Compound mitres are commonly associated with the installation of crown mouldings. Crown mouldings are manufactured at a predetermined angle designed to span the 90-degree intersection of the wall and ceiling. For this reason they are not recommended for installation in rooms with cathedral or sloped ceilings. The installation angle remains fixed but must be accounted for when cutting individual moulding sections to meet at inside and outside corners. Compound mitre saws have a predetermined angle setting, while allowing for the inside or outside corner angle to be independently set.
Determine if the mitre saw you will be using is a compound mitre saw. Compound mitre saws allow the blade to be angled vertically as well as horizontally. If the mitre saw is not a compound mitre saw, you can still cut compound mitres.
Draw the room's perimeter, showing all outside and inside corners. Interior walls generally intersect at 90-degree angles for both inside and outside corners. Typical exceptions are bay window extensions. Bay extensions are typically three sided, yielding two outside corners and two inside corners. In this case the angles will be 45 degrees each.
Cut two pieces of 1-by-2 wood at a 45-degree angle at one end of each board, to begin calculating the angles. Each board should be approximately 12 inches in length.
Place one board with the mitred end on one wall and slide it to the corner. Place the other board on the intersecting wall and slide it to the corner until the 45-degree-angled ends of the two boards meet.
Check the angled joint where the two boards meet and determine if any gaps exist. For inside corners, if the gap is consistent from front to back, the corner angle is 90 degrees, but some sanding may be required to remove the sharp pointed edge of the moulding before final installation. If the heel of the joint is tight but a gap exists at the outermost intersection of the joint, the corner is greater than 90 degrees and each 45-degree cut will have to be increased. If the heel of the joint shows a gap and the outermost intersection of the joint is tight, the corner is less than 90 degrees and the 45-degree cuts will have to be reduced. Record the angle on the drawing as either 90 degree, 90 degree +, or 90 degree -.
Calculate outside corners, in the opposite manner to that described in Step 5. If a gap is shown at the heel of the joint and the outermost intersection of the joint is tight, this indicates the outside corner is greater than 90 degrees and each 45-degree cut needs to be reduced. If the heel of the joint is tight and the outermost intersection of the joint shows a gap, the angle of the outside corner is less than 90 degrees and each 45-degree cut needs to be increased. Record the angle on the drawing as either 90 degree, 90 degree +, or 90 degree -.
Measure bay extensions by recutting the two pieces of 1-by-2 wood to angles of 22 1/2 degrees. Check each inside and outside corner as indicated in steps 5 and 6.
Measure the length of crown mouldings at walls and never at ceilings. All measurements are taken from inside corners to outside corners. Hold the tape measure firmly to avoid any sagging or bowing in the tape measure. For longer measurements, a second person holding one end of the tape measure may be necessary.
Mark all cuts along the bottom edge of the crown moulding where it will intersect the wall.
Make an angled mark at each cut mark to indicate the direction of the angle cut.
Set the tilt of the saw blade to 33.9 degrees if the mitre saw is a compound mitre saw. If the mitre saw is not a compound mitre saw, you will not be able to change the blade tilt to 33.9 degrees. Set the saw blade to the correct angle for the cut. For a true 90-degree corner, set the saw angle at 45 degrees. For a 45-degree corner, set the saw angle at 22.5 degrees. Increase or decrease the saw angle when needed as determined in previous steps.
Place the crown moulding flat on the horizontal bed of the mitre saw, if you are using a compound mitre saw. Place the crown moulding face up for one side of the mitred corner. Place the crown moulding face down for the other side of the mitred corner. Align the blade to the measured mark you made and make the cut.
Position the crown moulding so the edge that intersects the wall is flat against the bed of the saw and the edge that intersects the ceiling is flat against the vertical back of the saw, if the mitre saw is not a compound mitre saw. For one side of the mitred corner the face of the moulding should be facing you; for the other side of the mitred corner the back of the moulding should be facing you. This method simulates the 33.9-degree angle of the moulding.
Most tape measures are scaled in 1/16 inches. Professional carpenters don't record measurements in 1/16th inch increments. If the actual measurement is 9 7/16 inches, record the measurement as 9 3/8+. If the actual measurement is 9 5/16 inches, record the measurement as 9 3/8-. Carpenters use this trick because it is actually easier to call out measurements in 1/8 inch increments and either add or subtract the 1/16th inch fraction.
Never wear loose or improperly fitting clothing when working with power tools. Always wear eye protection. Keep hands and fingers away from saw blades and other moving parts.