I don't seem to need to cut circles very often, but when I do, I often resort to drawing them out on the wood I am cutting then cutting them out by hand with my Jigsaw. This method is ok, but the jigsaw always leaves a rough edge that is uneven, so after cutting I usually spend as much (or more) time cleaning up the cut with my belt sander. For one-ups, this is ok, but I know there are better ways ... like using my router and the circle jig I made for that quite some time ago, but there is still another way, using the bandsaw, and that's what I am doing today.
As we all do, I checked out the Internet to see what was available and there are a number of designs and all that I could find were designs that made fixed sizes. What if I want a circle made that is between those sizes? I need a variable distance circle jig, and that's what I made.
I decided the best way to make a variable jig was to create a sliding center, which means 2 pieces of wood, fastened together in a manner that lets the slide move uninhibited.
As happens with woodworkers who can't throw out any size of scrap wood, I did have a couple of MDF chunks around that would be suitable for the job. MDF is nice because it is very flat and stable ... well at least as long as it doesn't come into contact with moisture.
The next piece I needed was the sliding piece and figuring out how it would tighten down. In the past I have used wood for this and the problem with wood, is that when the moisture content in the atmosphere changes, the wood absorbs or releases moisture, which means the wood shrinks and expands slightly, and often it can expand just enough to make the slider part stick in a slot. This is the reason that I never use wooden mitre slot material for use on my table saw or router table. What does work great is extruded plastic material. It's stable, inexpensive, easy to cut with woodworking tools and has a multitude of uses.
After gluing the carcass of the circle jig together and fitting it to the bandsaw (which turned out to have some unforeseen challenges) the next step was to cut the slot for the sliding piece that would be the pivot point for the circle jig. The best way to do this was to cut rabbets on each side of the carcass and that way the pivot point would be centered on the jig. Centering is somewhat important as it makes it better for tightening down the pivot point plastic part and thereby making it a bit more stable.
After the slot was cut, the plastic that would be the pivot point was checked and a hole was drilled through the middle of it that would would line up with the slot. The hole had to be large enough to accept a suitable T-nut. The T-nut that is to be used needs to have the tangs filed down significantly as T-nuts are designed to be used in wood, not this dense plastic.
After filing the T-nut was inserted and a dropped into the slot, a matching tightening bolt was inserted from underneath the table and checked to see how well it tightened, and how smoothly it would slide up and down the slot and it all worked very nicely.
After all the parts were assembled and the jig attached to the bandsaw, time to try it out. A hole was drilled in a piece of wood I would use to make a circle from and it was butted up to the bandsaw blade, and pushed against it ever so slightly so that when I started the bandsaw, the blade would bite into the wood.
I was astounded at how well the jig worked. The wood spun easily on the jig and the bandsaw blade made a very neat job or cutting the circle. I could see partway though the cut that I would not have to get the belt sander out to fix the edges, for my application, they were perfectly fine and very accurate.
This is one jig I know I am going to get a lot of use from, it is quick to attach and use and does a great job ... I wish I had done this years ago, it would have saved me and my belt sander a lot of time together ...
Copyright Colin Knecht