Need Help!- Stepper motor RPM - too low?

[Aryantes]

I am running a chain drive, with no gearing using a 9 tooth .25" pitch sprocket.

This gives me 2.25" per revolution.

With a stepper motor going at 100 RPM, That is 225 IPM.

Chances are, I'll always be cutting around this speed or lower.

If I am cutting aluminum, I will probably be cutting at 10-30 ipm.

If I'm cutting at 20 ipm, that's going to be running the stepper motor at only 11 RPM or so.

11 RPM is 37 steps per second.

Is this too slow? I'm also running 1/16th microstepping using a HobbyCNC Pro board with 305oz unipolar motors if that makes any difference.

I'm wondering if I should gear down the system, 2:1? 3:1? I suppose 1500 IPM rapids are not necessary to keep

Do stepper motors like to be run at a certain speed? such as 200 rpm, 300 rpm, etc? I understand torque curves and not being favorable at high RPMs, but don't see any pages about running them too slow.

Also, about acceleration. I've tried a bunch of different settings in the motor tuning page and found that even one as high as 400 doesn't stall my motors or anything. (My table needs to be bolted to the floor for this to work though haha)

Whats the basis around a proper acceleration rate?

[johnmac]

Running steppers slow will not harm them. I probably wouldn't depend on microstepping to get the needed resolution, meaning I would gear down 2:1 or 3:1, but no more than 3:1. The gearing will also help with braking when the machine needs to slow down.

John

[Pandinus]

What kind of machine... mill,router,plasma,water-jet,laser??

Think about resolution vs speed... right now you have about 0.01125 inches per step, don't expect your micro-stepping to be linear between full steps... half step location might be close. Also, a low gearing plus high micro-steppingvalue means a really fast pulse rate for the controller, but at 20 IPM you should be OK with that.

For milling aluminum I'd think you want to be closer to 0.25"-0.50" per revolution but I'm not really sure on that figure.

[Aryantes]

This is a CNC with Hitachi router. Built using 80/20 and aluminum plate. I'll find time to take pictures later.

fast pulse rate for controller isn't a problem I don't think. the driver test said system excellent at 100k in mach3 ( not sure if this is significant enough? )

I'm starting to learn that I'm not too concerned with theoretical accuracy of my machine.

There's far too many factors that came into building it that are going to be much worse than having 0.001", for instance, making sure everything is level and square.

I'm not sure how everybody else is measuring their machine, but I don't have a square or level that will tell me if it is within 0.001" or even 0.01".

80/20 extrusion has a straight/flat tolerance of like 0.010" over a few feet. ( something like that )

Their cut tolerances are also only within 0.010".

I just want to make sure that I'm not doing something horribly wrong by having everything geared at 2.25" per revolution.

A theoretical accuracy of 0.011" would be ok with me but I'm almost certain my machine, which will appear square to my machinist squares and carpenter levels, will be more than 0.01" out of square over the 4 foot x axis.

If someone has tips on squaring/leveling down to the 0.001" level I'm all ears hehe.


johnmac, so if I don't gear it down, I run the possibility of the machine braking improperly? As in, doesn't stop when it's supposed to and goes too far? I imagine acceleration value has a lot to do here.

What typical acceleration values are you guys using?

This is the kind of issue that I want to address, as that sounds like it could a problem in the 1/20" inch magnitude instead of the 1/100" inch magnitude.

[johnmac]

After a move, the steppers need to act as an electric brake. Having the drive geared down some will act the same as when you are slowing down in a car with a manual transmission and shift into a lower gear.

In my searching and learning I have found that about 2 turns of the motor to 1 inch of travel seems to be a good target to shoot for. For your setup, gearing down more than 3:1 might get complicated, though.

John

[RomanLini]

Originally Posted by Aryantes ... [B]I just want to make sure that I'm not doing something horribly wrong by having everything geared at 2.25" per revolution. ...

I think that would be a big mistake.

A medium speed, good accuracy machine is probably around 1/4" per stepper motor revolution, for a larger faster machine (sacrificing accuracy a little) you might want to go 1/2" to 3/4" per motor revolution.

The big problem at 2.25" per rev is not just accuracy it's torque, if anything is a tiny bit wrong or starts to bind etc it will crash or lose steps. And since you have said you don't have a lot of alignment equipment then the last thing you need is a fussy machine that keeps binding and losing steps.

(edit)Sorry Johnmac - posted the same time...

[Aryantes]

I'm not quite sure I fully understand.

If I gear my system to move less per revolution, then the stepper motor has to spin faster, and therefore less torque, right?

Or does it simply depend on how much force it takes to move gantry?

or, what acceleration/velocity I am attempting to use?

[Pandinus]

I'm not sure about your drivers/motors... but my setup Gecko G540/40V power supply Rack and Pinion geared to about 1 inch per revolution was pushing upwards of 1800 IPM so 1800 RPM on the motor before I reached a stall condition.

So I guess it depends on a few things for the usable torque and speed.

[lunchtrayrider]

I think at the step rate that your talking about your going to get bit chatter and wear issues. at the 20ipm you gave as an example your talking about 37 steps per second. on the hitachi router (i have one too) the lowest speed is 8000 rpm. so figure that with a 4 flute cutter your getting 32000 cuts per minute. your only stepping 2220 times per minute. so maybe a two flute cutter would be better at 16000 cuts per minute. however, your looking at potentially slamming a cutting edge into the material and biting off a minimum of .01125" (which is probably a lot for aluminum) and then having 7 cutting edges do nothing while it waits to step again and then slamming it into the material again. So i would assume you'd have to microstep at least 1/8 step and probably all them way down to 1/16 step to get good cuts.

[Aryantes]

lunchtrayrider, that makes a lot of sense.

I will be running 1/16th microstepping and I guess I'll see how it goes as far as cut quality.

I'll plan on gearing down closer to 1" or 0.75" per revolution in the near future. This would give me roughly 400 rpm around the 200 ipm cutting speed, which is decent I think as far as torque curves go.

My math in original post was also off. 20 IPM would have been 9 RPM and not 11 haha. This makes the issue worse.

Thinking more about the speed of the hitachi as you said, if I'm using a 2 flute cutter at 8000 rpm, 16000 cuts per min, I'd want the steps per min to be equal or higher eh?

I have not researched anything like this, but I guess anything over 80 RPM is healthy cutting in general. (80 RPM being = 16000 steps per min at full stepping ) Just so that the bit is always cutting and you can have a consistent chip load per tooth.

Micro-stepping would just be a healthy bonus.

This could be the answer to my original question, thanks. I'll look into it.

[lunchtrayrider]

have you looked at onsrud's chipload rec's? i use them as baseline.

http://www.onsrud.com/xdoc/FeedSpeeds

1/4" cutter rec's are around .002-.004 for aluminum. so 8000 rpm 2 flute, 32-64 ipm. that doesn't sound too bad. 2.25" per rev, 1/16 micro being 200*16 steps per rev equals 3200 steps per 2.25" or .0007" per step. this seems like it would work alright. even though microstepping isn't perfect placement it would maintain movement while cutting.

for the record i'm using cncrouterparts rack and pinion so i've got around 1900 steps for inch with 1/10 micro (g540). it works really well for me. around 1 rev for inch. I do all wood and plastic, have not done aluminum.

[Aryantes]

I have looked at them before.

I was curious though about cutting depths since those tables use 1x diameter for cutting depth usually. However, I'll probably be using like, 1/5x diameter.

I figured it would be a good idea to just cut light and not worry about speed. Probably 0.03" per pass.

It would probably be in my best interest to get some very good aluminum bits to minimize chatter.

1" per revolution looks like a good spot to be. I'll probably invest in CNC router parts R&P in the future. Their stuff is pretty awesome.