It depends on the Crown... not all crown sits on the table with the top and bottom exposed corners parallel to the table. The best bet is to try to cut one good corner the normal way and then lay it on the table and line up the blade to the miter...
remember that the crown can be "rolled" up or down against the wall untill you get a good match. If it is solid wood you can "adjust" with a chisel or two. Hope this helps.
Try to picture the molding as a solid block that has one diaginal corner removed, this will give you the perspective of a simple miter cut. Then place to molding at the correct angle upside down on the miter. Not all crown molding fit between wall and ceiling at a 45 degree angle I try to fit the molding to my miter saw at the correct angle and then draw a line on the back plate so that the molding is placed the same each time I make a cut.
To miter a 45 you need to cut 22 1/2 degress, there should be pre set marks on most miter saws. I know with my delta miter saw there was a table of angles in the hand book to use in situations that were not true 90 degree angles. First determine if the corner is a true 45 degrees then adjust the angle of the cut. If you do not get the right degrees to make the corner then one profile will be longer and the molding shape will not fit.
I'm in the process of installing crown molding on a project in my basement. I can't seem to figure out how to match the molding on an inside 90 degree corner? Do I butt one end against the wall and cut/cope the adjoining molding?
Crown molding can be one of the trick-i-est stuff you'll ever encounter.
There are lots of formulas, lots of advice, and lots of it is damn confusing!
My youngest son help me at Xmas time with mine and here's some of what we learned:
Take scraps of the molding (or cut some MDF and bevel the back corners to EXACTLY match the back and width of your molding)and work on making your trial cuts and fitting them. Then make your adjustments in small incroments and retry the fit. You'll be closer or further off. Adjust and repeat.
Normal crown molding angles are 33.9 by 33.9 degrees for a 45 degree backed or tilted molding. Weird yes, but it fits. Most CMS's have these on the scale, I belive. This is your starting point.
You may want to build a small mock up of one of your walls on your benches edge and use an angle finder to fit your practice pieces and check your angles.
(We were dealing with crown molding on a vaulted ceiling of a house 37 years old and not a square corner in the house.)
Remember if it gets too harry or frustrating, to refer back to the 33.9 x 33.9 degree angles.
At times you will find useing 45's inside and outside will step you around an offset in the wall.
You are dealing with a house, not a picture frame. Chances are you won't get picture perfect results. Just do the best you can.
If you leave about a 1/8 to 1/4" gap at the top of the crown molding and only nail it to the wall, it will add a very pleaseing shadow, and the effect makes the whole ceiling look higher to the eye looking up at it from below.
I hope some or all of this helps you.
Many crown mouldings do not sit at a true 45 degrees - more
like 38 to 40. For these, set your bevel angle at 34 degrees and
your miter angle at 31.6. Set the stock flat and face up for
both cuts of an outside 45 degree corner. Most chop saws have
these settings marked. If you want the moulding angle at 45
degrees, try the opposite bevel and miter settings. Either way,
make test cuts on scrap (any) stock. For inside corners, the
best fit is to cope one side.
Finally, something I can actually answer on this site (intelligently that is)! I have over 600 ft of crown molding in my house. When I started putting it up, I had about 20 ft on the ceiling and 200 ft in kindling. It took 2 days to get that 200 ft of kindling I might add. I told my wife no crown, packaged up my brand new DeWalt 12" CMS and brought it back to Menards disgusted. Luckily, the guy pressed me on my return and he shared a great piece of advice for hacks like me. CROWN STOPS.
Cost me about 30 bucks and they are a dream come true. No compound miters. Kind of defeats the purpose of the saw, but makes for a SUPER easy job putting up crown molding. I highly recommend them. The moulding goes into your miter saw as it would sit on the ceiling (just turned upside down). Then, you adjust your CMS saw to the angle of your corner on the side to side miter. You leave the saw blade at a 90 to the table. Take the time and set them up correctly on the CMS. Work w/ a scrap piece to check it. Once you have them adjusted though, then you are on your way. Take a look at Amazon.com's link attached. They have crown stops (as should Home Depot, Lowes, whatever). With any luck, they make them for your saw. I have to tell ya, I can't live w/ out them. Good luck!
Note: the first two pictures, top to bottom, show the sample woods I bought to match: mahogany, walnut, oak, the bottom is the side of the table. The remaining 3 pictures are of the wood in question...