[kolias]

I can’t find an easy to understand method to size stepper motors and below is what my interpretation is of the various pages a read.

My new hobby CNC router will be made out of 1/2" thick aluminum plate and it will use 3/4" SBR supported rails, SBR pillow blocks and 0.63” ballscrews from overseas. The approx. dimensions of the machine are Y=36”, X=24”, Z=10” and the estimated weights of each axis is Y=100 Lbs, X=35 Lbs, Z=20 Lbs (incl.spindle and motor). Assuming a 500 IPM and the heaviest weight to be 100 Lbs, I calculated the following:

(500*100) / 531 = 94 Watts
Estimated drive Amps = 3A, 40V, or 4A, 30V

Do the above sound ok?

[ger21]

The simple method, is to do what everyone else does.
Get a Gecko G540 package with 381oz motors from Keling. 3.5amps and 48V. Pretty close to what your math says.
With rack and pinion, these would give you over 1000ipm.

However, you need to make the right choice on your ballscrews. It's usually safe to assume that the maximum usable rpm for steppers is about 1000 rpm.
A ballscrew with 5mm lead turns 5.08 times per inch of travel. 1000rpm/5.08 = 197ipm
A ballscrew with 10mm lead will give you about 393ipm @ 1000 rpm.
It's possibly that you can get more than 1000rpm out of your steppers, but it's also possible that you could get less.
It really depends on how fast your acceleration is set to.
The faster you accelerate, the more torque that's needed, which means your max rpm will be lower, because torque decreases as rpm's increase.
Using a lower acceleration rate can result in higher top speeds, if the machine doesn't run out of room before it reaches the top speed.

Generally, you'll be better of sacrificing top speed for higher acceleration.

There are two other options for your ballscrew choice.
1) find some screws with 20mm pitch, which will perform closer to a rack and pinion machine.
2) Use a belt drive to turn your screws, which will let the screw turn faster than the motor. This way you can play with the gearing to find the best balance for your machine.

[kolias]

Thank you Gerry for the valuable insight

Checking on eBay I can only find 16mm, 20mm or 25mm diameter ball screws with either 5 or 10mm lead. I can’t find any screws with 20mm pitch. So my option 1 is out and option 2, well I would love to do a belt drive system but I never built one and this again it’s a territory which I have no experience and I hesitate to get involved with it.

I would also like to add that acceleration and top speeds are not my main concern. Perhaps these two parameters are more of a concern in commercial applications. If I’m right, I think I will be happy with 393 ipm from a ball screw with 10mm lead.

The G540 sounds a good idea with a good price range. Keling has two motors in that range; a 381oz-in with 4 wires 3.5A and a 425oz-in with 8 wires 2.8A both the same price. Somehow I remember that 8 wire motors gives you more flexibility and so I prefer the 425oz-in motor. What is your suggestion?

[ger21]

The 381 oz motors will outperform the 425's, due to their lower inductance.

[OCNC]

... Keling has two motors in that range; a 381oz-in with 4 wires 3.5A and a 425oz-in with 8 wires 2.8A both the same price. Somehow I remember that 8 wire motors gives you more flexibility and so I prefer the 425oz-in motor. What is your suggestion?

The 425oz-in motor has a little bit more than twice the inductance (6.8mH) of the 381oz-in motor (2.8mH) so you might get better overall performance with the 381oz-in motor, ie. better accelerations and speeds. To be honest though I don't know quantitatively how much one mH effects performance. I should add that the 400oz-in stepper that GeckoDrive sells is 2.6mH and is considered to be ideally suited to their drive products.

Chris

[OCNC]

Checking on eBay I can only find 16mm, 20mm or 25mm diameter ball screws with either 5 or 10mm lead. I can’t find any screws with 20mm pitch.

You might want to look at Roton's Torquespline screws for the high pitch. Also their HiLead screws.

Chris

[kolias]

Much appreciated Gerry

Thanks Chris for the info, although the 400oz-in GeckoDrive is a bit more expensive that is fine since it’s considered to be ideally suited for their drive products.

Regarding the Roton’s product sorry but I will have to pass. I have searched extensively recently for linear products and the best price including delivery is from overseas and gathering from this forum people have been satisfied. In addition, they provide you with a complete package (supported rails, pillow blocks, ball screws with the nut and support bearings at both ends, etc) which “hopefully” they should mach and you have nothing else to buy

[ger21]

Gecko's website looks like it may have been hacked, but if I recall correctly, the 400 motor they sell will not work with the G540, as the current rating is too high.

As for the 425oz motor, using Gecko's formula, it needs 83Volts to get maximum performance out of it vs 53 volts for the 381oz motor.
Since rpm is proportional to voltage, the 381oz should be roughly 30-35% faster.

[kolias]

Had the same problem with Gecko’s site but after a few tries I got in

So might as well go with the 381oz motor and save the extra $$

[electric2u]

have you seen CNC Stepper Motor Driver Systems & Hobby CNC Routers:: PROBOTIX™ I have it. don't know if it's good or bad but they have been very good to me.

kevin

[kolias]

Thanks Kevin, I know Probotix very well and have purchase my previous 2 systems from them - excellent service

[jzak3]

what type of speed will I get useing The probotix 260/0z 24v steppers with 3/8 10-2 acme rod?