Probotix monster kit question

[timtje]

Hi guys,

i'm a cnc starter.

I've already bought a XYZ table (the parts) from IGUS.
My milling area is 1000x1000x300mm.

On probotix i noticed the monster kit including 400oz/inch steppers.
How do i know those steppers are strong enough?

I'm planning to mill plexiglass, MDF, plastics, light wood.

Is a 400oz/inch motor just a hobby stepper or is it already semi-professional?



Tim
Belgium

[mwood3]

Im no expert at matching steppers to the machine, but if im correct that kit is over kill for what you plan to cut... maybe some more experienced guys can step in...

[Jack000]

you need to match your steppers to your stepper drivers and power supply. Calculate the required voltage from the stepper's inductance (the formula's floating around somewhere), and match that with the max voltage of your drivers and power supply (with a safe margin!). You may find that based on your choice of any one of these components, the other parameters are already decided for you.

400oz/in is definitely enough, but they work best with a low-tpi leadscrew (multi-start 10tpi, or equivalently 2 tpi or so) for maximum speed. If you have a high tpi (turns per inch), like 10tpi, I think you're better off with a lower power motor, since these lose less torque as speed increases. Definitely do the calculations first and make sure your stepper/driver/power supply combo are compatible.

[jsheerin]

Those motors don't sound too big for your table size to me. Mine is slightly larger and that is similar to what I use. You can go with slightly smaller motors that have better high speed torque, but it depends on many factors. To calculate how big of a motor you need, you need to take into account the masses of the parts you're going to be moving, how fast you want to accelerate them, the specifics of your screws (or whatever you're using), and cutting forces. Then you can see how much torque you need from the motors and by looking at the motor's torque curve, you can see if it will provide it at the rpm you'll be running at.

[pminmo]

Typically the nema 23 400 oz in steppers are high inductance steppers that are problematic to get higher rpm's for rapids. Look at the Kelinginc KL23H276-30-8B with higher pitch screws. http://pminmo.com/mechancial-power
http://pminmo.com/which-stepper-motor

[jsheerin]

Hey Phil,
Nice webpage. However, I believe your calculations are a bit off towards the end... If you have torque from a motor, that will turn into force when applied to a screw and nut. That would give values in ounces or pounds, not oz-in. For example, with a 1/2-4 2 start screw with 75% efficiency and 140oz-in of torque, I get a force produced of 82.5 pounds (1759.3 ounces). The 1/2-10 2 start would produce 89.5 pounds, the 1/2-10 single start would produce 48.3 pounds using your numbers.

In the case of my machine with 425oz-in steppers, I could get more force at higher speed with smaller motors, but the 425's with 1/2-10 5 start give me more force at the somewhat slower speeds I run at. I forget exactly where the crossover point is, but I seem to remember that it's over 100ipm and I've done most of my cutting below that point so far.

[pminmo]

Your right, I goofed, senior moment... Torque is rotary and force is linear, now I need to go fix it.......... You are in the minority that understood that you needed the pitch to acheive your outcome. You would get bored with .1" pitch screws and those motors.

jsheerin, double check my grey haired math now...

[daliti]

Those motors don't sound too big for your table size to me. Mine is slightly larger and that is similar to what I use. You can go with slightly smaller motors that have better high speed torque, but it depends on many factors. To calculate how big of a motor you need, you need to take into account the masses of the parts you're going to be moving, how fast you want to accelerate them, the specifics of your screws (or whatever you're using), and cutting forces. Then you can see how much torque you need from the motors and by looking at the motor's torque curve, you can see if it will provide it at the rpm you'll be running at.

Which motors did you use if you say "mine is slightly larger"?
If you can turn the XYZ screws freely with your fingers, wouldn't these motors be strong enough?

I've seen the monster kit on Probotix, looks great, but is the supply heavy enough, i'm having my doubts.

[jsheerin]

I used the Keling 425oz Nema 23 motors with a Gecko G540. I have a 4'x4' router. The gantry is about 100 pounds total. I have dual motors to move the gantry, but that is one of the limiting factors in the design. You can turn the screws with your fingers - there is no binding - but that is still a large mass to accelerate. If you can't accelerate to your top speed in a reasonable distance, then having higher torque at high rpm's is pointless as you won't be able to ever get your load up to that high speed within the bounds of the table. I don't think anything being discussed here is that under rated, but it is something to consider when designing.

Looking at the Probotix kit, yes, the power supply looks like it might be too small. I use a 48V 8A supply with my 4 motors, and the motors look like they are very similar specs to mine.

[pminmo]

100 lbs is significant mass to accelerate/decelerate. That's another issue, regardless of motor/leadscrew sizing, the mechanics of inertia on wear and tear of accelerating and decelerating a large mass takes its toll if one tries to get to aggressive.

[jsheerin]

Actually I would say that is not correct - wear and acceleration are very specific to motor and leadscrew sizing. If your motors are too small, they won't be able to accelerate a given mass as quickly (for the same screw setup). If your leadscrews are small, they could buckle under the force generated by accelerating a large mass. If the threads in the nut are small, they could be sheared off. There are lots of other issues as well, but my point is that a large mass is not a problem in and of itself. You just need the rest of the design to take that into account.

[pminmo]

I agree if everything is done correctly it isn't an issue. I was tossing out the implications of leadscrew and motor just to defocus on them and focus on other parts. The last sentence of my post said "takes its toll if one tries to get to aggressive". Agressive in that they overload a mechanical stresspoint, for example a non backlash plastic leadscrew nut isn't an issue in itself when loads are properly tuned, but if the acceleration values are too agressive over time the wear will lead to ever increasing backlash. I think were on the same wavelength, just my poor commumication skills.... Did you get a chance to be a second set of eyes on my new numbers?