Here’s the 2.5″ drive bay in the initial iteration of the threaded-box Luggable PC design. It is a simple, basic place to hold one laptop-sized drive, but not a very efficient use of space.

The most obvious thing to do is to vertically stack another drives adjacent to the existing drive. Easy to do in CAD, but has problems in the real world.

Problem 1: How do you access the fasteners? The laptop storage market have mostly settled around two basic fastener schemes: four screws on the bottom of the drive, or four screws along the sides. If we use the side fasteners, they would not be accessible for drive replacement without taking the whole case apart. The bottom fasteners would be accessible, except that bracket would be impossible to print on a FDM 3D printer.

Problem 2: How do you connect the wires? While at first glance there is enough space to physically accommodate the drive and its plugs, it fails to take into account the wires. The wires for the existing drive barely clear the edge in the picture. Stacking another drive into that space (even if moving it another cm or two to the right) would demand wires make relatively sharp right-angle turns. This would place strain on the connectors including the fragile unsupported SATA connectors on the drive itself.

After some experimentation with the available space, keeping in mind the requirements above, I decided to angle the two stacked drives:

Now the Luggable PC has two independent drives: One for Windows 10, and another for Ubuntu Linux 16.10.

This design solves the fastener issue: in this arrangement, it is possible to 3D print the drive bay so both drives are held in place by the shape without use of any screws on the sides or on the bottom. Both drives are held in place by a single 3D printed clamp that is secured with just two screws. The downside is that the design only works when both drives are present. It is unable to hold a single drive in place.

This also solves the initial wiring issue: By angling both drives, the wires do not have to make sharp turns and would not place unreasonable strain on the connectors. However, this comes at a cost of usable interior volume. By angling the connectors, and avoiding sharp turns, the cables consumed more precious interior volume than the previous design.

Going back to the idea of drives stacked in the available volume, we revisit the problem of forcing wires to make sharp turns. The turns can be relaxed if we can find more room for the wires without angling it into precious interior volume. The solution turned out to be… turning the drives instead! By rotating 90 degrees a drive can be positioned to make room for the cables’ turns. It also allows two drives to exist side-by-side, allowing the bay to work with a single drive or dual drives.

This design was incorporated into the following iteration of the chassis, built using aluminum extrusions instead of threaded rods as the metal structure.

 

After some time using the assembled Luggable PC prototype, I wanted to adjust the screen ergonomics. While the screen has been raised to a height much more comfortable compared to a laptop, after a few hours of use I wished I could tilt the screen upwards a few degrees.

At first glance this would have been impractical – the screen hinge design is already complicated and adding a tilt adjustment mechanism would only make it more so. So instead of tilting the screen, let’s achieve the goal by tilting the entire Luggable PC by adding 3D printed feet that can be bolted to the bottom of the machine.

The easiest approach would have been to print a simple solid wedge with the desired angle, but I decided to be a little more experimental. I played with Fusion 360’s spline tool to sketch out feet that are designed to move and flex a tiny bit. The flexibility adds two features:

1. Shock absorption: We still need to be careful setting it down on a surface, but the minute bit of flexibility we gain will help cushion the harshest part of initial impact and make it feel less like we’re breaking the machine every time we set it down.
2. Flatness compensation: With a bit of flexibility in the feet, the Luggable PC can now conform to surfaces that are not perfectly flat. Previously, the solid bottom means it will rock on 3 out of 4 feet on any surface that isn’t perfectly flat (which is most of them.)

Here is the first iteration, which accomplishes the desired goals:

Unfortunately it also has a problem: by moving the contact points inward, the front feet are now uncomfortably close to the center of gravity. A slight push from the rear will send the whole thing toppling forward. The design needs to be adjusted to have a wider stance… but we are constrained by the fact the fastening nuts still need to be accessible.

In this case, the short-term fix is to move the feet as far front as possible while still allowing wrench access to fasteners.

The real solution is to redesign the bottom of the case to accommodate the feet while maintaining a sufficiently wide stance. This idea was incorporated into the following iteration of the Luggable PC. As it used aluminum extrusions for backbone instead of threaded rods, the new wider feet were installed on the bottom extrusion.

The use of threaded rods to hold the Luggable PC case together was borne out of necessity: The print volume of the 3D printer is much smaller than the volume of a PC case so it must be printed in pieces then fastened together. A threaded rod provides strength along its length, but how do we best handle the inevitable corners?

The key constraint is the strength of a 3D printed part, especially the adhesion between layers. This is an unavoidable fact of life for FDM-type printers: the part is weakest between layers, so designing for 3D printers must consider the layers similar to how designing for wood must consider the grain.

Version 1: basic asymmetric

In this design (as used in the “Easel PC” iteration) two of the rod axis are aligned with the direction of the layer. Stress along those two axis would mostly be held in check by the strength within each layer, but a fraction of the force would try to push the layers apart. To guard against this, the third axis is orthogonal so its fastening nuts would also try to hold the layers together.

The problem with this design is that the corners are asymmetric by nature. Not just in appearance, the loads it can tolerate are also asymmetric.

Version 2: symmetric but space consuming

The goal of a corner that handle loads symmetrically across the plastic layers means finding a way to make sure the plastic grain is equally strong across all three axis. The solution is to print at an orientation that lies at the same angle to all three axis.

While this design solved the problem of symmetric appearance and strength, it introduced a new problem: by printing this way, the hinge consumes a lot of the enclosed volume making it unusable. When the goal is to pack the computer components inside a minimalist PC case, every cubic centimeter counts!

This hinge was used in the “Threaded Rod Box V1” and the space it consumed severely hampered the packaging of that layout. It is definitely not the optimal solution so the search continues.

Version 3: Let’s All Huddle Close!

The previous two designs both depended on the plastic to take some part of the load and hold on to a few steel rods. These rods were a few centimeters apart because we needed room for a wrench to tighten the nuts. We needed the nuts to sit inside the corner because…

… um, why do we need them inside? The key for version 3 is the realization that we don’t need that. By offsetting the rods slightly, we can extend the rods past the corner so the fastening nuts are outside of the enclosed volume and not competing for space with the components inside the box.

When the nuts (and the required wrench clearance) are no longer inside the volume, it allows the rods to sit much closer to each other. Now the closest distance between rods are measured in millimeters instead of centimeters. It also means the three sets of fastening nuts help exert a clamping force across all three axis, compressing everything together. This compression means the alignment of the print layers become much less critical allowing significantly more freedom in designing the rest of the case.

This corner design was used successfully in Threaded Rod Box V2 as shown. (In these pictures, some of the threaded rods have yet to be trimmed to length.)

In the previous post we have established all the desired traits of the ideal screen layout, and how it’s impossible to meet them all simultaneously. The only solution is to design a mechanism allowing us to convert between two different configurations, each designed to provide the traits desirable for its corresponding condition.

• Closed: the travel configuration.
  • Compact: We want to be able to lug this around without too much worry of catching on things, so the screen should align with the rest of the case (vertical or portrait orientation.)
  • Protected: To protect the screen, it should be facing inward so the glass surface is less vulnerable to damage.
• Open: the computing configuration
  • Landscape: Unlike phones and tablets, desktop computer applications are not designed for the possibility of vertical/portrait orientation, so the screen needs to be in horizontal/landscape orientation.
  • Ergonomic: Unlike laptop screens that sit at table height, we can turn our extra heft into an advantage as support to hold the screen up to eye height. Ergonomically superior to the tabletop height of laptop screens.

To transition between these two states, we need movement along at least two axis:

• Flip: The screen needs to move from facing inward (protected) to facing outward (visible)
• Rotate: The screen needs to move from vertical/portrait orientation to horizontal/landscape orientation.

My ideal was to devise a mechanism that can execute both of these movements in parallel, so the user sees a single continuous movement from one configuration to another. After quite some thought and experimentation without success, I decided to postpone this ideal for later. For now, I’ll implement a hinge that has two separate degrees of freedom so the two desired axis of movement can be accommodated.

Originally the open configuration would have the screen up and centered relative to the rest of the body, and I had a few overly complex mechanical linkages attempting to make this happen. But then I realized it isn’t really necessary: the body has enough heft to hold up the screen even if it is not centered left-right. If we accept that the screen can be offset to the left, the rotation axis becomes a very simple hinge, leaving plenty of room to implement the flip axis.

This “ah-ha!” moment of realization, letting the screen be offset, greatly simplified the design. With the side bonus of reliability as simpler designs tend to be more reliable.

 

In the back of my mind, I will continue to dream of a continuous single degree-of-freedom unambiguous movement between open and closed. Maybe I’ll have another “Ah-ha!” moment to make it happen. I’m happy with this as the first draft.

The previous two posts discussed the design reasoning behind the positioning for the power supply unit and the motherboard. Now we get to the most interesting problem: Where do we want to position the screen?

The easiest approach is to line the screen up with the existing components, so I tried that first. A 17″ screen is almost the same length and width as the ATX motherboard plus PSU. But that means the screen would be at a vertical (portrait) orientation. While common for phones and tablets, it is not a typical layout for a desktop PC. (Historical trivia: The Alto by XEROX PARC, recognized to be one of the first computers with a graphical user interface, uses a portrait orientation.)

The easiest solution to that problem is to rotate the whole works 90 degrees. I tried it for a while and the upright screen sitting at table height level was ergonomically poor.

Laptops also have their screens at table height (one of my peeves against laptops) but at least their screens can tilt. I wanted to do even better than merely tilting: I aim for the OSHA ergonomic recommendation raising the top of the screen to eye height.

The wasted volume between the screen and the motherboard was another problem exposed by this prototype. The space looked small in CAD because the CAD model blocked out all the volume allocated by ATX spec. Since the actual motherboard consumed only a fraction of the allocated volume, the real world example had far more wasted space.

I had the idea to solve both issues by raising the screen high to eye level, oriented horizontally, and tilt it into the empty volume. I never got as far as building it. Looking at the CAD layout, it is quite clear that the horizontally-oriented screen sticks out on either side of the case. This makes for a shape awkward to transport and also leaves the screen extremely vulnerable to damage. The screen height was good, but everything else was bad.

Plus, there was one more problem not addressed by any of these ideas: The screen glass surface is exposed while in transit. Laptops fold closed to protect the glass while travelling, but all these designs leave the glass exposed.

It became clear that no single static arrangement will have all of the desired qualities. Similar to a laptop, we will need some kind of mechanism to switch between two states.

• Closed: A compact configuration for easy transport while protecting the screen from damage.
• Open: An ergonomically desirable screen position.

Next post: The mechanism to address these challenges.

To help optimize arrangement of Luggable PC components, I sketched them out in Fusion 360 so I can experiment with layout in CAD space. I was able to find the specification for the ATX motherboard and power supply, which allowed me to use official dimensions. Unfortunately I wasn’t able to do the same for the PCI-Express cards, because I needed to be a member of the PCI SIG to access the official specs. So I measured and guessed dimensions from the specific implementation I have on hand.

Power Supply Unit (PSU)

As the heaviest single component, I wanted the PSU at the bottom so the overall system is not top-heavy. The question is then: which way to orient the PSU? There were two considerations:

1. PSU cooling intake: The standard ATX case layout places the PSU at the top of the case, drawing air from beneath. I can’t do that with the PSU at the bottom since a downward-facing intake would be blocked by the table surface. I tried the upward-facing intake once, in the Mini-ITX “Easel Frame 2.0” design. That turned out to be a bad idea because every time I dropped something (usually a screw) it would fall inside the PSU and I have to retrieve it to avoid short-circuiting the internals.
2. PSU wiring: One side of the PSU takes the standard IEC AC cable. The opposite side is where all the DC wires go to the rest of the components. The decision is then whether to point them front-back or left-right. I didn’t want either of them to point towards the user, so I went with a left-right orientation for the wiring.

Taking care of those two considerations leave two good orientation for the PSU. One with the cooling intake facing front towards the user, or facing away from the user. In the current design, facing backwards allows an unobstructed air path so that’s the preferred position today.

Next post: Positioning the motherboard.