Jigs

Cutting past 45° on a mitre saw

A simple jig with a 45° notch that lets a mitre saw cut at 55°, 60° and beyond.

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3 min

A typical mitre saw only cuts to 45 or 50°. Sometimes you need more, for example when mitring boards of different widths, where the narrow piece needs 54.5°. A simple jig made from offcuts gets you there.

The idea

The jig holds the piece turned through 45° to the saw's fence. So with the saw at zero it cuts at 45°, and every degree on the scale adds to that.

The maths

setting = angle − 45°

55° is 10° on the scale, 60° is 15°, and 54.5° is 9.5°.

Angle you need Setting on the scale
50° 5°
55° 10°
60° 15°
65° 20°
70° 25°
75° 30°
80° 35°

Building it

You need two boards or pieces of MDF 18 mm thick, as long as the saw's table, for the base and the back board. For the block, take a piece at least 25 mm thick and 75 mm wide. Glue everything with CA glue and activator.

First set the depth stop so the lowest point of the blade stops about 12 mm above the table. The blade then cuts into an 18 mm base but not through it.

Side view: the jig base, 18 mm thick, lies on the saw table with a board behind it against the saw fence. The lowest point of the blade stops 12 mm above the table, so it cuts into the base but not through it
The blade stops 12 mm above the table and doesn't cut through the 18 mm base.

Then put the jig together in three steps:

  1. Glue the base to the second board in an L. That board sits against the saw fence. Mark how far the blade reaches turned fully left and right, and glue only outside those marks.
  2. Glue the block to the base, pushed up against the back board.
  3. Cut through the block at 45° left and right and take out the middle. That leaves a V-shaped notch with a right angle at its tip.
Three steps from above, saw fence at the top. One: the base glued to the back board in an L, glue only at the ends, with dashed lines for the blade's full swing either way. Two: a block glued to the base against the back board, with dashed lines where the blade cuts it at 45 degrees left and right. Three: the middle of the block removed, leaving a V-shaped notch
1: base and back board, glued outside the blade's reach. 2: the block. 3: two 45° cuts leave the V.

Cutting

Lay the piece in the notch along one face and pull it back slightly from the back board, so the offcut doesn't jam during the cut. Set the saw to the angle minus 45° and cut. Which way to turn the saw depends on which face of the notch the piece lies against, so make the first cut on scrap and measure the angle with a protractor.

A mitre saw from above with the jig on its table: the V-shaped notch with its tip at the middle of the saw fence, a narrow piece along one face of the notch held a little short of the back, a dashed line where the blade cuts at zero, and the blade turned 9.5° about the middle of the saw fence, just behind the tip of the notch, cutting a small offcut from the end of the piece
The notch gives 45°, the saw adds 9.5°. That makes 54.5°.

For a single cut, doing it by hand is quicker. Mark the line, cut just beside it on the waste side with a hand saw, and plane down to the line.

Mitring boards of different widths

The most common reason to reach for this jig is mitring boards of different widths. You cut the wide piece as usual, without the jig. The narrow one needs more than 45°, so it goes in the jig. The first column gives the narrow and wide widths. “Jig” is the scale setting for the narrow piece. The angles are rounded to half a degree.

mm Wide Narrow Jig
50/60 40° 50° 5°
40/50 38.5° 51.5° 6.5°
70/90 38° 52° 7°
60/80 37° 53° 8°
50/70 35.5° 54.5° 9.5°
40/60 33.5° 56.5° 11.5°
50/100 26.5° 63.5° 18.5°

With 50 and 60 mm the narrow piece needs exactly 50°. If your saw reaches 50°, you can cut that one without the jig.

Sources

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