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

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

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 :)

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.

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

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

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