Wednesday, 10 October 2018

Pizza Box C64 part II

A followup for my rough C64 case prototype. I came across ready-made 3mm thick, 400 x 300 mm smooth chipboards, and I got an urge to re-make the box already.

(For any DIY-ist out there, I'd remind this kind of case might be considered an added fire hazard!)

This time I put more attention to thinking how the cover would work. I would want to dismantle and assemble the box as often I want, without damaging or wearing out the actual connecting parts. So, although wooden screws would work for a while, they might loosen up eventually.

For this I revived an idea of using flanged, threaded insert nuts for connecting the cover. The major difficulty in obtaining these is remembering what they are called, and not every shop sells the flanged variety. So: flanged insert nut. (In Finnish: kaulusmuhvi)

These are made from zamac, apparently a material similar to what was used in those "metal" toy cars of my childhood. Live and learn.

After sketching the parts on paper I actually bothered to plan this in LibreCAD to study the detail in the millimeter scale. The 40mm interior height was decided on the availability of material in 20mm thickness.

The wooden blocks attached to the top and bottom parts are multi-purpose. Firstly, they keep the top positioned snugly by themselves, but they also hold the insert nuts, which help keep the top and bottom together and well aligned.

Also, the wooden blocks give strength to the corners. The parts are glued together, and with these chipboards the adhesion is not very strong - I could simply pull a part and off it comes with a layer of chipboard. But I'm hoping to get away with it as the box is meant to withstand weight, not tearing apart.

The weight

The thin material brought some challenges I didn't have with the previous box. The enormous weight of the 1084S monitor (in relation to the flimsiness of the material I use) caused some worry.

It's no problem at the front but as the C64 board is near the back I can't put large blocks there without obstructing the connectors. So I kept the space for the board rather small.

Looking at the results of a very complex weight simulation (see above) I also ditched the idea of using rubber legs and added a rigid structure below the box.

These are yet another set of parts that are simply glued together, but if I add enough stuff and wooden corner blocks they'll stick.

The weight ought to be held in check by the vertical "walls". At the same time this structure helps keep the circuit board from bending, as it has been screwed in with 3M machine screws to the bottom cardboard.

Not-to-scale sketch of the section from the side (circuit board in green)
My first test with the monitor resulted ok, the cover does not seem to bend, not even at the backside where the support is not so consistent. Nice!

About making stuff

It's not that easy to make really accurate objects at home, with limited tools and no workshop facilities. But it's not impossible with at least some ready-made parts and some organizing.

These boards are kind of fluffy, not very ideal for drilling. On the plus side sawing with this saw was quite easy:

Razorsaw S-290
The blade is well suited for cutting thin boards and mdf. The front has been rounded so it becomes easier to make the first trace, then start deepening it. Using different blocks and rulers as guides, it's possible to cut very accurate slices.

Start pulling along the ruler
It may seem an obvious point to labor, but if I have a ready-made board with guaranteed straightedges, it follows at least two corners remain 90 degrees after a cut.

I cut the box sides out of separate 400 x 300 chipboards, so all outer walls are already mostly machine-cut.

Perhaps surprisingly, positioning the SD2IEC took more effort the second time round. I cut the SD card reader opening with a paper knife as I was afraid of using a drill, and this took time. I tried to be more clever with positioning the device but ended up making mistakes I had to repair. It stays put anyhow.

The separate keyboard is still a bit of a fantasy. I now have the chip (MT8816 switch array) that would make an Arduino PS/2 keyboard adapter possible. It's not the only option, though. Until next time!