This is the joint! This is the jam...For building structural truss systems both simple and complex. It is made from two repeated sheet metal parts and off the shelf hardware. It has eight ball joints that make an octahedron for finding compound angles for curved/doubled curved surfaces with a rigid frame. This joint does all of the hard work so assembly is fast and easy!!
In short: design a truss system. make the joints. cut your rods. screw it all together by hand
Step 1: The Joint: Cutting the Sheet Metal Parts
I used an Omax Waterjet to cut these parts with 20 gage stainless steel. As shown in the diagram, the two parts (A & B) make one quadrant and 16 parts make a whole joint (8 of A and 8 of B). A couple features are optional due to what you are making. For example, the holes for tension cables can be used to anchor the system or add other things such as a 'net' within panels or your space frame surface but ...it can also be left out for faster cutting.
The wire frame sketch is a design I made to test them out. The scale is for furniture. It is a truss made between two surfaces, one flat plane on the bottom and a double curved surface on the top. This truss has 25 joints: Part A x 200 and Part B x 200.
Once you decide on your frame, then you can count how many parts are needed.
Step 2: The Joint: Bending the Sheet Metal Parts
Again, the diagram shows dashed lines where the bends need to happen. Part A has two 90 degree bends.Part B has three 125 or 55 degree bends. Luckily, I had access to a hydraulic sheet metal bender for making many parts.
It isn't a factory until you start throwing parts into a box one by one. Clink!
When I started, I was using a vice, small aluminum block and a rubber mallet. The 3D printed jig above was great for checking the angles. The long cut holes act as a perforation to make it easy to bend the in a vice. But if you can get your hands on a proper bender, then they are not necessary but do help to line things up.
Step 3: The Joint: Getting the Rest of the Parts
The numbers depend on your design.
Constant: Each joint always needs 8 x Part A, 8 x Part B and 24 rivets.
Variable: The number of ball joints vary from 3 to 8 in a joint and the same number of threaded rods. Once you have your design ready, count how many active ball joints you need. (The rest will be blanks as shown above). So in other words, each rod with have two drop in anchors, two threaded rods, and two ball knobs.
-1/8" diameter Pop Rivets: 24 per joint
-Ball Knobs (McMaster Part No. 61095K23): 3-8 per joint or Two per rod
-Threaded Rod 1/4"-20 threading (cut to 1 1/2"): 3-8 per joint or Two per rod
-Also grind the ends after cutting to clean them up! Hit the grinding belt and give it a twist...
-3/8" Lipped Drop in Anchors (1/4"-20 threading): 3-8 per joint or Two per rod
Step 4: The Joint: Assemby
Attach the threaded rods to the ball knobs. Rest them within Part A and Part B to make a socket. This envelope allows for a 90 degree pivot and 360 degree swivel!
Make two of these quadrants and start popping rivets. Keep adding another quadrant until you have four (One half of the entire joint). Make another half and rivet the two halves together.
When riveting, keep your blank quadrants adjacent to one another.
To stay organized, I made a list of the different joints needed. Some had 3, 5, 6, 7, and 8 balls and so I labeled them with tape / sharpie matched it with layout drawings.
Step 5: Cutting and Preparing the Rods
Make a cut list of all your rods... label and group each "pyramid" within the truss and keep them organized. I taped them together and labeled with sharpie :)
From the wire frame design, I had to subtract 1 7/8" from each side of a rod since the ball joints are spaced away from the true center of the joint. Then each rod is 3 3/4" shorter that the actual length in the design. It also gives the rod clearance from the sheet metal.
I ordered aluminum rods with an inner diameter to match the drop in anchors. The inner diameter of tubing can vary a bit. So, if you have to, you can drill out the inside of the rods, in this case, with a 3/8" drill bit. The anchors also expand when set with a bolt or rod, so you can get it locked in place.
Hammer your drop in anchors into the ends of each rod with a rubber mallet.
Step 6: Assembly
Now that your joints and rods are ready, you can screw it all together! I found it a little tedious to tighten the threading this way... because the they are all in the same direction. Once you screw in one side of the rod, the second is tricky because it can start to unscrew the first side. But it works if you fix one side and turn the whole assembly into the second joint.
So for another iteration, I want to tap the rods and drop the threaded anchors. This way both ends of the rods can have threading in two directions and the rod becomes a turn buckle. The tricky part is finding balls with reverse threading or just getting balls and taping them with the right threads.
At this point, I am still simplifying the joint for less parts! yikes... But this version has proven to be structural and fun to play with. You might wonder if the ball joints are fixed: For the structure that I made, it is rigid even with 'loose' ball joints. But there is a hole for putting a set screw to hold the ball in place after it is assembled. (Shown in the diagram at step 2)
Enjoy! I would love any thoughts, comments, improvements, thanks! :)