Day 6 had gone extremely well. Finally having a method to cut concave faces meant that I could now continue on with the more difficult elements which I had postponed. First however, I needed to finish off the reverse side of the lens which I had cut the day before. The element is a convex-concave lens, and so after parting off at the correct length, I hammered the element into a collet and secured it back into the CNC machine.
The reverse side cut as expected. The acrylic cut normally and after a rapid polish the part was ready to set aside for final polishing and assembly later on. Even only half polished, with a lot of scratches still visible, it was by far the most impressive element to have manufactured so far simply because of it's unique shape.
Next I decided to take one of the other concave-convex PMMA element. My brother was convinced that for every good day I would have a bad day, and vise versa. I have to agree. The first element had cut a breeze, and it finally seemed like things were getting well worked out. However I found myself troubled by the tools dimensions. I had programmed the profile cutting paths for the CNC party the night before and partly during the day, and simulated it as before. No collisions, no problems. I went to run it on the machine, straight onto the acrylic given that it had worked so well the previous day, and what do you know? Failure.
As you can see above, everything started out fine. Cutting the steps to hollow out the profile was going as normal, no problems there. However the deeper and deeper the incremental steps went, the closer and closer the tool edges ran to the opposite face of the billet. For a second I thought it would just clear it by a micrometer, but alas, on one of the final and deepest pass, the bottom edge of the tool collided with the billet face.
I watched it for a second in exasperation, and then hit the stop. With the tool away from the part the damage was obvious. The continuous friction of the non cutting part of the tool rubbing against the opposite side of the surface caused the PMMA to melt completely, not just a little. It honestly looked like a circular river of melted and then re-solidified plastic.
Designers have a word they say when stuff like this happens. It begins with an F. In the light the damage is even more potent. The problem I thought I had solved, now wasn't solved. This was a tighter radius than the previous cut, hence the first working fine and the second not. However the remaining concave faces had even smaller rads. Therefore, with this set back, the entire project was back in jeopardy with no way to cut the concave faces and therefore no completed lenses until a solution was found.
