[sdfeter]

Hi guys, I bought Chinese ball screws and HGR25 Hiwin linear rails to build a 4x8 plasma table, and now I need to choose the stepper motors. I have two ball screws 3220*3000mm for the Y axis, one 2020*1700mm for the X axis, and one 1610*400mm for the Z axis; I want to couple the motors directly to the ball screw shafts (direct drive) , and It would be nice to achieve speeds of 10,000mm/min (+/- 10%, it's ok); the gantry will weight 100 lb.
My concern is about the 2 stepper motors for the ball screws of the Y axis (32mm diameter, 20mm pitch, and 2700mm stroke). The more I read the more I get confused because there are lots of specifications and interchangeable terminology that I don't understand. For example if we talk about torque, we have: holding torque, driving torque, acceleration torque, load torque, required torque, RMS torque, dynamic torque, running torque, breakaway torque, etc. So for a plasma CNC machine I don't know what I should look for. About acceleration and inertia for those big ball screws, I am totally lost.

Please guys I need some suggestions and advice; If you guys had the ball screws and the Hiwin linear rails mentioned above, what motors and drives would you choose.

Do you guys think this kit would be ok?
Gecko G540 w Four NEMA 23 Steppers 620 oz in Motors | eBay
this one?
US Free SHIP 4Axis Dual Shaft Nema34 Stepper Motor 1600oz in DRIVERDM860A CNC | eBay
Thanks in advance.

[aarggh]

For a machine that size with the rapids you want, I would have thought servos would be the better solution?

Regarding steppers though, in practice torque is more or less inversely proportional to speed with most steppers. So finding the right balance to get a set torque at a set speed, and determining what PSU you need can be a balancing trick. Another aspect to consider is the inductance of the steppers, the PSU value will change dramatically depending on the inductance, and how close your PSU matches the steppers. The formula for that is 32 x the square root of the inductance in mH.

It's worth checking through the various docs on Gecko's site.

cheers, Ian

[victorofga]

for 20 mm pitch you need more than nema 23

the first set, sure don't works for you..

the second is ok, however I think a 1200 ounces would be sufficient.. but more torque never bad..

another thing is, if you use ballscrew for plasma cutting, you need to find a way to hide screws behind bellows, or set under table, sure the sparks can not fall onto screws..
same thing for rails...

the 20 mm screw can be fast.. I think you can reach 600 ipm with direct drivng and stepper motor..

also you need to build this machine, if routing comes on your mind, sure it can handle a spindle..

[awerby]

Yes, this sounds more like a router than a plasma cutter. You'll definitely need to shield those rails and screws; the stuff that comes out of a plasma cutter is amazingly gritty and sticks to everything. Building a water tank underneath the support rails will help some, but ball bearings of any sort are pretty delicate and it doesn't take much to ruin them.

Most plasma cutters, which aren't that particular about accuracy, use a simple V-rail system with rack and pinion drive. This is a lot more robust than ball-bearing rails and ballscrews. I'd say save that stuff for a router project and start over on the plasma cutter. I'd also say to go with servo motors on a machine this size, which are more likely to have the right amount of torque at the speeds you want to run it at. You will have to gear them down, though.

[sdfeter]

Some of you guys are suggesting servo motors, and I would like to know the minimum specs of the servos that I need. What about if I lower the speeds to 7,000 mm/min (+/-10%), I will still need servos? servos are expensive. I have to use the ball screws and the linear rails that I bought; I have no choice. Thanks guys for your replies.

[victorofga]

no

while need a certain speed

for this setup you don't..need servos...
stepper will provide enough speed..

as oi mentioned with steppers you can accelerate out to 600 ipm...

means 15meter per min


in deed look for torch height control..

[Mark_T]

500 rpm will do you 0.167m/s (10m/min) with 20mm pitch.

You can convert linear force to torque easily with a ballscrew since you can ignore friction (to a rough approximation), so a pitch of 0.02m means linear
force is (2 * pi / pitch) * torque (all in SI units), so for example 2Nm becomes 630N linear force (60kgf). The inertia will mainly be the linear inertia
of the carriage (if its 45kg), so the acceleration will be roughly force/mass (14m/s/s for these example figures, assuming (incorrectly) that the motor's
torque sustains it value of 2Nm as the speed increases)

You need to look at the graph of torque against speed for a particular supply voltage to see what torque you can expect at your design max speed.
(And treat with a pinch of salt - resonances and friction will work against you)

If you always work in SI units the sums are hard to get wrong, the only tricky bit is the 2 pi...