We have a hexagonal window that gets a lot of direct sunlight in the summer. I wanted to make a custom design shade for it. Due to the size, I’m planning on making six identical panels. They’ll still be too small by a bit, but I’m hoping that that will work into the design.
So here’s what I’m starting with:
As you can see, it’s really a 2D design. Probably a project I should take over to the Makerspace fifteen miles from here, and cut on their laser cutter. But I have a Nomad! No way I’m driving all that distance 
Since I need the full 8x8 inches, I’m bolting my plywood directly to the Nomad table. I use two sheets at a time, so the next one to be cut serves as the wasteboard for the sheet being cut.
I loaded the design into Meshcam, and made sure to make two adjustments (per other discussions here): I set exclusion areas around the bolt heads, and then set the retraction height to 0.5 inches so it would easily clear the bolts. Then I ran the default Carbide auto toolpath for soft wood and the 0.125 inch end-mill.
I generated the gcode, and it looked pretty good.
What I didn’t pay attention to was that detail on the upper edge. You can see that for the 3D roughing pass, that very top edge of the curve is too close to the edge of the stock for the settings, so it is not roughed.
When I ran the carve, that detail became an issue. Everything was going reasonably well. I could see that the feed speed was a little too fast for this plywood based on the splintering, but that’s something I could adjust for the next one. But when it came to the waterlining, things went off the rails.
My guess is that the waterlining gcode acts as if the roughing has taken place, even in areas where it didn’t. In this case, remember that top edge where the roughing didn’t happen because it was too close to the edge? Well, when waterlining started, the system bombed into that area much too fast and much too deep. Belts squealed, steppers juddered, and I had to dive for the shutoff. You can see the damage up top in this picture.
I’d done some previous experiments in harder woods where I roughed with a larger cutter and waterlined with a 1mm cutter. The 1mm cutter snapped. I didn’t understand what had happened. Now, I suspect it was similar circumstances - an area where the cutter doing the roughing was too big to clear the material, but the waterlining pass acting as though it had.
Now I started thinking. For a 2D design, the 3D roughing is only important to clear out material so that the cutter can access where it needs for the waterline cutting. If I’m essentially doing a simple outline through plywood that’s less than 1/5th of an inch thick, I could just do everything with waterline and pencil cleanup. But it was clear that the feed rate was too high, and maybe the depth per cut was also too high?
So I tried again, cutting feed rate by 30% and cutting plunge rate in half. The plunge rate change was an error - what I should have been adjusting was step down for the waterline. It turns out that the step down was fine; lowering the plunge rate just slowed down the process rather than helping (or causing trouble).
These settings were much more successful. It took longer than needed, but the results were pretty good. I’m still getting some splintering and minor delaminations of the plywood. It may be that this is just not the right material to work with.
Lessons learned:
- make the retraction height as low as you safely can
- don’t confuse plunge rate with step-down distance
- for some cases of outline cutting, you can omit the roughing pass
- in cases where you are doing roughing, look at Meshcam’s preview carefully. Notice if there are omitted areas, and make sure they won’t cause trouble in subsequent passes.
Samuel
