Designing for CNC is a fairly straightforward process. You start with an idea, draw it in your CAD software of choice, and then create a toolpath file with the instructions to fabricate the pieces for your project. You almost always have to do this process a couple of times to get it just the way you want, but the thing you finally end up with is the toolpath. And you have the design file so it’s easy to make changes and modifications.
But what if you came across a project where the only things that are available are the toolpaths?! Can you start at the end and work your way back to the beginning? That’s what I just helped a group do in resurrecting the DayStar Zome design by Rob Bell. You can read about the start of this project in a post on Medium.com called A loss in so many ways.
You might ask “Why don’t you just cut the toolpath files…why do you have to turn them back into design files?”. There are a couple of reasons, but most importantly there‘s no bit information in the files. We THINK they were cut with a 3/8″ bit, but can’t be sure without reconstituting them back into design files.
We’re also not sure that we have toolpaths for all the pieces, and that they are all correct. Without being able to ask their creator…Rob Bell…we have to do a little detective work. Fortunately, we have some tools in our arsenal that help us resurrect a toolpath file into its design.
The first tool we used is the ShopBot FC file convertor. It’s installed with the ShopBot control software and lets you convert cad formats into ShopBot files, and vice versa. Since we already had folders full of ShopBot sbp files, and they are text files, all we had to do is select a ShopBot file and convert it to a dxf formatted CAD file. Rinse and repeat until all the files are in a format that our Design software, Vectric VCarve, can understand. It’s a little tedious and repetitive, but once you get in the zone it’s not all that bad. All you have to do is open the piles of dxf files in VCarve and you’re good to go, right? Easy peasy!
Not quite. The first problem is that many of the files are cut in multiple passes, so we have vectors piled on top of each other. You have to go entity by entity and delete any extra vectors, until you get a good clean design file. Once again, it’s just a matter of rinse and repeat until they are all done.
So now you’ve got drawings of your parts, right? Not so fast. Do you remember how toolpaths work? They describe the motion of the center of the bit, so outside cuts are oversized and pockets/holes are undersized. We need to offset our vectors by the radius of our bit to get accurate parts.
Hopefully our toolpaths have the size of the bit written in comments, but what if it’s not? It requires some educated guessing. Are there slots in the parts that interlock with other parts? Measure those slots and see how much the slots have to be adjusted to fit well.
I didn’t have any experience with how the DayStar Zome went together, but fortunately there were members of the Zonetopia Burning Man crew that were able to jump in and take my converted dxf files and turn them into something useful. I got a test file back from Chris Palmer that had a sample connection that I could bring into VCarve and do all the steps that are critical to fabrication. I cut a sample, massaged the files just a little bit, and got a very cool sample to take to the upcoming 2024 Vectric User Group Meeting in Austin Texas, along with the model described in the medium blog, “A loss in so many ways”.
The goal of all this design resurrection work is not just to create models and samples, but to eventually see Rob’s creations appear again at Burning Man. We’ll keep you updated as we work toward that!