2D design for base piece
AutoCAD DXF - 27.29 kB - 03/25/2016 at 21:45
2D design for top piece
AutoCAD DXF - 159.76 kB - 03/25/2016 at 21:45
First things first - I don't have M2.5 bolts, nor an M2.5 tap, so I always drill my Pi mounting holes to 3 mm and use M3 bolts. If that doesn't suit you, you'll need to modify the designs a bit to fit M2.5 instead of M3.
Second things second, before embarking on trying to make one of these cases, you will need to run some calibration cuts on your laser using your material of choice to determine the power settings required to get the right cut depths. The depth of the HDMI and USB port cut-outs is reasonably crucial - if you go too deep you weaken the plastic too much, and the engraved letters destroy the area around the HDMI socket.
Thirdly, work carefully! For the top piece, you need to cut both sides to fairly precise depths and maintain registration between them. The concept is quite straightforward - but for some reason executing on it took me a few attempts!
(OK so not all of these were failures on my part - some were prototypes or calibration runs)
Finally, sorry for the crappy pictures - my ageing Nexus 4 camera isn't up to much
We'll start with the base piece because it's easier. Download the "gadget-case_base.dxf" file, and adjust it for the kerf of your laser. In the DXF file, the holes are exactly 3 mm diameter. We're going to tap for M3, so you need to reduce the diameter a smidgen to account for the laser's cut width.
Also, it makes for a slightly nicer fit if you expand the outer edge of the base to account for laser kerf too, otherwise it comes out marginally smaller than the PCB and looks a bit ugly.
The green hatched areas need to be engraved to around 1.5 mm depth. Annoyingly the HDMI socket has some tiny pins protruding from underneath the board, which is what the four small holes are for.
We only need to engrave one side here, so you can do everything in a single pass.
The top piece is a bit more tricky, if you want to include the pinout text, because you need to laser both sides of the plastic whilst keeping registration. If you don't want the pinout, then you can ignore the 'jig' and 'text' layers and jump straight to engraving the underside.
The DXF file is a "top view", looking down through the top of the case. This means you need to horizontally mirror when you do the engrave for the underside
As with the base, adjust the outline for laser kerf. This time we don't want to adjust the holes, because we want them to accept M3 bolts easily.
The pinout text should be engraved in the surface as lightly as possible to avoid cutting right through when the connectors are engraved out on the underside
To achieve registration, I blu-tacked the acrylic sheet to the laser bed, and added the yellow 'jig' line. If you cut layers '0' (white), 'text' (magenta) and 'jig' (yellow) in your first pass, then you can carefully remove the piece and flip it over and it will sit back in the correct place in the sheet. Assuming you can make the laser start from the same position, you can then get the underside engraved in the correct place.
For the underside, remember to horizontally mirror, and then you can cut the port holes - 'usb' (orange) should be engraved to around 1.1 mm deep and 'hdmi' (cyan) to 1.5 mm deep.