I needed to cut two 24" circles and didn't have an easy way to do it. I didn't like the router or band saw jigs I saw, so I decided to make one instead. I figured that the easiest way to go was to have a groove in plywood that I could use as a way to feed the wood into the band saw blade, and have the end of the groove act as a pivot point. The end of the groove lines up with the front of the blade, giving me the proper distance for the diameter I want and the whole jig just slides onto the band saw from the side. The pictures above show the after effects of using the jig and a picture of it on my bandsaw and the YouTube video shows me using it and the finished product.
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Step 1: Materials
one 2'x2' 3/4" thick plywood sheet ($7.65 at HD)
1/4" dowel ($0.88 at HD)
one router with 1/4 inch bit
Band Saw with blade no wider than 1/4"
Straight edge with clamps
spare piece of wood
sandpaper to clean the routed channel
Optional: Small C clamps ($2.50 at HD) or quick clamps
Total I spent on new materials for this: $14, but only because I didn't have scrap plywood that would allow me to cut a 24 inch circle. If you are doing smaller circles you can probably make do with a different size of plywood - possibly even with scrap you have around!
Step 2: Measuring
Place the plywood on your bandsaw table so that it overlaps the back of the table about 1/2 inch. Mark the front and back of your bandsaw blade on the inside edge of the plywood. Move the board so that it is sitting in front of the blade, overlapping the table 1/2 inch towards the arm of the bandsaw. Mark where the blade is on the board.
Using your square, and the line you marked for the location of the blade, draw a line straight up the board, from front to back, marking blade location. Next, coming in from the side where you marked blade length, draw straight, parallel lines across the board to the first line you drew (draw the line that indicates the front of the blade all the way across the board - this will be important later!). The space between the parallel lines is the space that will be cut out - this is where the blade will go. Because of the design of the band saw I couldn't cut the lines out the way I needed, so I had to transfer the cut lines to the back of the board. I used a speed square to help line up the lines, but eyeballing it will get you almost the same results as long as you use your carpenters square to ensure the lines are straight and perpendicular to the edges of the plywood
Step 3: Cut Out Saw Blade Area
Use your band saw to cut along the inside of the two lines marked for cut out (I cut a third line in between the other 2 just to help with cutting it out. You could do more if you wished). Use a chisel to cut the end flush.
Step 4: Set-up and Route the Grooves
Use the square to measure 1/2 the distance needed for the final diameter, minus 1/4". I need circles at 12" and 24", so I measured 6 3/4" and 11 3/4". We are taking off the 1/4" because we are going to leave 1/8" from the edge of the band saw blade and the body of the jig and 1/2 the size of the router bit is 1/8". This gets us closer to the needed final distance from the blade and will give a more accurate diameter of the circle. Draw a line straight up from the bottom to that point using the carpenters square. Make sure the line is straight and accurate. Use your router, and a guide, with the 1/4 inch router bit, to make a 1/2" deep groove to the first line equaling the front of the bandsaw blade. I recommend doing it in at least two passes to make it easier and smoother. Do any additional lines that you might want to make. After you have cut the groove, sand the groove to make it smooth and allow for easy travel of pieces. I used a thin piece of plywood with 100 grit sandpaper and just softened up the edges of the cuts.
My guide has clamps and adjustable ends, but a piece of wood with clamps at each end will work as well, as long as you are accurate. I also used a stop block at the end to ensure I stopped at the appropriate point. In order to stay on my line I measured the distance from the edge of my router base to the edge my router bit and set the guide that distance away from the line and stopping points.
Step 5: Aligning the Jig
Place the jig on the table of the band saw. Slide it on from the side so that the blade goes up the groove. Make sure that the grooves are parallel to the blade. Clamp the jig to the table and make sure that it is true to the table. I did this by measuring to make sure the back side was a consistent distance from the table all the way across the table. Attach a piece of scrap wood to the bottom side of the jig, flush against the front of the band saw table. This helps make it easy to attach jig, set it up consistently, and make sure that the jig doesn't move in relation to the blade. Alternate step, add a quick clamp or two to the bottom of the jig to attach to the bottom of the table - I didn't include this in my cost because I happened to have one and it is not necessary. You can clamp the table to the the board you attached to the underside of the jig.
Step 6: Set Up of Cutting Material
Cut a piece of the 1/4" dowel approximately 3/4" long. Find the centerpoint of the board to be cut and drill a hole into the bottom side of the wood, but not through it. My board was just over 16 inches wide and 17 inches long, so I marked a point that was 8 inches in from one end and 8.5 inches in from the other. I used a quick 45 degree jig to make a hole perpendicular to the board and approximately about 3/8" deep because my wood is 3/4" thick. Drive the dowel into the wood and you are ready to cut!
Step 7: Cutting the Circle
Hold the board upside down and place the dowel into the groove for the diameter you want to get and push the towards the blade. For example, I wanted this wood to be 12 inches diameter so I'm using my 6 inch groove. Adjust the height gauge on the band saw to your wood thickness, and turn on the band saw. Slowly push material in towards the blade until you hit the back of a groove in the jig and the board stops moving forward, then begin turning to cut the circle. Monitor your speed so that the saw blade doesn't bind. After a few moments you should be done and have your perfectly cut circle. Congratulations!
As to the additional thoughts I promised - here are a few:
Add grooves every inch to be able to make whatever circles at whatever diameter you need
Add clamps to the underside of the jig to hold it to the table
T Tracks instead of grooves and a dedicated holder for the holes