Ok. The ring take two. Sounds like a movie sequel doesn't it? The ring I have at the moment is one I'm not happy with, The castor wheels that it rolls on are to stiff and flimsy. The edge of the ring is cut close with a jigsaw, but not accurate enough, so I want to get it right.

Yesterday I picked up two lengths of 50mm x 5 mm x 6 m, mild steel flat bar from the benders. It cost me about $40 plus GST, to get them both bent. I used the jigsaw to cut some  19 mm MDF (Craft wood) into arcs to make up a double layered ring as I did earlier with the 6mm plywood. My concern is that the current plywood is too thin and will eventually sag over time.

After carefully cutting the arcs, making sure I left a little extra outside the line, I laid them all out in a circle, using a tape measure from a nail knocked in the concrete to accurately check it for true all the way around. I then lay down a squiggle of liquid nails, placed the second layer on top of the first, re-checking it with the tape all the way. I placed some chocks under all the joints and screwed self tappers through all the way to provide clamping for the glue. This makes for a much better bond than just gluing and placing.

The next bit is a neat trick. Lay some thin board down in a cross under the ring and centre it all up with a tape measure. Keep it the same width as the chocks already under the ring, propping it up off the ground. Find and mark the centre of the cross and tap a shallow nail hole in it. Next, unscrew the bottom plate off the router. Use it as a template to mark out a new modified plate on some thin ply or sheet metal if you have it. Make sure you leave a good sized tab, protruding from one side to later attach a long plank. Attach a plank to this tab, after measuring it out to the size you need. Make it a bit longer so you can drill a few different holes for your adjustments.

Make sure the ring is screwed down to the crossed planks so that you can keep the whole thing from moving. Set the router to the thickness of the ring. Place a nail through your measured hole at the other end of the router plank, and tap into the centre of the crossed planks. Carefully swing the router round the inside of the circle, to make a nice true circle. This is why you should have cut just outside the lines with the jigsaw earlier. The router will fix it right up. Adjust the distance, then do the same for the outside.

WARNING Make sure you know how to use a router first! It is a high-speed power tool and can easily grab, run away and cause SERIOUS INJURY!

Next, work out where you want the steel track to go. Make sure you check and measure the ring on top of your walls, where the wheels will mount, so the two will match up later. Set the router about half the depth of one layer of MDF. Adjust the nail again and carve a trench in the ring for the steel ring to fit into. It helps if the router bit is the same width as the steel ring. This will ensure that the steel track runs in a true circle. Next, lay the two bent halves of steel down in the trench, check, adjust and cut off any excess to match the two halves. Then its just a matter of a quick weld. If you don't have a welder, then use a strip of thinner metal, place across the join and then drill two self tappers through each side of the join.

You can run some liquid nails in the trench, then weigh down the ring or clamp until set. If you want to get fancier still, then attach some short "L" brackets on the side with screws or weld, then screw into the timber. From there it is a matter of getting help to flip it over and your dome can go on top.

I made some wheels and countersunk them into the wall's top ring, on an axle. The steel track will sit on these. Much better and easier than the old castors! If don't own a lathe, you can get them made up or have a hunt around for small pulley wheels and the like.

Next I MIG welded linked motorcycle chain all around the inside of the metal ring. I have a Front sprocket of a motorbike to engage on this chain later with a crank handle or directly onto a small electric motor.

Rollers again.

I have had my fair share of problems with the rollers. Even though the metal ring is round and true, because of the sprocket trying to push the dome from a single point, the track still rides up off the wheels. This can be alleviated by making larger wheels with much deeper flanges. I didn't particularly want to do this again, so instead I welded some guide wheels onto the brackets, to run on the inside of the ring. This could also be done on the outside, but I had more room on the inside in this case. This fixed the problem.

Driving the dome.

In order to turn the dome, you need something to drive it with. The dome at this point, can be quite heavy, so pushing it around manually can be nearly impossible on your own. This part is soooooo much trial and error, its not funny. No really...it isn't!

I have so far broken three mechanisms, all because the dome kept getting stuck on something, ran off its rollers or the mechanism was simply too flimsy. Now that I have fixed all the roller and guide wheel problems, it is time to completely rebuild the crank mechanism.

I started with two heavy 25mm internal diameter roller bearings, sandwiched together. NICE AND STRONG! Next, I welded together some thick gauge mild steel bar and angle, to make a bracket to bolt it onto the wall as sturdy as possible. After that I had to consider that I might need to pull it apart later for adjustment of modification, so I bent up some steel bar in the vice and made a bracket that could hold the bearings on, and clamp down on an adjustable bolt. Then it was just a matter of pushing a 25mm round steel bar through the bearings, welding the sprocket on one end and the crank handle on the other end.

I bolted the bracket to the wall, loosened the clamp bolt, adjusted the sprocket until it lined up with the chain under the ring, then tightened it all up again. It worked first go!