[royzturbo]

Hello, I'm working on building my first 4 axis C&C Mill. Currently I have acquired two 22 inch(travel) kuroda precision ground ballscrews that have 10 mm pitch. And one 8 inch long kuroda z axis ball screw with the 5 mm pitch. *All are 15mm thick. *I will be milling aluminum and plasma cutting as well. I'm using THK linear slides and hoping to acquire some 8020 extrusions for my chassis. *

I've decided to use stepper motors due to the ease of implementation and cost effectiveness. After doing some research on the cheaper motors found available on eBay by various companies like Wantai motor and Long's motor. I noticed that the inductance is a little bit higher than the American equivalents produced by companies like Keling. *I was considering some of their 1600 ounce inch nema 34 motors they had for sale but noticed that the inductance was 22 mH. *Do you think a motor that size is bound to be slow? The company's representative said that advancements in their new digital drivers provided for faster speeds. *

I called Keling and spoke to one of their representatives. They said that for my application even a 570 ounce inch motor would be too big. They recommended something more along the lines of 300 ounce inches. They also stated that their new digital drivers could achieve over 180 ipm. *but if I wanted to achieve higher speeds I needed bigger motors. *

I guess my questions are these. *Are bigger motors still faster if the inductance is higher like the 22mh 1600 oz in. Chinese motors. Or are smaller American made motors with lower inductance better for my application and also capable of providing higher speeds due to lower inductance? *Would I be better off speed wise buying motors and using a gecko 540 driver instead?

The Chinese motors are over three times the torque at the same price. *I don't know if there is some hidden cons to their products. *Thanks for listening

[royzturbo]

I have no idea where those asterisks came from sorry.

[doorknob]

Just a few comments...

High inductance motors will want high voltages to achieve their speed potential.

The G540 can be damaged by voltages over 50 volts. As noted in http://www.geckodrive.com/images/fck...4%20MANUAL.pdf the recommended voltage for a motor with 2 mH winding inductance is about 45 volts. So, using a G540 with a motor that has much more than 2 mH winding inductance will mean that you will not be able to use a high-enough voltage to obtain the motor's ultimate potential speed.

BTW, while I am not certain, my impression is that the Keling motors are manufactured in China rather than in America. (Their regulated power supplies also appear to be of Chinese origin. I don't know the origin of their drives.) Keling often recommends motors in the range of 380 oz. in. because their low winding inductance is a good match for the maximum voltage rating of the G540. Usually those motors are paired with a 48 volt power supply.

I do not have direct experience working with a machine of the size that you are building, so I am unable to give advice on what type of motors you should be using.

[ger21]

Are bigger motors still faster if the inductance is higher

Bigger motors are almost always slower than smaller ones. Especially when you're talking 1600 oz motors, which are way overkill.
If you look at all the machines built on this site using steppers, my guess is that 98% use 600 oz-in motors or smaller.

Those ballscrews need about 2oz of torque to provide 1 lb of force, so even a 300 oz motor will give you 150 lbs of holding force.

What you need to do is determine how fast you want to go, and how much torque you need to go that fast, then choose a motor.

And as was mentioned, Keling steppers are Chinese.

[royzturbo]

Thank you both for your informative responses.

@Doorknob:
Using the formula in the g540 manual to determine the proper voltage to be supplied to the stepper motor (32 * sqrt(Inductance in mh)), It seems the g540 is suitable for smaller motors only (with approx. 2mH or less). Thanks for bringing this to my attention since the voltage limit on the G540 would have rendered it the wrong controller for bigger 1600 oz in. motors with 22mH inductance. Now my problem is still this, if 1600 oz in. is overkill, what size should I go for to get the maximum performance without missing steps??

@gerry
The ball screw manufacturer states that the ball screws I have are above 90 percent efficient. I used an online linear force calculator (here: Linear Force Calculator) with the following settings (10mm pitch@ 1 start = lead .41", motor oz in 600, efficiency .9) it says my linear force is 517 linear pounds. That sounds like one big sumo wrestler standing on my router axis :wee: Just curious where did you find the 2oz in to 1lb ratio? Is that a more accurate real world formula taking gantry weight and other variables into effect? I am judging speed based on youtube videos at this point, I don't have a huge surface area to cover with only 22" x 22" x y travel and 2.4 rotations per inch but I still want it to be fast since my spindle can do high speed. Maybe 200 ipm or higher? What type of motors should I get? I've been stuck at a stalemate for weeks pondering this but the more I research the better I feel. P.S. your videos on your webpage are very informative and I greatly appreciate the time you put into it. Thank you.

One other thing, I just bought a lot of multiple sized cnc gears and timing belts on ebay since they were sorta cheap, just in case I find a killer deal on some servo's.

It would be nice to get a motor solution for less than $1000. That can do closed loop and fly at the same time.

Any suggestions on motor kits?

[ger21]

Just curious where did you find the 2oz in to 1lb ratio?

I went to Nook's website and found a similar sized ballscrew, and they give you the torque required to lift 1 lb. I think the screw I looked at was a 5/8 with a .5 lead. I'm a bit surprised that the calculator you used shows so much more force.

Now my problem is still this, if 1600 oz in. is overkill, what size should I go for to get the maximum performance without missing steps??

You have to define what you consider maximum performance to be.

You'll only miss steps if you try to go faster than the motors are capable of.

My guess is that a G540 with 380 oz steppers will probably get you around 400ipm?

If you want closed loop, then you need to go with servos.