How To Get Angles to Match When Cutting Molding
When cutting molding at an angle to be joined with another piece of molding, getting a precise match on the angle can be challenging. The casing around a doorway is a good example. Three pieces of moulding must be cut to go around the door. Across the top, all three pieces have 45° angle cuts. If any of the cuts is the slightest bit off, the pieces will not match exactly, it will either have a gap at the outer edge corner or the or the inner edge corner.
The first thing to know is, don't assume the door frame is square. It isn't unusual for a door frame to be a degree or two out of square. So even if you do cut the angles on the molding to align properly, they will not align with the door. As you adjust the moulding to align with the door, the joints will not longer be tight. You need to determine the angle of both corners of the top of the door frame before doing any cutting.
When cutting two pieces of moulding, or any two pieces of material, the method to get a perfectly matching joint is to divide the angle in half. So around a door frame that is a little out of square, let's say by 1° making it 89° , you bisect the angle (89 divided by 2 = 44.5°).
If you are using a power miter saw, cutting the angles is a lot easier. You can set the angle on the saw and then make the cut. Although, most saws are a little out of alignment, so that doesn't actually work perfectly. So if you can't rely on your saw's accuracy or if you are making the cut by hand, how can you get the desired results?
The trick to getting a perfect cut is to position the two pieces as they will be when installed, overlap the ends to be cut and then cut through both pieces at the same time. This does not guarantee that you will perfectly bisect the angle but it does guarantee that the joint between the two pieces of moulding will match. Also, since the upright pieces will need a simple cross cut to achieve the finished length, this should be saved for last. Errors on angled cuts are common but cross cuts are easy, so save the easy cut until after you have satisfactorily made the more challenging cuts. This will require that you position the top piece an inch or two higher than its final position when setting it up to be cut. After the angle cuts are done, then you can measure and cut the uprights to the proper length.
To do this practically, such as in the door frame example, position the first piece and then secure it temporarily with a nail at either end. Leave the nails part way out so you can remove them. Position the top piece to overlap the end of the first upright and secure it in the same way with a couple of nails. If the moulding thickness varies across its width, it will help to place a length of wood of the same thickness as the moulding under the cross piece, to give it a solid base to support it while making the cut.
With the corners overlapped, use a straightedge to draw a line from the inside corner to the outside corner. Now position your saw and cut along that line. Because the two pieces were cut in the same pass, they will be "reciprocal" angles. If you were trying to cut a 44.5° through both pieces but you were a little off, one piece might have a 45° angle and the other piece would have a 44° angle. But these slight errors compliment each other and so still yield the 89° angle goal from the example above.
Repeat these steps for the other upright, then cut the uprights to their final length with a simple cross cut at the bottom. By saving this step until last, it gives you the opportunity to recut your angles until they are just right. Otherwise, if the lengths were already cut, the piece would become too short for you to recut the angles.