If you've ever worked on a project where you needed another person to hold something against a ceiling while you screw it in, and that person is not going to be available when you have time to do the job, you can make a "3rd hand" out of two (2) 2x4's, some scrap 3/4" pine, two (2) four inch long x 5/16" carriage bolts, four (4) washers, and two star knobs available from Rockler.com (part # 23812).
For less than $10.00, you can put the whole thing together in about 30 minutes.
I made it at TechShop! I have a router table / saw at my house, but my shop is cramped and it's tough to deal with 8' 2x4s. Doing it at TechShop made it a breeze to put this together!
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Step 1: Route Two 3/4" Slots in Two (2) 2x4s
Using a 3/4" router bit, adjust it so that it cuts a "dado" type slot about 3/4" into a 2x4. Use the fence on a router table to adjust the location of the cut so that the slot is straight.
Cut one of the 2x4s to a length of 3 feet.
Route two slots along the entire length of the 3' 2x4.
On the other 2x4, route two slots about 3 feet from the end.
Step 2: Route a 3/8" Slot Down the Middle of the 3' 2x4
Use a 3/8" router bit to route a through slot down the center of the 3' long 2x4. Have the slot start and stop about 3" from each end. This will enable the 3' long 2x4 to slide up and down.
Step 3: Cut "keys", Drill Holes, and Screw on the Top and Bottom.
Cut two 11/16 x 1 3/8 pieces of pine that are 3' long. Make sure that they fit in the slots and that the two 2x4s slide easily.
Drill two holes in the 8' 2x4 approximately 6" and 30" from the top. Make sure that the holes align with the 3/8" slot that was routed.
Insert the carriage bolts and place a washer on the other end, then attach the star knobs
Cut two 18" long pieces of 2x4 and affix one to the bottom of the 8' 2x4 and the other to the top of the 3' 2x4
Step 4: Use the 3rd Hand to Hold Items to Ceilings
You can now use the 3rd hand to hold items that need to be screwed to the ceilings. Just tighten the two knobs and you can now do work that would require two people.