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.

 

Luggable PC Project Complete!

The Luggable PC project page on Hackaday.io has been fully documented for anybody to build their own home-built 3D-printed computer chassis. All the components required for assembly have been listed, and all the steps of assembly documented with pictures taken at each and every step.

I expect to continue to make small tweaks to the design, improving little things here and there, but the machine is usable enough that I should stop tinkering with it and actually start using it. This means no new versions rebuilt from scratch for the foreseeable future. But if inspiration strikes, there will be!

When I’ve taken my Luggable PC to various maker events in the local area I’ve received generally positive reception and appreciation. Now I wait to see if anybody actually takes me up on the information compiled and build their own.

LuggablePCAssembly640

Luggable PC Drive Bay Revisions

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.

IMG_20170228_090310 - Copy

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:

IMG_20170228_091421 - Copy

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.

DriveBayV3

 

Luggable PC Feet Design Considerations

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:

IMG_20170228_173547 - Copy

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.

IMG_20170228_174620 - Copy

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.

FeetV3

A Tale of Three Corners: Design Evolution

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.

Corner 1 B
Two of the rods are co-planar with the print layer. (Rod pointing left, and rod pointing down.) The nuts fastening the third rod (Rod pointing away) also exerts a clamping force on the layers.
Corner 1 A
Same hinge viewed from a different angle.

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.

CornerCura
The corner laid flat on the print bed for slicing in Cura.
Corner 2 B
Results in a corner that is equally strong across 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!

Corner 2 A
Angle showing the problem with this design – it consumes a lot of space inside the enclosed volume.

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.)

Corner 3 BCorner 3 A

Luggable PC Screen Hinge

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.

front-open
The open in-use configuration, with the screen offset to the left instead of centered

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.

front-closed
The closed travel configuration

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.

 

closelid
Demonstrating the open-to-closed transition. (Animated GIF by Shulie)

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.

Luggable PC Screen Layout: Challenges

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.)

threadrodboxisoThe 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.

spaceThe 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.

screenwingsI 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.

Luggable PC Motherboard Layout

a360mobopsu2The previous post described how I decided to position the PSU (Power Supply Unit). Once the position was decided, the next task is to determine the motherboard position.

The first challenge is my desire to accept a full-sized ATX motherboard. Full-sized boards are the easiest to work on and has the best feature set. They also have highest sales volume, which usually mean less expensive. I knew my project would be easier with a smaller microATX or Mini-ITX motherboard, but I wanted to accept full-size.

However, accepting the full size board doesn’t necessarily mean I intend to use all the expansion slots. In fact, I am happy to block the majority of them, leaving just the primary PCI-Express slot available to the GPU.

selectcardsThe GPU itself is the next challenge. The primary slot is close to the CPU, which means it is going to stick up in the middle of the board, making the whole assembly awkward to fit. Again, I have an escape if I want it: there are PCIe extension ribbons available for purchase that allows more positioning flexibility for the GPU. They range from $89 well-regarded units from Digi-Key to $7 roll-of-the-dice units via mystery retailers on Amazon. I want to make this idea work without use of an extension, and avoid the variable that introduces to the system.

While researching the layout, I learned the primary slot is not in the same position across all motherboards, adding to the challenge. While most boards position them in the slot closest to the CPU (all of the Mini-ITX boards have to by necessity) some of the boards place it in the second position. And since high-powered GPUs are two slots wide, that means I need to allow for three expansion slots worth of space.

selectcomponentsThe GPU in the middle of the board leaves two rectangular volumes on either side: Both volume are candidates for use. One volume sits over the remaining expansion slots, and the other volume sits over the CPU.

The volume over the expansion slots are predictable. ATX spec restricts height of motherboard components in order to maintain clearance for expansion cards. If I’m OK with the absence of cards, that entire volume can be reclaimed.

In contrast, the volume over the CPU is less predictable. While the ATX spec allocated volume to CPU and accessories (most significantly, the CPU cooler) that volume is highly variable. Stock CPU coolers typically take much less volume than allocated, and many fancy CPU coolers exceed the volume.

Given those two choices, it was an easy choice to snug the PSU up against the motherboard in the volume allocated to expansion cards that won’t be there.

The last factor in positioning the motherboard is which direction I wanted the ports to be accessed. Pointing down is inconvenient to access. Pointing up makes ports vulnerable to damage from dropped items. So that leaves pointing left or right. Since the PSU power cable port is already on the right, I decided to face all the ports that way as well so everything the user needs to plug in is facing the same way.

All of the above considerations resulted in the PSU+motherboard layout I used.

Next post: Positioning the screen.

Luggable PC PSU Layout

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.

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Luggable PC Gets Fancy Screen

closelidThe latest iteration of the home built luggable computer gets a fancy rotating screen to protect the screen while in transit and hold the screen up while in use.

The time pressure of making it ready for show-and-tell at the February Hackaday LA meet meant I hadn’t been documenting my lessons learned here.

Which is a shame, because there were quite a few 3D printing lessons learned while building this thing. I briefly mentioned a few of them on the project log update up on Hackaday.io but I intend to find the time to expand on those ideas as future posts on this blog. I just hope I can get them all written down before I forget.