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Author Topic: Jogging distance varies. Set to .001, jogs .0005 then .001, then .0015, etc  (Read 3845 times)

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Offline simpson36

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Mach 3, Gecko 203V, Keling NEMA34, 4650 steps per inch.

With jog set to .001, the actual move varies between .0005 and .0015. Maybe three will be .001 and then it goes off again.

It always catches up and does not loose or gain overall distance . . .i.e. it repeats exactly to the same spot with multiple 1" long test moves, but I would not be able to use the jog for accurate depths.

Is this normal behavior? Is it coming from the Gecko drive or from MACH 3? Do I have something set wrong?

Offline Hood

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Using your steps per figure and taking into account the microstepping of the drive each step is  0.000215". As microsteps are not accurate, they will be about there but never exact. Even the full steps of the motor will not be exact although they should be close.  The higher the steps per unit you have the better chance you will have of getting close to where you want but it will never be spot on. If you increased the ratio so that you were 46500 steps per then you would have a better chance of being able to jog 0.001 accurately.

Hood

Offline simpson36

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Thanks for the reply.

For now I am using a timing belt to drive a pulley in place of the handle on the rack/pinion Z axis on the little X2. Down and dirty, but it got me running quickly. I looked at available retro fit kits for the X2, but the X2 has no quill, so in order to be useful for tapping, the ball screw would have to have a quick release of sorts and I didn't find any with such a feature.

The table has ball screws and the steps work out to 20,000 and I have not noticed the same issue . .  although those motors are NEMA23 so that is another variable.

I have finished a quick release design for the Z axis ball screw so I'll be converting that soon and it will also be 20,000 steps. After that the problem will likely go away, but I'm still lacking an answer as to where the problem comes from. Is it the Gecko, Mach3, the motor?  Is it solvable, or is it just intrinsic to the method used to calculate microstepping?


My understanding is that the control software (Mach3) simply sends steps and the Gecko (or other drive) is where the pulses are totalized and the microstepping is calculated. Maybe I have that wrong.

If this is intrinsic to stepper motors, that would be a compelling argument for servos.

I'm planning to sell the X2 at the end of a current project and launch into another CNC conversion using an X3 . . and perhaps servos.















The issue is introduced by microstepping the stepper motor. Microsteps are not necessarily even divisions of full steps, as you've experienced. The rack and pinion drive likely intoduces some translation error as well.
Joe
« Last Edit: October 30, 2008, 07:34:48 AM by scudzuki »

Offline RICH

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simpson36,
I would quess that the rack and pinion on your mill is not adjustable such that you can adjust the mesh of the gear to
the rack. And if you could and did you would probably end up with a tight spot because the rack and pinion is not precision. You have the equivilent of screw backlash in the rack and pinion. You can take out some of it when change in direction occurs using backlash compensation, it will help minimiize it, but not eliminate it.  I have the same problem with my quilled mill directly driven via the fine movement gearing. I jog it down to take the play out, but on retract, even with backlash compensation, the Z accuracy is not repeatable.
Yes, a few sweet spots, but in general not in the one to 3 thou range. So for general milliing I can work around it,but,
 for engraving requiring very precise movement, forget it.

Positioning the head via ball screw should eliminate your problem. Make sure that part of the system has adequate torque
to hold position and microstep.
Now just as an option to consider and if you can live with the tolerance of the belt driven rack......you can add another
removable compact / short ball screw movement driven spindle for fine work. Just a thought.
RICH
 

 

Offline simpson36

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Thnaks for the comments.

The behavior I am talking about is in one direction.

I do have a lot of backlash in the Z, but that should not be effecting the movement in one direction. Certainly the standard X2 rack is not accurate, but I did shim behind it to take out most of the play.

I'm waiting on the timing pulleys to get the ballscrew installed on the Z axis. I have a new 640oz-in NEMA34 motor for the Z. I am going to end up with exactly 15,000 steps per inch on Z.

Currently the setup has 20,000 steps on X and Y with 425 oz-in NEMA 23 motors.

Offline RICH

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I do have a lot of backlash in the Z, but that should not be effecting the movement in one direction. Certainly the standard X2 rack is not accurate, but I did shim behind it to take out most of the play.

Try this. Using an indicator to measure movement,  jog in the same direction down in 0.1" increments say for 2 or three inches. Note the indicator readings for each of the .1" movement. Are they all the same or do they vary? 1/current steps = resolution whats the difference as compared to what the update would be? May be wrong but the gear and the rack is not precision stuff.
RICH