I looked online, and sparkfun (who I love), sells a third hand that takes advantage of this (yay!), but theirs is just a rectangular plate with some threads in it.
We can definitely do better!
Today I'll show you my version of a third hand. It was made as a gift for a good friend of mine (sidenote: flate rate shipping boxes from USPS are awesome! This thing ended up weighing a TON, but it was still cheap to mail it halfway across the country).
The "body" of the 3rd hand here came from some remnant stock I picked up at my local metal supplier (been sitting on the scrap shelf in the lab for at least a year now), and the head and legs (bolts and nut) came from a really awesome scrap yard in Phoenix called Davis Salvage (the only place I know of in town that still lets people dig through scrap).
I built this at Heatsync Labs in Mesa, AZ. If you want to come there and build something like this with us, come over! We have open hours almost every day of the week! (come in and use the lab: as always, it's free).
Step 1: Gather Your Materials
Step 2: Sketch It Out
Step 3: Mill Some Holes for Threading
Consult your tap-and-die set for a drill-size. For size #14-1.5, I needed a 1/2 inch mill.
I plunged a .5inch deep hole into my piece for this. This gave enough room for my hoses to thread into, as well as some "run out" space for the tap (I'll explain in the next step).
Sidenote: here is a great chart of tap:drill sizes: http://www.carbidedepot.com/formulas-tap-metric.ht...
Step 4: Tap the Threads!
I used a little thread cutting oil for this. To do it, get your tap, put it in the hole, and twist! Super simple stuff!
A tip for tapping: your hole needs to be a little bit deeper than what you actually need to thread into. Keep in mind that your tap is tapered; it starts out not very aggressively cutting threads into a hole, and gradually tapers up to your full thread size. Give it a little extra room at the bottom to account for this.
Step 5: Clean Your Materials
No questions here: gotta clean these up.
Wear a respirator when you're grinding. One time I didn't wear a respirator (don't need no respirator!), and the next day, I found some grey slurry coming out of my nose. GROSS! I learned that lesson!
You don't want to breathe old paint and corrosion into your lungs, let a cheap-o respirator catch those particles instead.
I started off by cleaning these up with a bench grinder, but it was taking longer than I wanted, so I switched to a wire brush mated to an angle grinder. The metal brush worked a LOT better, although seeing clouds of old paint and corrosion poofing off of them every time grinder met bold was a solid reminder about that respirator
Step 6: Weld Everything Together
Weld your bits together. On our machine, I used maximum current (there wasn't a setting for 2" square stock, haha).
Step 7: Combine Alligator Clips and Tubes
Super glue worked great. Really, really strong hold on these, I don't think they're coming out any time soon.
Don't get superglue on your hands!
Step 8: Admire Your Work
(Last picture is proof that old and new can still be friends)