In search of the perfect bandsaw circle jig
Steve Maskery – 14th November 2001
There have been many designs published for circle jigs, and not surprisingly, they all rely on spinning the workpiece about it’s centre whilst cutting. And therein lies the problem. Not only must the pivot point be the centre of the circle to be cut (not difficult to arrange) but it must also be the centre of the circle which is actually cut (rather more tricky). Let me explain. Many bandsaw blade do not, in practice, cut in a straight line, at least, not in a line which points straight ahead. All kinds of forces contrive to steer the cut to the left or to the right. This can be due to the tension setting, to the way the blade has been sharpened, or just to the fact that the teeth on the inside of the band are always pressed against the wheel, while those on the outside are always being stretched over the wheel. To compensate we usually skew the rip fence, and, with trial and error, we can get our machine to cut in line with the fence. Until we change the blade, when we have to do it all over again.
The same problem arises when cutting circles. If the line of cut is not exactly tangential to the circumference of the circle, the blade will be pulled in or pushed out, thereby cutting a spiral rather than a circle. To compensate for this, the pivot point must be moved closer towards us or further away, to alter the tangent angle. I wanted to design a jig which would overcome this problem, and be easy to use too. There are other problems with some jigs too. If the jig consist of a simple pin in a baseboard, it can be difficult locating a large blank onto a small pin, because you cannot see what you are doing. I made a jig some years ago with a swinging arm which helps in this respect, so I decided to incorporate both this and a scale for easy setting up into the “perfect” jig.
The adjustable arm consists of a captive slide in a slot, with a pivot pin which can be set anywhere along a scale. Whilst having good length (thereby allowing large circles), it is still quite narrow, so you can see where you are putting the pin into the blank. This arm itself pivots on a wooden peg in a fixed baseboard, and this makes it easy to start the cut, by swinging the workpiece into the blade. Tracking is effected by sliding the whole jig back or forth in the machine table slot, locked down with a thumbscrew.
My bandsaw has the column on the left, so if you have and Inca or other machine which is the other way round, you will have to reverse my description.
The arm.
I started by making a T-shaped slide from hardwood, and two mating sides. The flanges of the T can be kept small, so that there is a large area in which to locate the locking screw. This is fitted into the right hand end of the piece nearest to the operator. First drill a shallow hole large enough for the pressure pad. Then a smaller (10mm) hole into which to pull an M6 nut, then through at 6mm. The pressure pad can be anything – a brass disc, a small coin filed smooth (one of those that makes you a millionaire on holiday), or even a wooden plug. I used an old watch battery which I was throwing away. Anything to stop the screw chewing up the arm. Glue the two sides onto a piece of 6mm MDF so that the slide is an easy but not too sloppy fit in the groove. Make sure that the slide doesn’t get glued down too, that’s very embarrassing. Fit a sharpened pin to the slide. I cut a very shallow rebate (0.5mm or so) on the top edge of the front piece for the scale – that way is won’t get rubbed off by the workpiece – into which I glued a scale which I printed on my PC. I made it to read diameters, that is to say, 100mm on the paper was labelled 200mm. It’s not critical where this is glued on, we’ll calibrate it later.
The baseboard.
This is a piece of MDF which covers the table from just to the left of the blade to a few inches overhanging the right of the table . A hardwood strip is carefully sized to be a Goldilocks fit in the mitre slot (not too big, not too small, just right) and screwed to the underside. On the side away from the user, find an area under the table where there are no webs, and drill and tap an M8 hole in the machine table itself. Slide the baseboard into the saw until it is in place (the saw will cut it but that’s OK) and mark on the baseboard from below, through the hole. Rout a 10mm slot in the board at that point. Now when the board is on the machine table, the slot and the hole will line up. Make a thumbscrew with a short length of M8 thread, a nut and wingnut and a washer. If the total height above the board of the thumbscrew is greater than the total thickness of the swinging arm, you will have to recess the thumbscrew into the baseboard. It must not foul the workpiece as it spins.
Fasten down the baseboard with the screw roughly in the centre of the slot. Clamp the arm to the baseboard such that the pin is in line with the front of the blade, and drill through both the arm and the baseboard (but not the table). The best position for this is near the right hand end of the side away from the user, overhanging the table. Glue a dowel in the arm, it will pivot in the baseboard.
Finally screw a stop block behind the arm, so that it stops at 90 degrees.
Setting up the jig.
As I pointed out earlier, 90 degrees is not necessarily the right position for the pivot. If you have set your rip fence accurately to compensate for drift, you can use this to set the jig position. Simple lock the ripfence in place to the left of the blade, place a try-square against it up to the blade, and lock down the baseboard when the pin is in line with the square. The jig should now be compensated for drift. If you don’t have a ripfence set up, than you must resort to trial and error. Cut a few squares of scrap MDF and drill a small hole for the pivot in the centre. Set the slide to roughly the radius you wish to cut, lock it in place and remove the whole arm. Locate the pin into the centre of the blank and replace the whole lot onto the baseboard. Start the saw and slowly swing the job into the blade, up to the stopblock. Then start to rotate the workpiece on the pin until the circle is cut. If the jig is cutting true you will have a perfect circle. If you have cut a spiral (and you will know because the start won’t meet up with the end), you must unlock the baseboard and slide it towards you (if you were spiralling out) or away from you (if you were spiralling in). If you wish, you can reduce the radius setting and use the same test piece again, or just use another piece of scrap. When it is cutting true, measure the actual diameter of the piece cut, and make a pencil mark on the slide at that position against the scale. You can test if it is accurate by moving the slide to another reading and seeing if the circle actually cut matches the setting on the slide. It should do.
Making a test cut each time you set up the jig is wise, but it needn’t mean using up lots of scrap. Just set the jig to cut a bit bigger circle than needed. If it cuts that accurately you can re-dial the right diameter and cut again, knowing that you will have a good circle. Anybody got any more improvements?
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