Need Help!- Another motor question(s)

[redccm]

Hi folks,

I've decided to build myself a homebrew CNC machine which I'll mainly be using for wood, acetal and aluminium. It's only going to be a small one, about 300mm x 300mm x 150mm (or 1' x 1' x 1/2'), made out of aluminium and using stepper motors direct driving 3/8" 20TPI threads with lead nuts. I reckon the max weight (including trays) will be around 40lb's and as this is purely for hobby use I'm not overlly bothered about how fast it does the job...would 10 IPM be unreasonable?

I've managed to get my head around a lot of the concepts and terminology but I've come unstuck with the stepper motors. I've spent the past few days trawling the forums and I can't seem to work out whether the stepper motors will work or not. It seems I can't fill in all the details of the formulas and logic I find on the forums with the details supplied about the stepper motors.

The motors I'm looking at are NEMA23 motors (http://en.nanotec.com/downloads/pdf/...out1%20(1).pdf) run through a Gecko G540. I'm not sure what power supply I would need or whether these motors would do the job as (like I mentioned above) the stats don't seem to marry up to what the formula's need. Can anyone help and advice please?

Cheers,

[ger21]

would 10 IPM be unreasonable?

Yes. even on a small machine like that, you'd want the ability to cut at 100ipm at least.

The motors should be fine, though. The inductance is a little high, but they'll work OK. Just wire them bipolar parallel.

[hemsworthlad]

Hi,

First 20 tpi will be very very slow, i'm metric so it will be 1.27mm per rev and even the best steppers struggle after 1100/1200rpm with very little usable torque left.
If you get 1000rpm that is usable your fastest rapid speed will be 1200mm/min. Obviously 10tpi will double this, unless your 20tpi is multi start then I would even look below 10.

Regards the steppers, yes they will work ok for a small machine like this and fit within the G540's 3.5a limit. They will work best if wired Bi-polar parallel.

To work out the PSU Voltage requirements for best performance there are a couple of ways, first the rule of thumb way which is approx 20x the motors rated voltage upto a max 25x.
Then theirs the more accurate calculated way which is 32x Sq/root of the motors Inductance rating. . . . So with these motors wired Bi-po Parallel the voltage for best performnce would be sq/rt 4.6=2.14 x 32=68V

To work out the amp's you just take 68% of the total of all the motors combined amp's.
If mixed size motors you calculate the voltage from the smallest motor used.

Obviously with the G540 your restricted to 50V so you will need a 48V supply with enough amps to cover, 48V @ 8A will work fine for these motors.
All the lower voltage means is your motors will not be working at there ultimate speed, but they will still work fine.

Something to consider thou.? Because your using the G540 you have 4 axis drives each capable of driving 3.5A motors, if you plan on upgrading your motors at a later date then I would buy or build a supply that can handle the full capabilty of the G540. So I would size the PSU for 4 x 3.5A/68%=9.5A this will mean slightly more cost initally but give far more upgrade potential and save you money further down the line.

Another thing to consider is building your own PSU, it's very easy to do and the motors/drives work better with an unregulated PSU than they do linear supplies. If you want to know how to do it and size the components then PM me, I can also point you where to buy the parts in UK.

Hope this helps.

[redccm]

That's great thanks for the help

[johnmac]

My first router table was 10"x10", used 1/4"x20 threaded rod and a stepper driver board from Stepper World. That combination resulted in a machine that would only run at about 12 IPM. While the Stepper World driver is slow, the biggest mistake was using the threaded rod. Even on a small table, consider using Acme 1/2"x10 2 start screws.

John