CNC Joinery Notebook: Update 1

CNC Joinery Notebook: Update 1
"Mantis Chair" by Wildetect.
Mantis Chair by Wildetect.

For about ten years, I have been collecting various clever ways of cutting flat stock to design 3D shapes that slot together in space. Back in April, I posted a long, rambling brain-dump from this personal file under the title “CNC Joinery Notebook.” If you pick up a copy of MAKE’s just-released Volume 33, you’ll find a much-polished version of that article on p.59.

Since then, a few more patterns have come to my attention, and I thought the publication of MAKE’s new issue was a good opportunity to share them with you.

Three Basic Approaches

The design vocabulary of CNC panel construction is evolving rapidly. We’ve been tracking the story at least since 2007 when, in MAKE Vol 11, Bruce Sterling used the term “router aesthetics” to describe the emerging pattern language of furniture, and other forms, designed to be assembled from flat parts cut from sheet stock by robot tools.

In his article, Bruce mentions a taxonomy of three rudimentary CNC panel-construction techniques that he attributes to Scott Klinker at the Cranbook Academy of Art: the “stack of sections,” the “grid of sections,” and the “graphic profile” technique. Here an illustration is very helpful:

Three approaches to designing a router-cut lounge chair. From left to right: "stack of sections" (), "grid of sections" (), and "graphic profile" techniques ().
Three approaches to designing a router-cut lounge chair. From left to right: “stack of sections” (Suave Armchair, Julia Krantz), “grid of sections” (Worm Chair, Design Guild), and “graphic profile” techniques (Elephant Lounge Chair by Mediodesign). Note increasingly efficient use of materials from left to right.

The first two “sectioning” techniques are amenable to algorithmic design. Software exists that can model arbitrary shapes as either stacks or grids, with various parameters that can be tweaked, as needed, to fine-tune the plan without a human being having to manually draw any part profiles. The “graphic profile” technique is, IMHO, more interesting from a design perspective, and I think of the material I’m collecting as the “bag of tricks” for doing this type of digital carpentry.

The Inside Corner Problem – A Better Solution

From left to right: "outside divots," "inside divots," and "point divots."
From left to right: “outside divots,” “inside divots,” and “point divots,” aka “overcut corners.”

In writing about what I called the “inside corner problem” with rotary cutting tools, I missed what may be the most elegant solution to the problem. I described “outside divots,” where the tool’s overcut hemicircles are on the outside corners of the joint (and visible when it is assembled), and “inside divots”, where the hemicircles are on the inside corners of the joint (and invisible when it is assembled). For aesthetic reasons, the “inside divot” is obviously preferable, but in practice the range of normal cutting tool diameters and normal stock thicknesses does not often permit it—usually, the bit diameter is too wide.  Thanks to commenter geotek for hipping me to the “point divot” method:

Regarding the corner clearance when using a router: If your CNC router uses Mach3, you can set “corner overcut” (in profile) to true. This will automatically generate a tiny divot in the corner at 45 degrees. This tiny divot is hardly visible, yet allows the clearance needed for square corners.

Here, the overcut parts of the circle are evenly divided between the inside and outside corners of the joint. When the inside divot is not practical (as it often is not), the corner overcut method is probably the optimal solution.

The Oblique Edge Lap

Basic (left) vs. oblique edge laps.
Basic (left) vs. oblique edge laps. Source: 67 Prizewinning Plywood Projects

The rudimentary slotted joint, in which two slotted members are “halved” into one another, is often referred to as an “edge lap.”  In the basic edge lap, the slot’s width and the stock thickness are essentially the same, and the two joined members are disposed at right angles to one another.  In the oblique edge lap, both slots are wider than the thickness of the stock, the resulting edge lap is “loose” or “wobbly,” and the joined members can be disposed at acute or obtuse angles to one another.  In this configuration, the edges of the slots do not lie flush against the surface of the stock, and the joint will be unstable without the presence of at least one other member.  Nonetheless, the oblique lap can be indispensable for breaking out of the “boxy” limitations imposed by using only basic ninety-degree edge laps.

Andy Kem’s “Breakplane” Technique

Andy Kem's Breakplane Furniture

Becky posted about Andy Kem’s striking CNC-cut plywood furniture designs back in 2009. As CNC joinery, Kem’s work is notable for the use of long, large-radius bends to apply tension against a second part indexed by slots and/or tabs to the bent member. In his article, Bruce Sterling describes the graphic profile technique thusly: “[t]abbed and slotted structures of this kind have a Nipponese Superflat look, very angular and planar, perfect for the inhabitants of a pop-up book.” Techniques like Andy Kem’s “Breakplane” designs, and the amazingly flexing and fluid work of Gregg Fleishman, below, are especially interesting landmarks in the evolution of router aesthetics because they break out of the slab-sided “flatland” that characterized earlier design.

Gregg Fleishman’s Captive Spiral Seat Cushion

Gregg Felishman's Nebula II chair, and plan. Note spiral-cut flexure to cushion seat.
Gregg Fleishman’s Nebula II chair, and plan.

No survey of router aesthetics is complete without a mention of Gregg Fleishman. Fleishman, whose “Sculptchair” series dates as far back as 1975, makes extensive use of flexural elements in his designs, and is really leading the pack when it comes to pushing the limits of what router-based digital carpentry can do. Here, I wanted to call special attention to the spiral-cut feature in the seat of his “Nebula II” chair design, which allows it to flex in response to the weight of an occupant without the use of any actual upholstery. Also notable in this design are the large “snake” flexure that forms the chair back, and a pair of load-bearing oblique edge laps in the base.

What Have I Missed?

There are undoubtedly lots of “words” in this vocabulary remaining to be discovered or invented.  Of particular interest to me, recently, are patterns of the type I have been referring to as “inclusions,” which is the use of clever CNC joinery to attach common hardware elements like nuts, bolts, or other fasteners.  If you know of a clever CNC panel design trick that I have not mentioned here or in my first notebook post, please do let me know, and I’ll add it to the growing file.

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I am descended from 5,000 generations of tool-using primates. Also, I went to college and stuff. I am a long-time contributor to MAKE magazine and My work has also appeared in ReadyMade, c't – Magazin für Computertechnik, and The Wall Street Journal.

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