It was really starting to bother me that the lid was perfectly round. It really doesn't make a difference other than aesthetic. Since I'm waiting on some fabrication for the lid hinge, I decided to try to figure out how to cut the lid into a better circle.
I decided to use my circular saw. (It had to be the circular saw since I haven't used it yet on this project.) I have a small saw which only accepts 7.5 inch blades. So, that meant I would have to do the job in two passes, one from each side. (And still not get all the way through.) This was a bit of a problem since it meant finding the centre from both sides. I had to find the exact point above the point I used for the first side. I just did it with some really careful measuring. I used the same centre on the hot side as I did for the trimming, and noted that it was 1/2 inch to one side of a row of bricks. Then I just measured how far it was from each of the two ends on either side of that row. I used those measurements to find the analogous centre point on the cold face.
So, the very first step here was to replace the saw blade with a grinding disk. Fortunately I didn't have to buy one. Then it was time to figure out how to do it. Of course, I could just do it freehand, but I didn't give myself very good odds at doing very well that way. I played around a bit trying to attach a swing arm to the table of the saw, or replacing it all together, but, then the blade would not cut deep enough for my satisfaction. I elected to make a jig in which the saw is held freehand in a cutout the shape of the table of the saw. This jig is then attached to the centre of the slab for the pivot. picture. I had to hold the saw in the correct position and rotate at the same time. It wasn't too tricky though.
It worked out pretty well. The two things that are important, is that you take into account the size of the saw table when you locate the hole in the jig (should be obvious), and that you align the hole with the centre of the blade. If you put the hole in the centre of the jig with the blade offset it makes it much more difficult to figure out how far the hole should be from the blade, plus the profile will be a scallop (I think) rather than a nice cut face.
I didn't get all the way through the lid on the second pass, but it was very easy to use the drywall saw and some sandpaper to smooth it up. It was very nice to put the aluminium back on and see a nice perfect circle.
