fast. Can someone double check my math?

I have 400 step/rev motors. I'm not sure what their torque is. They are only rated for .4 amps. I plan on making a foam cutter with them, so they don't need to move super fast anyway.

If I mate these up with a 1/4 - 20 threaded SS rod, you get 20 rev/ inch on the rod. So 20 revs * 400 steps / rev = 8000 steps / inch.

I'm using a UCN5804B chip on each motor to drive it, using 12V and a 22ohm resistor in series with the motor (motor is 16.5 ohms) to get around .300 amps. I realize that is probably pretty butt weak for a CNC? (Although as I mentioned I am going to be doing very very lightweight work. C'mon a hotwire foam cutter really has no mechanical load!)

I'm currently experimenting with KCam software, and I can't seem to really go faster than 1in/min. If I go faster it seems like the motors start to miss steps.

Could it be the software (talking thru XP maybe it can't keep the pulses up correctly) or do I just need bigger motors to go faster...

The reason being is if I use a larger lead screw, like instead of 20th/in if I could get like 13/in I could go faster and the motor requires less steps/inch so it doesn't have to be driven as fast. Yeah accuracy per step decreases, but 8000 steps/in comes to .000125 inch per step, I really don't need that much "accuracy" anyway...

And so KCam driving at 1in/min would be 20rpm.. to go up to 10in/min it would be 200rpm and would be 1333 steps/sec..wow can't believe motors that can take that.

I'm pretty sure my math is right.

-niko

[HobbyCNC]

Originally Posted by nikolatesla20 Hi, Cnc newbie here I posted in another forum about some steppers I have and I remarked about the fact that I didn't think I would be able to go very fast. Can someone double check my math? I have 400 step/rev motors. I'm not sure what their torque is. They are only rated for .4 amps. I plan on making a foam cutter with them, so they don't need to move super fast anyway. If I mate these up with a 1/4 - 20 threaded SS rod, you get 20 rev/ inch on the rod. So 20 revs * 400 steps / rev = 8000 steps / inch. I'm using a UCN5804B chip on each motor to drive it, using 12V and a 22ohm resistor in series with the motor (motor is 16.5 ohms) to get around .300 amps. I realize that is probably pretty butt weak for a CNC? (Although as I mentioned I am going to be doing very very lightweight work. C'mon a hotwire foam cutter really has no mechanical load!) I'm currently experimenting with KCam software, and I can't seem to really go faster than 1in/min. If I go faster it seems like the motors start to miss steps. Could it be the software (talking thru XP maybe it can't keep the pulses up correctly) or do I just need bigger motors to go faster... The reason being is if I use a larger lead screw, like instead of 20th/in if I could get like 13/in I could go faster and the motor requires less steps/inch so it doesn't have to be driven as fast. Yeah accuracy per step decreases, but 8000 steps/in comes to .000125 inch per step, I really don't need that much "accuracy" anyway... And so KCam driving at 1in/min would be 20rpm.. to go up to 10in/min it would be 200rpm and would be 1333 steps/sec..wow can't believe motors that can take that. I'm pretty sure my math is right. -niko

Try 10 ohm resistors and forget Kcam. It takes alot of CPU. You'll at best go to maybe 5" per minute.

Dave

[abasir]

You indicated that the motor is rated at .4A with 16.5ohm resistance thus voltage rating of about 6.5V. You'll need to increase the supply voltage to increase the speed. I would personally use about 36V or 48V but check the maximum voltage allowable by your driver

[mdreitzusa]

the ucn5804b has a sustained output of 1.25amps and 35volts.thats the same chip i'm using.i'm now building a 32volt/2amp supply and a driver board to get better speed and more torque.here is the data sheet for your chip.

[ViperTX]

Well the threaded rod is probably doing more to keep you from going faster...switch to an ACME screw.....

[nikolatesla20]

I threw together a cardboard test bed (yes, cardboard) since I have all my materials to start the cutter but I unfortunately was missing some tools, so I will have to wait until next week to make the "real" cnc. For now I made a cardboard mockup with the motor and leadscrew and put down a ruler next to the moving X axis.

For now it looks like I can run up to 8in/min before the stepper starts to choke, so I think I should be ok. The motor gets dang hot after a while though, so I think I'll use metal motor mounts instead of MDF so the heat can dissipate.

Thanks for the extra information guys, yes, I know the UCN chip can put out quite a bit of oomph but the motors I don't think would handle it! hehe.

And the lead screw yes, that is what I was referring to .... it almost seems better if I used a ACME with a lower turns/in. Then the motors don't have to rev as fast to move the screw as fast.

For now thought I think it should work out ok since it seems even with a crappy cardboard mockup I can go up to 8in/min so depending on the torque load of my real cnc it should be similar. If everything is properly aligned so the screw can turn freely I might be able to hit 9in/min (at 10 the motor starts to spit back at me). This is way fast enough for foam cutting.

-niko

[ViperTX]

Standard screw profiles are designed with fricton for a reason....so things don't loosen up....