So I've gone through about 3 third hands from the local electronics store (whose name rhymes with "The Force"). And each has had alligator clips come off, lose grip in the arms and other assorted annoyances. So I set out to design my own and I immediately found this epic instructable by rstraugh.
I do not readily have access to coolant tubes where I am and I am not quite yet willing to order them so I thought about what the basic principle of the coolant tube was that makes it so good for this and I figured out, it was ball and socket joints. So I thought what do I have readily that I can use as ball and socket joints? Then it hit me.
I HAVE A BUNCH OF LEGO BALL AND SOCKET PARTS
They were popular in bionicle lego sets and are still used in other sets. If you don't have any available then you may be able to find the odd set that's an ok price with some of them. If not then try ordering the coolant tubes because they're probably cheaper and better than ordering the lego parts. The major plus with the lego though is it does make customization of the arms very easy.
I'll show you how I built my 3rd hand but I'll also make a list of alternative things at the end
Step 1: Materials and Tools
-Assorted Lego ball and cup joints
-Small plastic clamps
-4'X4' plate of aluminum
-Double sided foam tape
-4mm screws (or closest imperial/standard measurement)
-Measuring tools( I used a combination square and a fine point sharpie marker)
5/32" or 4mm drill bit.
1/8 or 3mm drill bit.
3/32 or metric equivalent drill bit.
-Hammer and center punch
-M3 or #4 threading tap
Step 2: Layout the Baseplate
Since I'm doing a co-op at a local metal shop before going off to university for engineering, I have some access to an excellent scrap pile and also a hydraulic shear. So I managed to get a very accurately cut 4 inch by 4 inch square of aluminum. HOWEVER It doesn't need to be that fancy at all. It could be a scrap piece of plywood or steel. As long as it is reasonably weighted then it should work.
I measured half the width of my base plate and then made a mark with my center punch about half an inch in. Do this again for the other edge.
You can use a sharp screw, or even the sharp end of a thread tap as a center punch (only use an old or broken one, its inadvisable to go breaking your taps).
Step 3: Drill the Parts and Tap the Baseplate
So I set the baseplate in the drill press and used a 3/32 drill bit to drill out the two marked holes (3/32 works for both M3 (3 millimeter) and #4 taps.
I used a 5/32" drill to drill holes into the clamps so I could attach them to the Lego pieces.
And I used a 1/8 drill bit to ream the cross shaped hole on the Lego piece to make the screw fit more comfortably as to not snap the Lego.
Then tap the holes you drilled in the base plate with your #4 or M3 tap.
Step 4: CLEAN UP CLEAN UP!
To prevent yourself from slicing your meaty parts on the base plate it is highly advised you clean up the edges and holes to make them a bit easier on the skin.
I used a file to clean up the edges of the holes and edges of the base plate. No more (or heavily reduced) chance of me cutting my meaty bits now!
Check over other parts like the clamps for any areas that need cleaning.
Step 5: Assemble It, Assemble It All!
Time to assemble your bits. Start by attaching your clamps to the blocks with your 4mm or 5/32 screws. My screws were salvaged from a space heater so luckily they had nice flat bottoms so I wouldn't prick myself.
Once you've screwed on the clamps, move on to securing the arms to the base. Luckily #4 or 3mm bolts should readily pass through your average cross shaped connector on Lego technic pieces. so simply screw them into the tapped holes and tighten them so the arms are snug.
Step 6: Finishing Touches
At this time your third hand is effectively finished however it may be a little wobbly. so I decided to give the base plate a little bit of padding and since I lacked any kind of rubber feet or felt pads so my compromise was to use double sided foam tape.
I cut the tape into smaller squares, about 20mmX10mm squares, and unmasked only one side. I stuck them on the base plate and left the other side of the tape masked. This made the base much more stable. I also added a small weight, which in my case was the concrete chunk at the base of a balloon bundle, this helps the third hand from moving while you work.
Step 7: Test It All Out!
And that is that. I clamped two wires in and lined them up with relative ease. Since I've made it, it has been an excellent help in any of my electrical work. Plus you can also modify it with different Lego parts to suit your needs. Show me what you've put on!