Cutting crown flat
Calculating the angles to do it right. October 30, 2003
How do you cut crown flat?
(Cabinet and Millwork Installation Forum)
From contributor A:
Cutting crown moulding flat is how I most often do it. It's easier to handle and I get more consistent results because I don't have to worry about bedding the crown on the same point every time. When you cut crown flat, you're simply cutting the crown on a compound mitre that is equal to bedding it and cutting on a 45. That's not really a big deal because most mitre saws have detents to set up the bevels. First lay the saw over to 33.9 degrees, then set the mitre to 31.5 degrees (someone correct me if that's not right - too used to using the automatic detents on my saw). Once that's locked in, you're all set. Just lay the crown in flat arranged for your mitre or cope cut and you're good to go.
From contributor B:
Bosch makes an electronic angle finder that allows you to place it in an inside or outside corner to read the true angel (not always 90%) and then it gives you the miter and bevel setting. Quite a nice tool and we have used it often when cutting larger crown moldings flat.
From contributor C:
I saw a new setup for coping crown or whatever type moulding you need to cope at the Atlanta show last August. It works like a key cutter - you put the moulding in a machine and it copies the profile of the moulding you need to cope. If I did this type of work much, I would sure look at it closely. It looks simple, fast, etc. I don't remember the name of it (maybe Copemaster or something), but it won the Challengers award there, so should be easy to find. I think it cost a couple grand, but like I said, if I did lots of moulding installation it would pay for itself pretty quick. However, you still need to miter outside corners.
All the angle settings used when cutting crown flat are dependent on the angle of the back or the face of the crown. Some are at 40 degrees, some are at 30 degrees, some at other angles, so there is no set rule on the saw settings unless you do the same moulding over and over, in my experience. If the chop saw is big enough to cut the crown standing upside down, then that is the easiest way to handle this chore for me. You only need to set the saw for 45 degrees right or left miters. I screw a stop on the bottom of the chop saw table to keep the moulding in the same place every time when cutting - that simplifies the job for me.
From contributor B:
The electronic angle finder that I mentioned in the above post sells for around $100 and does the math for you for cutting the crown flat. Sometimes, especially when cutting larger crown, there is not a saw big enough to do it standing against the fence. Also, corners out of square can be troublesome and this allows you to do any angle. You input the spring angle, which is the angle which the crown or other molding is set against the wall and then take a reading of your corner, i.e. 90% or whatever and input it. It then will give you the miter setting and bevel setting for the saw. If you do it properly, it is a snap and the cut is perfect the first time. It is great when using odd angles, etc. and it works for both inside and outside angles. I don't know what I did without it before. We have cut up to 12" crown with good results.
From contributor D:
Here's how to figure the mitre and bevel using a scientific calculator:
To find the mitre angle
1. angle divided by 2=
2. press tan.(answer=X)
3. X x 5.5=Y
4. 3.88 divided by Y=
5. press inv. tan.
To find the bevel angle
1. angle divided by 2=
2. press cos.(answer is X)
3. X x 3.88=Y
4. Y divided by 5.5=
5. press inv. tan.
Make sure to use the actual angle of the wall. Took me some time to get it right but it is guess-proof. I've used it to run huge crown with perfect results.
From contributor E:
To cut most commonly available crown mouldings, set the angle at 31.6 degrees and the bevel at 33.9 degrees. Always have some extra stock to practice with. Cut a couple of short pieces and try them in your application. A little common sense will tell you if the bevel or angle needs adjustment. I recently installed some that required 29 angle and 37 bevel. As far as coping inside corners, particularly on complicated profiles, pre-finished or man made materials, I say forget about it. Miter inside corners once you have found the correct angle and bevel and test the fit with your setup pieces. If your stock and work conditions are not excessively moist, a well-glued butt joint at the inside miters will stand up well.
From contributor F:
Yes, it is called the “Copemaster” and the thing is amazing. I had to put myself on a waiting list but it finally arrived about a month ago. A four and a half inch crown can be perfectly coped in less than 30 seconds. Wow!
From contributor G:
45 degree crown is 30 for the bevel and 35.26 for the miter. 52/38 crown is 33.9 bevel and 31.6 miter. I put up a lot of crown and I don't see much other than these two types.
From contributor B:
In an earlier post, I wrote about the electronic angle finder. The reason I liked it so much was that regardless of the wall angle or type of molding, I could get a quick answer without having to go through a formula (learned all that in high school, but it is a pain in the neck when you are out on the job). Not all corners are at a perfect 90% angle and what happens when you have a wall that is at 22.5% or 30% or some other angle? You can figure it out mathematically, but this instrument gives you a true reading and then allows you to find the miter and bevel at the same time within a couple of seconds. It is made by Bosch and is available at most Home Depot or Lowe's stores.
From contributor H:
As a carpenter who makes a living installing mouldings in high-end residential construction, I have found that crown stops and coping saws can't be beat. For larger crowns we use a 12 inch slide saw with the appropriate detents and some Kentucky windage. A jig saw makes coping a snap. A coped corner will always provide a better fit than an inside miter.
From contributor I:
I agree that a coped inside corner provides a better fit. However, some profiles do not lend themselves to coping and an inside miter is faster and cleaner looking.
From contributor J:
It is true that some of those moldings with a bead can't be coped, and still slide together. To the average observer, coped or mitered joints will pretty much look the same when done well. Mitered sections, though, are very finicky about having accurate lengths, as you don't tuck one behind the other. A piece 1/16" short one a coped joint will not show up, but a 1/16" short mitered stick will look horrible. 20 ft ceilings do seem to improve the looks of some joints I've seen.
From contributor K:
I have been a carpenter for more than 25 years. I have installed miles of crown molding, (not kidding - more than tens of thousands of feet). I have had many a mechanic frown and shake his head at me with a look of disapproving malice. I miter inside corners. If I can fit it, I put a brisket or two in the joint. I always take the time to jig up the chop saw to hold the stock (broom handles in the holes of a 15" Hitachi with a rip of plywood on the back fence and across the front will handle crown up to about 8"). Building a reverse fence and bending back some of the blade shroud will take you to a little better than 9". When it gets bigger than that and I have to resort to a compound miter saw, then I use at least two saws and a jigged chop saw with smaller stock to set the angles on the compound saw. I admit that I "dance" a little with the reveals, and at times, I only tack the build-up parts to open up the dance floor and find a compromise. Carpentry is a controlled series of screw-ups. If floors and ceilings where perfect (and stable) there would be no need for molding. In a shop environment out of square should one cut one kill philosophy should take a look at how some not so old work is holding up. Not be an issue. My priority is tight, lasting joints. I feel I can achieve that faster with miters. I get the angle right with flat stock, cut the crown from the bottom dimension and tap with a wood block and hammer until airtight. I do cope simple trims like ogee and quarter round. Primarily because it saves the effort of finding the angles (it seems that most inside corners in drywall construction are less than square). Strangely, it does not matter to me whether the teeth of the coping saw face up or down.
Here is another can of worms. When it comes to splices in the field I butt cut the joints. Yep, that is what I do. I can feel the heads moving left to right - more of that disapproval. Long lengths of crown molding are sprung into place and under an ever-changing amount of pressure. I have seen even 22.5 degree scarfs walk out of flush after five years.
The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).
Comment from contributor L:
For cutting crown molding on a flat, I've created a calculator that gives you the angles and bevels for any angles.
Comment from contributor S:
I had a heck of a time installing crown in my study. I went through (5) 8' sections before I got the angles right. What I ended up doing was setting my compound miter saw at a 33.9 degree bevel and a 31.5 degree miter - then I cut the right side with the crown lying flat and facing up (the top of the crown that sits on the ceiling tight against the fence of the saw) and the flipped the piece over and cut the left side with the crown lying flat and facing down (the top of the crown that sits on the ceiling still at the fence, but the pretty side is facing down.)
This prevented me from changing the bevel and angle on the saw for every cut and produced a nice clean edge. There were a few gaps that were easily fixed with caulk, but that was because I didn't get the lengths 100% accurate. The best advice is to practice on some scrap before you delve into the real stuff.
Comment from contributor M:
One of the best ways to put up and figure cutting angles for any crown cutting flat on saw is with a free palm Pilot program called "Compound Angle Finder" programmed by Gary Wiese. The program is very simple. It shows a picture of crown moulding. You enter the angle of the crown on the bottom where it fits against the wall and then use a protractor from any lumber yard to see if the corner is a true 90 degree.
You put these two numbers in and then hit the calculate button and it tells you the miter angle for your saw and the bevel angle. This will work with 45 degree crown, 52/38 or any other odd style crown you may come across. None of the charts that come with some of the miter saws helped me since I needed to cut flat 7.25 inch crown that had an odd "spring" angle and not the normal 45 or 58 degree.