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View Full Version : What size motors? I'm confused again :)



Darren_T
11-17-2003, 12:18 PM
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
11-17-2003, 12:25 PM
What size machine? I think 100oz are OK, you can gear it also to get more torque.

Darren_T
11-17-2003, 12:48 PM
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
11-17-2003, 05:40 PM
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
11-17-2003, 05:46 PM
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 ??? :confused: 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
11-18-2003, 02:32 AM
Here is a CNC running, this will give you an idea of the speed.
http://www.cnczone.com/showthread.php?s=&threadid=2130

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

Hager

balsaman
11-18-2003, 08:39 AM
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
11-18-2003, 09:29 AM
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
11-18-2003, 11:09 AM
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
11-18-2003, 11:13 AM
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
11-18-2003, 11:18 AM
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
11-18-2003, 11:24 AM
PS drive voltage has little to do with holding torque.

Darren_T
11-18-2003, 11:36 AM
Ok, so here is what I'm considering right now... I won't be buying the driver board or motors until I have the machine built but I like to think ahead a bit.

I'm looking at the XS-3525/8S-3 from Xylotex and their package deal on 3 - 116oz Stepper Motors for $60.00. I'm also looking at the hobby CNC 125oz kit which looks like a great package.


I haven't looked around much at stepper motors and prices because I really don't know much about them and what is good or bad. If anyone has suggestions for other steppers to check out that would be great. I'm not opposed to paying more for quality. I'm trying to stay relatively low cost but recognize quality isn't cheap either.



Thanks.

umrk
11-18-2003, 10:22 PM
Balsaman, yes, higher drive V through a PWM or Resistor drive isn't going to increase the Hold Torque, unless you up the amps.

Let me clarify, I ran out of time this AM. We dont want the steppers to remain stopped, we want them to move something at a good usable rate of speed. Some drives actually drop the A a few seconds after the motor stops because its easier to Hold than to move and keep good torque. Upping the drive voltage through a resistor helps, with PWM it helps more to be able to move faster and/or keep more torque at speed. Hold Torque is more of a reference point to help determine a steppers Run Torque. If you up the hold torque by 50% by increasing A, run torque will follow suit.


I'm skeptical about the HobbyCNC NEMA 23 Single Stack motors shown being anywhere near 125oz/in unless their being driven well above the 7V 1A rating. A typical Vexta of that size is closer to 83oz/in driven with 24V. Put 7V 1A on it and I bet its closer to 60oz/in. Also 7V is going to limit your top speed with a drive rated at 35V max, like the ones you mentioned. I'd recommend finding some 3V or less motors, or as low of a V as vou can find and still stay at or under the drives max A.

The PacSci motors that you mentioned from Xylotex use stronger magnets to get a higher rating. Rule of thumb is, if the motor is NEMA23 size or larger, square or hex instead of round, they use the stronger magnets. IIRC they usually are rated for a higher top speed. Also being 8 wire you have more options on how to hook them for unipolar or bipolar drives, series or parallel coils. Also the PacSci's windings are probably rated at 3V or less since they have a max input of 65V (based on 25X V with PWM drive).

I'd also recommend going with a PWM drive instead of a Resistor limited drive, for better top speed. IIRC the Xylotex drive is PWM and rated at 2.5A.

One thing to consider about top speed, your going to have to Jog your router (or whatever) into place. On a 36" axis with a 30IPM top speed its going to take over a minute to go the full length of the bed, before you even start cutting on the opposite end.

Check out Jones on Stepper Motors (http://www.cs.uiowa.edu/~jones/step/) for more info on steppers, drives...


If your not in a hurry, read, read, read...! Knowledge is Priceless.

I've been gathering parts for a CNC Router for a few years, reading all that I can find on them... Bought and sold/upgraded my parts a few times. This winter I'll put it all togeather and start learning the software, G Codes...
My goal is to have a first machine that will be fairly fast and upgradable, no need for a second machine to replace it. 7A Gecko microstep drives, 40"x20" cutting area, able to hold a 4' wide piece of wood one way, 24" the other, slid through it And big enough to cut out parts for my later machines with. CNC PC Board router, CNC Vinyl Cutter... all smaller. Yeah, thats kinda of BassAckwards to what most people do. But the cost has been spread out, and of course theres the bargains I've found (see below) by waiting longer. But thats me, unusual at best. ;)


Ebay is a good source to find stepper motors. I won 4 of the motors like 116oz/in that Xylotex sells (Pacific Scientific) for abt $40 over a year ago. I also won a 250-300oz/in (double stack, 2.7V, 4.5A) NEMA 34 frame motor in excellent shape for under $15 almost 2 years ago. You just have to keep checking and looking for bargains.

Also check out local scrap dealers, theres a goldmine of usable parts at some places and usually they have no idea what linear slides and bearings, steppers, 8020 type aluminum structural tubing... is really worth. I have several feet of 8020 and hardware for it that I got for $.50 a lb scrap price. A couple of nice 1" rod linear shaft assemblies (2 shafts, carrage with bearings and end clamps) with 30" movement (Y axis after I cut it down), that I got for $20 ea. A small 12mm shaft assembly like the above one with 4" fo travel (Z Axis)... Ect.


Any of the parts you mentioned would make a good first machine if thats what your after. For most people thats the way to go, get the first one built and use it, learn from it, building a bigger, faster... machine later on, possibly going to servos on the second machine.

Sorry that I don't have more good links to post. My old computer crashed and took them with it. But Jones is a very informative read.

mike
Sorry for the length.

ViperTX
08-31-2004, 03:07 PM
Well it isn't only how fast you want it to traverse across a path....but also how deep a path in what type of material with what type of cutter. Think in terms of size of cutter, type of material and you can determine the optimum material removal speed...I would dig up a machinist handbook and work from there or plan on experimenting a bunch....or just pick up some 300 or 600 oz-in motors and then play with the gears to get the correct torque.

arvidb
08-31-2004, 04:26 PM
*snip* 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. *snip*

Just to clarify, it is oz*in, not oz/in. So if you drill the holes on 2" centers and can lift 127 oz, you have 127*2 = 254 oz*in of torque (not 127/2).

Arvid