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First 2D Plot – Proof of Concept Using Pen and Ink

My first 2D plot might not be very creative, but as a “proof of concept” it’s perfect.

In my last article, I described how I created my first G-Code for engraving with my Mostly Printed CNC. I chose a bulls-eye pattern with several concentric circles, and added some text. The pen was told to lower itself by 1 mm, which placed the tip of the pen on the paper.

With the MPCNC tuned up and the model loaded into Repetier Host software, pressing “print” starts the action.

In this video, you can hear the motors turning and the machine moving along its three axis. It’s actually pretty quiet.

First 2D Plot – Fine Tuning the Z Axis

I only ran into one problem with my fist 2D plot. The Z axis stepper motor gave a squeal once in a while, and the Z axis (vertical location of the pen tip) lost its calibration. It turns out that a common problem with the MPCNC is trying to turn the threaded rod faster than it can handle.

When the G-code tells the machine to raise the pen between printing by two millimeters, the controller expects this to be accomplished in roughly one quarter of a second, based on default settings. The MPCNC design cannot meet this expectation, so the stepper motor skips a few steps. This results is the pen or any other tool being raised by less than the expected height. Then, the next time the Z-Axis lowers the tool, it goes too far.

One way to correct this would be to issue a slower speed for the Z-axis move in the G-Code. But this would require a lot of manual editing for every job. The other way to correct this is to reduce the maximum movement speeds in the firmware. That is what I did, reducing the Z-axis speed from 8.7 mm/s to 7.0 mm/s. I also reduced the acceleration slightly. That solved the problem. The MPCNC now runs about 20% slower on its vertical moves, but with complete accuracy. The speed of the horizontal moves is not effected.

Overall, this engraving took a couple of minutes to complete. I can probably speed up the machine once I learn more. Baby steps.

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