Cutting out Circular Parts on a Bandsaw

by Melvyn Wright

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Cutting out Circular Parts on a Bandsaw
The wheels and circular discs on the John Smith organs can easily be cut out on a bandsaw, using this very useful and easy-to-make Circle Cutter.  You will see various complicated and cumbersome circle-cutting jigs on You Tube, but this one is simple, compact, easy to make, costs next to nothing, and does the job perfectly.

You will need a square piece of MDF about 12mm thick to make a sliding table that fits on top of your bandsaw table, and slides in the runners.  Cut 2 long strips of MDF that fit in the runners, they need not be an exact fit.  Apply glue to the top of the strips and place the sliding table on top, carefully lined up square with the edges of the bandsaw table.  The result is a sliding table that slides along the bandsaw table.  It does not need to be an accurate fit, in fact a tight fit will help to hold it in position.  Your sliding table only needs to cover about half of the bandsaw table, depending on what size circles you intend to cut.


2 long strips cut out of MDF and placed in the table runners


Underneath view of the sliding table, showing the strips glued on.

Now put the slider on the bandsaw, slide it slowly along the runners and let the blade cut a slot in it.  The length of the slot should be about.3", so that the table extends 3" behind the blade.  The part of the table behind the blade supports the waste as it is being cut out, but it doesn't need to be more than about 3 or 4".

That is the basic jig finished.  All you need to do now is to drill holes at various positions on the table corresponding to the size of the circles you wish to cut out.  These holes will hold a 3mm pin at the centre of the circle to be cut out, so the distance between the blade and the pin will be equal to the radius of the circle.

It is possible to have a moveable pin, but this is complicated by the need to recess it and the locking mechanism underneath the surface of the sliding table; and there must also be nothing protruding underneath the table either.  These restrictions make a moveable pin somewhat impractical to implement easily.  So what I decided to do was to cut a whole matrix of holes at 1mm intervals, so that I could use the jig to cut out circles of any size, as shown in the photos.  This also has the advantage that you do not need to take any measurements when using the jig, you can just push the pin straight into the required hole.  You do not need to drill all the holes that I did, you may wish to only provide the 5 or 6 positions needed to cut the parts for the John Smith organs.  You can always add more holes later as you need them.  The holes are 3mm diameter and a 3mm pin is placed in the chosen hole.  If you do not have a suitable 3mm pin, use a nail and drill the holes to suit the diameter of the nail.  Do not make the pin any bigger than 3mm as this could be a disadvantage if you want to make a wheel with a hole in the middle which needs to be smaller than 3mm.


Finished slider with matrix of holes


The slider mounted on the bandsaw table.
The pin must be in line with the blade

How To Use
Cut the workpiece into a square just a bit bigger than the circle you want to cut, and drill a 3mm hole in the centre.  Insert the pin into the matrix board and position the table on the bandsaw so that the pin is in line with the blade.  (This is what the pencil lines are for.)  Mark this position with a pencil, because you will not be able to see the lines when the workpiece covers them up!  Now put the workpiece over the pin and advance the table into the blade until it lines up with the pencil mark you just made.  Hold the table in that position, and rotate the workpiece to cut out a perfect circle.


You can start the cut like this....


....or like this.  It doesn't matter!


The workpiece is advanced until the pin is in line with the blade......


....and then rotated to cut the circle

Hints:
You can clamp the table into position if you want to, but I found that it stayed in place without that.  Just a light pressure should hold it in position, as long as you don't try and force the blade.

Instead of marking the position with a pencil, you could fix a clamp to the bandsaw table to act as a stop, as in some of the contraptions shown on You Tube.  Again, I found this to be unnecessary.  I suppose it depends on what thickness of wood you are cutting.

If you make a single permanent mark on the R/H side of your band saw table in line with the blade, this can be used against the marks on the slider to line it up with any of the rows of holes. This saves having to mark the position with a pencil each time.


Close-up of the hole matrix in my table. Yours can be much simpler than this, depending on your intended purpose.  The horizontal lines on the right allow the table to be easily aligned with a fixed mark on the bandsaw table.

You can cut surprisingly tight radii on the bandsaw.  The way to do this is to remove as much of the waste as possible before cutting the circle.  It is the waste wood that causes the blade to bind.  To do this you will need to draw a circle on the wood.  First cut out a square around the circle.  Then cut off the 4 corners, then cut off the resulting 8 corners, etc. until you are nearly down to the circle (this can all be done freehand on the bandsaw in just a few seconds).  Then you should be able to use the Circle Cutter to cut out the circle.  You will find that the waste wood breaks away as you go round, rather than binding on the blade and trying to bend it!  Of course, you can always use a narrower blade, but it's a major hassle changing the blade just to cut out one circle.

If you make a Circle Cutter like this, you will find that it's much quicker and neater to make the idler wheel and other pulleys by cutting 3 discs and glueing them together, rather than trying to file a groove in a single disc.

Making a Take-Up Spool from Multiple Discs
When making the take-up spool, it is very difficult to cut the drainpipe accurately so that the ends are square.  I have ruined a lot of drainpipe trying to get the ends square and the length exact.  Builders in America are also complaining that they cannot obtain the required diameter of pipe specified in the plans.

Well if you make my circle cutter, the take-up spool can be made quite quickly by simply cutting out multiple circles and glueing them together in a stack until you obtain the required length of spool.  This may sound like a laborious method, but is in fact much quicker than trying to cut out the drainpipe and true the ends up.  You can also choose any diameter of spool that you like.

Choose your spool diameter and use my circle cutter to cut out enough discs to make up a complete spool of the required length.  Once you have set up the circle cutter, you can turn these same-size circles out very quickly.  If you are using 18mm MDF, then you will need to cut 7 discs, plus one 15mm disc to get the required length of 141mm.  This is only theoretical, so you will need to measure the actual length as you go along and make adjustments. With MDF being available in lots of different thicknesses it should be relatively easy to obtain an exact length of 141mm.  Fine adjustments can be made by inserting cardboard or paper spacers.

Of course, you can reduce the number of circles required to be cut by glueing several layers of MDF together before cutting the circles out.  Drill out the centre holes of the discs you have produced to fractionally more than the size of shaft you are using (6.1mm), apply glue and thread them onto the shaft, until you have built the length up to 141mm (or 111mm for a Universal organ).

WATCH YOUR FINGERS!

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