Portable External Monitor 1.0

LCD Enclosure 1 piecesOnce the LCD panel and matching frame had been salvaged from the laptop, it’s time to build an enclosure to hold it and the associated driver board together. Since this was only the first draft, I was not very aggressive about packing the components tightly. It’s merely a simple big box to hold all the bits checking to see if I have all the mounting dimensions for all the circuit boards correct.

It was also the first time I had the chance to try acrylic sheets in a color other than clear. There was a dusty stack of 6 mm green acrylic that I enlisted into this project. Since this is just an early draft project, I valued ease of construction over appearance or strength (6 mm is more than sufficient) and so I used the interlocking tab design for self-aligning assembly.

The resulting box was functional, but not very interesting from a design viewpoint. I just wanted to prove that all the components worked together before I proceeded to the next draft.

I did not design this enclosure to stand by itself. Instead, I had designed this enclosure with a VESA standard 100x100mm mounting pattern in the back and intended to tap those laser-cut holes to take M5 fasteners. Once so prepared, I can mount this enclosure on any existing stand that conforms to the standard. That little design detail – independent of the LCD panel and driver board – sent me off on a little side exploration of plastic construction techniques.

That is a story for the next update.

Simplified Acrylic Box Fixture

After being humbled by my ambition overreaching my skills, I abandoned the idea of an articulated build fixture. To keep tolerance variations under control I wanted to build a simplified version just to make sure I can do at least the simple thing. Also, doing simple fixtures will be an useful skill for times when I want to build a one-off project that needs a fixture but doesn’t justify the investment for a complex fixture.

The simplified fixture is a stack of acrylic plates, made of a mix of designs depending on the task for that layer. The common thread along all the plates are strategic cutouts to keep away from the cement surfaces. This ensures any overflowing cement will not seep into the gaps between the box and the fixture and ruining everything.

The box is built upside-down with the side pieces going into the fixture first. Once they are in place and glued together, the bottom of the box is added last. The bottom-most plate in the stack keeps the box panels aligned vertically. The top-most plate locates the square panel that serves as the bottom of the box.

This fixture tells me the kerf compensation I had been using is a tiny bit on the aggressive side. In the previous fixture, the various errors masked this fact, but in this simplified fixture there is no escaping the truth. The four side pieces of the box inside the fixture have a very tight fit. So tight, in fact, that capillary action was unable to wick enough cement into one of the joints, which promptly fell apart after the box was removed from the fixture.

First run of the stack-of-plates fixture, with the very precise (but one corner not sufficiently bonded) box it built.

Which brings us to the advantage of the simple design: I could make an adjustment, cut replacement pieces, and have a better-fitting fixture in a fraction of the time of building the overly complex articulated version.

Laser Cut Acrylic Fixture Exercise

So after the successful kerf compensation and the reminder that thickness is important, the resulting construction fixture was much better than the 3D printed version but sadly still not good enough.

Fixture4 Open
Box assembly fixture in open position.

The holder for each of the four sides worked well – I’m especially happy at the fact they can grip the panel with just enough force to hold it in place. This was a super encouraging result of the kerf compensation math. If I were a tiny bit off one way, the side piece would be loose. If I were a tiny bit over the other way, the side piece would be gripped too hard and cause scratches. (Or wouldn’t fit at all.) Feeling the pieces fit “just right” was very satisfying.

Fixture4 Closed
Box assembly fixture in closed position.

The problem came from the multi-piece articulated design. Even though the kerf compensation was close to exact fit between two pieces (+/- 0.1 mm) the overall dimension of the fixture depends on perfect alignment of acrylic pieces across assemblies of 5-10 pieces. I was close, but each little error adds up and the resulting box built by this fixture has errors of up to 0.5 mm. Easily detectable by the eye.

Fixture4 Result
Close-up of box built with the fixture.

And, as should be obvious from the pictures, this fixture took a lot of work to assemble. Generally speaking, it is OK (and actually fairly typical) for mechanical design of a fixture to be more complex and time-consuming than the mechanical part itself, so the complexity itself is not a problem. The problem is that I have yet to learn all the ins and outs of designing the fixture so the desired tolerances can be maintained when my fixture starts getting complicated.

But that’s OK, learning from experiences like this is exactly why I’m doing it.


Building with Acrylic: Thickness Variation

Thickness failIn the previous post, the laser cutter kerf was successfully compensated, admittedly in a way that left plenty of room for improvement in the future. This post will look at a different challenge of building with acrylic: variation in thickness of acrylic sheets. So far experience showed different sheets of “6 mm” acrylic can actually be anywhere from 5.31 mm to 6.03 mm.

Since most laser-cut acrylic projects are 2D in nature, any variation in acrylic sheet thickness usually goes completely unnoticed. But when building 3D structures out of multiple interlocking pieces, the thickness dimension has a significant impact.

Fortunately for us, while thickness can vary across different sheets, the thickness is relatively consistent within a single sheet. There may be some variation from one corner of a 4′ x 8′ sheet of acrylic to another, but once cut into smaller pieces that can fit in a laser cutter, the thickness can be reasonably treated as constant.

This allows us to treat thickness as a parameter in a Fusion 360 CAD file. Any slots cut for acrylic pieces will need to reference the parameter. So that when it comes time to generate the cutting profile, the thickness parameter can be updated with the thickness of the actual sheet of acrylic, and Fusion 360 will automatically recompute all the slot widths to match.

Which brings us to the attached picture illustrating human error: the assembly on the left is built up to the proper dimensions. In contrast the assembly on the right was too thin. I made the mistake of measuring on one sheet and cutting on a different sheet that turned out to be 0.29 mm thinner. 0.29 mm is a small difference, but when the assembly is built by stacking seven pieces together, it results in a significant dimensional error of over 2 mm.

Building With Acrylic: Kerf Compensation

After learning my 3D printer’s inability to hold dimensional tolerance, I went back to practicing building with acrylic. Laser cutter kerf may be annoying but it is at least consistent. Now that I know my choice is between a consistent kerf or an inconsistent extrusion width, I choose to deal with consistency.

A bit of Google confirms laser cutter kerf compensation is a fairly common problem people have sought to deal with. What’s less common are actual practicable solutions for designing 3D structures intended to be built up from laser-cut pieces of acrylic. While 2D work on a laser cutter is common, construction for 3D structures appears to be less so.

A laser cutter workflow usually ends in a series of vector graphics commands. Common formats are DXF, DWG, SVG, and PDF. All are good for describing lines, but they only describe where to cut. They don’t contain information on which side of the line is the desired output. So while it is possible for an automated script to offset all lines, it doesn’t know which direction is “inside” vs “outside” in order to perform the proper offset for kerf compensation calculation.

The CAD software (Fusion 360) knows this information, so I thought it’s an obvious place for such mechanism to exist. Google knew of people who have devised some very clever workarounds to make it happen, but not an actual feature in the CAD software itself. Before I started using other people’s workarounds, I thought I’d try to do it manually first, adding to the kerf amount to the dimensions of individual components to CAD.

The result was very encouraging, the laser cut pieces came out at the desired dimensions and pieces fit together with their edges well aligned. This validated my manual process but added mystery. What I did was tedious for a human, simple for a computer, but for some reason the software doesn’t do it. Perhaps I will find out why as I continue learning about laser-cut acrylic construction.

Successful kerf



3D Printed Acrylic Fixture

3D Printed Acrylic Fixture CADSince my last fixture project was foiled by laser cutter kerf, I thought I’d try 3D printing the next fixture to avoid laser cutter kerf spoiling my fixture accuracy.

I started with the same idea as the previous project – just put two pieces together in a right angle joint. This time I put a hinge in the fixture. The idea is that the work pieces can be put in place separately (with acrylic cement already applied to joint surfaces) and then I rotate about the hinge to bring the pieces together.

I could have stopped there, but a single joint doesn’t do anything. If I’m using up acrylic, I prefer building something that can be nominally useful. So the ambition grew to building a little box: 5 pieces (four identical for sides and one for bottom) joined together by simple right angle joints. This is only a small box, just big enough to be useful for things like holding little screws, nuts, and washers. It seemed a suitable baby step since most of the projects I have in mind for acrylic (starting with the FreeNAS enclosure) basically boil down to acrylic boxes as well. So the fixture was designed in CAD, then multiplied to create three additional copies at right angles to each other, to create my box building fixture.

3D Printed Acrylic FixtureThe end result demonstrated that, even though a 3D printer does not have cutter kerf to compensate for, it introduces other errors in the system. Maybe expensive industrial 3D printers would have enough accuracy to make this fixture work, but my little hobbyist level printer definitely did not. The corners of the box did not mate together as precisely as it did in my mind. The gaps are too wide and uneven for acrylic cement to bridge.

After this experiment, I decided I should go back to laser cutting and learn how to compensate for kerf and/or design around it.


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.