Bandsaw Circle Cutting Jig for Porthole

after trying to cut a plywood circle and a plywood ring with a jigsaw for my instructables 'Porthole Sign' I realised that I needed to make a circle cutting jig for my bandsaw. I looked at a few ideas on Utube and came up with a variation of my own.

I needed a stout base so I used up an old oak desk top. It is plywood but the old style quality where every layer is full ply, no fillers like a lot of modern ply. this makes it very strong and stable. I needed a track and pin system to give infinite variable circles within the max and min of my design. I tried a few ideas but whilst routing through my 'dont throw it away - itll come in handy' cupboards I found a UPVC window hinge. the kind that works from the sides of the window. I disassemble it and it was ideal apart from the pin. I replaced this with a flat head countersunk screw, directly threaded into the plastic part of the hinge slider. It didnt seem robust enough but it proved to be ok for the job in hand. I may replace it in the future with something stronger. perhpaps an all metal slider and pin assembly. I had a perfect piece of steel that I used for the mitre slot but i wouldve made one from ply otherwise. I routed a grove for the stainless steel guide and countersunk some small woodscrews to locate it in the base. the base needed a stop block inderneath so that it can be offered into the blade before commencing a circular cut. this contacts with the front edge of the bandsaw table the line up the center of the slide with the front edge of the bandsaw blade. I used a 1/4" blade to allow small circles without jamming. the center of the metal slide, and hence the centre of the adjustable pin lines up with the front of the bandsaw blade. if i use a larger blade then I would need to adjust the front stop block. I was pleasantly surprised how well it works and i used it to cut the perspex for my instructable by riding the perspex on a wooden sled to avoid having a centre hole. held in place with double sided tape.