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What is the best way to calculate the angle that crown moulding should be cut in the compound miter saw to fit an inside corner?

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See my answer on this question. –  Doresoom Aug 10 '11 at 13:35

7 Answers 7

up vote 7 down vote accepted

Most crown molding is going to have to be cut flat on the table. the angles are going to be 52 degrees on the angle, and 38 degrees on the bevel. For your corners, you are going to want to cope them instead of trying to fit these angles. Coping is the proper way of installing any type of trim (crown mold, base, shoe molding, cherry rail ect.)

How you would cope your corners is by getting your measurement to the wall. You will be running the coped end into another piece that is cut square into the wall. Your wall measurement for your coped piece is going to be the long point of your 52 degree angle, and the long point of your 38 degree bevel. You will then take a coping saw and cut back into the piece cutting along the short edge of your bevel cut. Following the pattern of your molding. A good technique to installing to ensure a tight fit is to:

  1. First make sure your corner fits well into the other piece.

  2. After you have to corner matched nail the molding about 4-5 feet back from the corner to hold the piece up for you. Also, leave your square cut loose as well. This way you can manuver both pieces to fit right incase the corner is out of square.

  3. Match your corner and nail where it fits snug.

  4. Nail in the middle of you first nail and the corner. If there is any slack, push the middle in and nail. This will help seat your corners and give you a nice fluid look.

If you choose to try to angle you corners you are going to spend all day trying to get them right. It will not look good, and you will waste a lot of time. With coping you can get away with being a degree or two off without it opening up at the base or top of your corners.

I have done trim carpentry for 8 years and this is the best way to put up your molding. It will look great, and you will not have to worry as much about the expansion/contraction of your molding due to season changes. Also another tip. On your long runs, make sure you bevel the ends together when joining two pieces in the middle. Do not ever but up square ends when installing any type of trim. Happy DIY 

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For an inside corner, don't concern yourself with mitering the moulding, instead cope the crown moulding. Basically what you do is cut one piece so it goes to the end of the wall. Cut the other piece at about a 38 degree angle and then cope the angled piece. It's a little hard to explain in text, but here's a picture and a link to the corresponding instructional video.

Coping moulding

As you can see by the red circled area, the author has emphasized the miter cut with a pencil. You would make a steep angle cut into the moulding along this pencil line following the contour of the moulding. After doing this, the coped piece should fit snugly against the flat piece and look like a nicely mitered joint.

Here's the corresponding instructional video.

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+1 for copping internal corners, is "generally" the only way to go with internal corners (IMHO). –  Mike Perry Aug 10 '11 at 14:36
    
It is not going to be at a 45 degree angle. Most are at 38 degrees read my answer below. –  lazoDev Aug 10 '11 at 14:41
    
The video linked shows a mitered corner, not how to cope it. –  gregmac Aug 10 '11 at 16:14
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45 degree angle would work if the piece was flat against the wall. Crown molding sits at a 38 degree angle from wall to cieling. When you pull the crown mold out and try to fit a 45 degree angle i bet you $100 that your bevel will be open on the front, and you will have a gap at the bottom of your piece. When you pull the piece off of the wall you are opening up the angle. –  lazoDev Aug 10 '11 at 16:46
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I'll defer to lazoDev on the angle to cut. A 45 probably is too steep a cut. 38 probably is the better choice. I'll edit my answer. –  billoreid Aug 10 '11 at 19:09

You could pick up a Bosch Miterfinder™ Digital Protractor

enter image description here

Features

  • Anglefinder — Determines the exact angles of jobsite or workpiece, eliminating guesswork.
  • Compound Cut Calculator — Automatically determines the exact miter and bevel settings necessary to make each crown molding cut fit precisely.
  • Protractor — precisely positions the workpiece or intended cut line at the desired angle
  • Level — Built-in vertical and horizontal spirit levels, accurate to 0 +/- 0.05°
  • Digital readout displays on front and back — For easier use
  • Hold Function - To freeze measurements

It will calculate the angles you need based on the actual angle of the corner, so you won't have to guess. You might not get a perfect fit every time, but you'll be very close so there will be less of an adjustment to make.

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Note that you don't need to cut crown moulding flat, you can just place it upside down with the top on the base of the miter saw and the wall side against the back of the saw.

As ChrisF mentions, your walls won't be a perfect 90°. For inside corners, the mud from the drywall install will push the corner out, which will result in the back corners of the moulding touching before the visible ones. Therefore, I typically cut closer to 43° or so for an inside corner. For outside corners, I'll reverse that to 47° or so.

And as others say, coping an inside corner has the best look if you're good at coping. And when joining pieces in the middle of a long wall, make sure the 45° you cut points into the wall instead of into the room where any gap will be visible. (Same rule goes for installing vinyl siding, make sure the pieces overlap so you aren't looking into any gaps from the front of the home.)

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Miter-Bevel Settings Chart - Attached Below

Miter-Bevel Settings Chart - Attached Below

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Your answer would be much better if it also included how to use such a chart. Also, do you have a source for said chart that you could link to? –  Doresoom Jul 18 '13 at 17:20

The angle you need to cut is simply half the angle of the corner. So assuming it's a 90° corner you want 45°.

However, most houses don't have exactly 90° corners (at least not in my experience) so you'll probably find that this produces a joint that doesn't fit.

You'll have to resort to trial and error on some scrap pieces (as I outline here) to get a more accurate fit.

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I was looking for a reliable way to do this and came upon a phone app called Crown Molding King (Android OS - but I'm sure there are others for other OS). The app is free. Enter the angle of your wall and the app will calculate the Miter and Bevel.

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