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Author Topic: OFFSET ERROR !  (Read 717 times)

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OFFSET ERROR !
« on: October 26, 2018, 07:02:03 AM »
I have a small 3-axis CNC machine and I design my work with AUTODESK INVENTOR HSM and export it for MACH3, in simple work with less g-code lines it has acceptable error but when machining a large file we have error in both X, Y axis.
let me show you:

it's my X axis: you can see each Z level it get more offset to right on X axis!



it's Y axis: it's so better but still it have have same problem to left


you can clearly see this error from top view


my machine have three 1.8 degree stepper with 16 level microstepping
X axis screw is ball-screw 5mm/turn
Y axis screw is lead-screw 2.5mm/turn
Z axis screw is lead-screw 8mm/turn
my controller is MACH3-LPT

please help me in this matter

Offline RICH

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Re: OFFSET ERROR !
« Reply #1 on: October 27, 2018, 04:57:19 AM »
Your axes must be  calibrated properly. Repeatability of the axis movement will only be as accurate as the axis components.
Here what you need to do:
1. Test the axis for  backlash. You need to do this in a number of positions along the total movement available. You also test the axis for backlash in both directions.
    Ideally the backlash should be the same, but you will find that it probably varies, so use a backlash value that best represents what you have found.
2. Test each axis for proper steps per unit. Test with backlash first removed, then base steps per unit on the longest axis movement available.
    You can use the axis calibration feature of Mach to do this. Once you have the steps per unit correct for the longest move, then test equal smaller moves along
    the full axis travel. You do this for both directions of movement.
3. The above basically is profiling an axis to see just how accurate YOUR axis moves. You will then have confidence in you steps per unit settings.

In actual machining spindle run out, riqidity of the machine, feedrate, type tooling , etc come into play on actual results of the finished work.
That can be addressed but need to have the axis movement correct to begin with. You have what you have, but at least you will know what you have!

RICH

Offline Graham Waterworth

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Re: OFFSET ERROR !
« Reply #2 on: October 31, 2018, 01:04:23 PM »
You are loosing steps, you need to lower your velocity and acceleration until you find the sweet spot where.

Or

you may be trying to feed too fast into sharp corners and stalling the move.

Or

I could be totally wrong
 
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