miter crown "into" existing crown

Try a coping saw. Cut your crown as though you were going to join two miter cuts at right angles. Then follow the cut with your coping saw. A bastard file may come in handy for a little touch up.

-- Darrell, making more sawdust than I know what to do with

devann is correct. Cut the 45 degree cut then cope the line where the cut begins. If the box is not 45 degrees then the saw might need to be adjusted to something other than 45 deg. I like to use a protractor on a machinists square. I can read exactly what the angle is then I can adjust accordingly (half of the angle you read). I try to cut the back shorter than the front of the cut. use a half round course file and shape it a bit if necessary. Once you establish that cut then you can mark and cut the outside corner to length. I also use the protractor to read this angle so I can make it perfect. If the molding is not exactly the same as the original it will not fit. I have seen pieces bought out of different lots that just didn’t work together. Sometimes it can be different mills or the blades have been ground or whatever. It has to be exactly the same molding.

45 degrees may be the wrong angle. Crown comes at
different “spring angles” and 45 is not the most common
from what I’ve seen. In order to cope the new crown
into the old, you need to know the spring angle of
the old crown…. well, there are other ways, but I suspect
you may not be grasping some key principle of crown
installation and geometry.

Mitering your new crown at the outside corners will challenge
you as well… if you get the spring angle wrong.

I make scrap boards with the same back-side bevels
if crown is not making sense to my brain… then I play
with the scraps until I think I’ve got it figured.

This is true. If the molding is not on the corner at a 45 degree angle then things change again.

Did you cut your mitre (on the chop saw) with the molding upside down on the table? Then use your coping saw.

-- Ben

The typical crown molding seen in houses today is a 38-52 degree crown. It does not sit at a 45 degree angle to the wall and ceiling, it sits 38 degrees from the wall to the ceiling and 52 degrees from the ceiling to the wall. My late father in law and I installed hundreds of feet of crown in old houses with uneven ceilings and out of square walls and coping the corners made it look square. If you a small piece of the crown, 4”-6”, place it upside down on your mitre saw and check when the ceiling portion (flat on the table) and the wall portion (on the fence) are both flat against there respective surfaces. When they are, mark a line on the fence and and the table with the mitre set a 45 degrees both ways and when it is square. This way you will have a references that will keep the molding at the right (correct) angle. If it is primed or stained, when you cut the mitre, you then take your coping saw and cut down the profile between the the surface and the meat of the molding. Angle the saw back into the molding so you cut out enough that it does not interfere when you place it in the corner. A 4in 1 rasp or file helps clean up any leftover that may be in the way. Once you get the first good joint, the rest will be easier. I does have a bit of a learning curve so be patient and good luck.

b’wooof… I presumed a simple three-sided wrap of crown would take five minutes with a compound miter saw and an air nailer. However, my experience coupled with my inexperience and my naiveté guaranteed it’d morph into a multi-tooled project with too much scrap.

The consensus to my post is that the angle of the existing crown was the source of the problem. It was.
So I got the mounted angle of the existing crown, then let the computer determine the profile for me… virtually. (...If only a CADD-CAM driven tool existed to make the actual cut, eh?) The required profile cut of the new crown appears in the illustration below. I set my saw to match the mounted angle of the existing crown, then used my Dremel to form out the profile shown in the illustration. To my amazement, the pieces fit.

Thanks a whole bunch for the help!