Make Your Own
Band Saw
Circle Jig
There are a lot of band saw circle jig designs available, but my complaint about most of them is that they damage the surface of the circle. They do this because most jig designs hold the workpiece by poking a shaft of some sort into it. My design does not require that you drill into the workpiece or leave a hole of any sort. Rather, a dowel glued to the workpiece fits into a pre-drilled hole in the jig to provide a pivot point. The dowel is later removed and the surface sanded clean.
The general idea behind the band saw circle jig is really simple: establish a center and a radius, then just spin the workpiece against the blade. The jig consists of an MDF cover over the existing band saw table with a throat cut for the blade, and holes drilled on a line perpendicular to the fence and the blade.
Figure 1
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To make the cover for the existing table, cut a piece of MDF 3" wider than the length of the table top, and as long as the largest radius that you would like to accommodate plus about 8 - 12 inches. For my Laguna band saw table top, I chose a 24" square to accommodate my needs in every direction. (Figure 1)
Figure 2
Cut a throat in the table cover to accommodate the blade. My saw accepts 1" blades, so the throat on my cover has to be at least 1". I allowed a little extra wiggle-room — there's no harm in doing so. The depth of the throat helps determine how far apart the holes will be drilled in the cover. You will adjust the radius of the circle by selecting the appropriate hole and clamping the jig at the correct radius. My throat depth is 6 inches, which means that if I drill holes about 2 to 4 inches apart, I can safely adjust the jig to any radius, and leave enough of the cover on the table to clamp it in place. (Figure 2)
Figure 3
Use 2x2s to build a frame on the bottom of the jig that will fit snugly around the table, and will allow clearance for the blade in the throat. That is, the distance from the base of the throat to the back of the frame should be slightly less than the distance from the edge of the blade to the edge of the table. If your table is thinner than a 2x2, you can resaw the 2x2 to an appropriate thickness using your band saw :) (Figure 3)
Figure 4
I attached a scrap of 2x4 to create the pocket where the jig wraps around the table and fits snugly. (Figure 4)
This is really the most important part of the process and deserves attention. Depending on your saw and your table setup, you might have to modify the framing underneath the MDF cover. In my case, I had to trim one side of the "U" to accomodate the fence. I also had to dado a couple of grooves in the "pocket" piece to allow it to slip over some bolts and part of the fence rail.
I found that the easiest way to lay out the framing was to clamp the MDF cover to the table top, then trace the table on the bottom of the cover. Then, while still clamped, use a piece of scrap to see if you'll need to modify the frame-cover (the "pocket"). Lay the scrap across the two sides of the "U": if it doesn't lay flush with the framing, then you'll need to do some modifications.
The tighter you get the pocket, the less you'll have to rely on clamping later. (My jig is very tight, but I still clamp it because 1) you never know what might happen; 2) it reduces inevitable vibration and keeps the cut cleaner.)
I attached everything with drywall screws. I found that if you resaw the framing too thin, you can shim it up with washers — otherwise the jig will not completely slip over the band saw table. If your framing is too thick — so that the jig rocks on the table top — your best bet is to unscrew the framing and resaw the pieces to the appropriate width.
When you're done with the framing and the pocket, square the jig up to the fence and clamp it in place. (Note that the clamps will not necessarily get in the way when you rotate the workpiece later: this depends on the radius that you're cutting and on the depth of the throat on the jig. If the clamps do get in the way, you can chop off the corners of the workpiece to reduce interference.)
Figure 5
Draw a line perpendicular to the fence-edge of the jig, and in line with the front of the blade. Drill holes 1/16" larger than the diameter of dowel that you intend to use as a pivot. I use 1/2" dowels, so my holes are 9/16" — this will provide sufficient room for the workpiece to spin freely. If you have a 6" throat, drill holes about 2" - 4" apart — this will help to keep your options open as far as radii are concerned. (Also, this design has the advantage that the radii are all adjustable from the top of the jig; some designs require adjusting the radius by adjusting the pivot in a groove underneath the jig. This can be awkward given that the table might be in the way.) (Figure 5)
Completed
Most other circle jigs that I've seen include a scale that is supposed to determine radius of the circle. This is not necessary on my design, for my design presumes that you'll start with a perfect square: the only measurement that you need to make, in my scheme, is when you cut the square. The rest is geometry. Furthermore, should it seem necessary to make a measurement, I find that a tape measure or combination square is just as effective — and it keeps the jig simpler.
Since the holes are drilled some distance apart on the jig, this leaves the possiblity of using different sized dowels for different projects. You can drill larger or smaller holes between the 9/16" holes (if you chose to set your jig up like I set mine up.) This might be useful for smaller or larger circles — it depends, as always, on your purposes and preferences.
Cutting a Circle
First, start with a square workpiece: the length of each side should be the diameter of the desired circle.
Figure 6
Locate the center of the workpiece by drawing diagonals. The center is where they intersect. If you are concerned about hitting the clamps, then cut off the corners of the workpiece, being careful not to cut what will be part of the circle. (Figure 6)
Figure 7
Cut a small piece of dowel, making sure that it is no longer than the thickness of the jig, and making sure that you have cut a perfect cylinder. To cut a perfect cylinder, clamp the dowel in a vice with the amount of stock sticking out that you would like to cut. Use the edge of the vice as a guide and cut with a dovetail saw (or some other fine-toothed saw). I do not recommend using a power saw for pieces so small! (Figure 7)
Figure 8
Glue the dowel to the center of the workpiece. My wife, Tanya, is the resident woodturner, so she has some very good glues. We use CA glue (Cyano Acrylate) and an accelerator to dry it instantly. When the glue is dry, you're ready to make the cut. (Figure 8)
Figure 9
Insert the dowel in the appropriate hole on the jig and slide the jig over the table until the edge of the workpiece touches the blade, making sure to keep the jig squared up to the fence too. (Figure 9)
Clamp the jig to the table.
Figure 10
Turn on the saw and rotate the workpiece on its pivot. Be careful! If you torque the piece too hard, you might break off the dowel. This has happened to me: the piece will start to drift — you'll notice quickly — and you'll likely lose some of your diameter. (Figure 10)
When you're done, just snap off the dowel (or cut it off if it's stubborn) and sand.
Tips
For safety reasons, I stand to the right side of the saw and guide the piece at a good distance from the blade. I find that my left hand wants to push the workpiece, but then the left hand is on a path toward the blade! So, I sort of leave my left hand to keep the piece firm against the jig and rotate with my right hand.
Know your blade's tolerances! Don't use a 3/4" blade to cut a 2" circle, for example. I tend to just use my 1/4" blade and the final cut is fairly smooth. Experiment with some scraps for a while and get the hang of using the jig and seeing what kind of finish you can get on the edges.
Make sure that the blade guide, as always, is as close as possible to the workpiece. This way, if for some reason the jig rocks upward (if say a clamp comes loose or you need to un-clamp an edge to allow a large workpiece through), you'll not irreparably damage your workpiece.
Be very careful if you unclamp a side of the jig during operation. It is possible to do this in order to clear large workpieces where you have not cut off the corners, but I do not recommend it as a general practice. If you do find yourself in this situation, always leave at least one clamp on the jig at all times. The more snugly the jig fits to the table, the safer this operation will be. If you plan ahead, unclamping will never be necessary.
If you would like to make beveled edges, you can tilt the band saw table. The Laguna band saw tilts in two directions, so I don't have to worry about the pocket hitting against the saw. If on your saw, the pocket does hit, you can round over the part in question, or slide the circle jig back slightly on the table.
Completed
As always, practice safety first! Don't rush through machine set-up or through the final cut. And wear your safety glasses!
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