Making a Jack Miter.

Well some of you may not know what a jack miter is, this picture pretty much says it all. It is used when trim or molding needs to butt together and still have the part or all of the profiles continue around.

I was putting together a bid for a trim job that required jack miters, so I needed to make a few test pieces so I would know how long it would take to make a jack miter. I thought I'd take a few photos to share with you. First, here is a drawing of the trim detail for the widows on this job.

I saw Gary Katz did a write up about making Jack miter on his site a year or two ago, so I thought that would be a good place to start. Here is a link to his Jack Miter page, but sure to check it out. He made a jig from wood, but I didn't want to spend time on making a jig so I used the MFS. Like Gary, I started by altering a chamfering bit, I removed the bearing and flatten the tip of the bit with a grinder. I used a guide bushing to ride on the inside of the MFS. Note: I would not recommend altering router bits yourself, find a professional capable of this kind of alteration.

Here is a shot of the router setup.

The next thing was to rout some scraps, I used a beading bit in my router table.

I marked the edges and then the center of the intersection.

What will be the center trim leg, it gets the corners clipped off on the miter saw.

With some careful measuring you can set the MFS to a rough width. Set the bit the approximate depth. Measure the offset of guide bushing to bit. The offset times two, plus the width of the trim leg, this will give you a place to start. Now set the 2 angle stops the same distance from the edge of the MFS.

Clamp the MFS down with the stops on the work piece, then clamp the work to edge of the table.

With everything in place the next step is to set the depth of the router bit. Carefully rout out the work piece, I used the find adjuster to creep up on the exact depth.

Slowly nibble away the martial.

I'm testing the fit in this picture, you may need to make an adjustment to get the right fit.

The two pieces should look like this.

To keep everything perfect aligned two 5 mm dominos and one pocket screw will be used in the joint.

Here is the end result, it took less time than I thought it would to have this joint turn out perfectly.

The last thing I needed to work put was to be able to make the cutout in the top head where I wanted. What I did was made a mark where I wanted the center of the joint to be.

With the stock clamped in place, I used calipers set to 1/2 the width of what the MFS is set to. In this case the MFS is 102 mm and the calipers to 51 mm.

This really is easer than it looks, the whole test took a half hour and that was while taking the pictures too. Jack miters can be used on case work, faceframes and the like. It's a real shame you don't see this kind of craftmanship anymore.