Month: June 2017

Roger Cheng

Nexus 5X, Hand Warmer

Power drain
Red rectangle marks the problematic hot area.

I’ve been pleasantly surprised at how far I was able to revive a Nexus 5X soaked in a public swimming pool for 30 minutes, but I think I’ve reached my limit.

All the phone functions I tested had worked, but using the phone for more than an hour uncovered a serious problem: something related to battery power management is very ill.

The first symptom was heat: the phone got far hotter than it used to, and trying to run the phone while plugged in to the charger resulted in a lot of heat and not a lot of increase in charge level.

I shut the phone down, unplugged it, and set it aside to wait for it to cool. It remained warm for many hours and never cooled. When I turned the phone back on briefly to check, I see the battery charge level has dropped dramatically. Something was draining battery power and turning it into heat even when the phone was turned off.

I knew most phones had two charging programs: One while the phone is on, running under the main operating system. And a second while the phone is off, run by a very minimal piece of code. I’ve already tried the first without success, so I turned off the phone and tried the second. Fortunately this was able to charge the battery though the phone still got pretty toasty.

After the battery was charged again, I disconnected the battery. Only then was the phone able to cool off. When I reconnected the battery, it was quickly obvious the heat source is under one of the pieces of metal shielding, marked in the picture by a red rectangle. I don’t know how to remove that metal shield without destroying the components underneath.

Two hypothesis are open: (1) this circuit was damaged earlier trying to run a ruined battery, or (2) the shield isn’t as watertight as it looks: there’s a lot of pool water chemicals in there corroding parts. Either way, it is beyond my current ability to address, so I have to stop here.

I can’t use this as a phone for more than an hour or two on battery. And I can’t store it with the battery connected because of the drain. So I’m going to disconnect the battery and put it aside in the hope I’ll have better ideas later.

Roger Cheng

Nexus 5X Reassembly after Cleaning

The Amazon Prime truck has delivered the aftermarket replacement battery and a syringe of thermal compound, so it’s time to put the phone back together.

The first order of business is applying a dollop of thermal compound on the frame where it accepts heat transferred from the CPU. Previously, there was a piece of pliable tape sitting in this square depression in the frame, but it was damaged by the alcohol and removed. The gray compound is intended for DIY PC builders to be applied between their computer CPU and its associated HSF (heat sink + fan) assembly. It is designed to bridge only a very small gap, since the CPU and the heat sink of a desktop PC are in physical contact and this compound only needed to fill the remaining tiny air gaps. Here it is asked to bridge a much large gap of almost 1 mm with no physical contact between parts.

Thermal compound

The reassembly went smoothly until I installed the aftermarket battery and found the connector cable is too long by a few millimeters. It is a large distance in the tight confines of a cell phone’s guts. Looking on the bright side, I guess it is better to have a connector cable that’s too long rather than too short.

Too long

With the connector plugged in to the appropriate location on the main circuit board, the cable is bent at an angle that won’t fit under the rest of the phone.

Bent

As much as I hated to do it, I pressed down the cable and made a sharp fold in it. This is a recipe for metal fatigue and the cable won’t last long if I keep doing this. Well, at least the battery is cheap.

Folded

Folding the battery cable allowed the rest of the pieces to be fully reassembled. The phone can power on and launch into Android, so the basics look OK and clear the way for more testing. Let’s see if the phone works well enough to justify replacing the screen assembly.

Roger Cheng

Powering Up the Waterlogged Nexus 5X

After waiting overnight for the re-cleaned parts to dry, I reassembled the phone and pressed power. The lack of response was no surprise. I then plugged the phone into the charger and was very encouraged to see the screen light up with the depleted battery icon. This tells me the some of the phone made it through the ordeal in at least partial working order.

But after a few hours, the situation did not improve. So I took the phone apart again to take a closer look at the battery. When I first took the phone apart to soak in distilled water, I measured the voltage across the connector terminals and got 3.0 volts. It was lower than healthy for a lithium battery but not necessarily fatal. I measured it now – after charging for several hours – and it read zero.

So, that’s not good.

I thought the battery cell might be dead, so I got another one to test my hypothesis. I wanted the connector from the existing battery so I cut apart the plastic wrap to extract it. I was surprised to find that there’s a tiny circuit board inside. I’m not sure what that circuit board does… but looking at its current condition, it’s not doing that job anymore. Since I didn’t know it was there, it didn’t receive the water + alcohol cleaning treatment received by the rest of the phone electronics. It has been under attack by swimming pool chemicals for the past few days.

Battery Board Toast

Well, I wanted the connector, and now I can access the connector. Let’s use it to wire up the lithium cell straight up. There were four soldering contacts on the connector, two with large conductors and two with small conductors. The two large ones were helpfully labeled “V+” and “V-” so that’s how I soldered the lithium cell.

Battery hackAnd it worked! This setup was sufficient to get into the Android OS. Now that the screen is showing more than the “depleted battery” icon, I could see that it was damaged in this adventure. Thankfully it was still legible, and the touchscreen still worked, so I could run the phone for about 40 minutes. Long enough to access the multi-factor authentication app so I could transfer my MFA security to another phone.

Since the phone appears to be running, I ordered a proper replacement battery. I don’t know if the corroded battery circuit board did anything bad to the rest of the phone. The charging circuit may have been damaged trying to charge a zero volt battery for hours. I’m going to see how the phone works with the replacement battery before spending money to address the damaged screen.

Roger Cheng

Drying Nexus 5X Off From Swim

My Nexus 5X phone took a 30-minute swim in a pool due to my negligence. It was unsurprisingly dark when retrieved from the pool. I’ve already ordered a replacement phone but I was curious: could it be brought back to life?

The first order of business was water removal. A public swimming pool has all sorts of chemicals unfriendly to electronics. The first thing I found upon return to home was a jug of distilled water originally intended for the car’s battery. Good enough for a starting point, I left the phone soaking in distilled water while I went online to read up on Nexus 5X disassembly on iFixit.

The information is promising – by modern phone standards, this model is very easy to disassemble and repair. Following the instructions, I disassembled the phone into its major components, performed a second round of rinsing, and laid the parts out to dry.

Disassembled

After drying overnight, it was obvious soaking in distilled water was not enough. There were enough chemicals remaining to leave a white residue on many surfaces and corrosion began eating many components. Here’s a close-up picture of the SIM slot and a few of the surrounding components. The brown stuff building up in the lower-right is especially worrisome.

Soak in Distilled

If gentle soak in distilled water wasn’t enough, it’s time to step things up. Isopropyl alcohol is easily available as a first aid disinfectant though at a lower concentration than ideal. First aid rubbing alcohol is 70% alcohol and electronics cleaning usually specifies alcohol content of 90% or higher. Since time is of the essence, the first aid stuff will have to do. Once the parts are soaking, I also ran a small plastic bristle brush over the surfaces to dislodge any remaining pool chemical and the corrosion that is accessible.

It’s not clear if the alcohol or the brushing was more useful, or if they were both required, but things look much better after the alcohol dried off overnight.

Alcohol and Brush

Some printed numbers were erased by the alcohol, which I wasn’t worried about. Some adhesives were dissolved by the alcohol, and I’m worried about the tape that used to sit over the CPU. I will need a replacement heat conductor to help transfer heat generated by the CPU to the chassis frame for dissipation.

Roger Cheng

Portable External Monitor 2.0: Stacking Plates

Enclosure2_CADPortable External Monitor version 2.0 (PEM2) explored a different construction technique from PEM1. Instead of building a box by assembling its six side pieces (top, bottom, left, right, front back) the box is built up by stacking sheets of acrylic.

With this construction technique, it is much quicker to place components in arbitrary locations in 3D space. Control along the X/Y laser cutting axis are trivial. Control in the Z axis takes a little more effort. The components can be aligned to the thickness of the sheet of acrylic, but if that’s not enough, it is possible to use engraving operations to precisely locate the component in Z.

In contrast, when we want to locate components inside a box at a specific coordinate, we’ll have to design additional pieces – supports and brackets – to mount the item at the appropriate location in the box.

It is also very easy to assure alignment between the parts of the box. Cut a few fastener holes at the same location across all the sheets. After they are stacked up, inserting the fasteners to align all the sheets.

The downside of this approach is that it is very wasteful of material. Each layer will consume an acrylic sheet of the overall X and Y dimensions. And if we only cut away the parts we need for the components, there is potential for a lot of unnecessary acrylic in the final assembly. They add weight without usefully contributing to the structure. Putting in the design time to cut away those parts reduces the time savings of this technique, as it starts approaching the work needed to design supports and brackets in an empty box.

If there’s an upside to the wasted material, it is the fact that this glue-less technique can be easily disassembled. When we’re done evaluating this prototype, every sheet of acrylic can be reused as material for future (necessarily smaller) projects.

Lesson learned: This “stacking plates” construction technique offer a trade off of reducing design time and effort at a cost of reduced material usage efficiency.

Enclosure2_Complete

Roger Cheng

Testing Heat-Set Inserts in Acrylic

As a beginner playing with plastic fabrication on a 3D printer, I hadn’t known about heat-set inserts for putting durable and reliable threads in plastic construction. In all my projects to date, I tapped threads into the plastic directly and made sure to be careful when tightening a screw threaded into plastic. The inserts look like a much, much better solution and they are easily available from hardware vendors like McMaster-Carr.

Before I put in an order, though, I wanted to do a quick experiment. I salvaged some M2.5 heat-set inserts from the dead Dell laptop, and I laser cut holes of various diameters into a scrap piece of 3 mm acrylic. When the hole is too large, the result seems to be obvious: insert will be unable to grip tightly. It’s less obvious to me what happens when the hole starts becoming too small. Recognizing the symptoms will help me determine proper diameter for future applications.

HeatSetTests

For their M2.5 inserts, McMaster-Carr recommends drilling a hole .152″ in diameter. This translates to about 3.86 mm. The largest hole in this test piece is nominally 3.75 mm, but with laser kerf will end up closer to 3.91 mm. The hole labelled 3.7 would, after laser kerf, end up right on the dot at 3.86 mm.

The experiment showed that they will all suffice to hold the insert into the acrylic, so in practice there is some amount of tolerance for the diameter precision. As the holes got smaller, more heating is required to install the insert, and more acrylic is visibly distorted around the insert due to the additional heat. Fortunately optical clarity seems to be mostly preserved, the distortion is barely visible in the above picture.

Once I got down to around “3.5” (actually ~3.66 mm with kerf) I started seeing the insert pushing plastic out of the way during installation. This results in a small ring of excess plastic around the base of the insert, which is undesirable. This is a good enough marker for “too small” and I stopped there. The holes smaller than “3.5” remain unused.

Experiment complete: In the future, the combination of optical distortion and excess plastic at the base will serve as my first warning sign that I’m installing heat-set inserts in too small of a hole.

Roger Cheng

Thread Tapping Failure and Heat-Set Threaded Inserts

Part of the design for PEM1 (portable external monitor version 1.0) was a VESA-standard 100 x 100mm pattern to be tapped with M5 thread. This way I can mount it on an existing monitor stand and avoid having to design a stand for it.

I had hand tapped many M5 threads in 3D printed plastic for the Luggable PC project, so I anticipated little difficulty here. I was surprised when I pulled the manual tapping tool away from one of the four mounting holes and realized I had destroyed the thread. Out of four holes in the mounting pattern, two were usable, one was marginal, one was unusable.

AcrylicTappedThreads
Right: usable #6-32 thread for circuit board standoff. Left: Unusable M5 thread for VESA 100 monitor mount.

A little debugging pointed to laser-cutting too small of a hole for the tapping tool. But still the fact remains tapping threads in plastic is time-consuming and error-prone. I think it is a good time to pause the project and learn: What can we do instead?

One answer was literally sitting right in front of me: the carcass of the laptop I had disassembled to extract the LCD panel. Dell laptop cases are made from plastic, and the case screws (mostly M2.5) fasten into small metal threaded inserts that were heat-set into the plastic.

Different plastics have different behavior, so I thought I should experiment with heat-set inserts in acrylic before buying them in quantity. It doesn’t have to be M5 – just something to get a feel of the behavior of the mechanism. Where can I get my hands on some inserts? The answer is again in the laptop carcass: well, there’s some right here!

Attempting to extract an insert by brute force instead served as an unplanned demonstration of the mechanical strength of a properly installed heat-set insert. That little thing put up quite a fight against departing from its assigned post.

But if heat helped soften the insert for installation, perhaps heat can help soften the plastic for extraction. And indeed, heat did. A soldering iron helped made it far easier to salvage the inserts from the laptop chassis for experimentation.

Roger Cheng

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.

Roger Cheng

LCD Panel Frame From Laptop Lid

Drilling out plastic rivetsMy Luggable PC display was a LCD panel I had salvaged from an old laptop, which I’m doing again for this external monitor project. When I pulled the Luggable PC panel out of the old laptop, I left most of the associated mounting hardware behind. During the Luggable PC project I wished I had also preserved the old mounting hardware.

The first reason is dimension data. When I mounted the screen to my Luggable PC, I had to measure the panel and design my frame to match. A Dell engineer did this work years ago, and when I threw away the mounting hardware, I threw that away as well.

The second reason is strength. A LCD panel is fragile, but when backed with its sheet metal frame, it becomes quite a bit stronger. This is usually a worthwhile trade off against the increased size and weight.

The third reason, less obvious than the previous two, is to manage heat. The back light assembly across the bottom of the screen would get quite hot when the panel is just sitting by itself. However, when the panel is mounted in its frame, the frame served a secondary purpose as heat sink.

The metal frame I want to reuse is attached to the plastic outer cover of the laptop lid. The attachment is done via small plastic rivets: bits of the plastic lid cover melted into the metal frame. Pulling off the frame with brute force is likely to bend and damage the frame, so the assembly is put under the drill press. After cores of all of the plastic rivets were drilled out (above), the metal frame easily pops off the plastic lid cover (below).

Frame freed

The metal frame can now be used to build the rest of the enclosure. The frame can be cut, drilled, and generally manipulated in ways that I would never do to the LCD panel itself. And when I’m done with all the prep work, the panel itself will drop right in to the frame. This should be much easier than what I had to do for the Luggable PC screen.

Roger Cheng

Portable External Monitor Project

Thanks to a friend’s generous donation of a nonfunctional Dell Inspiron E1505, I have another LCD panel to play with. (And distract me from FreeNAS Box project.) The eventual ambition is to upgrade my Luggable PC to a multiple-monitor system but as a first step, I’ll learn to work with the new panel by turning it into a portable external monitor. If phase 1 is successful, it becomes an optional additional accessory I can lug alongside the Luggable PC. Then, if I’m feeling ambitions, I can move on to phase 2 of integrating everything into a multi-monitor Luggable PC.

The first order of business is to extract the panel and look at its specifications. Dell laptops around that vintage offer resolution as low as 1024×768, which wouldn’t even be worth the effort to resurrect. Fortunately, this LG Philips LP154W02 panel has a decently respectable 1680×1050 resolution.

Since the computer doesn’t work, next I have to see if the panel does. Off to Amazon to look for boards that claim to drive this panel. The first board (*) pictured here was able to present all the right info to a computer, and it can power the back light, but no picture showed on screen. At this time I was worried – did I get a bum board, or is the display dead?

After some diagnostic chatter with the seller, I was pointed to another board they carried. The first board was computer-focused, the second board was more TV and media focused. The upside is that it worked. The downside is that it seems to have trouble preserving 1:1 pixel information at 1680×1050. I wonder if its media-focused nature meant it up scale all signals to 1080p and then down sample to the panel resolution. That would explain the minor visual artifacts.

The artifacts does take a bit away from the success. But it’s lit, it shows a picture, and that’s good enough to proceed.

(*) Disclosure: As an Amazon Associate I earn from qualifying purchases.

Roger Cheng

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.

Fixture5
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.

Roger Cheng

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.

 

Roger Cheng

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.

Roger Cheng

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

 

 

Roger Cheng

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.

 

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

Vacuum Table – Spoilboard and Gasket

Now that we have a baseline on the vacuum table performance, time to start performing modifications to see what happens.

The easiest thing to reduce air resistance is to remove layers – we don’t strictly need the spoilboard in this setup, so it is removed. We then added some rubber gaskets to improve the seal between the plenum and the fixture, which should reduce air leaking past that particular junction.

8 Plenum Gasket Fixture small
Spoil board removed, gasket added – 25 inches

These modifications did not drop the vacuum of an empty fixture – in fact vacuum was boosted by 2 inches to 25. This implied having the spoilboard in the setup was letting a lot of air slip around the fixture. Removing it was a good call.

9 Plenum Gasket Fixture Blanks small
Adding work pieces increased to 26 inches.

When the work pieces are in place, the vacuum went up another inch to 26 inches. Less air is leaking past the work pieces, and they are now held by about one inch of mercury (roughly half a pound per square inch.)

We haven’t put any effort into improving the sealing between the work pieces and the fixture. How much gain can we realistically expect from the effort? In order to get a rough estimate of how much more we might gain, we draped a plastic sheet over the fixture.

10 Plenum Gasket Fixture Sheet small
Replacing work pieces with a plastic sheet boosted to 27.5 inches.

Looks like we have about 1.5 inches of mercury we can gain from better workpiece-to-fixture sealing.

This is a promising start, as this tells us we’re in reach of a decently high level of vacuum for work-holding. We now need to put some effort into the other side – improve the path for the vacuum to reach the work pieces and hold them in place.

More improvements to come!

Roger Cheng

Vacuum Table – Baseline Measurements

The CNC router at Tux-Lab has been under-utilized partly due to its under-performing vacuum table. It has a poor track record on an existing project, and we want to understand why (and hopefully fix the problem) before doing more projects on the CNC router.

To narrow down the cause, we will record the pump’s vacuum gauge reading at various configurations. We use a phone to take a picture identifying the vacuum configuration. We then hold that picture up next to the gauge and take a picture of the phone and the gauge together.

Establish the bounds

First, we get the upper bound: once the pump is up and running, close all the zone valves. The reading – nearly 30 inches of mercury – confirms the pump itself and the majority of the vacuum piping is in good working order.

2 Zone Valves small
Maximum vacuum – nearly 30 inches, good!

The lower bound is obtained by opening all zone valves and place nothing on the spoilboard. When in working configuration, the vacuum will never be weaker than this 7.5 inch reading.

6 Spoilboard small
Minimum vacuum – roughly 7.5 inches.

Most of the tests confirmed that the vacuum setup itself appears to be in good working order. We only started seeing problematic numbers once we started involving the spoilboard and the project fixture. Good news since these are the easiest pieces to fix.

4 Spoilboard Fixture small
Fixture without work pieces – just under 23 inches.

Past runs of the existing project has been done with the fixture mounted on the spoilboard. The vacuum reading of this configuration is surprisingly high at a hair under 23 inches. Indicating a lot of air resistance despite being carved from low density fiberboard.

We then added the work piece blanks on top of the fixture and measured again.

5 Spoilboard Fixture Blanks small
Fixture with work pieces – just over 23 inches.

The vacuum barely changed, to just a hair over 23 inches. This is a problem: it tells us the air can easily find a path around the work pieces so very little of atmospheric pressure is applied to hold pieces in place.

Now the objective is to modify the setup to (1) reduce the vacuum reading of an empty fixture and also (2) increase the vacuum reading of the populated fixture.

Increasing difference between these two readings should increase the holding power.

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.