Compound Miters on the Radial Arm Saw #2: Outside and Inside Corners - Crown Moulding

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Blog entry by KTNC posted 05-06-2018 10:15 PM 4484 reads 0 times favorited 0 comments Add to Favorites Watch
« Part 1: The Broken fence method Part 2 of Compound Miters on the Radial Arm Saw series no next part


Reference: Part 1 of this blog series – The Broken Fence Method

The broken fence method for making compound miters that I described in Part 1, creates only inside corners. That is what you need for picture frames and similar projects.

If you want to add moulding to the top of an armoire or install crown moulding at the wall/ceiling juncture of your house you also need to make outside corners.

This post explains how to use a modified positioner (see the reference above for terminology) to create both inside and outside corners with the radial arm saw. I’ll take you through an example I did to teach myself how to use those modifications to do all the cuts.

Below is a picture of the final result: crown molding all around this mock-up. For this discussion, I’ll be referring to the flat surface at the bottom as the ceiling and the vertical surfaces as the walls. Yep – it’s an upside down house.

These are the rules of the road for this method of cutting crown molding. As you go through the example, they will become more clear.
1. The surface that touches the ceiling will touch the surface of the saw table.
2. The keeper pieces will always be the the part that is on the table in front of the blade or to the right of the blade. It will be supported in the channel.
3. The good side of the moulding will always be up so that the blade enters the good side and exits the back. This avoids splintering and tear out on the visible side of the crown moulding.
4. An inside corner is cut with the workpiece leaning against the fence or the broken fence.
5. An outside corner is cut with the workpiece leaning against the positioner.

Start by creating a numbering system for the pieces of moulding to cut. In the picture below, I started from the left side with number 1 and the final piece is number 5.

Measure the length need for each piece and make a cut list. For each end, note if it’s a Butt end (B), inside corner (IC) or outside corner (OC)

For outside corners, you have to add some material so that when you cut the corner you’ll have the final length desired. For inside corners and butt ends, you don’t need to add material. The pictures below show a way to figure out how much extra to add for the pieces with outside corners.

- Establish a line on the ceiling that comes out from the corner at 45 degrees.
- Place the moulding against the wall
- Mark the intersection of the moulding edge and the line
- Measure the extra length you need.

Once you’ve calculated the lengths for each piece of moulding, cut and label the pieces.

I later found out that I mislabeled #5. The left side should be IC for inside corner.

You’ll need to make two spacers to get the positioner in the right place. An easy way to place the saw blade for the rip is to put the moulding in the proper position with one face on the table (the face that goes against the ceiling) and the other face against the fence (the face that goes against the wall). Just move your blade up so that it touches and you are ready to rip your spacers.

Install the broken fence board and positioner. Two rails have been added to the positioner. They are parallel to the fence and the broken fence. They are the same height as the fence and broken fence: namely as high as possible and still clear the bottom of the motor. This is the only addition needed to be able to cut outside corners. To help avoid mistakes, I’ve labeled the fences and rails with OC and IC.

Safety Note: You might think all you need to do now is remove the spacers and place the mouldings in the channel and cut away. This is not a good idea because the cutoff is likely to get caught between the blade and one of the fences.

To do the job safely, get set up for what Mr. Sawdust calls left hand miters first. Replace the regular fence with a short fence and scrap so that the workpiece can extend past the saw blade. Rather than use a short fence and scrap, I chose to make a new full length fence with a large notch: it’s easier to get into position.

Place your workpiece in the channel. If it’s an outside corner, lean the top against the rail on the positioner as below.

If it’s an inside corner, lean the top against the broken fence as below.

After you finish all the left hand miters, set up for right hand miters by removing the broken fence board and replacing the notched fence with the regular fence as below. The positioner always stays in place.

For an outside corner, place the top of the workpiece against the rail on the positioner as below:

For an inside corner, place the top of the workpiece against the fence as below:

After you finish all the right hand miters, return the saw to the crosscut position to cut the butt ends.

Below are a couple close-ups of the resulting joints.

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