Stepper Motor Sizing???

[w102acd]

All,

I am building a homemade cnc router. The structure is MDF and the ways are all linear bearings (thk and iko). I am using 1/2"-10 1 start acme thread with a delrin nut for the drives. The machining area is 30" x 20" x 10".

I want to use the hobbycnc pro kit, but need some opinions on the size steppers to use. I would like to get 100 ipm out of the router which is 1000 rpms out of the motors. I was leaning toward the most powerful motor (305 oz-in) that they offer. However, last night I hooked up some small vexta (54 oz-in) using a simple ULN2003 circuit and the drives worked well. I would like as much power as possible without sacrificing speed. I have the torque vs speed curve for these motors and the curve tells me I will get ~80% of stall torque at 1000 rpms.

My question is: Can I easily get 1000 rpms out of the 305 oz-in motors using the recommended power supply??? I have found other posts around, but all use qualitative terms like "fast" and "really fast". I would like some quantification of these terms.

Thanks in advance,
w102acd

[BobF]

Call Dave and ask him.
I have the 305s on my machine. I use 1/2-8 2 start screws with effective pitch of 4tpi. With these screws I can cut at 100ipm and get 200ipm rapids. This is 800 rpm at rapid speed. I have experimented with rapids a little higher, but I don't think I would cut at that rpm.
To answer your question, yes you can get 1000 rpm with the recommended supply. I have no idea how much torque is still available. It is enough to push my gantry, but I don't cut at that speed. I don't have a torque curve for this motor so I don't know how much fall off there is.
With most steppers the larger the motor, the slower it wants to run. High torque motors usually loose torque sooner up the speed curve than smaller ones.
Sounds like you could use the motor you tested depending on what you cut and wieght you are pushing.

[w102acd]

Thanks for the reply BobF!!!

I am leaning toward the larger motors like you have. I really like the set-up and kits that hobbycnc has so I was planning on just buying a kit (board and steppers together). I still have to get my y-axis lead screw installed and can check this out with another motor I have laying around. The z-axis is a THK KR33 so that is already done, just need to install the motor.

I think 100 oz-in (33% of stall torque) at 1000 rpms would be nice. All of this may be a mute point if my leadscrews start whipping. I guess this is just a hobby thing so if a part takes 3 hours instead of 1 hour, it is really not that big of a deal. I figured I can always run the motors slower, but there is no easy fix to get 300 oz-in out of a 200 oz-in motor.

Thanks again!!! Sure do love the zone!!!

[BobF]

w102acd,
When you say you have the torque vs speed curve for these motors, which motors do you refer to?
If you have the curve for the 305s I would like a copy as I don't have it.

Thanks

[w102acd]

BobF,

I do have the torque vs. speed curve for the hobbycnc 305 oz-in motor in bipolar operation. I would prefer to make sure Dave is alright with me posting it first. I emailed Dave and got it within an hour or so.

I figured the bipolar curve will look very similar to the unipolar curve. I suppose if you take the curve and multiply it by the ratio of unipolar to bipolar stall torque it would be pretty close. Anyone have any experience or comments about this as I am shooting from the hip???

The hobbycnc pro board only has one limit switch terminal. I assume this is wired to all 6 limit switches (3-axis machine)??? This essentially stops the machine correct???

Has anyone ever used a relay to control the spindle with a hobbycnc board??? At first glance it should not be that difficult as long as one digital output is left unused.

Thanks,
w102acd

[BobF]

If you use Mach you can use any input for the limits, since you define them in Mach. Probrably true of other control programs too, But I don't know any others.
I wire all my limits together in series. This way it only requires one input, and any limit trip will stop the machine. Also, if you ever happen to cut a limit wire or one breaks due to wear and tear the machine will shut down until you fix it. This prevents more serious mishaps from broken limit issues.
My spindle is just a router and I control it with the built in switch.

[w102acd]

BobF,

I like that method for the limit switch. I didn't realize Mach had limits on it also. I assume they are software limits. Meaning if the machine were to get lost (missteps, bad home, etc) the machine could still try to drive an axis too far and could potentially destroy the machine. The limit switches done the way you have them will save people like me from destroying their machine.

I have seen that one can limit the current allowed to go to the motors. Is this true for each axis??? I basically have a moving gantry style router (probably the typical type design). If all three motors are the same size, should one limit the current (and thus the torque) to the different axis. Obviously, the x-axis has to move the most weight and should thus have the most current. I suppose the z-axis could also be relatively high due to the wieght of the spindle. However, the y-axis should move with the least resistance so should it have less torque put to it in order to acheive the same cutting force as the other axes.

Thanks,
w102acd

[w102acd]

BobF,

Just got a reply from Dave. I forgot to email him this morning, but emailed him just before my last post and (just as you read all over the place) he replied within a few minutes.

He would prefer you to get the curve directly from the source. If you email him, he will probably get it to you faster than you can download it.

Thanks,
w102acd

[BobF]

Originally Posted by w102acd BobF, I like that method for the limit switch. I didn't realize Mach had limits on it also. I assume they are software limits. Meaning if the machine were to get lost (missteps, bad home, etc) the machine could still try to drive an axis too far and could potentially destroy the machine. The limit switches done the way you have them will save people like me from destroying their machine. I have seen that one can limit the current allowed to go to the motors. Is this true for each axis??? I basically have a moving gantry style router (probably the typical type design). If all three motors are the same size, should one limit the current (and thus the torque) to the different axis. Obviously, the x-axis has to move the most weight and should thus have the most current. I suppose the z-axis could also be relatively high due to the wieght of the spindle. However, the y-axis should move with the least resistance so should it have less torque put to it in order to acheive the same cutting force as the other axes. Thanks, w102acd

Yes Mach3 has soft limits as well. They can be used, but are user set items, so you can screw them up just as easy as screwing up the program. Also, lost steps can make the software limits just as off as the rest of the machine. They are better than nothing, but switches are better.

I set my current the same on all the motors. You may need max current for acceleration or a heavy load. I don't think there is any benefit to limiting the current lower than the motor rating. The motors will only generate the torque required to move the load. Having more available will not change how the machine performs, but not having enough will.

Some of the drive systems have idle current reduction which will help keep the motors cool when they are not moving. Gecko has some great drives that do this and Mariss is updating some of the to be even better at this.
Read this
http://www.cnczone.com/forums/showth...t=38763&page=3
you might want to browse around the Gecko site, there is a wealth of information there there about steppers and drives.