Laser Cutting

Roger Cheng

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.

Roger Cheng

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.

 

Roger Cheng

Mini-ITX Server Box

Mini-ITX Server CADTux-Lab had components on hand for a completely fan-less bare-bones Mini-ITX system. A small board with a passively-cooled CPU, a small 12V DC to ATX DC power supply that didn’t need a fan, and a solid state drive for storage. All it needed was a simple basic box to keep everything in – which made it an ideal learning project as a follow-on to FreeNAS box V1. (Well, it can be argued that this simple box should have come first… but that wasn’t the order things ended up being.)

This time there was no design challenge with hard drive placement or power supply fan clearance. Just a simple box with two sets of holes so convection will pull cold air from the bottom and let hot air out the top. The back plate had opening for the standard ITX motherboard port plate, plus two holes: One for the 12V DC power input, and the other for a momentary-on power switch.Mini-ITX Server

The result was an upgrade from its previous placement, which was the bare circuit board sitting on top of a cardboard box. Now it has some minimal protection against accidents like an errant dropped paperclip shorting things out.

This machine is now set up with the Xen hypervisor and ready to run the server-side code of whatever future projects arise at Tux-Lab, as long as that code can run in a Xen virtual machine.

Roger Cheng

FreeNAS Box V1 Prototype

FreeNASv1With the concept designed, it’s time to head over to Tux-Lab to build it!

To be honest, it was not fully designed for use as a computer case. Since this was my first effort designing for acrylic construction I expected to run into some amateur mistakes very quickly. As a result I had left some known design issues open to be resolved in future prototypes. One example: I had not designed any kind of door or hinge. The prototype panels are mostly glued together, except for the front panel which is held in place by tape.

Aesthetically, I am pleased with how the clear acrylic looked though I am not pleased at how much of a rat’s nest the power supply cables became. Taming wires is a perpetual challenge. I now understand why Apple enclosed all the ugly guts of the G4 cube in shiny aluminum shell inside the clear acrylic shell.

Other than the messy computer wires, the clear acrylic does hint at the illusion of a computer floating in mid air. I’m pretty happy with that, but it’s not enough to offset the tangle of wires. Next prototype will either have good cable management (takes effort) or have some dark colored acrylic to hide the interior (much easier.)

The cooling functionality worked as designed: the intake air is drawn from the bottom, past the two hard drives keeping them cool, and out the top.

Similarly, the designed goals of tilted-PSU (power supply unit) space optimization was successful, as did the gentler turn demanded of the wires. However, there was an unforeseen deal-breaker of an issue.

Uh-oh!

FreeNASv1_FlawOn the back side of the tilted-PSU, we see that the tilt has pressed the bottom of the PSU up against the wire bundle at the top of the motherboard. The tight quarters mean individual wires of the bundle tried to relieve the crowding by moving into the space for the PSU fan preventing it from turning. Since the PSU fan is the primary air-mover for this enclosure, a stopped fan is obviously not acceptable.

Other notes

Space utilization efficiency has room for improvement. Some of this was caused by the desire to emulate the Apple G4 cube and have a square footprint. (20 cm squared!) The squareness was completely unnecessary and future iteration will likely have a rectangular footprint for space efficiency.

There was uncertainty about how well 3mm acrylic can hold the weight of the power supply unit. It proved to be surprisingly capable once the two top sheets reinforced each other at right angle.

Amateur Hour: A laser cutter only cuts vertically. There was no way to cut a 30 or 60 degree edge for the tilted PSU section! For this learning exercise, the cornered edges are simply left open and unattached.

The angled PSU was a novel idea to solve specific problems, but it caused new ones and also unsuitable for laser cut acrylic construction. That was a fun experiment, but we’ll have to leave the angled PSU concept behind for the next prototype.