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

[RRLedford]

Originally Posted by buchanan

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Echo that, inquiring minds really want to know - the $$ difference is about 3x.

CORRECTION MADE. IGNORE WARNING BELOW.
WARNING FOR CHECKING POSSIBLE RADIUS VERSUS DIAMETER ERROR ON THE SUGGESTED ITEM (spherical washer)!!!

MOSTLY OFF THE SHELF H/W CAN ALLOW BOLTING YOUR CONDUIT TOGETHER & WITHOUT GUSSET PLATES NEEDED.

Here is how to save even more $$$ and time, esp. if in-house welding not so good or not available.

You can buy mostly off-the-shelf hardware from McMaster Carr to sturdily bolt your pyramid together.

The only special items are the making of (12) 1.25" OD conduit tube inserts, for which we are using 1.25" OD bar stock of 2011 aluminum (~$10/ft @speedy metals.com), which pieces then need to have two holes drilled/tapped @5/16-18 (or M8-1.25) into their OD for the assembly bolts to thread into.

Then ~1-1/4" long flanged button socket cap screws protrude out from the IDs of the horizontal tubes, first passing thru a spherical washer against the tube's ID, then going through a 5/16" hole in backside of the horiz. tube wall, then through a pair of flat-to-flat (curved ends out) nylon tube spacers that will give an ~1/2" tube spacing between the slope tube and the horizontal tube, then thru the slope tube 5/16" hole, and finally they thread into the tapped holes of the custom made aluminum inserts fitted inside & holes aligned to the slope tube.

So to have the inserts work as corner joint anchor points at each level, pairs of 5/16" holes are drilled into the slope tubes at proper angle and elevation. The 1.25" OD aluminum plug inserts (~2" long w/ pair of tapped holes) are then slid up into the slope tube and aligned with that level's pair of tube holes, and the cap screws are fitted through both tubes, along with the rest of the H/W stack to give a reasonably strong joint.

I forgot to mention that small holes are also needed on the outside of the horizontal tube for the allen wrench to fit through while tightening the joints. These can be drilled as a pilot holes in the process of doing the backside larger holes. Since this wrench hole size is so small, it doesn't affect much as far as interacting with the robot differently than a proper pyramid will, which is another reason for choosing the flanged button socket head cap screws (smaller wrench size & curved profile head top).

Keep in mind that the spacing between horiz. conduit tube and sloped tube can be adjusted wider by adding a washer or two between the pair of curved end nylon tube spacers.

Hope this assembly helps teams get their test pyramids built a little more quickly and easily than otherwise might have been possible for them.


Flanged Button Socket head bolts:
Min. length needed is ~1-1/4" and McMaster only goes to 1" on 5/16-18 so we went M8-1.25 x 30mm instead of 5/16". Other places have the longer 5/16" cap screws. Stay with button head (not socket head) or length gets too long to fit easily thru hole from inside of the horiz.tube ends.
http://www.mcmaster.com/#button-head...screws/=kztpuj


Note that the curved end nylon tube spacers are made to match a 1-1/8" OD tube size, but they should crush down and flex enough to still give decent stability of the horiz. tubes.
If we find they wobble too much, with all the money we saved, for plan B we bought a 1.5" dia. ball end mill (3/4" shank, to do some reshaping of the curved ends. We can also use larger dia, or square, nylon bar stock to make some one piece spacers with the two curved cuts in the ends made at the proper skew angle to each other, something that is not an issue with the smaller radius spec., off-the-shelf pair being able to swivel relative to each other.

http://www.mcmaster.com/#unthreaded-spacers/=kzterj


CORRECTED FOR BEST TUBE ID RADIUS MATCH => Male spherical washer with 15mm radius (gives match to 1.2" ID tube) - expensive at $1.70 each & may be cheaper elsewhere, or perhaps not even required.
CORRECTED SPEC. ID- 10.5mm OD-21mm; Note for thin wall conduit use next size up Metric washer with 17mm radius (either size will still work OK with 5/16-18 or M8 cap screws) => http://www.mcmaster.com/#98148A103