The Ring

There are not many parts of an observatory that are truly critical to its operation but the running ring is one of them:  It must be strong; it must be round; it must be level and it must be centred.

This then is the most important thing we have yet worked on, we need to get it right.

I spent a lot of time fretting over this part of the build.  

The only feasible material to build the ring from is plywood.  There is no way you can get the entire ring out of a single piece of plywood, plywood comes in sheets eight feet by four so the ring needs to be cut in sections then joined together to form the whole circle.   If your budget allows there are many companies that will laser cut sheet plywood in whatever forms you need, this is significantly quicker and vastly more accurate (and therefore easier to assemble) than cutting it yourself, however my budget did not allow so I had to cut them myself.

As I would have no choice but to buy the plywood I did actually try to plan out how much to buy,  I broke out Autocad and sketched out the arcs I needed on a template the size of a sheet of plywood, I figured I would get nine arcs from each sheet, and that I would need 10 arcs to make a complete circle.  I wanted 3 layers of plywood so I would be needing four sheets.

Now we meet another of my friends who can do Things.  I was talking over the problem of accurate building the ring with my friend Rob and described how many arcs I thought I would get out of a sheet and he said "I don't think so... get me some paper, I have a pencil" What followed was a lot of complicated maths ending with the declaration of "you'll get thirteen."   Well now we had to settle it didn't we?

See that big grin on his face?  Yeah, that was as he started drawing the thirteenth arc. Never bet against a maths professor (he really is)!

As you can see in the above photo we had a bit of a production line going that day and after a few hours we were able to layout the first layer of "Rob's Ring":

Not bad, its the most circular thing we have built yet!

So that was the first layer done.  Having lost my helpers I modified the drawing jig we had set up to take a router instead of a pen this made the process a lot quicker but I did have a couple of minor accidents.  The jig works by having the main plank slide up and down against a peg.  Depending on which way you are swinging the arm the torque of the motor will push the arm against the peg or pull it away, if you are swinging from the wrong side the router will dive away from your intended cut on the torque of the motor.  I found this out the hard way.  The way I set mine up I needed to start the inner arc cut from the right hand side and the outer arc cut from the left that way the torque was pushing against the stop on each cut which meant I didn't have worry about following a line anymore.

So after a few hours of routing we have the the following piles of wood:

It took a couple of weeks to glue this lot together I built it up into two semi-circles which (with an eye to any future house moves) I then joined together with nuts and bolts so that they could potentially be separated later on.

Looking through my photos archives it seems that I never took a picture of the completed ring.  That's probably because I came to feel that it was something of a millstone round my neck.   Having built a strong, round circle I now spent months worrying about how to get it level and centred.  If I can't achieve those two things the observatory simply will not function.  The whole summer went by and I hadn't committed to fixing it down on top of the walls.

Towards the end of the summer I was describing my worries about this task to another friend, Richard, (Richard is a builder but he has so far studiously avoided getting involved with this project!) he asked me one simple but oh so important question:

"Do you still have the routing jig?"

Of Course!  how could I not have seen this gloriously simple solution?   All I needed to do was wedge a bit of timber into the pipe in the centre of the observatory and fix the jig arm to this at the right height, then as I turn the jig arm round I can immediately tell when it is off centre and adjust!  Even better, if it turns out that the final ring isn't properly round I can just turn the router on and make it round in situ!  How had I not thought of this before?!   

Richard also pointed me towards frame packers as a way of levelling the ring.   So I set up a post in the pier, spun the router jig round a few times, nudged the ring this way and that and within half an hour had a centred ring!  Amazing, several months had gone by while I worried about this task and here it was, done between getting home from work and having dinner.   

The next job was to level it up, I used a laser spirit level for this, the technique went something like this:
Step 1: place a piece of wood on the top side of the ring
Step 2: shine the laser level at the piece of wood
Step 3: mark the level of the laser and the position on the ring on the piece of wood
Step 4: move the piece of wood to a different location on the ring
Step 5: repeat Step 2, 3 & 4 until you return to your original position
Step 6: identify the difference in height between the lowest and the highest point
Step 7: pack the low point up until it is the same height as the high point
Step 8: go round the ring packing any gaps which are wide enough to cause sagging
Step 9: go round with the laser level again make any adjustments as needed

With the levelling done (which took a couple of hours) I went back around with the jig to make sure everything was still centred, then I screwed the ring down on to the top of the walls.  

What a moment that was!  The sense of relief!  No only did this moment stiffen my resolve to get on and finish the project it also significantly stiffened the nature of the walls, now there was no movement in the walls or the floors, now we have a load bearing structure upon which we can place a rotating dome!