What size motors? I'm confused again :)

[Darren_T]

I posted some pics in another thread below showing the materials I have so far for my CNC router project. I did some research a couple months ago here on what I'd need to build the CNC router I wanted.

I have most of the materials needed now to begin building and am researching motors etc now. My router will be used for all sorts of things from 3/4" MDF, Aluminum, Foam, balsa, and plotting.

I have no idea what size motors I'll need. For some odd reason I had thought 100oz motors would be fine but am seeing people discussing 600oz motors now. Was I on another planet?

I'm trying to keep the cost down like everyone else but don't want to skimp.

Darren

[CNCadmin]

What size machine? I think 100oz are OK, you can gear it also to get more torque.

[Darren_T]

What size machine? I think 100oz are OK, you can gear it also to get more torque.

I was going to go pretty small but hit the jackpot on aluminum for construction.

I'm planning on going a little bigger at 28 X 36. I'm hoping for 4-6" clearance for the Z axis. I'll be mounting a 1.5hp router on the Z axis.

[balsaman]

How fast do you want to go? If the answer is 100" per minute, then maybe 100 oz motors are too small. If you can live with 15 to 30" per minute then 100 oz will be fine. It also depends on friction, weight of the moving parts etc. With 100 oz stepper motors my machine went ~30" per minute. It was 12x24".

[Darren_T]

Originally posted by balsaman How fast do you want to go? If the answer is 100" per minute, then maybe 100 oz motors are too small. If you can live with 15 to 30" per minute then 100 oz will be fine. It also depends on friction, weight of the moving parts etc. With 100 oz stepper motors my machine went ~30" per minute. It was 12x24".

Hi Balsaman,

Yeah, I remember talking with you about this. I don't need a formula one CNC router I don't want it to take forever either. I have nothing to compare 30" per minute to as I haven't seen a machine operate at that speed. I'm going to assume it isn't terribly slow ??? I'm picturing a yardstick and having the router travel along it for a minute and reaching the 30" mark. I get the idea of course... just harder to apply it to cutting out patterns

I would definately be happy with a slower low cost machine for my first that will mill aluminum, MDF, and solid woods. I can deal with a slower machine as long as it isn't too painful. I was looking at the "3-fer" deal on the 116oz motors. I suppose I could always swap the motors out later if I want more speed. I just need to figure out which controller would support which larger steppers.

I'm not sure how much my gantry will weigh yet but it won't be terribly light as I'm building this sucker as sturdy as possible.

Thanks for the info.

Any more advice?

[Mr.Chips]

Here is a CNC running, this will give you an idea of the speed.
http://www.cnczone.com/showthread.php?threadid=2130

He said that it was 7 - 10 inches per min.

Hager

[balsaman]

Its a balance between torque, stepper drive, and mechanical. Once you build, swapping to 200 oz motors won't make it faster.

10-30" per minute is "normal" for a first machine. I was very excited to go 30".

Eric

[Mr.Chips]

Balsaman
Is the move speed (IPM) software controllable?

Or is it a fixed rate depending on your steppers, gearing, and lead screw?

I can see where you would want one speed for cutting balsa wood and o slower speed for oak.

Thanks,
Hager

[Darren_T]

Thanks for the info guys. If that video was 7-10 ipm then I'd be perfectly happy with anything at or above that.

I'd love to go 30 of course If I can approach that with the 116oz stepper motors then that would be perfect.

Balsaman, you suggested I go with 1/2 - 10 acme thread a while back when I was beginning my research. Would you recommend I stick with that? I suppose you would

I'm getting close to starting the build. I have a few other projects to get out of the way before I begin so I was going to order the acme thread rod and try to get my rails worked out so I'll have all that ready.

Darren

[umrk]

IPM is controllable via software and Gcode.

The max speed is limited by your steppers, gearing, and lead screw, also by your drive voltage (10 to 25X the motors rated V is preferred, as is PWM drives).

You can run good steppers at 125% of their rated current for more power. For example I've read reports of people getting ~250oz/in out of 150oz/in NEMA 34 frame steppers. 100oz/in steppers should be able to get close to 150oz/in if driven hard. But only if the oz/in rating is a true one, meaning taken at the steppers rated voltage and current. BUT many modern steppers are rated at a higher V with PWM drive, Vexta likes to rate theirs at 24V giving better numbers.

Steppers with lower inductance (lower V and higher A ratings) are capible of higher speeds.

To test your steppers oz/in, make an arm with 2 holes drilled on 1" centers. One hole to grip the motor shaft (with a slot and clamp bolt) the other to hang a bent wire or rod fixed to a coffee can. With the motor mounted, shaft hanging off your bench, arm sideways, apply the rated V and start adding weight until you make it skip steps. Reset the arm sideways, remove a bit of weight, retest until it holds, then weigh the can/rod/weights. 127oz weight = 127 oz/in torque. This can also be done with the motor driven with your drive (stopped, or rotating) to see where it maxes out. It will sounds like it strips when it misses steps, this will not hurt the stepper motor, its just jumping magnetic poles. The few I've tested this way actually held more than their Mfg rating.

Hope This Helps.

mike

[umrk]

116oz/in? PacSci motors?
I think they are rated at a high drive voltage already. At least the ones I have are the size of 40-50oz NEMA 23 true rated motors (5v, 1a typical), and the Superior Electric 50oz/in that I have tested closer to 60oz/in when tested as above, at rated V and A.

mike

[balsaman]

PS drive voltage has little to do with holding torque.