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Get perfect fitting mitre joints every time on the table saw!
One of the most common joints a wood worker might cut when building a project is a mitre joint, such as when two pieces of trim meet at a corner or when making a picture or mirror frame. Cutting the two pieces so they meet perfectly is a simple matter using a compound mitre saw which would create a straight cut that mates perfectly with its neighbour.
However, using a compound saw requires you to set up your saw twice, once for each piece, and if you are not exactly dead on with the angle setting, the two pieces will not meet at 90 degrees. On a single corner this may not be as noticeable, but when you are building a picture frame, by the time you get to the fourth corner, the pieces will have no hope of closing properly.
This jig employs a principle taught in junior high math class, and while it is not as sexy an addition to your shop as a nice new Compound Mitre Saw, you will find this jig quickly becomes your choice for all 90 degree mitred cuts. By cutting a perfect square to act as the fence on the jig, the principle of complimentary angles assures that when one of the pieces is cut on one side of the jig and its mating piece is cut on the other, the two pieces brought together always form an exact 90 degree angle. If the angle on one piece is not exactly forty five degrees dead on, dont worry, the jig will ensure that the second cut is the exact difference between the cut you just made and 90 degrees.
Build the jig
For this jig to be accurate the piece of plywood (fence) that the material registers against must be cut at an accurate 90 degrees; use a cross cut sled for this
Make the perfect cut.
To cut a perfect mitre, lay your pieces out and mark one piece with an A and the other piece with a B. Place the piece marked A on the A side of the jig and make the cut. Place the other piece, marked B on the other side and make the cut. The result will be a perfect 90-degree mitre joint. Most mitre joints will be cut on stock that will adequately register against a shallow fence as shown, if you need to cut a number of joints at the full depth of the blade you might consider building a jig with a taller fence.
One of the most common joints a wood worker might cut when building a project is a mitre joint, such as when two pieces of trim meet at a corner or when making a picture or mirror frame. Cutting the two pieces so they meet perfectly is a simple matter using a compound mitre saw which would create a straight cut that mates perfectly with its neighbour.
However, using a compound saw requires you to set up your saw twice, once for each piece, and if you are not exactly dead on with the angle setting, the two pieces will not meet at 90 degrees. On a single corner this may not be as noticeable, but when you are building a picture frame, by the time you get to the fourth corner, the pieces will have no hope of closing properly.
This jig employs a principle taught in junior high math class, and while it is not as sexy an addition to your shop as a nice new Compound Mitre Saw, you will find this jig quickly becomes your choice for all 90 degree mitred cuts. By cutting a perfect square to act as the fence on the jig, the principle of complimentary angles assures that when one of the pieces is cut on one side of the jig and its mating piece is cut on the other, the two pieces brought together always form an exact 90 degree angle. If the angle on one piece is not exactly forty five degrees dead on, dont worry, the jig will ensure that the second cut is the exact difference between the cut you just made and 90 degrees.
Build the jig
For this jig to be accurate the piece of plywood (fence) that the material registers against must be cut at an accurate 90 degrees; use a cross cut sled for this
- Cut the base of the jig to size for your saw. This version is sized to fit a Delta Unisaw and you may need to adjust the base size and placement of the runners to suit your equipment.
- Cut the triangular fence piece.
- Place the triangular fence piece on the base and fasten it in place. If you choose to use brads or screws ensure that they are not located in the path of the saw blade. Using glue alone ensures there is no chance of damaging the teeth your saw blade by hitting a nail or screw.
- Cut a blade guard and fasten it over the centre section of the fence so that the blade is completely buried in the wood after the cut.
- Fasten a set of toggle clamps (Lee Valley) to the fence section close to the cut line to hold the material in place during the cut.
- To keep the jig running snugly in the tracks without any play we use Quarter-sawn White Oak or UHMW for the runners. Fasten these to the underside of the base plate so the cut line of the blade runs right to the point on the fence. Mark one side of the jig A and the other side B
To cut a perfect mitre, lay your pieces out and mark one piece with an A and the other piece with a B. Place the piece marked A on the A side of the jig and make the cut. Place the other piece, marked B on the other side and make the cut. The result will be a perfect 90-degree mitre joint. Most mitre joints will be cut on stock that will adequately register against a shallow fence as shown, if you need to cut a number of joints at the full depth of the blade you might consider building a jig with a taller fence.
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