Week 2: Computer-controlled cutting

The first step of this project was characterizing the laser cutter. My lab partners and I discovered the following metrics for a PLS6.75 laser cutter:

We found a focal length of 2.0” when we placed the chuck key under the laser and adjusted it until the chuck key started to tilt back slightly. We found that ±0.2004 from the 2.0” focal length also worked well.

We discovered the laser cutter kerf was around 0.005”. We made a test cut of a square and measured the difference between the theoretical square length and the actual square length to get the kerf value.

We found that the ‘Balsa wood’ setting was the closest to cardboard. The first time we cut we used a speed of 16%, and as a result the laser didn’t cut all the way through the cardboard, so we experimented with speed. We found that 13% speed effectively cut through our cardboard. (with the power set to 100%).

During testing and when making other projects, I found that joint clearance needs to be really accurate. The joints need to made just right, not too tight and not too loose. The thickness of the cardboard is about 0.165 inches or 4.19 mm, and the joints need to be made within a max of +/- 0.04 of 4.19mm to fit.

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One of the projects this week was to cut something using a vinyl cutter.

Here is the setup for the vinyl cutter using mod software:

I realized quickly that small pieces are really difficult to peel afterwards when vinyl cutting from watching other students spend 5 minutes cutting and 45 minutes peeling. Given time, I chose something reasonably small with a medium to high level of complexity.

I chose to make the Cadillac logo for my laptop which consisted of a few square pieces to peel out. The outer edge of the logo is supposed to have lines, but the vinyl cutter actually failed to cut the lines because they were so thin, so it just looks like dots (which I actually thought looked cooler), where the dots represent where the lines met.

I cut out a sheet of transfer paper and stuck it to the vinyl cut and used a plastic tool to smooth out the edges and bumps to get the vinyl material to stick to the transfer paper.

I then took a pair of tweezers and began peeling the pieces out, which only took around ¹⁵⁄₂₀ minutes for this particular piece.

I peeled the transfer paper apart and stuck it to my laptop and used the same plastic tool to then transfer the vinyl cut to my laptop from the transfer paper. This is the final logo on my laptop!

The next project we worked on was using the laser cutter to make a press fit construction kit.

My first attempt was in freeCAD- the tool was pretty good at first, but when it came to parameterizing variables, I ran into a lot of problems. I wasn’t able to find the spreadsheet mode for 30 minutes, and then realized you have to leave ‘edit sketch’ mode in order to access the spreadsheet. The other main issue was struggling with constraints. I had a lot of constraints because I was making a castle and there were 18 tabs per side of each castle. I kept getting errors for unsolvable constraints and I wasn’t able to debug easily and locate my errors. I asked for help and the advice was to switch to solidworks! So I did, and it turned out to be a much easier process. Nevertheless, here’s my freeCAD castle model.

In solidworks, I still struggled to parameterize my designs and get the hang of CAD (because there is quite a steep learning curve), but I was able to create a parameterized design with constraints for each side of my castle, with enough tabs to hold the system together. I also realized the importance of balancing constraints and parameterizing. At first I was trying to make constraints for every little thing, but in reality I wasn’t really saving myself any time because making constraints tends to complicate things and I didn’t have a very complex design so not all the constraints I had were necessary. I got rid of a few constraints and instead parameterized those specific variables with the same name, so that those parts would be constrained together, but through parameterization and not actual solidworks constraints.

Here are my castle designs in Solidworks this time:

I proceeded to save the DXF files and solidworks part files to a USB and then plug into the laser cutter monitor. I set up my laser using the metrics my team and I characterized earlier, meaning 100% power, 13% speed, accounting for 0.004” kerf, and placing the chuck key correctly underneath the laser. I had to run through several iterations because the first, second, third, etc times I tried cutting, I had to re- adjust the width of the tabs. The joint clearance was not quite right and it needed to be really accurate for it to fit. I changed my parameters in solidworks and was able to get the pieces to fit together well. I then changed the height of the tabs, along with other corresponding parameters to make different sized castles, shown below.