There’s nothing like a deadline to drive progress, so I’ve imposed deadlines on myself to keep things moving. For Luggable PC Mark II Revision B, the self-imposed deadline was to get it finished (enough) to show at the Hackaday LA August Meetup. It was a mad scramble towards the end, cutting fancy feature ideas in favor of simple ones that can be done quickly within the deadline. But I made it! I took it on the Metro Gold Line train to the meetup venue SupplyFrame DesignLab. Here’s a picture somebody took of rev B sitting on the projects show-and-tell table.

Luggable PC Mark II Revision B sitting on the projects show-and-tell table along with project from other people. I’m not visible in this picture by [SpencerSkelly]

I always forget to take pictures of my own project while at a Hackaday meetup… I’m too excited talking about my project. I’m not visible in this picture. The location I spent most of my chatting time is blocked by the person with white polo shirt on the right.

Many Hackaday LA regulars are familiar with my Luggable PC Mark I, and might even be getting tired of it. Mark II was a fresh take and attracted a new wave of attention. It is always fun to share my projects with like-minded people.

A downside of mad scrambles to meet deadlines is overdose. I’m enthusiastic about Mark II and there’s still lots to problems I need to fix with rev B…. but after the deadline scramble I’m ready for a break before I start working on rev C.

I was planning to go back to learning Python, but they had a giveaway at this meetup and I was granted a Microchip MPLAB Xpress PIC16F18345 Evaluation Board. Getting some basic familiarity with these low-power (in both computation power and electrical power) microcontrollers had been on my to-do list for some time. Now that a PIC microcontroller board has dropped in my lap, I might as well run with it!

Due to real-world inconveniences like gravity and manufacturing tolerance, the monitor sags relative to the aluminum extrusion frame of Luggable PC Mark II Revision B. We’ll have to compensate for this by adding something to help hold the monitor in the frame.

This Lenovo L24q-20 has barely any bezel around the screen, which was a tremendous plus when I was shopping around monitors. The tiny bezel makes for compact dimensions which makes it easy to package, and the lack of excess material contributes to weight. But now the lack of bezel means I need to be careful with the bracket that we’ll need.

When there’s physical stress on a LCD screen, it distorts the layers inside and show up as visible color distortions on-screen. It isn’t good for the screen and doesn’t look good, either. We want something that can spread this stress evenly over a large area. Ideally something flexible so high-stress areas can give way to balance the load.

I started designing rigid 3D printed brackets with stick-on foam strips for flexibility, but then remembered “vase mode”. This is an option in 3D printing where, instead of printing a solid object, the plastic is only extruded on the perimeter. This results in a thin shell of the shape, the thickness of the wall is the 3D printer extruder nozzle diameter, and the center is empty.

Thingiverse had a few objects to be printed in “vase mode”. It was good for showing off something 3D printers can do easily that is difficult for other manufacturing methods. But while it was good for these Thingiverse trinkets, I didn’t see a functional use for this technique… until today!

I designed the shape I wanted in Fusion 360 (as a solid) and printed a short segment using vase mode to prove the idea is sound.

Once the short test piece proved successful, I proceeded to print enough segments to cover all available space on the extrusion bar. (Everything not taken up by the handle or the corner pieces.) They hold the monitor in place while distributing that pressure across almost the full width of the monitor.

On today’s episode of “why we build prototypes”: chassis flex.

In the digital CAD world, all features are exactly as drawn. Their dimensions always perfectly match the specified value. All surfaces mate perfectly. All fasteners are aligned to their holes. All dimensional values are static and never change, regardless of the physical stresses applied on them. Multiple objects are allowed to occupy the same space.

None of these things are true in the real world.

Digitally simulating all the messiness of the real world are hard. There exists software tools for engineers to simulate specific aspects. Interference checking can try to find objects occupying the same space, but it can be deceptive because they rarely take into account all the other factors such as manufacturing tolerance and physical stresses.

Finite element analysis can help understand how objects move in response to physical loads in the real world. It takes some level of expertise to properly set up an analysis, beefy computing resources to run the simulation, and then human expertise again to interpret the results. A badly set up simulation will tell the wrong story, a bad interpretation can do the same, and manufacturing tolerances can throw everything off in unexpected ways.

For a hobbyist project that is quick to build and failure is cheap, it is faster and easier to find out how things act in the real world by just building it in the real world. Hence the construction of Luggable PC Mark II Revision B. Seeing everything in the physical world highlighted some problems. Most of them are trivial, but one stood out.

The Lenovo monitor is attached in the lower back, using a metal plate I pulled from the original display stand. The plate goes to a 3D-printed spacer, which attached to aluminum extrusion bars, which attach to another 3D printed part, before it is attached to the bottom of the aluminum frame. All those less-than-perfect joints add up to a clearly visible problem. The monitor is supposed to sit within the aluminum extrusion frame, but when all the little errors accumulated, the top edge of the monitor does not sit in the frame like it did in CAD, it actually juts out over 20 mm from the frame.

Next: how to help the top of the frame and the top of the monitor stay together.

In the previous post I described how to keep individual M3 nuts in place on a Misumi HFS3 Aluminum Extrusion. After I started using those little 3D-printed holders to keep the nuts in place, I ran into a related but different problem.

Some large parts require more than one fastener to hold them to the extrusion. And some of these parts get moved around as I revise the details of my design. A specific large bracket required four nuts and, after pushing the M3 nuts around (all four every time I moved the bracket) I started thinking about how to improve this process.

The first answer was to scale my existing design upwards. Instead of a tiny object that holds a single M3 nut, create a longer strip that holds multiple nuts in place. In practice, the friction of a longer strip causes many problems. When pushed, the strip will want to bend instead of move, which increases the pressure on the sides of the rail, which made it even more resistant to moving. And when pulled, the strip is not strong enough to stand up to the strain and would break apart.

The second answer is to reduce the size so there’s less frictional stress to bend or stretch and generally break the strip. But then the old problem came back: with less friction, the nuts would move around if the frame is jostled or tilted. It’s nice that they all move together, maintain proper spacing, but that’s not terribly useful.

The third answer is to combine elements from the previous two: the strip inside the rail is still loose and free to move, but I added a tab that sticks above the rail. This tab is large enough to provide friction against the rail edge. As the friction is at tab, friction would not cause the rest of the strip to bend or stretch. Since such a strip is customized for a particular part, the tab is also specialized to the mating part. When the tab is pushed up against the side of the mating part, all the nuts on the strip are at the appropriate places.

With the help of this strip, it is now much easier to move the brackets around to try out different ideas.

While putting together the exterior extrusion frame for Luggable PC Mark II Revision B, I got frustrated with another recurring headache. Aluminum extrusions (like the 15mm Misumi HFS3 I’m using) are shaped so I could put fasteners (in this case, standard M3 nuts) in a rail to fasten things at arbitrary locations along the extrusion. The fact the nuts can slide anywhere along the rail also meant they don’t stay still. If I place the nuts at the desired locations, I have to be careful not to bump or tilt the assembly or the nuts will go sliding out of position.

After too many episodes of nuts moving out of place, I decided to put some thought into the problem. I ended up with a small 3D printed part I can insert into the extrusion along with the M3 nut. It is large enough to rub against the edge of the rail and thereby holding the M3 nut in place, but small enough that it can still be moved with a little push.

It was a challenge to dial in the exact dimensions. The acceptable range is very narrow – in fact almost too narrow for a consumer-level 3D printer like mine to handle. Within the same batch I printed, some are extremely tight and some are too loose. If I print at a different time of day, some are entirely unusable. I also ran out of one spool of filament during a print, and the new spool (even though it is the same type from the same vendor, probably even the same manufacturing batch) returned different results.

So I have to keep adjusting dimensions and generate different files between batches. Fortunately, since they are small, it is not a huge loss of material to just throw away the unusable pieces. It solves my headache and that’s all I really ask of them.