I am making the plastic the parts for my Mostly Printed CNC, starting with the MPCNC Corner Block.

The Mostly Printed CNC (MPCNC) runs on thin walled steel conduit. The frame is held together with 3D printed parts. My starting point was the MPCNC Corner Block, shown on the left side of the above picture. I thought these pieces would be a good place to start for two reasons. First, each is a lengthy print. The MPCNC Corner Block and its associated lock bracket take about six hours, when you print slowly for increased strength and accuracy. Second, they would be a good test of printer calibration, with lots of holes for the conduit, screws and nuts that need to be sized right.

Before starting, I printed a 10 mm calibration cube, and fiddled with my printer’s firmware to make sure that I was getting the right dimensions within less than 1%.

The parts will be made of PLA, with infill generally ranging between 40% to 75%. This should provide lots of strength. My first MPCNC Corner Block was printed on a cold bed made of blue painter’s tape and a bit of glue.

First MPCNC Corner Block – Good but not Great

While the part was printing, I drove to Home Depot and brought back a ten foot length of 0.75” EMT conduit. The actual outer dimension of this conduit is 0.92 inches, or about 23.4 millimeters. The MPCNC Corner Block model on Thingiverse is sized for 23.5 millimeters.

As you can see from the top right picture, my printed block provides a nice snug fit.

I did have a problem with too much plastic oozing or stringing as the print head travelled. You can see this effect clearly in the lower right picture. Fortunately, these strings are very fine and not hard to remove and clean up. Pushing the EMT into the  hole removed most of them.

Most of this oozing is caused by not retracting sufficiently while crossing open space. Initially the retraction was set to 1 millimeter and I have since increased this to 3.5 millimeters. Interestingly, the so called “retraction” mainly retracts the plastic filament that is still cold inside the extruder. However, it does reduce the pushing pressure on the melted plastic in the nozzle. Anyway, increasing the retraction and reducing the temperature slightly eliminated most of the oozing on future prints of the three remaining MPCNC Corner Block parts.

It’s always a thrill for a six hour print job to run without failure.