DRIVING MY PLASMA

[macca_loto]

Im in the process of planning my own CNC plasma table. I have most of the mechanicals sorted out but no idea on what size stepping motors i will require or or the power supply i will need for them.

[Robin Hewitt]

The usual idea is you panic about the gantry bending, make it enormously heavy in steel, then fit the biggest stepper motors you can buy to compensate for the weight, then make the torch lift assembly overly massive because anything else would look silly.

This seemed a bit odd to me so I took a 3 meter length of relatively light 4" box section aluminium, supported it at either end and used a DTI to see how far it bent down when I put a 10kg bag of lead shot in the middle.

10kg bent it 0.5mm so lightweight gantries are possible. The bending force is worst when you accelerate the gantry with the torch in the middle, fortunately the sums are easy if you do my little bending test.

Keeping the gantry weight down has the enormous advantage that you can use a smaller size NEMA23 steppers and you can get MUCH faster rapid transits with no loss of accuracy if you pick the right one. Moving the torch head across requires a lot less power than moving the gantry.

[macca_loto]

Yes the gantry will be as light as possible i plan on making it all out of aluminium. My biggest head ache now is working out what size stepping motors i need and then the power supply etc, electronics are not really my thing!

[Robin Hewitt]

Motors are easy to calculate if you keep the rolling resistance low, ball bearings throughout. Then it's all about accelerations, a tiny motor can move a huge gantry but the acceleration will be horribly low.

You need to decide 2 things. The total weight of the gantry and how far it will move when the motor turns one revolution.

Stepper motors force you to compromise step resolution against top speed. Your motors will comfortably give you 400 half steps per revolution, but the power falls away as the speed increases. Most NEMA 23's are good for 5 rps before the power drops away dramatically, some can do 10 rps.

Assuming a zero rolling resistance we can now calculate the acceleration for a given motor. First get the motor torque at your chosen top speed then convert that to foot pounds. Here's a handy conversion site... Online Conversion - Convert just about anything to anything else

Here's my formula

a = t*200 / d*m

a = gantry acceleration in ft/s/s
t = motor torque in foot pounds
d = distance moved in feet per motor revolution
m = weight of the gantry in pounds

Incidentally, 200 = 2*pi*G approx

Let me do one.

Motor SY57STH51-3008B is rated 50Ncm at 10 rps (Half step 30V 3A)
Convert that to foot pounds, t = 0.4 ft.lbf
Guess 1" per motor rev, d = 0.083 feet
Gantry weight 40 pounds

a = 0.4*200 / 0.083*40 = 24 ft/s/s

I usually assume 50% losses so I'd say this combination is probably good for 12 ft/s/s.

You actually need about 3-4 ft/s/s for plasma so this motor is comfortably over spec for a 40lb gantry at 1" per rev. Assuming the motor can actually deliver what it shows on it's torque/speed graph and assuming you don't add any massive flywheels, keep that cross axle light.

[macca_loto]

thanks for the advice! Its a start but i really have alot of learning to do, i think the gantry will be more like 70 pounds!

[Robin Hewitt]

thanks for the advice! Its a start but i really have alot of learning to do, i think the gantry will be more like 70 pounds!

Perhaps this might help you get your head around it...

If it weighs 70 lbs then a 70 lb push will accelerate it at 1G (32 ft/s/s)

That is what would happen if you dropped it

Plasma only requires 0.1G so you need a push of at least 7 lbs