3. Power Cards

Initially, I suggest cutting only one of these cards from some of your thickest 3 mm acrylic. Use that card to test fit your circuit boards and test fit in the card slots before cutting the rest of your cards.

I’ve published a laser cutter tips guide along with these instructions. I highly recommend reading the guide and taking all of the tips to heart before attempting to cut this or any of the other plastic parts for this project. Additionally, I share tips on this page regarding how to align the parts used in a lot of this build. If you have questions about how to manually align parts before gluing, refer back to this step.

Step 1. Cut and Prepare Your Parts

Cut your parts, then peel them and wash them before moving on.

Power Card Top - Freshly Cut Parts Power Card Top - Freshly Cut and Peeled Parts

Step 2. Glue the Acrylic Card

I always recommend test-fitting your parts before gluing them. In this case, it’s a simple matter of laying one part on top of the other just to see if it looks like it’s in the right place. Etched guide lines are provided on this part, which should make it easier.

Power Card Top - Gluing Process 1

Also, find some good clean pieces of plastic, metal, or wood to use for right angles. MDF would probably be best, since it tends to absorb the acrylic cement, but doesn’t deform or melt from it. Also, the right angles don’t have to be perfect; close is still a lot better than guessing.

Power Card Top - Gluing Process 2

With your first part in place, apply enough acrylic cement to fill the thin gap between the parts you wish to glue.

Power Card Top - Gluing Process 3

With the glue still fresh, slide your right angle tools up against the piece, with their other surface still firmly on the table. This will line up the parts you’re trying to glue together. Slowly pull your right angle pieces away as soon as it’s lined up. so you don’t accidentally glue them too.

Power Card Top - Gluing Process 4

Also use them to line up the parts in the other direction. Cycle between these steps until you have the parts aligned well. You’ll have to do this carefully and fast, because the cement gets difficult to work very quickly when you’re gluing this much surface area.

Also, bubbles will tend to smear as the parts are moved relative to each other. So try not to move the part a lot unless it’s simply necessary to get it oriented correctly.

Power Card Top - Gluing Process 5

The can of cement I used says it takes 48 hours for the parts to recover 80% strength after gluing. However, we’re working with a lot of contact surface area here and the wet cement soaks into the plastic very quickly. I’ve found that you get a pretty solid bond after about 10 to 20 minutes with this kind of joint.

So, of course, the safest thing to do after you’ve glued two parts and lined them up perfectly would be to set them aside for 10 or 20 minutes of drying. But, if you’re careful not to disturb the part you just glued, you can continue to glue other smaller parts to the bigger piece you were working with, then set it all aside for a few minutes after you have both layers together. The bonds will be reasonably firm after you’ve worked with other parts for a while.

Power Card Top - Gluing Process 6 Power Card Top - Gluing Process 7 Power Card Top - Gluing Process 8

In case any glue seeps out of the joints, use a couple scraps or extra parts to stand the part off the table. Don’t put them directly beneath joints that are still wet. Note that the leftmost joint in the picture below was glued a few minutes prior and the adhesive has already soaked into the plastic, whereas the rightmost joint is still fully wet. As such, having the standoff directly beneath the leftmost joint is OK, where it would not be safe to put one under the rightmost joint. If you put standoffs below wet joints, you’re likely to end up gluing your standoffs to your piece and you’re likely not to notice it until your piece is ruined.

With standoffs in place, set your part aside for the adhesive to soak in a little bit.

Power Card Top - Gluing Process 9

Now, glue the rest of this part. Use the right angles again to line up the two layers of it.

Power Card Top - Gluing Process 10

After the part is lined up well, apply adhesive to tack the “fingers” of it. Adhesive application holes have been provided in this part for easy tacking.

Power Card Top - Gluing Process 11

Wait few minutes for the tacking adhesive to dry.

Power Card Top - Gluing Process 12

Then apply some adhesive to this part of the joint. We wait for the tacking adhesive on the other areas to dry because there’s a risk of bumping the part with the needle when you apply adhesive to this joint, which carries a risk of ruining the alignment of the part.

Power Card Top - Gluing Process 13

Wait a few more minutes for the adhesive to soak in. Then fill the remaining areas of the joint with adhesive as well. Apply the adhesive slowly and steadily to get as much air out of the gap as possible. Remember, once you’ve enclosed an air bubble with adhesive, it’s very difficult to completely remove it.

But, don’t worry too much about air bubbles at this stage. When you’re gluing the cards, consider it to be practice to learn how to leave behind as few air bubbles as possible. When the entire cluster is fully assembled, the air bubbles in the cards are going to be impossible to see. Air bubbles in the outer case parts and sides will be more obvious.

Power Card Top - Gluing Process 14

Now, line up the other half of the part and glue it to the top of this one.

Power Card Top - Gluing Process 15 Power Card Top - Gluing Process 16

With the entire part glued, set it aside and stand it off the work surface. Give it at least a few hours to cure before test fitting parts in it.

Power Card Top - Fully Assembled

Glue the other half of the card using the same techniques I provided above. Then, set it aside for a few hours as well. Let the parts cure well before finally fitting the completed card together.

Step 3. Assemble the Finished Card

When the parts of the card are done, you’ll have an acrylic top, and acrylic tray, and the power card electronics.

Power Card - Fully Assembled Parts Power Card Electronics - Underside Detail

I recommend testing the electronics one last time before sealing them in the card. But it really isn’t that hard to get the card apart again, so that isn’t necessarily required.

Place the electronics in the tray and secure the top with zip ties, as shown.

Power Card - With Zip Ties

Now, trim the zip ties with a pair of wire cutters or nail clippers. I’ve found that nail clippers are great for this, because you can get cut the end of the tie nearly flush with the rest of it.

Power Card - Trimming Zip Ties

Once the ties are trimmed, your power card is finished.

Power Card - Fully Assembled

Test it again one last time to be sure it works, then pack it safely away for later.

Future Development

In the design presented here, 8 of the 10 available dumb USB ports run indirectly off the 12V rail of the main power supply. I designed it this way because the 5V rail of a consumer-grade ATX power supply is typically not adequate for driving 40 Raspberry Pis and 12 hard drives. Running most of the Pis off the 12V rail is effective, but the buck converter modules I used in the design are not very compact. A purpose-built design could be far more space-efficient and, I think, more energy efficient. Also, if I had to do it over again, I would probably just run all the USB ports off the 12V rail.

If I ever redesign this part, I intend to build a custom version with 24+ dumb USB ports per card (with the whole card running off the 12V rail). Each port would have a maximum current capacity of about 2.0-2.5 amps. Cumulative capacity would only be about 45A, for the sake of the power supply, but I would never go near that if most of the devices are still Raspberry Pis.

If I had a 24-port version of the power card, I could power 48 devices from just two power cards. Since the power cards and Pi cards use the same form factor, that would free up 2 card slots for something else. I probably wouldn’t install Raspberry Pis in those slots, since there aren’t enough open network ports. However, I have a wireless router in this thing and there are already several Android sticks with wifi. For example, I could probably add eight GK802s. That would be fun.

2. Circuit Boards 3. Power Cards 4. Raspberry Pi Cards

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