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Author Topic: X Axis clunking - Skipping Steps  (Read 1153 times)

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X Axis clunking - Skipping Steps
« on: March 17, 2019, 10:27:26 PM »

I have a 3 axis CNC that I have just built using Nema24 Steppers on all Axis.  The X axis has 2 steppers slaved via Mach3.

All axis work fine except X.  It seems to miss steps (you can see and hear it) in a certain direction while running a program.  I can't replicate it while jogging.

The unit uses Mod1 Racks with 20T gears for both X and Y.

Does anyone have any ideas?

Thanks in advance.

Re: X Axis clunking - Skipping Steps
« Reply #1 on: March 17, 2019, 11:56:28 PM »
there are several possibilities that can cause missing steps. The trick is to make a list of all the things it could
be and devise some tests to rule them out until only one is left.

The most likely reason that a stepper loses steps is because it is overloaded. From what you have described though it sounds like
it goes OK in one direction but not the other? Is that with the spindle running and the tool engaged in cutting the material?
If the tool is cutting it may be that the cutting forces are much stronger in one direction than the other. That is the difference
between climb and conventional milling.

It is also possible that the gantry has got out of square which can cause a lot of drag in one direction but not  the other.
Can you disengage the two motors from the pinions and establish that the gantry is free to move in both directions? Would be
a good time to intentionally drag the gantry out of square to get a feel for how much drag an out of square situation causes.

May I suggest that you run a piece of Gcode  program (that causes the fault to show up) and run it with the spindle running but no
tool in it. If the X axis works as its supposed to then that would prove(?) that cutting forces are overloading the stepper.

Another possibility is that electrical noise, most likely from the spindle, is affecting the X axis direction signals.
A test to try is to run a piece of Gcode without the spindle running. If it performs OK it points to electrical interference.
I would guess that if electrical interference is responsible that it would affect the axis in both directions rather than just one.
Having said that its not impossible for such uni-directional interference. Its a simple test to try......and may give you some
insight into your machine.

Do you have any gearing between the pinions and the stepper motors? If not then the steppers will be required to deliver
strong torque to accelerate the gantry. If that is the case then the faster you go the worse the problem will be. Given that
you cant get it to fault while jogging, and jogging is usually conducted at a percentage of maximum speed, that it faults
when running Gcode, potentially at maximum traverse speeds, but not at lower jogging speeds it not impossible.

May I suggest trying the reduce the max velocity of both master and slave motors down to 25% of what they are currently.
Then try running the Gcode again to see if the fault has disappeared. It will give you an indication as to whether its a speed
related issue. That it happens in one direction but not the other is perplexing. This is however a very simple (motor tuning)
test to perform....why not try?

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