Simple Acrylic Fixture Foiled By Kerf

CornerFixtureThe current goal is learning how to join pieces of acrylic without introducing tabs that weaken the acrylic pieces. I started simple: a simple corner join between two small pieces, and a fixture to help me do it.

Initially I thought that I should make the fixture out of something other than acrylic. This way, if the acrylic cement should seep into unfortunate locations, my fixture is not stuck to the work piece.

Then I realized if I wanted to make good looking joints, wayward glue would still be unacceptable in the result anyway. So for extra challenge I built the fixture out of spare scrap pieces of acrylic. It’s all part of the exercise: if it fails and I end up bonding my work piece to my fixture, learn what went wrong and incorporate into the next exercise.

Acrylic or not, the fixture needs to be designed so it stays clear from the features being joined. At least far enough that capillary action won’t wick the cement into places it shouldn’t go. I find this a pretty new and interesting constraint to designing geometry. Adding a lot of extra little slots and gaps to make sure no part of the fixture contacts the joint.

The fixture was successful at keeping the cement from wicking into places it shouldn’t be. The glue joint looked clear and beautiful, unmarred by wayward glue. But it had a pronounced lip. What went wrong?

I debugged my fixture’s flaw to the cutting laser’s kerf. The gap in my thinking is literally the gap cut by the width of the laser beam. This is something I neglected to account for when designing the geometry of the part, and it throws off the alignment of the work pieces in this particular fixture. Not by a whole lot – the caliper says less than 0.1mm – but enough to make the joint misalignment detectable by touch.

Acrylic Joint Evaluation

Acrylic JointBefore diving into building FreeNAS box #2, I thought I’d take a pause and take a closer look at the acrylic construction results of experiment #1. This is purely about learning to build structures from acrylic – independent from the positive or negative aspects of the project as a computer enclosure.

Since laser cutting acrylic is a fairly popular construction technique, there is a wealth of information on the internet. (To be taken with the usual grain of salt.) After getting some first-hand experience I now have context to better understand the information people have shared online. My favorite single page so far is on Makezine. After reading some of these again (with better understanding due to the new experience) I re-evaluated my design and decided my corners are bad.

For the corners of the enclosure, I had designed tongues for one panel to fit into another. On the upside, this helped with aligning pieces for assembly. On the downside, it made the design more complex to draw up and arrange. And even when well joined with acrylic cement, it is an visually unsightly interruption in the clean clear joint.

Even worse, this has introduced stress points that would otherwise not been there. As I recently learned building the Luggable Frame #1, a sharp internal corner laser cut into acrylic concentrates stress from surrounding components and is liable to start cracking from the point of the corner. Each of these tongues introduced two new stress points in each of the two sheets.

Since the only real upside here is making construction easier, I’ve decided this is not the way to build with acrylic. I should keep the edges for corners joints smooth and clear, free of these tongues, and figure out other ways to keep the pieces aligned during construction.

I’ll spend some time and effort to improve my acrylic joints before proceeding to build FreeNAS box prototype #2.