What size stepper motor do I-need.

[massimomb]

Thanks a lot for the excelent advice,

I'm just concerned about the microstep selection, actually I've found the right balance in my cnc, I run 3 2M2280N coupled with 3 12NM nema 34 motors, I could run my cnc as fast as 10m/min in rapid and 2,5m/min while working (mach3, parallel port), but in a work of 4 hours it loosed the step and I had to trash the work out, so i reduced the microstep resolution to half step and it seem to work right, but the motors do a hell of a noise.

I've read different opinion on the web about microstep resolution, someone says that the more you set higher this value the lesser will be the torque, others says that in the modern drivers, microstep res. almost do not afflict the torque,

Who do I have to listen to?


Can you explain how this value can be taken in account?


Thanks a lot if you can reply.


M.


P.S. I'm wondering to move to servo with brushless motors,maybe this can be tha solution?

[Robin Hewitt]

How long is the cable on the parallel port.

A TTL signal might get you a reliable 10 feet without handshaking at that speed, if you were lucky

[magno_grail]

I have a couple of questions about your motor calculation spread sheet. The motor inertia is input in gm-cm^2 (B32). The rotor inertia given in I27 must be Kg-m^2 since the total inertia is the sum of the screw inertia, load inertia and motor inertia, the others are in Kg-m^2. I27 = G35 which is a lookup to convert units of the motor inertia.
The problem is 1000 gm = 1 Kg and 100 cm = 1 m so gm-cm^2 should be divided by 1000*100^2 = 1*E7 not 1*E8 as in G35. The motor inertia in the sum is 10 times too small.
I cannot find the time used for the acceleration torque (I34). The equation is G32*I28. G32 is labeled as V but it is really the number of pulses/second of the stepper based upon a 200 pulse/revolution motor and the screw speed (which is the Max. linear speed divided by the screw pitch). From looking at the equations, the running torque is small compared to the acceleration torque unless there is a lot of friction in the system. I have not found any references as to what range of acceleration is needed for a CNC. Obviously, when a stepper system is tuned it will limit the acceleration so positioning steps are not dropped due to the loads. Too small motor torque will result in a very slow CNC due to acceleration limitations. At some point a larger motor torque results in small performance gains for the cost increases. Are there any guidelines for this? Thanks.

[masinecc]

How can i calculate what motor size i need for my gantry if i use rack and gear on both sides. This example is for ballscrews. Looking forward for your answer.

[Boyan Silyavski]

How can i calculate what motor size i need for my gantry if i use rack and gear on both sides. This example is for ballscrews. Looking forward for your answer.

Backwards. Choose the RP size, gearing so that you have as a result normal rapid speeds with acceptable resolution. Then figure how many RPM you need in reality so you have mostly that in mind. Then you go back to the gearing ratio and gantry weight and from there you decide Nema 23 or 34.

Obviously as the RPM is the important part here cause most possible it will be well geared, then you finish with low impedance motors driven by high voltage.

And knowing that nema 23 motor driven at 70VDC moves a gantry max 50-60kg with gear 1:1 without problem at 10m/min via ballscrew, you can figure the motor you need for your case.

[Robin Hewitt]

Nobody works it out, they just assume that the biggest motor they can find has to be the best option. Then they buy a set of 3 because that saves even more thinking and they end up with a Z axis that could raise the Titanic and is so heavy the gantry has to be made from RSJ

[Boyan Silyavski]

Nobody works it out, they just assume that the biggest motor they can find has to be the best option. Then they buy a set of 3 because that saves even more thinking and they end up with a Z axis that could raise the Titanic and is so heavy the gantry has to be made from RSJ