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Tool Z offset suddenly changes in the middle of CNC toolpath

Message 1 of 5
225 Views, 4 Replies

Tool Z offset suddenly changes in the middle of CNC toolpath

Good morning, I have been using Fusion 360 with a CNC machine for 4 years. I machine various materials almost everyday. Today twice the tool offset suddenly changed, resulting in the first case in the work being cut in half.This did not show during Simulation and I tried to simulate again after my work was broken and it did not show either. The offset just jumped on the machine.


Then afternoon, twice the Z offset increased by about 8mm, resulting in air milling. These are multiple setup parts that take half a day to mill with a stock that is not too cheap so it is highly inconvenient.


Looking at the G-code I can't find where the offset changes. Here in the attached file it suddenly changed around line N20000 but I could not find where exactly. The tools library both on the machine and on Fusion are exactly the same we've been using for years. Tool offset is set with a tool sensor on the machine. CNC is a german model from CNC STEP running their own sort of Mach called Kinetic NC. Up until last week everything was working great, I suspect some of the changes in the Fusion Post Processing introduces some invisible (to me!) offset.


Any help would be immensely appreciated!

Tags (3)
Message 2 of 5
in reply to: micheloudeg

Hey, there is nothing in the code that is going to give you an offset change, the problem more likely lies with your machine.


Looking at the type of machine it has stepper motors which can lose position in some circumstances if overloaded.

I would also be checking any couplings from the steppers to the ballscrews aswell as thrust bearings etc.

Message 3 of 5
in reply to: micheloudeg

Thanks for the answer a.laas

Message 4 of 5
in reply to: a.laasW8M6T

@a.laasW8M6T  I think you nailed it, indeed apparently when overloaded the Y and Z axis sort of slip and the machine thinks the bit is a couple millimeters higher or lower. I am machining Staron/Corian so this is almost has hard as aluminum on the machine and if you go in too fast or too deep the machine can't push through. Obviously I fix the feedrate as I go along but there are 12 operations over 2 setups for each piece so it takes a while to get it right.


Do you think I may have busted one of my stepper motors and it comes loose? Any idea on how to check that?

Thanks again

Message 5 of 5
in reply to: micheloudeg



You probably haven't damaged anything, stepper motors can "lose Steps", basically when there is too much load the controller keeps telling the stepper to move but its stopped or stalled, you then have to re-home the machine as the controller has no idea that the stepper stopped, and you see it cutting in the wrong place.


you just have to program the correct feeds and speeds that your machine can handle as you have found out.


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