Flat laser cut parts can be utilized to create 3d forms by slotting them together. This construction method is most commonly used for boxes.
Because sheet materials have thickness tolerances, getting the fit right can be challenging. Adding some of the following recommended features to your designs will help prevent breakage and improve the fit of your parts (and therefore the overall success of your design).
Please see the Ponoko materials pages for the thickness tolerances on your chosen material under technical specifications. The widest variety of thickness is listed here, but typically sheets are closer to the middle of the range. Keep in mind, that even within a sheet there can be some variation.
Also shown on materials pages are the average kerf values for materials (how much material is burnt away during cutting). Kerf is centered on your cutting lines.
Design tips for interlocking parts
Add nodes - best for wooden materials
Nodes are little bumps located in the slots or on tabs in your parts that are there to help compensate for material thickness variations and the laser kerf. The idea being that the slot is made a little wider than needed and the nodes ensures friction and a tight fit. Nodes work best with wooden materials, because they tend to be soft enough for the nodes to compress down. See more about nodes here.
Add radii - best for plastic materials
For materials such as acrylic (that are hard, but can be brittle when force is applied), adding small radii to the corners of slots reduces the likelihood of these sections cracking under the pressure of use. See more on designing for acrylic here
Add slot allowances - best for plastic materials
When creating slotted designs, adding additional cutting lines around your slots allows for extra give and improves fit on hard materials such as plastics (which can be fairly unforgiving for fit otherwise).
This design was created by Wendy, manager of a local Fab Lab:
The black rectangles in this design were engraved, which resulted in a very satisfying click when the parts went together (the engraved section aligns with the node in the second piece).
Consider your material choice
Materials such as MDF wood and Delrin plastic tend to be more consistent in thickness than say plywood and acrylic plastic, but each material has it's own high and low points when it comes to cost, surface finish and strength. These things need to be weighed against one another to determine the right choice for your project. Please see the Ponoko materials pages for images, descriptions, and for more in depth material properties comparisons MatWeb is a great resource.
Fastening - if possible, use bolts or clips
The tolerances for friction fit parts are incredibly tight and relying on this often leads to parts not fitting well. Gluing is also problematic because you can’t open boxes back up, it can look messy and you have to wait for the glue to dry. Here are two commonly used fastening techniques for laser cut boxes:
Bolts - the easiest most reliable way:
Nuts and bolts are commonly used for laser cut boxes as they are nice and strong. If your box contains electronic parts, this is important. Common box design generators such as makercase can quickly create 'T-slot' boxes made to utilize nuts and bolts.
Clips - the more elegant way:
If you don't like the look of screws, adding little hooks into your design to clip parts together means you don't need any other parts to assemble your box. Here is an example of a box that clips together (assembled version pictured at the top of this article):
This method requires a bit of testing to get the clip size and shape right (the first time we made one of these we broke the clips trying to assemble it). It's worth checking open source files for a template that has already been tested for this type of design. You'll see some of these on thingiverse when you search for laser cut boxes.
For equations on the stress and therefore clip size your chosen material can handle, check out this analysis done on flexible plastic clips.
Prototype and iterate
It can take a few tests to get the a design just right, as described by Adafruit when they made this very clever clip together Pi Box design (this is their 14th version, and it's pretty awesome). Testing a series of variations in parts, nodes and allowances can be invaluable in creating the best version of your product in the fastest time possible.