My goal is to add a 5rpm DC motor to drive a full length piece of all-thread to move the carriage for time lapses - but have not found any locally yet, so after I figure out the linkages eBay has a few to choose from and we'll give that a go.
The shopping list (save for the wheels) is a Home Depot/Lowes/Ace Hardware trip away, and approx. $15.00 of parts/pieces.
2 - U-Post fence posts (3')
1 - 1/4" All Thread (3')
1- 1/2" square steel tubing (3')
1 - Simpson T-Strap (6")
4 - 1/4" Cap Nuts
12 - 1/4" Washers
3 - Inline Skate Wheel Assemblies
(In the picture are some 3" x 1/4" bolts and coupling nuts, these are/were to make leveling legs but so far have not been needed)
UPDATE: The motorizing of the dolly is complete, the instructable for it is here: http://www.instructables.com/id/Yes-Another-Camera-Slider-NOW-MOTORIZED/
Step 1: Dolly Assembly
The T-Strap is rigid enough, dirt cheap, and has perfect clearances for the wheel diameter - the mounting holes are large enough to allow the wheels to move in and out 1/8" or so for fine tuning tension in the frame.
A couple washers help take up the space the skate frame provided, and the bearings and axles all go together as easy as could possibly be.
Step 2: Frame Assembly
Using the assembled dolly I laid everything together to get a length to cut the square tubing. Once cut, a few taps of the hammer "squished the ends slightly so they would fit into the post channel nice and tight. The thoughtfully provided 1/4" holes pre-drilled in the fence post ends made positioning easy.
*NOTE: The flat piece that is welded onto the post is for securing the post in the earth. There are two big rivets on each of them. When you grind off the rivets, nothing happens - I'm guessing the plates are welded as well - and as such did not see the immediate need to remove them - and still haven't.
Next thread the rod through the posts and tube, tighten it down with an open nut on the long end, then trim the rod to size. Remove the open nut and replace with a cap nut for a feeble attempt at making it look nice.
Repeat on the other side and check the dolly for tension.
Now, good, bad, or otherwise - there is just enough "give" in the posts to squeeze the dolly in and out. If yours are slightly bowed like mine were, a little muscle will fix them right up. Adjust the wheels to tension as you like it.
Step 3: Final Observations
It can also just be laid flat on the ground, the original intent when I started this project. Those flats on the one side raise it up nicely, and a magnet leveler on the other end balanced it out and leveled it up. It rolls silky smooth this way as well.
I did not see the need to drill and mount a 1/4"-20 screw for a camera mount as I already have a sturdy magnet mount that fits perfectly. Also, in the future, something new might come up, and having a bare flat surface allows any possible way of mounting.
I'm still not decided on how to manipulate the dolly: so far have just used my hand, a string, and gravity - until I motorize it I'll just keep experimenting. The attached video shows various shots using just finger pressure.
As always, any and all comments, critiques, suggestions and improvements are always welcome - happy trails!