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Foamcore Rabbet Cutter

Foamcore Rabbet CutterFoamcore can be joined so that the foam doesn't show by cutting a 'rabbet' at one edge (see the picture to the right). You can use a craft knife and a ruler to cut rabbets. The trick is to cut through the first layer of paper and then run your knife along the cut again to go through the foam while avoiding cutting through the bottom layer of foam.

If you plan on cutting a lot of rabbets then it's a good idea to use a rabbet cutter and while commercial versions are available, Phil Graham (wwizzard on the forum) made his own for a fraction of the cost.

Foamcore Rabbet CutterPhil's cutter was made from a piece of wood, a Swann-Morton 10A blade, and three brass pins. The picture to the left makes it fairly obvious how it should be assembled but you should note that Phil is left-handed; so right-handed people will probably want to make a mirror image of the one shown i.e. with the blade the other way around.

For his cutter, Phil used a piece of wood measuring 19mm x 19mm x 130mm but this is not crucial. What is crucial is that the blade should be positioned such that it will only cut through to top layer of paper and the foam while leaving the lower layer of paper untouched.

Phil started by inserting the lower pin. The wider part of the Swann Morton blade will go over the head of the pin, the narrower part won't so the blade is held in place. After adjusting the angle/depth of the blade Phil used the other pins to hold in to place. Note that the two lower pins are not pushed all the way in while the 3rd pin is. This allows the blade to be changed without needing to remove any of the pins.

The cutter can be used in this form by marking a line the appropriate distance from the edge of the board and running the cutter along this line. The blade is 'automatically' held at the required depth. The waste strip is then removed with a scalpel. This technique is shown in the photos below.

Foamcore Rabbet Cutter Foamcore Rabbet Cutter

Foamcore Rabbet CutterA more sophisticated approach is to make a jig to use with the cutter.

The jig shown in the drawing to the right is also for a left-handed person. It consists of a wooden baseboard (A) a wall (B) a stop (C) and a spacer (D). Note that there is a gap between the stop and the spacer to allow the blade of the cutter (which rests on the spacer and is drawn along the wall towards the stop) to pass through it. The spacer is the same width as the wood that was used to make the cutter and is of a thickness such that when the cutter is sat on top of it, the blade does not touch the baseboard. In fact the blade is held the thickness of a sheet of paper above the baseboard as illustrated by the drawing below.

Note that if you plan to make the cutter and jig at the same time you will find it easier to set up the blade last so that you can use any thickness (over 5mm) for the spacer and adjust the blade to suit. If you make the cutter first and then want to make a jig at a later date you can make the spacer from a piece of 4mm MDF and then use layers of paper or card to build it up the the correct thickness before glueing the whole lot in place.

Foamcore Rabbet Cutter

Because the spacer is the same width as the wood used for the cutter, when the cutter is placed on top of the spacer and pushed against the wall, the blade will run along the edge of the spacer. Placing a piece of foamcore of the thickness for which we are cutting the rabbet vertically between the wall and the cutter will move the blade away from the spacer by the distance that the rabbet needs to be cut into the foamboard.

Cutting a rabbet is now as simple as lying the piece of foamcore that is to be cut on the baseboard, such that it butts up against the spacer and the stop, putting the piece that will go into the rabbet between the wall and the cutter, and pulling the cutter along the spacer. Remove the waste strip as before and you have the perfectly sized rabbet with no marking or measuring required!

It also works for 3mm foamcore without any need for adjustment!

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