pic: Just finished our pyramid. Conduit seems to work fine.

[RRLedford]

Originally Posted by Jeffrafa

Dick,

Have you built a pyramid bolted together this way? It sounds appealing, but I'm curious if stability is an issue at all, like it is with FIRST's low cost design.

I had noticed comments about the low cost one twisting here, and I was looking for alternate construction methods before we sink money into materials.

For us, welding is not ideal as we would not be able to get it into or out of the room we need to use it in.

First off, we are using the 1.51" OD by nominal 1-3/8" ID tube that is named "1-1/4" conduit" at Home Depot.

We are still machining the threaded aluminum plugs that go inside the conduit, but the initial single joint mock up with actual H/W stacked pieces shows proper contact with tube IDs and ODs to ensure no tube wall bending of the edge pipes will hppen when the horizontal rung pipes are loaded with single robot weight.
BTW, watch out for getting any dings in the conduit used for the edge tubes, as they make inserting the plugs in the edge tube nearly impossible if they are slip fit.

The dual threaded insert plugs' OD spec is 1.368" +.000/-.005 for a snug fit to the ID of the conduit that Home Depot sold (Allied brand). We know the two holes must be drilled in the edge tubes at 104.5 degrees apart going around the edge tube's OD to match up with the side to side dihedral angel, but we are still trying to calculate at how much of an angle --- skewed away from being parallel to a plane crossing the pipe plug at 90 degrees --- will these holes need the hole needs to be oriented?

Also, the rung pipes' bolt holes need to be angled downward ~22 degrees from horizontal, so that they pass through the imaginary tangent point where the edge & rung tubes would touch (without the spacers), and the angle of the rung bolt holes are thus offset down, skewed away from being at a right angle to the vertical 45 degree cuts of the pipes' mitered ends.

We are deliberately leaving extra edge pipe length at BOTH the lower and upper ends of the edge pipes until the equally spaced rung holes are all drilled. Once that is accomplished, we will then fine tune the edge tubes' 60 degree miter cuts at their bottom & top ends to get the first rung's top side elevation right at 30 inches and goal elevation at its proper height.

We drilled a 3/8" hole down the centerline of the plugs so that when the tapping drill and the tap cut in from the OD they both hit this 3/8" center hole, rather than having both holes intesecting each other asymmetrically & possibly breaking the tap.

Will update with PICs in a day or two. If anyone can CAD up the angles of these bolt hole axes relative to the pyramid pipe axes and the insert plug shape, it would be quite helpful. It would have made so much sense if these insert plugs could have been made available for purchase at kickoff day, and teams could have quickly built a sturdy metal pyramid that closely matched the welded design. The PVC design suggested by FIRST was a very bad joke to play on us.

Here is a graphic of my assembly joint concept, but angles are not exact. It is clear that the neon green nylon spacers will be thicker than the welded version's gusset plate, so if you are planning a gripper design that engages right at this spot near the edge tube, they will need to be made less thick by filing, or by making a notch in your gripper there, interference with it can be eliminated.



-Dick Ledford