Cutting crown moldings can be simple and it can be difficult.
First you want to check if the corner you are putting the molding into is in fact 90 degrees. If you don't check that you can get real frustrated with your mitre saw. 88 or 92 degree walls require different mitre cuts.
Over the years I have touched on the complicated job of cutting crown moldings various times. Lower down in this entry you will find the chart for the mitre bevel settings for cutting these moldings flat in an electric mitre box. You may want to visit some of the older entries for more tips: A Great New Tool -- the Angle Finder or Getting Tight Mitre Joints In Molding or How do you Cut Crown Molding in an Electric Mitre Box or Cutting Corners on Crown Molding or Cutting Angles On Crown Molding
See this entry in video
Especially useful is my new videos on this task: Cutting Crown Molding in the Learning Curve tab above, part of the whole video series on Miter Saws. Some people like both text and video, so I have kept the text version below which spells out what you see in the video.
The simple Vertical Chop
The best way to cut crown molding is by not using the bevel nor the sliding action of a mitre saw at all, but just using a straight 45 degree mitre vertical chop cut. The only time I will abandon this good old technique is when the molding is too big to sit up in my saw, when the blade or the bottom of the saw won't clear. Look at the third photo.
Think of the saw bed as the ceiling and the fence as the wall, which means that you place the molding up-side-down in the saw.
Clamp a stop block on the fence so that when you slide your piece of molding up to the stop block, it will be sitting exactly as you want it on the wall. This automatically compensates for the difference in different moldings that may be 45/45 or 52/38 degrees in the corner or maybe some other angle.
Now you simply cut left and right 45 degree cuts.
The best way to get those stop blocks in the right place is to make a wooden fence with integral stop blocks that gets screwed to the metal fence. Identify this fence jig as being for a certain type of molding and you will quickly be right every time you need to cut that same crown molding.
When the molding is too big to fit into the saw standing up, then you have to lie it down on the table and use a compound mitre/bevel cut, and even a sliding cut if the molding is very large. It can be done, but it is far more difficult to get it right.
The all important GUIDE SAMPLE SET
Take a small molding and cut it in the traditional way to get a good corner, then lay it down flat and play with the mitre and the bevel angles to see how you will have to place the wood to get the cuts you need. The flat back of the molding always goes down on the table but whether you cut to the mitre right or the mitre left and with the wood on which side of the blade can get down right confusing. I always have a small pair of moldings which make a good inside corner, and another small pair for the outside corners, with each piece clearly identified, inside left, or inside right, or outside left, or outside right.
I hold the proper pair up to the wall where I want to install my real molding, identify which piece I want to cut now and then put the rest back in my pocket. I then take the definitively correct piece to the miter saw and play with all the possible positions until it will fit the saw cut. That is how I place my final piece of molding in the saw without wasting precious inches cutting the end of the board off two or three times. If you are having trouble following that, watch the video.
Finding the Angles
Now you have to consult a very complicated table to figure out what mitre angle and what bevel angle is required. Most new saws have marks, if not stops for the most common 52 degree by 38 degree molding which certainly helps the operation. But if your wall is 92 degrees, without the table you would be out of luck.
In the table, first determine which type of common molding you have, 45/45 or 52/38.
Then determine the wall angle.
Then you will find the mitre and the bevel angles for that combination in the table.
Setting the saw is never as precise as the table which tells you the angle to .01 inches precision, so you must do some test cutting. If your joint is too open, or too closed, move towards the next or previous row of settings listed in the table. Remember that you may have to adjust both the mitre and the bevel to get it right.
Here are the table entries close to 90 degrees. When I have the time, I will add the full table for you.
For 52/38 degree molding:
-- Wall Angle 88 deg. -- Mitre 32.52 deg. Bevel 34.53 deg.
-- Wall Angle 89 deg. -- Mitre 32.07 deg. Bevel 34.20 deg.
-- Wall Angle 90 deg. -- Mitre 31.62 deg. Bevel 33.86 deg.
-- Wall Angle 91 deg. -- Mitre 31.17 deg. Bevel 33.53 deg.
-- Wall Angle 92 deg. -- Mitre 30.73 deg. Bevel 33.19 deg.
For 45/45 degree molding:
-- Wall Angle 88 deg. -- Mitre 36.21 deg. Bevel 30.57 deg.
-- Wall Angle 89 deg. -- Mitre 35.74 deg. Bevel 30.29 deg.
-- Wall Angle 90 deg. -- Mitre 35.26 deg. Bevel 30.00 deg.
-- Wall Angle 91 deg. -- Mitre 34.79 deg. Bevel 29.71 deg.
-- Wall Angle 92 deg. -- Mitre 34.33 deg. Bevel 29.42 deg.
The bevel stays fixed for both the right and left cuts, but one side will be cut on a right mitre and the other on a left mitre and some cuts on the right side of the blade and other cuts on the left side of the blade. Use those little sample pairs to keep you straight.
A special thanks to Porter Cable for the compound mitre/bevel table that you see here. Click on this hot link for information on the new "Dual Bevel Mitre Saws".