which forces i should consider in my calculation to size stepper motor for homemade 3 axis cnc machine that uses ball screws on its axis
which forces i should consider in my calculation to size stepper motor for homemade 3 axis cnc machine that uses ball screws on its axis
Consider the weight of each axis, how much force it will take to move it when cutting and how fast the rapids will be. But remember that holding torque (the most common number given to distinguish one stepper from another) isn't the most important thing. You need a motor with low inductance in order to run it at the speed it's capable of, given the power you're supplying. This is usually limited by the drivers you're using. If your motors have 2 mH inductance, you'll need to give it 45 volts to run it at optimum speed. If your drivers can only handle 36 volts, you'll want to get motors with less inductance than that. If you want more torque at the peak speed, get motors rated for higher amperage but the same inductance.
Hi,
I strongly support awerby's post.
Most first time buyers see the holding torque and chose the stepper with the highest rating. That overlooks the inductance and inductance
will determine how fast those steppers can go.
All the stepper manufactures know that many buyers just look for high torque, so they make high torque steppers but also with high inductance.
Only after sale does the hapless buyer realize that he has bought a lemon.
The facts of physics are that ALL steppers lose torque the faster they go. If you have a low inductance (1.2mH) stepper, say 23 size (the flange is about 60mm square),
it will retain about 40-50% of its torque at 1000rpm with a good high voltage (60-80V) driver. Another 23 sized stepper but 6mH will struggle to retain 10% of its torque
at 1000 rpm, in fact it will probably stall at 250-350 rpm.
One thing you can do to counteract the degradation in torque with speed is to use high voltage drivers. At this time 80V is about the maximum voltage available drivers
can handle and I would suggest that is what you get. Don't settle for less. If you buy well you'll have these drivers for years. Leadshine AM882's are pretty
good value but the gold standard in reliability, quality and backup is Gecko. Their 80V drives range from about $100-$150 per axis, nice but not cheap.
The next thing you'll hear about is closed loop steppers and how they 'are like servos'. If you believe that then I got some bottom land I want to sell you....
the bottom of what????.....would I put you wrong???
The manufactures do a good job of advertising their closed loop steppers playing on your fears about losing steps. What they don't tell you is that closed
loop steppers are STILL STEPPERS, and they lose torque as they go faster and a closed loop doesn't change that. It must be said there are some advantages
that come with the closed loop but well short of the manufactures claims and hype. You could do better by spending the extra that you'd pay for closed loop steppers
on even better open loop steppers and drives.
I have Vexta five phase steppers, open loop, and they have not missed a step in the six years I've had them except when I do something stupid.
If you want closed loop performance then get servos, I'd recommend either Delta (Taiwanese made in China, very good indeed) or DMM (Canadian
made in China also very good) as being two quality brands at very fair prices. You can buy cheaper but risking quality and backup. If you want
the best and don't care how much they cost then there are plenty to chose from like Yaskawa or Sinumeric.
If you are buying 23 sized steppers look for 1mH-2mH, 1 mH preferred. Reject anything over 2mH. They tend to have high currents relative to their
holding torque because they use fewer turns of a larger diameter wire, that results in high current (for the torque) but also low inductance, very desirable.
If buying 34 size (flange about 80mm square) look for 2mH-4mH, 2mH preferred and reject anything over 4mH.
Another trick when looking at a catalogue is that there will be several models of identical size, lets say 23 size but 100mm long. One will have
high torque say 570 oz.in, whereas another the same 23 size and 100mm long is 425 oz.in. It is almost certain that the lower torque version will have
lower inductance and perform better overall than the high torque one. Whats the bet that the manufacturer made the high torque one just to capture
those people who don't know about inductance. You do know about inductance now so don't be duped.
Craig
Hi Adna - search for this thread and entry. It has a typical screw calculation in it. Peter