The Dome (part 2)

In the last post we built the skeleton of a dome now we need to make it turn and make it weather tight.

It will be much easier to do the rest of the work if we can turn the dome to suit our stable work position (remember my observatory is on a slope, there aren't many places I can safely put a ladder!) so the first job is make it turn.   There are a number ways to make that happen: 

1. You can use non-swivel castor wheels, but you must make sure that they are very accurately lined up at a tangents to the ring otherwise you will find it difficult to turn or worse the dome will derail.

2. You can use swivel castor wheels but reversing direction comes with a swing of the wheel which could be difficult with the weight of the dome and if they don't all reverse simultaneously the dome could again derail.

3. You can use transfer bearings.  These have the advantage of universal movement with no backlash but they do need a strong running surface (as I discovered during the build).

I went for the transfer bearings for that feature of universal movement. You can get them very cheaply from Amazon.

Transfer bearings

I put one of these under each joint in the base ring of the dome, initially I only placed them under the joints in the lower of the two layers of plywood but the ring gradually began to sag at the joints in the top layer so I later added a second set to support all the joints in the ring. Also initially I ran the bearings directly on the plywood of the running ring but then every engineer I know on Facebook told me I was an idiot so I cut up a load of sheet aluminium and screwed that onto the running ring to make a better running surface:

Bearings on the aluminium running strip.

Great! So now we can push the dome around in any direction we want... Including right of the ring...  We need some way of keeping the dome on track and since the plan is reuse and upcycle where we can:

Three of these should do the trick!

Skateboard trucks ahoy! They work pretty well too:

Round and round we go!

In that video clip you can see that I have stapled a membrane skirt to the base of the dome, this was to try and stop wind/rain coming under the dome, unfortunately I made it too tight so it tends to ride up and slip between the dome and the running ring, at some point I will redo this with some slack in it but it is currently relegated to a "fettling" job for when I can be bothered.

Moving on swiftly we have arrived at another decision point we can no longer ignore: What is the outer skin going to be made of?

There is no doubt that fibreglass is the material of first choice, its relatively cheap, easy to shape, very strong, very durable, weather tight... However, for me it has one major downside:  it is very unpleasant to work with.  The glass fibres are extremely irritating requiring the use of extensive protective clothing; the resin is also quite nasty, very sticky and difficult to clean up spills.

I have had enough experience of working with small amount of fibre glass in the past to know that I wanted to avoid it if at all possible, this wasn't easy, I spoke to every architect, builder, engineer and artist that I know desperately trying to source an alternative product eventually, inspired by a visit to Castle Drogo in Devon I started to investigate scaffold shrink wrap.  I was initially somewhat dismayed to find that it is normally only sold on huge roles covering hundreds of square metres, I only needed about fourteen square metres so this seemed pretty wasteful... But I figured nothing ventured nothing gained so I called Tufcoat and very kindly they agreed to sell me a "small" offcut.  This product may not have the strength and multi decade durability of fibreglass but it is undoubtably a lot nicer to work with and is very lightweight, it is designed to last at least five years (by which time I will likely have moved house!) and I figure that it can be pretty easily patched if needs be!

We had a fun time fitting the skin which took pretty much a whole afternoon:

You can see that I have put a layer of breathable membrane over the dome first, this is very important: without insulation or the membrane you are going to get a lot of condensation forming on the inside of the skin which could be bad...

After fitting the shrink wrap we fixed the running rails for the shutter, these were made from three metre lengths of "L" section aluminium (left over from the mk1 observatory).  To get them to curve nicely we cut away most of the short side of the "L" leaving four tabs equally spaced along the length so we could fix them to the dome with screws.

Then the next morning was spent blowtorching it just enough to shrink it, not enough to set fire to it!  I'm not going to lie to you, it was a really nerve wracking task to start with but it turned out that you could get pretty gung-ho with the torching, so we did! 

My dad, just about managing to avoid setting the observatory on fire!

An interesting demonstration of Boyle's gas laws,
guess where the phase transition from liquid to gas was happening?

We now have a waterproof skin on a dome that spins!  This thing is really starting to come together!   Now we just have to do something about that 1.5M wide hole in the middle of it... We need a retractable shutter.