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Author Topic: Z-Axis Repeatability: Small Changes  (Read 4927 times)

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Offline Tweakie.CNC

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Re: Z-Axis Repeatability: Small Changes
« Reply #10 on: October 30, 2015, 02:34:29 AM »
Hi Richard V,

Try halving the Velocity and acceleration you have set for your Z axis in Motor Tuning (save the changes).

If this cures the problem then you need to find the optimum reliable settings for this axis.
If it doesn't cure the problem then I suggest you thoroughly check the mechanicals - such things as loose set screw in couplings, binding or tightness of slides or screw etc.etc.

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.
Re: Z-Axis Repeatability: Small Changes
« Reply #11 on: November 28, 2015, 10:46:47 PM »
Hello Tweakie. What you suggested is a good idea: halving the speed, and checking the mechanicals. I took apart my machine and thoroughly cleaned the moving parts. I also tuned the motor to give me a slow response to G00 commands (rapid move). I also made sure my feed rates were set low (like 20 mm/min). I even tried using painfully slow speeds, but that didn't help.

Despite all of this, I still keep losing counts. It's always in one direction; my cut depth is set to z = -0.01 mm. I can hear that something is going wrong as the milling takes place. At first it is good, but after repeated up and down commands, it keeps cutting deeper. If I pause the run, and re-zero the Z axis, I see that my Z axis is off by 0.23 mm (roughly). So instead of cutting to z=-0.01 mm, I'm going to z = -0.23 mm (or so). If I were to let it go, it would probably get worse.

In reviewing Part 3 of the Tutorial on YouTube, I saw a configuration for extending the pulse duration. The video explains that that older controllers might have problems with reversing the direction a lot, as I do in Z axis (making a PCB). The video explains that 99% of users don't need anything other than a "0" entry. Does anyone think this will help?

Thank you, Richard V
« Last Edit: November 28, 2015, 10:48:21 PM by rrsquez »

Offline Tweakie.CNC

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Re: Z-Axis Repeatability: Small Changes
« Reply #12 on: November 29, 2015, 01:26:02 AM »
Quote
In reviewing Part 3 of the Tutorial on YouTube, I saw a configuration for extending the pulse duration. The video explains that that older controllers might have problems with reversing the direction a lot, as I do in Z axis (making a PCB). The video explains that 99% of users don't need anything other than a "0" entry. Does anyone think this will help?

I think you need to try changing the value and see if it improves the situation.
(Checking my set-up I use 2 for step pulse width).

Is your Z axis stepper motor getting hot (approx. 60 deg.C) or is it running cold ??

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.
Re: Z-Axis Repeatability: Small Changes
« Reply #13 on: November 29, 2015, 01:30:17 AM »
Hi rrsquez,
what is the resolution of your setup ie steps per mm. What you have described sounds like the machine is driving down by say 100 steps
but only retracting 99 steps. After a number of moves the depth of cut will increase.
I cut PCB's as well using PCB-gcode plugin for EAGLE, are you doing similar? I had to do quite a bit of work to get the bed of the machine
flat with respect to the z axis and can maintain cut depths within 8um ie 0.008mm and was quite pleased to achieve that. I would have thought
cut depths of 10um ie .01mm quite a challenge, certainly not one I can match.
For most stuff I do I use a 0.4mm two flute endmill, I find the clean cut by comparison to a V bit preferable. For really fine quad flatpacks I have
to use a V bit tho. Doing this I can achieve trace spacings/widths of 0.2mm, I had hoped to get to 0.1mm but found the results not reliable enuf
to warrant populating the board.

Craig
My wife left with my best friend...
     and I miss him!

Offline RICH

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Re: Z-Axis Repeatability: Small Changes
« Reply #14 on: November 29, 2015, 07:24:43 AM »
Hmm....

0.01mm ( 0.0004") depth of cuts which requires a vey accurate SYSTEM which includes tuning and setup.
 
What kind of machine are you using?
Post your xml file.

Quote
If it were a "motor power" as someone suggested, it takes more power to dig deeper into my PCB material. Maybe it's when I lift the Z axis?

As Terry replied it could be the motor. For very small moves the motor  needs to be capable of delivering high acceleration and thus at 0.01mm
move it's almost instanteous.Thus you can loose steps which could be random or consistant.

You need to meticulously tune the steppers and then find your max
vel and accel that delivers repeatable results. Very seldom will the machine be able to move at it's resolution. Practicaly speaking "zero"
backlash is not to be had.

It gets very difficult to find those very small things that affect accuracy since many things can be the cause of it.
You should be able to say, with out doubt / absolutely, that it is not a speed problem.

First tune the steppers, do tests, to verify movements, etc. to know exactly what each component of the system is contributing or limited to.

BTW,
Resolution is 1/steps per unit...so for say 23000 steps per inch
resolution is 0.000043".

RICH