This dolly is designed for light-weight cameras and tripods only, and because you must walk behind it, it is mostly only useful for cameras with a flip-out LCD. This was my original dolly design, and I didn't keep detailed photographs or notes. For more construction details for this dolly design, see the Ron Dexter's tutorial which inspired this design. I'll only discuss here the things I did differently. Please note that this particular track dolly design does not permit the cameraman to stand on it, nor will it support a heavy camera system (such as a crane). The means to create a more solid track dolly are self-evident from this tutorial.
Here is the basic dolly, built out of 1/2" Schedule 80 Dark Gray PVC Pipe:
Be sure to use Schedule 80 with an inner diameter of at least 1/2". The Schedule 40 (the most common PVC available) is too flimsy and will warp easily. Some states in the US do not require Schedule 80 in their building codes, so you may not be able to find it, in which case, I recommend you build a metal dolly instead. If you live in California however, you can easily find it. (Note: "Schedule" refers to the gauge or thickness of a material. Also note: all diameters given for PVC components are always by inner diameter.) The dolly has a thread-less Tee-fitting at the central joint and holds each of the arms of the dolly with a bolt and wing nut for each arm. The arm's length is determined by the tripod used, and so careful measurement is required before one starts hacksawing their raw PVC pipe. Each arm's end has a threaded Tee-fitting with its one end pointing upward. As you can see in the next picture (showing the dolly holding a leg of the tripod), a 45º thread-less angle with an inner diameter of 3/4" fits snugly onto the outer rim of the 1/2" Tee. The friction alone should be sufficient to hold the tripod in place.
For the arm shown above only: after the Tee used to hold up the tripod leg, the next piece along each arm is a 1/2" female-to-female adapter (in other words, this little adapter has a male threaded end on each side). Then there is another Tee, with each end having a flat 1/2" threaded plug at the end.
For the other two arms: after the Tee used to hold up the tripod leg, the next piece is a flat 1/2" threaded plug at the end.
The following applies to all three arms: a bolt is then locked with a lock nut through a hole centered in the 1/2" threaded flat plug, and continues on to a square block of wood. Each block of wood has had a section removed to allow for the bolts that are sticking out of the angle iron. The angle iron has been screwed into the blocks using wood screws. The skateboard wheels, which have two ABEC-5 bearings each and a bearing spacer, are bolted through the bearings into the angle iron (the angle iron comes with a 90º bend in it) with a 5/16"-18 (Coarse Thread) bolt of 2" in length and nylon-insert lock nut. The bolt head is a hex head and, as is obvious from the picture, the head is on the outside of the wheel. Due to the width of the wheels and the angle at which both wheels are to each other, the wheels can actually accommodate a range of sizes of track. Mine accepts a track from 1/2" to 2". The 1/2" track you see here was just something used during testing. A separate section explains the construction of a heavy duty track system.
The total cost to build this dolly was approximately $75. Most of the expense is in the wheels and bearings. The wheels were $18 for four, or $36 for all eight. Be sure to get the poly-urethane wheels (they are softer and run smoother and quieter), and don't waste your money on designer wheels. For the ABEC-5 bearings (or you can get ABEC-7, but don't get ABEC-3), buy them where they sell bearings for inline skates and you'll get a pack of 16 bearings (the number you require). Mine were about $20.
Be sure your bearings are the same size as the wells for the bearings in the wheels. Also, be sure that when you put the bearings in, you have them ALL the way in (they go in deep). If you buy them at a sporting good or skateboard shop, they can put them in for you, otherwise you can use the largest socket of a wrench and socket set that will fit to force them in. Finally, bearing spacers are optional but a good idea to get considering they are only a few dollars (they go between the two bearings of the wheel and thus are sitting in the very center of the wheel).
If you construct a heavier duty version of this dolly, the wheel assembly of this design is sufficient for a much heavier load, so you shouldn't need to change that. Only the arms and support would need to be changed.
Be sure to construct with as little glossy metal as possible, since shiny surfaces reflect lighting used in filmmaking, often producing negative results. Hence most photography and cinematography equipment is black.
Finally, use nylon-insert lock nuts whenever possible. These usually require a good socket wrench to put on, but the advantage is they need a socket wrench to get off as well. In other words, they won't work themselves loose over time.