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• part 1: Design tool

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Okay in the video about the skateboard design tool, I stated that this project might be put on the backburner for a little while, but guess what; here I am already with an update.

The reason why I didn’t want to do this right away is because I wasn’t sure yet how to go about making the plywood-forming molds. I do have all the means at my disposal, but I was planning to use MDF and I realized a sizeable project like this would have taken a LONG time to finish on the CNC.

But as it happened I was leisurely strolling down the alley of my local hardware store when my eye fell upon this thick isolation foamboard. With evenly applied pressure this material should be able to hold the plywood into shape, but with a mere fraction of CNC time spent on carving the molds. So this project was now back on top of my projects to-do list!

I started by preparing the ‘blanks’. I carved a shallow guide groove with the CNC and then cut all the way through on my small bandsaw. I glued them to an MDF backboard just to add some general stiffness. I left the MDF oversized in places, just so I would have a surface to clamp to later on.

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I trammed my wasteboard just in case (this is a large area project after all) and as you can see on the pictures I nearly maxed out the work area of my machine over all three axis. I had to work diagonally just to get the board within my X-Y limits, and on the Z axis I barely had a millimeter to spare.

Because I used foam, I could easily use the full flute length of my endmill as a roughing stepdown value. In this case: 20mm. Additionally, the feedrate I set was quite aggressive, so roughing didn’t really take very long. That said, I did encounter a few mishaps along the way. Some were my fault, others I couldn’t have foreseen.

Anyway, Here’s a quick timelapse video. I apologize for the sloppy editing ;-).

Turns out I was a bit too trigger happy on the feedrates, especially the retract rate. My Acme-powered Z couldn’t keep up, missed some steps and gouged the surface … Luckily this wouldn’t affect the mold’s function.

Another issue I ran into (and initially stumped me) was losing steps along the X axis as well. I know my machine is mechanically and electrically sound, so I conducted an array of tests to home in on the problem. It turns out, the foam ‘chips’ are being statically charged; probably due to the rotation of the endmill. They got pulled towards the gantry (watch the close-up in the above video), and started building up on the X-axis V-rails.

As you can see in the picture below, this build-up was quite dramatic, and ultimately made the carriage choke, miss steps, and once again gouge my part. This time with worse consequences. I didn’t really want to start over, so I just glued in a replacement piece of foam and called it a day.

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On the finishing pass you can’t really win as much time using foam because you still want a small stepover value for a smooth finish. By now Murphy had left the building and I didn’t run into problems of note anymore.

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I finished the molds with some steel profiles for clamping purposes, and pre-drilled the hole placement for the skateboard trucks. I guess I’m ready to do this!