The design
Coming up with the design requires pre-planning. You can’t just sit down in front of Inkscape and get creative. The first thing to consider was how the box would fit together. Would I cut screw holes into the corners and if so, what would they screw into? In the end I went for a fixing strategy of finger joints and t-slots.
In the diagram the interleaved steps are the finger joints and the deep recessed slots are for captive screws. It is possible to make a case using finger joints alone but that won’t be enough to hold the sides of the case together. You’d have to use some form of glue or cement to fix them together and that might be messy and could leave you with an unserviceable case that you could never open again.
T-slots are neat idea that works by having a slot designed to accept a screw with a captive nut at the bottom. There’s a good discussion over on the Ponoko blog about joints in general that features some images of t-slots.
When the screw is tightened the nut (which cannot rotate) pulls the joint taught. Insert these at strategic points around your case and you’ll have a design that should hold together snugly and can be opened later for servicing if needs be.
Now I need to decide on a material. Razorlab offer quite a wide range of materials and I went for clear acrylic. Acrylic is a stiff plastic that is prone to cracking if put under stress so I decided to go for the 5mm thick variety instead of the thinner 3mm thickness that you see in many designs. This should help the side where the power cords are inserted to be more resilient to the insertion force of the plug.
After getting my ruler out and measuring how large I thought the box would need to be it was time to get something down into Inkscape. I was not looking forward to manually drawing all the finger joints and t-slots and thankfully I didn’t have to because there’s a highly useful automated case designer at the makercase website.
That website saved me hours of work. I entered my dimensions and then it asked me the details about how thick the lines in the SVG output should be and what colour they should have and critically it takes into consideration kerf.
You create the lines in your design using an arbitrary line thickness specified by your online service but in the real, physical world the laser beam has a measurable diameter and cuts by melting through the material. Kerf is an engineering term for the width of the slot created by the laser as it cuts. The beam width of the laser used by Razorlab is 0.2mm and so I stuck a wet finger in the air, assumed that lasers were not going to melt beyond the beam width and entered 0.2mm as the kerf size. If I get this wrong then the finger joints will either be too tight or too loose, neither of which would be a good result.
The SVG output from makercase loaded straight into Inkscape and I was able to transfer the design to the template provided by Razorlab.
Razorlab offer three templates of increasing size and mine fits into the P2 size. Your design is charged first for the size of the template you used and then for the amount of work that the laser has to do so it makes good financial sense to squeeze your design into the smallest template you can and then to reduce the amount of cutting that the laser has to do to the lowest possible.
One technique for reducing cutting is to place parallel straight lines back-to-back to make then into one line and you can see that makercase has done this for me with the finger slots which is a really nice touch. You can see that there’s an empty space in the top-left of my design. I didn’t actually leave this space empty when I had the design made, I added a small design for another project that I’ll blog about in the future. It pays to use all the material you’re charged for.
Here’s a tip for new Inkscape users. Razorlab require cut lines to be 0.01mm thick and blue (RGB 0,0,255). These lines are so thin that they’re not visible when you first open my design. Select View -> Display Mode -> Outline and everything will come into sharp focus. This is the mode that I work in permanently in Inkscape. Razorlab want the lines to be blue because the laser can also do engraving and those lines should be coloured black in the design. There are no engraved parts in my design.
The Ponoko blog that I referred to earlier shows some alarming images of cracks that have formed in the corner of a t-slot joint. I really want to avoid this so in my Inkscape design I’ve modified the corners where the screws will apply pressure to be slightly rounded.
This should serve to distribute any stress caused by the nut being pulled up against the edge as the screw is tightened. I’ve also resolved to use acrylic screws and nuts instead of metal.
Now I need to measure up the holes that will need to be made to fix the various sockets, screws and fan to the case. First is the mains power inlet.
This is a typical IEC ‘kettle plug’ 10A connector and it comes with an an-off switch and a built-in fuse. The fuse is nice, but overkill here in the UK where we have sensible polarised wall plugs that feature a built-in fuse across the live terminal. Next is the oven outlet connector
This is a simpler, unswitched version of the same IEC 10A kettle connector. Now the rotary encoder.
