A design-pattern for wood as a material.
The primary goal with click-together fabrication is to use the "free" complexity of a CNC machine to create strong, accurate parts that mate together without fasteners. Mating edges can be achieved with 'tenonless' slots, dashed perforations, and even flush glued surfaces aided by locating tabs or holes. Ideally, the parts that come off the CNC machine can be given a quick edge finish on a router (chamfer or fillet) and then assembled without special tools or fasteners. Clamping provisions for gluing can even be built into the design.
However, the promise of such ease is defeated by several problems:The following document contains my notes on using this design pattern to create designs and methods to avoid associated problems when manufacturing.
TODO
A tremendous amount of time can be absorbed correcting imperfections after the CNC operation on a sheet of material. Quality tooling and material is the only way to avoid this. See cutting tactics with sheet for a deeper discussion of doing this right. Most notably for edge finish quality, a proper compression bit should be used at sufficiently aggressive feeds and speeds.
Delamination (between sheet layers) and post-operation surface finish both take post-CNC correction time or even cause scrap parts. Using a higher quality sheet is the only real solution to this. I've found that the money saved using construction grade plywood (like 'plytanium') is simply not worth the time it takes to make it behave.
Without a vacuum table, something must hold the interior pieces of the work while the mill does it's final pass. Traditionally, either tabs or a thick 'film' (e.g. the remainder at the bottom of a non-thru slot) is used to achieve this.
For this design pattern, film should be minimized or eliminated, as it prevents mating of tabs and slots. Using the bottom-zero method (here) with only an offset equal to machine Z-tolerance achieves this. Tabs, therefore, do all the workholding and can not be avoided.
Tabs must be considered during the design phase. They should be placed on long, straight lines that are reachable by saw afterwards. A handheld jigsaw can break parts out of the mold. and then all parts should ultimately be run down a tablesaw to cleanly remove the tab remainders.
The simplest way of removing tabs is to make the tabs wide, but sufficiently shallow that 1/8" of the slot is always free of tab. That slot edge above / below the tab becomes a mating surface that can reference off a flat plate on a tablesaw.
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If aesthetics allow, a highly repeatable way to remove tabs is to leave a couple thru-holes at an offset a fixed distance from the tab cut plane. A jig can be constructed which slides along the tablesaw with the work located via pins through those thru-holes.
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The desire of wood to warp especially effects thinner sheets and will cause bowing or misalignment in individual pieces during the assembly and gluing stage. Glue requires solid contact for strength, so this can be an issue. Fortunately, assembly speed can be achieved by building in simple thru-holes at key points along a mating edge. This especially matters for dashed-slot-and-tab edges.
This concept is difficult to explain without good images, which I have not yet produced. I'm leaving these notes here as a reminder to come back and elaborate.