Choosing a step rate?

[Rhodan]

New to this stuff and I'm trying to think everything out prior to actually putting a machine together.

I'll be looking to route PCBs with the smallest traces being 10 mil. What would be the best step size for something like this? A good compromise between speed and precision.

I have a leadscrew with 5TPI, motors that do 200 steps/rev, and a controller that can do full, half, quarter, and eighth stepping.

If I go full steps, thats .001" per step. Half-stepping gives me .0005". Half stepping sounds like the perfect rate for this to me (the newb). At least for the X and Y axis. I do all my PCBs using either a 10 or 12.5 mil grid and 0.0005 mills per step would fit evenly into both.

At 1600 steps per revolution (.2 inch) thats .000125" per step. Which seems awfully small to me (being a newb). Would take forever to travel the limits of the axis. Might be good for the Z Axis if I start trying to machine non-square surfaces into plastics or aluminium or something.

So, I'm I on the right wavelength here or should I change frequencies?

[ger21]

Change frequencies Read this thread here first. Are all steppers microsteppable?

What kind of drivers do you plan on using. In your example above, you talk about .000125 @ 1/8 step, which is about 8000 steps per inch. Mach2 can deliver up to about 45000 steps/second. thats over 300 inches per minute.

I would gear your motors down 2:1, or get different leadscrews with 10tpi, and use 1/8 steps. You'll get smoother movements, and still get the resolution you need. You'll also double your available torque.

[Rhodan]

So if I understand this correctly, going with the smaller stepping rate is more for smoothness of motion than precision? Although a half step is usually quite precise and micro-steps are not (steps are not equidistant), using four 8th steps to make that half-step is still a half step, just smoother and quieter getting there?

[ger21]

Basically, yes. One thing is that most steppers exhibit resonance at certain low speeds where there will be a large drop in the torque curve. A lot of times this can cause missed steps because while the motor is accelerating it passes through this resonance band where the torque suddenly drops. Microstepping helps to get rid of this resonance giving you a much smoother torque curve.

[Rhodan]

Ah ok thanks for the info! Getting closer to understanding all this 8)