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1.  Miter Saw "Clamping" Fixture:

When crown molding is cut per method 1 (Part 1 Crown on Horizontal Ceilings), it is helpful to hold the part securely to the saw table using an indexing or support fixture. Holding it by hand only will cause distortions and shifting of the part just when you don’t want it.

I have made a clamping fixture out of ¼ inch plywood and two thin strips of wood as shown below. My good old 10 inch Delta saw has holes drilled through the back of the fence that allowed me to fasten a sheet of plywood to increase the height of the fence. I used four ¼ inch flathead machine screws with wing-nuts for quick release. Once the full sheet was bolted in place, I used the saw to cut away some of the material to provide clearance for miter and bevel cuts. Then I glued on some sandpaper to provide friction.

The "clamp" part was also made out of one piece to start with. I glued a ¼ x ¼ inch piece of wood along the back so this raised edge would provide extra support. Two holes in the saw’s casting were plugged with some wooden dowel and clearance hole for ¼ inch bolts were drilled through. These bolts with wing-nuts hold the fixture in place. Note that the slots allow the units to be moved back or forth for different size molding. Once the whole piece was clamped in place, I used the saw to cut the piece in two and provide clearance slots for various miter settings. I also trimmed up a few corners.

To use the fixture, I ripped some rectangular blocks to the width for each type of molding to be cut. These gauge blocks are positioned between the back fence and the raised back edge of the fixture. This allows me to set the two-part jig exactly to the right position. The blocks are not shown in the photos above.

Note that the molding still rests on the saw bed, not on a piece of plastic. The two-part design provides a good reference and support on both sides of the blade. Keep in mind that this is not really a clamp but an indexing guide. The part still has to be held firmly in place while cutting. However, the sandpaper and "clamping" parts help to hold the crown in the correct position. On hindsight, I should have made the backboards longer.

I have not provided any dimensions for this concept because every saw will have a different set of mounting holes. However, I believe these holes are present on all, or most, saws on the market.

2.  Marking and Cutting Gauges:

If you are planning to cut more than a few pieces of molding using Method 2, you might consider making the following simple fixtures. They help to identify how to orient the molding on the miter saw and to line up the cutting planes.  They are intended for 90° inside and outside corners only.

I made mine using a twelve inch section of ¼ inch plywood and a piece of 1/2 x 3/8 inch thick stock. The half-inch block is glued to the plywood as shown in the sketch below and serves as the reference edge. I use ½ inch thickness because that is just a bit smaller than the thickness of most crown molding. Since the gauge is placed on top of the molding, you want to have some clearance. The width of the plywood is such that it can span the overall width of the molding you are using. I made mine just a bit narrower than the crown so that I can place it on the molding without hitting the backstop of the saw. 

After gluing the two pieces together, set the miter saw for the correct bevel and miter angle and cut each edge as shown. Glue a strip of sandpaper to the bottom surface so that when you place it on the molding it cannot slide away too easily.

One template will be R1 and is used for the right hand molding pieces. One side is for inside corners, the other end for outside corners. Likewise, the second template, L1, is for left hand pieces.

3. Wall Angle Gauge:

In order to get a nice sharp edge with crown molding corners, it is helpful to know what the exact angle is between the two walls. One way is to use a square and judge the deviation from a right angle. A better way is to use a gauge with some means of reading the actual angle. Although tiny commercial units are available, I have made one using a six inch diameter protractor and some ¼ inch plywood. I didn’t think it wise to buy a unit with only a two inch diameter scale with divisions that are hardly readable. For those of you who like some woodworking, I am providing the following details. You can adjust the size to your own liking.The unit consists of a base and arm that can swing around a bolt in the center of the protractor. The arm rotates over the base and closes into a compact unit six inches wide. The protractor is fastened to the arm. A small piece of wood (with a hairline scribed as shown) is attached to the base. I have notched the arm to clear this block and also to clear the small bumps typically encountered with outside corners of two walls.Likewise the outside radius of three inches will clear the usual buildup of tape and "mud" on inside corners.

The length of the arm and base is up to you. I choose 16 inches overall. The large protractor has divisions of one half degree which is better than most bevel settings on compound saws.

Note that the unit can measure inside and outside angles from 0 to 360 degrees. Not only that, it can be used to measure vault angles and other intersections of walls and ceiling. I have also used it for layout work when cutting plywood sheets at various angles.

4. Nailing Blocks:

For wide molding you might consider adding support to the back side of the crown. Wooden blocks, 1.5 inches wide can be cut from a 2x4 (or 2x6 if necessary). First you need to measure dimension L of the crown molding. For standard 32/52 crown multiply this number by 0.85. This will be the width of the board you need. Rip the 2x4 or 2x6 to this width. Then using the miter or table saw, cut the blocks as shown. The angle corresponds to the spring angle of the crown. Note the dimension, L, should be just a bit less than the width of the crown face (say 1/32 inch). Knock off the sharp corner that would go up against the ceiling/wall intersection.

Find the wall studs and nail these blocks against the wall, making sure the angle is in the right direction. Put a small pencil mark on the wall below each block so you know where they are when dangling the crown in place.