Introduction: Oblique Tenon Stool
Often times simplicity at the surface belies a complexity or rigor not immediately apparent - the Oblique Tenon Stool is one such instance of deceiving appearances. At a glance, there is nothing special regarding this unassuming piece. Upon closer investigation, the construction of the stool begins to reveal the carefully resolved connections which enable it’s novel yet intuitive assembly.
In fact, the detailing of the joints is perhaps only possible with the capabilities of a 5-axis water-jet cutter. The intersection between each leg and the neighboring brace results in an oblique condition which is reconciled with a through-tenon joint. Five-axis cutting allows for an integral shoulder and a flush detail all from one single operation. Furthermore, the pin which ties the joint together (in this case a flat head cap screw) intersects adjacent parts at another oblique angle (limited by the maximum tilt of the water-jet head) and results in an elliptical (or conic-section) countersink - an operation that would be difficult if not impossible with traditional means.
While this project references traditional joinery details, it builds on past knowledge by adapting the techniques specifically for contemporary fabrication methods.
Step 1: Overview
¼” Aluminum Plate
5/16-18 Flat head cap screws and allen nuts
OMAX Waterjet cutter
Random orbit sander Tap
Initially my inclination for this piece was to make a really quickly designed, easy to fabricate, and fairly lightweight project. I knew I was was going to use aluminum plate - thick enough to be structural - and I knew I wanted to cut the aluminum on the waterjet here at pier 9. I figured I'd quickly draw up some profiles, cut em and weld em and have myself a small practical stool in no time....however my priorities began to shift as I explored the five axis capabilities of our OMAX....
Step 2: Design
I began by sketching 2D profiles in Rhino (this could be done in virtually any decent modeling program - Inventor, Solidworks, etc.) As I said, I thought initially this would be a really quick exercise, so the design focused on some some really straightforward tab and slot connections with the seat, legs and a separate support 'ring' - all the pieces could be cut as simple profiles. I was almost ready to cut them out...however around this same time I discovered (from another artist in residence) that the 5-axis capabilities of our waterjet were a lot more accessible than I assumed....
Step 3: Redesign
After a few simple cuts to test the head tilt and tolerance for an obliquely angled joint, my design strategy for the stool changed. I was now really excited about the potential to create some really interesting joints/connections. Rather than being constrained to profile cutting, I could resolve more complex intersections between pieces. I looked to precedents, such as Japnese wood joinery with many variations on joint types.
Step 4: Modeling
Eventually through trial and error of manual modeling I came to a configuration which seemed simple yet sophisticated in terms of overall form as well as details. An oblique through tenon at the leg / brace intersection 'pinned' together with a flat head cap screw and a through tenon at the leg/seat with flat head cap screw / allen nut to tension the joint both seemed like clean ways to assemble the stool.
Step 5: Troubleshooting
Testing the exported.step files in Intellicam however revealed that some of the cutting angles were too steep for the head tilt! This was great - the design was pushing the capabilities of the machine. On further investigation, I found that the max tilt angle was 59.9 degrees - the intersection between the cap screw and the leg brace produced a 60 degree cut (.1 degrees too steep)!!
One solution would have been to adjust the angle of the 'pin' cap screw by the very small (perhaps unnoticeable) amount. This could however come across as a mistake. Instead i decided to express the angle further and by doing so leverage the unique capabilities of a 5-axis waterjet - taking what could normally be done by hand (a countersink into a surface at a perpendicular 90 degree angle) and tilting the angle of that countersink to the point where it would be virtually impossible to do manually. I feel the resultant elliptical cutout to be a beautiful expression of machining process.
Step 6: Machining
With the design and detailing resolved, it was time to cut the parts on the OMAX. For creating the 5-axis toolpaths, Intellicam takes .step files, but unfortunately it doesn't seem to like processing more than one part at once - this meant I had to cut one piece at a time, not a huge deal, but was definitely not as efficient as nesting the parts and cutting them at once....otherwise the process was pretty quick and straightforward.
Step 7: Finishing
If your settings are correct, the waterjet leaves perfectly square, if not slightly sharp edges. I eased all the edges first by hand with various files. I tapped the steeply angled holes by hand while holding the workpiece in a vise. Then on the seat part, I used a small edge trimmer with a 1/16" round over bit to radius the edges for a softer feel. Lastly I used a random orbital sander with 220 grit to give the surfaces a consistent finish prep before taking the pieces to the anodizer.
I chose a 'bronze' anodized finish to highlight the brass hardware I planned to use. After getting the parts back, assembly took a minute or two.
Step 8: Assembly
The assembly of this stool is incredibly intuitive and straightforward - only one Allen wrench needed! I'll the the images speak for themselves here....
Step 9: Conclusions
This was a great quick project and I learned a lot about working within the capabilities of the 5-axis OMAX waterjet - it was also the first time I've had any parts anodized and I was pleased with the finish. If I move forward and decide to develop new versions, I'd probably make certain aspects parametric for easily exploring new design iterations.