Strange Z axis behaviour

Hi
I have just added a rotary axis to my xyzx CNC router (I have 2 steppers running the x axis).
The rotary axis worked well before I added the second X axis motor, but now the z axis is acting strangely. Every time the machine makes a rapid move and then plunges into the job it goes a mm deeper, so after 10 rapids and plunges the cutter is about 10mm deeper than it should be. I have checked the .ini and .hal files, but can't find a problem. The same file works well on a 3 axis configuration. Can anyone tell me why this is happening?
Thanks
Aaron

If you share your new and old ini files we can look for anything obvious.

To me it sounds like missing steps on up travel with the force of gravity against the stepper. But why is another story. The only thing I can think of is if your power supply can't provide enough current to feed 2 steppers at the same time the amount of torque is limited by lack of current.

Thanks for you answer.
Is there any way I can test this, what if I reduce speed and acceleration? It seems odd because each stepper/driver has its own power supply.

Try halving your acceleration. Still strange if it used to work.

Hi rodw

Sorry I should have got back to you earlier. You hit the nail right on the head. I reduced speed and acceleration and I got the job done. So thank you very much. However it was running very slowly and I would like it to work at the original speed. Could I ask you to look at the two ini files, the original xxyz and the xxyza please, just to see if I have overlooked something?
Kindest regards
Aaron

I'm not sure if it's the cause of your speed problem, but the recommendations for STEPGEN_MAXACCEL is MAX_ACCELERATION + 25%

You might want to bump it up a tad:

MAX_ACCELERATION = 380.0
STEPGEN_MAXACCEL = 475.0