# How to Calculate Compound Miter Cuts for a Table Saw

Written by timothy burns
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Compound mitre cuts are required when making many types of decorative wood projects. For example, compound mitres are used when installing crown moulding around the perimeter of a room, between a wall and ceiling. Compound mitre cuts are also used when making multi-sided bevelled planter boxes and gazebos. In most cases, a table of values is the simplest way to obtain these settings. For custom applications, compound mitre settings can be calculated in a few steps.

Skill level:
Easy

### Things you need

• Protractor
• Carpenter's square
• Carpenter's bevel or similar angle measuring device
• Table saw
• Table saw mitre slide
• Scientific calculator

## Instructions

1. 1

Determine the three dimensions -- drop, projection and slope -- involved in your wood project. The drop is total vertical distance, or the height of the piece. The projection is the total distance the piece of wood will project from a vertical line. The slope is the angle at which a piece of wood will descend from a horizontal plane lying along the top edge of the wood piece.

2. 2

Determine the fourth dimension needed for these calculations, which is the flat mitre dimension. The flat mitre dimension is the angle at which a saw would be set if cutting a perpendicular cut along the edge of the wood board or moulding. For example, if cutting the mitres to make a square four-sided box, the flat mitre dimension would be 45 degrees for each edge of each board. In general, the flat mitre would be one-half of the total angle to be achieved when joining the two boards.

3. 3

Apply the following equation to determine the mitre angle for the piece of wood: Mitre = ArcTan [ Tan [flat mitre] x projection / width]. Determine the angle setting for this miter with the scientific calculator. Set this miter angle on the table saw's miter gauge guide.

4. 4

Apply the following equation to determine the bevel angle for the piece of wood: Bevel = ArcSin [sin [flat mitre] x drop / width]. Determine the angle setting for this bevel cut with the scientific calculator. Set the table saw's blade to this angle.

5. 5

Remove the table saw's rip fence. Carefully cut each piece of wood needed for the project with the mitre saw slide gauge.

6. 6

Test fit each piece of wood after every cut. Small adjustments can be made to either the bevel or the mitre angles to ensure a perfect fit for each piece.

#### Tips and warnings

• Always use caution when operating power equipment. Wear safety glasses and make sure to not wear loosefitting or baggy clothing, which can become entangled during the table saw's operation.

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