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Author Topic: Movement of Z axis is great in down direction but is incorrect in Up direction  (Read 1693 times)

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Offline CES

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Hi TPS,

  Any setting less than 10000 steps per inch and I get no movement or completely incorrect movement.

NEMA 23 steppers on all axis with 5 TPI zero backlash ball screws. Doing the math, I get 0.0017 inch per step but in Mach3 is can see 0.0001 on a 0.0005 per tick dial indicator. I should get 5 steps per tick and I do and 1 inch is close to correct.

I tried setting the steps per inch to 100 and my DRO moved 100 inches before table moved 0.01 inch showing 10000 is correct (My table is 15 inches in X, 4.3 in Y and 9 in Z).

CES

Offline ZASto

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I guess that your acceleration is too high.
You are loosing steps because the mechanical resistances in your screw/nut combo, friction oh guides and mas of Y axis (gravity :) )
Gravity assisted going down is ok (everything falls down) but way up :)
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Offline CES

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ZASto,

  Too low acceleration causes lost steps or sluggish steps and lost steps. The high acceleration seems good on all three axis giving around 0.0001 inch per step. When lowered, the movement is not even and is sluggish.

Offline RICH

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Like I asked, and what TPS reinforced in more detail,.... post a picture of the Z axis.
What size steppers ie; in oz do you have? A motor curve would even be better.

You should calculate the steps per inch or use the calibration feature of Mach to get the step setting correct.
Then you test for a reliable velocity followed by testing for a reliable accel value.

RICH

 
« Last Edit: December 03, 2018, 08:29:19 PM by RICH »

Offline CES

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Hi Rich, the calibration value from Mach3 is for my X and Y axis is 10027.*********x and 10025.*********x so the Z value will be very close. I am not concerned about the 10000 value. What I am concerned about is the fact that movement in one direction is not the same as movement in the other.

Offline ZASto

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Just rectify the rising motion :)
Obviously you are loosing steps in the Zup direction.
Probable causes: high acceleration or impotent motor :D (sorry couldnt resist) :D
Make no mistake between my personality and my attitude.
My personality is who I am.
My attitude depends on who you are.

Offline CES

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I found that my X axis is doing the same but the cause is the X-Axis Screw nut. I guess it has become loose. The Z axis has a screw nut of sorts, supplied by the kit manufacturer with the CNC kit. The Z axis nut is very secure and can't be the issue. I'll set down the acceleration and put up with the sluggish response for the calibration of Backlash and steps per inch, as requested by the commenters. My pulse width will need to be modified to allow for the sluggishness of the axis. Can anyone post their X, Y or Z calibration screen from Mach3?

Thanks,

Offline ZASto

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Every machine is different, so calibration screens would tell you nothing relevant.
It is YOUR machine that should be calibrated.
Nut, beeing secure does not tell you anything about the friction/binding etc.
You should first start with low accel and incrementaly increase max speed to the point when stepper stalls. Back off approx 20%. Then start increasing accel, again to the point when stepper starts loosing steps/stalls, back off and test UNDER LOAD. Your Z axis should be calibrated this way in the UP direction.
Make no mistake between my personality and my attitude.
My personality is who I am.
My attitude depends on who you are.

Offline RICH

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Sorry, if you can't post what was asked or take some simple advice  won't be able to help you.

Have  Happy Holiday and good luck,

RICH

Offline CES

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Hi,
additionally I notice you are using incremental moves. What is your step per unit vale for the Z axis?

Craig
JoeAverage,
  My single motor movement is 0.0001 inch on all axis. That is the best I can get out of this machine and seems good enough for the parts I make.


I compromised on the calibration of my Z axis. Going up can be 0.0003 different than doing down as long as the down direction is < 0.0002. All axis are not calibrated to less than 1/5 of by 0.0005 dial. 1 inch or 0.015 inch, the return is to zero.

The up direction has to move the weight of the head, spindle, motor, Collette and bit so I see how it would put a lot of stress on parts that would not have the same stress in down direction. I do not have a zero weight Head.

Thanks, all how commented.

p.s. my machine is slow for a CNC. In the next week, I'll turn up the acceleration.