[Pinhead]

After days or reading on a subject I know very little about, I still need help.

What I have is a gantry router 14x25x5 10tip NEMA23 mounts, I have decided to go with the Gecko540 but for the life of me I cannot find a stepper match that makes sense to me.

I work mostly with wax and model board on small 4" square areas sometimes larger but only when cutting 20lb foam

Please help me find the right steppers and power supply.. I want to try and get the best match possible for my router and my application but with zero experience all I do is get a headache....

If you have suggestions on what the best match would be or some insight please help stop my obsessing.

[123CNC]

It is tough to find the perfect match. Keling was offering a package deal , 540, 3 motors, and 48 Vdc(IIRC) power supply in which the motors were near optimum cost and performance. Those particular motors were/are difficult to obtain, back ordered, or on the slow boat or something or other. Search the forums, it was a posted announcement.

Someone else may have made a 'new' find, which may appear in a search. Keling has some alternates with a little more or less compromise.

Take a look here, http://www.kelinginc.net/CNCNEMA23Package.html

Do you already have the g540? Or taking advantage of customer appreciation?

[Mariss Freimanis]

Perfection is an elusive goal no one can ever reach.:-) Know when good enough is good enough. "Good enough" means most any 3A per phase NEMA-23 motor used with a 48VDC power supply. You have a 10 TPI screw; you will get 135 IPM with Mach3 during a rapid. The G540, motors and power supply would be perfectly happy to go twice as fast but you run out of Mach3 at 135 IPM with your screw (based on a 45kHz kernel speed).

Don't go nuts on picking a ridiculously high torque motor either. A 200 in-oz holding torque will be more than adequate. Higher torque motors sacrifice speed and motor smoothness for that high holding torque; there is no free lunch, everything you get comes at the expense of something else.

A 200 in-oz motor will give you 785 lbs of thrust on your screw (100% screw efficiency). A 200 in-oz motor will run the pants of off a 400+ in-oz motor at higher speeds and be much smoother at low feed-rate speeds where it matters.

Motors are cheap. Buy a single 3A, 425 in-oz motor, buy a single 3A 200 in-oz motor. Put one, than the other on an axis. compare the performance. I'll bet the smaller motor will be the winner in a side by side slap-down contest.

Mariss

[Pinhead]

123cnc:
Yep, I was looking at Keling as well, debating this as one of my choices.

Maris:
You make some good points, do you have specifics as to a good stepper? my main concern with going to small is that I will suffer from missed steps

[ger21]

Don't go nuts on picking a ridiculously high torque motor either. A 200 in-oz holding torque will be more than adequate. Higher torque motors sacrifice speed and motor smoothness for that high holding torque; there is no free lunch, everything you get comes at the expense of something else.

A 200 in-oz motor will give you 785 lbs of thrust on your screw (100% screw efficiency). A 200 in-oz motor will run the pants of off a 400+ in-oz motor at higher speeds and be much smoother at low feed-rate speeds where it matters.

Mariss, will the smaller motor be able to accelerate faster than the larger one? For instance, would a 200 oz motor with a 4tpi screw accelerate faster (or even as fast?) than a 400 oz motor with a 2tpi screw? Looking for the quickest acceleration possible for a Z axis.

[Mariss Freimanis]

Generally speaking yes, a smaller frame-size motor can accelerate more quickly than a larger one. It only stands to reason; inertia increases as the 4th power of rotor diameter while torque increases as the 2nd power.

Never mind the math. All you need to do is to run a torque to inertia ratio on a motor. That ratio is its ability to accelerate. Simply take the published holding torque and divide it by the published moment of inertia. It doesn't matter what the units of measure are (in-oz, Nm, kg m/sec^2, etc.) so long as the units of measure are the same from motor to compared-against motor. Think of the ratio as a dimensionless (no units of measure attached) number. The bigger number wins.

Don't get too excited because here is a sobering reality: Say you are really lucky and you find a motor that has a torque to inertia ratio of 10:1 versus a motor with only a 1:1 ratio. Unless your application is an essentially a an inertia-less load (you are designing a laser scanner and your 'load' is a sliver of a mirror attached to the motor), you will have a significant inertial load connected to that motor (maybe a 5/8" dia leadscrew 8' long).

The external inertial load is an equalizer that levels the differences between motors and makes them far more equal.

Rule #1: Moment of inertia adds: Say you find a wonderful motor with a 10:1 torque to inertia ratio (torque = 10, moment of inertia = 1) and you compare it against a dog of a motor with a ratio of 1:1 (torque = 10, moment of inertia = 10). You attach a screw or other load that has a moment of inertia of say 100.

Now add the inertias. Motor 1 inertia becomes 101 (1 + 100), motor inertia 2 becomes 110 (10 + 100). Motor 1 torque to inertia ratio becomes 10:101 and mangy dog motor 2 torque to inertia ratio becomes 10:110. Not such a big difference now with only 9% separating them.

I have been working intensively on the stepmotor servodrive. I can report I have an unloaded step motor reliably accelerating from zero to 3,000 RPM in 1/2 motor shaft revolution (180 degrees) closed-loop. Very impressive and guaranteed to raise a stir but for the fact it's unloaded and not attached to a real-world inertial load. Real-world loads are a great equalizer and drain the life's blood out of any hype.:-)

Mariss

[123CNC]

I have been working intensively on the stepmotor servodrive. I can report I have an unloaded step motor reliably accelerating from zero to 3,000 RPM in 1/2 motor shaft revolution (180 degrees) closed-loop. Very impressive and guaranteed to raise a stir but for the fact it's unloaded and not attached to a real-world inertial load. Real-world loads are a great equalizer and drain the life's blood out of any hype.:-)

Mariss

Salivating, I see another 'cheap drive' thread' on the horizon, perhaps 'stepmotor servodrive.' But maybe keeping your cards ($) a little closer to your chest.

Please take your time, I can't keep up, not even enough time to implement as fast as you develop (203V and g100 in process, 320's and g540 still wrapped up as shipped and received).

My planned g540 application is borderline and thus trying to optimize the bang. And yes, perfection is just a form of procrastination. But it sounds nice.

I know I won't be alone checking in on the new(?) thread.