To make movies you usually need to move smoothly your camera on a flat surface, like a table or the floor. There are many commercial tripod dollies but you can easily make your own with a few wood boards and three wheels.
Step 1: The Design
To build this dolly you can choose different materials and dimensions. I made a small one, but you can adapt this project to build a big dolly from pallet wood boards.
Here you can see how I decided to cut an mdf (medium density fibreboard) board to obtain the pieces of the right size. Since I already had a 70 cm long board I made a little dolly, which is good for a little tripod or for a classic tripod with un-extended legs.
Step 2: Recommended Improvement
Anyway I noticed that I had some problems in finding three wheels with small plates. Indeed as you can see in next steps, with my 3 cm wide wood pads I barely found fitting wheels. Then I suggest you to keep at least the dimensions indicated in this step schematic.
Step 3: The Three Arms
When you have cut the three wood arms you can drill an hole on one extremity of each board, and smooth the edges with sandpaper.
Step 4: Glue
Now with a proper glue attach the small pads on the longer boards. As you can see you need some space between them so that you can fold the dolly arms. To make that space you can smooth the extremity of each arm after gluing the pads, so to obtain a very smooth and a bit rounded surface.
Step 5: Drill Foot Seats
I drilled three big hole where to fit the tripod foot. Then using a spherical drill bit and then a spherical abrasive bit, I rounded them, and I finished the smoothing with a small piece of sand paper.
Step 6: The Pivot
The pivot is a simple bolt with nut and washers. When you are sure that the dolly is finished you can glue the nut so that it will not un-screw opening and closing the dolly arms.
Step 7: Wheels
Choosing the wheels can be fundamental to determine the behavior of the dolly and the tripod movement.
I had some furniture wheels, but they had a large base plate, and they didn't fit on my 3x4cm pads. So I found different wheels, bus some needed a cavity in the wood, and I have one thin mdf arm where there is already a cavity (on the opposite side) for the tripod feet. At last there are some nice spherical wheels with a long screw, but in this case too I have not enough depth to screw them.
Finally I found the wheels in the first picture, which barely fit on the three pads. Unfortunately I didn't check that all three wheels were identical, and as you can notice, one of them is specular, and it fits a bit awkward with closed dolly. I will substitute it.
Step 8: Attach Wheels
Set up four screws for each wheel, then drill four holes in the boards, and attach the wheels in place.
Step 9: Completed
As you can see now the dolly is completed. You can paint it in black to make it appearing more professional.
After opening the dolly you can place the tripod with foot in the dedicated seats. As you probably noticed the height of each seat id different from the other ones. This is not a problem since the tripod will not be exactly horizontal, but usually tripod have an adjustable head. Anyway you can extend two legs of a small length (about 1 cm one leg and 2 cm the other one), and your tripod will be planar.
So, good luck now with your new easy DIY tripod dolly!