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Author Topic: Strange shift in Z-axis during machining  (Read 3545 times)

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Offline Chet

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Strange shift in Z-axis during machining
« on: April 04, 2012, 08:47:07 PM »
About 4 hours into a 5-1/2 cut, the Z-Axis shifted an additional 0.1875 inches down.
This was not mechanical slippage, as the pattern continued to be cut correctly, just at a deeper depth.
It sounds like the zero Machine Coordinates for the Z-axis somehow shifted to my depth of cut - 0.1875
A 2nd attempt to cut the pattern turned out without any problems.
Anybody have a thought on this?
Especially on how to avoid this anomaly in the future?
(I tried to include a photo, but can't seem to figure out the system here.  Yes, the photo was small enough in size...)

Offline Hood

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Re: Strange shift in Z-axis during machining
« Reply #1 on: April 05, 2012, 03:48:32 AM »
Sounds like you lost steps on the Z axis on an upward rapid. Mach would then think the axis was higher than it actually was so when a move down again was commanded it went further down than Mach sent it.
Hood

Offline Chet

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Re: Strange shift in Z-axis during machining
« Reply #2 on: April 05, 2012, 07:55:53 AM »
Thanks, Hood, sounds like a reasonable analysis.
But why would this effect only happen intermittently?
I can have hours of correct machining before this anomaly presents itself...
Chet

Offline Hood

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Re: Strange shift in Z-axis during machining
« Reply #3 on: April 05, 2012, 08:16:56 AM »
If you are using steppers and they are tuned near the edge of their limits then it can happen some times and others not, just takes something to be slightly different such as a wee bit muck on the screws or slides or whatever, could even be a bad pulse I suppose just at the wrong time.
Reduce the accel and Vel of the Z in motor tuning by say 20% and see if it still happens.
Hood

Offline Tweakie.CNC

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Re: Strange shift in Z-axis during machining
« Reply #4 on: April 05, 2012, 08:18:17 AM »
Hi Chet,

Just a thought, but ambient temperature plays it's part in determining the maximum speed at which rapids can achieve without loss of steps.

I live in the south east of England and we never really get cold weather here (unlike Hood in Scotland  ;D ) but I have just started using aircraft oil (Arctic grade) as a lubricant and this has made a tremendous difference to my router's reliable rapid speeds.

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.

Offline Hood

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Re: Strange shift in Z-axis during machining
« Reply #5 on: April 05, 2012, 08:21:12 AM »
The weather is nice and stable up here, I mean  last week it was mid 20's C,  this week it was 200mm of snow and -5 C at times, ;D

Hood

Offline Chet

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Re: Strange shift in Z-axis during machining
« Reply #6 on: April 05, 2012, 10:04:54 AM »
Thanks to all for their thoughts!
I use 3/8-12 Acme threaded rods and nuts for my drive systems and a high-temp lithium axle grease.
The CNC router is in the basement, so weather conditions tend to be standard.  ; - )
Motor Tuning Acc & Vel are actually set below maximum already.
So the random Act of God may be the best explanation...
However, I did discover another artifact that may, or may not, have some bearing on the operation...
In the Config File, I typically have the program switch off (Enable command) my router and vac at the end of run or M30.
Because of other surprises in the past I also checked the Emergency E-stop as another Enable trigger.
During one run, the E-Stop was activated by a single line of code, yet the router was in a safe area!
And it would only happen at that line of code...
Nothing mysterious, just numbers for the X, Y and Z axis motors.
Turned off the E-Stop notification, and everything ran correctly.
Perhaps an exorcism is more appropriate...
Re: Strange shift in Z-axis during machining
« Reply #7 on: April 05, 2012, 08:09:07 PM »
Chet,
Maybe you are experiencing some random electrical noise.  Do you have and limit/home switches and other inputs connected?  If so, try adding a .1 uf ceramic capacitor between inputs and ground.

Regards,
John Champlain
www.picengrave.com
Re: Strange shift in Z-axis during machining
« Reply #8 on: April 05, 2012, 08:24:20 PM »
Just a thought.  Noticed a reference to "vac" in the previous message.  I used to run a shop vac when cutting MDF material.  Several times - at different and unexplained intervals and in different codes my mill would suddenly take off in a completely strange direction.   This generally occurred in the X or Y axis, but recalling back, it may have happened to the Z also.  As I said it was completely random, but I noticed it always happened when the vac was running.  Although the vac motor was not near the mill, the suction hose was.  When I quit using the vac the problem went away. 

As I said, just a thought.

Bob@BobsShop

Offline Hood

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Re: Strange shift in Z-axis during machining
« Reply #9 on: April 06, 2012, 03:50:57 AM »
If you have home switches then the next time you have the problem do a verify and see if the Z is out.
Hood