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z axis deep problem
« on: November 09, 2012, 07:39:32 AM »
Hello everyone!

I'm new to CNC I'm using VCarve Pro and Mach 3.
I have a problem with the Z axis cutting deep that I can't solve, I have read thousands of posts and searched a lot and played with speeds and feeds and I don't thinks that it is a missing steps problem.
I attached to this post 3 files, a calibration toolpath I made and it's reference sheet and part of a pocket toolpath, both created in VCarve Pro.
When I run the calibration.tap it works very precisely, but adds 0.3 mm to the deep of each pocket, I can't understand why...
The pocket.tap is stranger yet, it is all made to cut at 0.5 mm deep and cuts with 2.0 mm each time I run it.
I don't think it is a missing step problem, because when I stop it or it finishes and I press the goto zero button it goes precisely to zero even the z axis.
I have tried with a piece of paper between the tip of the end mill and the workpiece, with the paper barely moving and after running the pocket.tap and pressing goto zero it returns I check with the paper and it is ok.

Please help!

Offline Tweakie.CNC

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Re: z axis deep problem
« Reply #1 on: November 09, 2012, 12:26:35 PM »
Your pocket.tap file cuts 0.5mm deep here when setting Z zero at the top surface of the work.

The only thing I can suggest is that you check the calibration of your Z axis (steps per unit).
Move your Z axis near to the bottom of its travel and zero the Z axis DRO, mount a steel rule vertically next to your axis, note the reading and in the MDI line type G0 Z100 (or whatever) and check that your Z axis does indeed move up 100mm.

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.
Re: z axis deep problem
« Reply #2 on: November 09, 2012, 01:16:47 PM »
Thanks Tweakie, but I set my steps per unit using Mach3 settings and a digital paquimeter and the result is very accurate.

Offline Hood

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Re: z axis deep problem
« Reply #3 on: November 09, 2012, 01:30:11 PM »
Does the DRO read where the cutter is or does it read where the code says it should be?
Hood
Re: z axis deep problem
« Reply #4 on: November 09, 2012, 01:33:58 PM »
Hello Hood,
The DRO reads the value in the G-code.

Offline Hood

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Re: z axis deep problem
« Reply #5 on: November 09, 2012, 01:54:36 PM »
Ok that would tend to suggest it is mechanical or some offset. As your code doesnt seem to have any offsets then mechanical seems likely.
I would do as Tweakie suggested and check with some MDI moves and see if all corresponds, if that checks out then try pulling down on the tool holder and see if there is movement there.
Hood
Re: z axis deep problem
« Reply #6 on: November 09, 2012, 04:57:53 PM »
Hello again,

I did the test with the rule and it worked as expected many MDI moves with a correct value on the rule. 
The tool holder is very tight also it does not move up or down.

What else can I check? Please...

Offline Hood

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Re: z axis deep problem
« Reply #7 on: November 09, 2012, 05:02:08 PM »
I dont know what could cause that, if it were Mach then the DRO would reflect it, if it were noise giving extra pulses then the axis would not return to the same position.
Only thing I can suggest is swap the drive from X to Z and vice versa and see if the problem persists with the Z or moves with the drive.
Hood

Offline BR549

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Re: z axis deep problem
« Reply #8 on: November 09, 2012, 08:25:08 PM »
What you are describing is a 3mm offset applied to Z somewhere.

All the symtoms revolve around a 3mm offset of the Z gcode

I notice the code does not set the fixture offset is it G54,G55,??? Check you fixture offset table and see IF there is a offset for Z. In the Fixture you are running.  Look in the mode line for the Number.

You are not using a G43 BUT was it used erilier and never cancelled ???   Offsets can create many odd things IF you do not cancell them or keep track of them.

Just a thought, (;-) TP