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berend

Z positioning questions

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All,

Every now and than, i experience Z positioning problems.

The height of my prints is not reliable, sometimes caused by positioning overshoot, sometimes by missing steps.

For troubleshoot purposes i'd like to play with Z positioning/speed/acceleration.

1. Is there a way to play with these parameters in my gcode generators (SF/Netfabb) or must i do this in the firmware (Marlin)?

2. I've got a old type of Z-spindle, the ordinary 8mm thread. Does this need special attention in settings ?

3. How is Z motion defined ? is it 'just' making an amount of steps/distance or is there a control loop with a position/speed/acc profile ?

Thanks,

Berend

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Z positioning just at the start of the print or throughout it?

Several people in the google group reported that prints were ending up too tall. I think that turned out to be fixed by setting ExportGcode Small in SF, though I don't know why that'd fix it. My best guess was floating point drift adding up to an occasional extra step but I also didn't think that was a very good guess..

 

For troubleshoot purposes i'd like to play with Z positioning/speed/acceleration.

1. Is there a way to play with these parameters in my gcode generators (SF/Netfabb) or must i do this in the firmware (Marlin)?

2. I've got a old type of Z-spindle, the ordinary 8mm thread. Does this need special attention in settings ?

3. How is Z motion defined ? is it 'just' making an amount of steps/distance or is there a control loop with a position/speed/acc profile ?

1. In SF, you can set the Max Z Feed Rate. It's in Speed in SF44 and Limit in earlier versions. Acceleration is something usually done in FW, not in the slicer.

2. I doubt it - mendels and makerbots are fine with that sorta deal, I think.

3. No idea.

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Thanx Dave,

My reaction and some new questions ;)

 

Z positioning just at the start of the print or throughout it?

The starting Z position is fine and reliable, the total object height can deviate up to +15%.

 

Several people in the google group reported that prints were ending up too tall. I think that turned out to be fixed by setting Export\Gcode Small in SF, though I don't know why that'd fix it. My best guess was floating point drift adding up to an occasional extra step but I also didn't think that was a very good guess..

Personally, i don't know about Gcode Small but it is and was always ON and i've got the same issues with Netfabb.

A rootcause can be found by overshoot from Z position: the energy thats needed to stop the platform from moving down cannot be delivered

- Electronics: poor stepper driver adjustment.

- Mechanical: speed too high

- Software: Z feedrate too high

- Firmware: PID not optimal (Do the PID settings cover the 2 different Z spindles?)

- Firmware: Does the PID take into account the gravity as present force ?

 

1. In SF, you can set the Max Z Feed Rate. It's in Speed in SF44 and Limit in earlier versions..

Acceleration is no part of slicing but defining speed is. So it's good to know there is at least 1 user parameter to control Z motion.

If only a limit can be set, who/what defines the operating Z feed Rate?

What is a fail-safe Z Feed Rate Limit?

(actually Acceleration is something usually done in FW, not in the slicer although Netfabb has in it's Machine/Extended Setup menu a Z Acceleration limit. Confusing)

Thx

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we have an ultimaker in the lab as well with the 8mm rod

we had a problem several months ago, the nut was becoming a ring, the thread was totally worn out, maybe this gives problems in the big variations

solution is to replace the nut and grease the rod very well

maybe it helps

cheersjoris

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Acceleration is no part of slicing but defining speed is. So it's good to know there is at least 1 user parameter to control Z motion.

If only a limit can be set, who/what defines the operating Z feed Rate?

Z is just part of the G1 gcode line. When Z changes, the firmware just moves it along with the other axes. I think.

 

What is a fail-safe Z Feed Rate Limit?

Dunno. Whatever max-z-feedrate value is in the firmware, maybe?

 

(actually Acceleration is something usually done in FW, not in the slicer although Netfabb has in it's Machine/Extended Setup menu a Z Acceleration limit. Confusing)

Setting the acceleration parameters isn't the same as doing acceleration! All I meant was that the fiwmare itself is what decides to tweak the speeds at various points, based on what those parameters are..

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