Mortise Jig – Modified Tage Frid

I needed a new mortising jig because the first one I made didn’t work as well as I wanted. So I searched my Fine Woodworking magazine digital library and found what I wanted. Issue #30, Sep/Oct 1981. Title: Routing Mortises Author:  Tage Frid.

T-Bones mortise jig.

Using the mortise jig

The jig is used for routing mortises of any length (depending on jig length).  The workpiece is set inside the jig against one side, on top of shims of various thickness cut to length of the jig. The top of the work piece should be set just below the jig sides. I set mine to approx. 1/16″ below the top of the sides.  Once the height is set position the go-bars to hold the work piece in place.  After checking that the work piece is at the correct height and there is enough room on each side for the router stops then you can start routing the mortises.  Place the router on top of the jig and adjust the router guide so that the bit is where it needs to be to cut the mortise. I wasn’t intending on this article to be so detailed as to describe how to cut mortises. Maybe I will do another posting someday that goes into laying out and making mortises.

About the go-bars

The go-bars are a nifty clamping solution, however, there are some things to consider. They have to be made and you need a set for each different stock thickness that you plan to mortise. Having said that you don’t have to make each set all at once and you only need 9 or 10  for the first mortising project provided all the pieces are the same thickness.

This does 2 things,

1.  it keeps the hardwood from marring your work piece and

2.  it allows for adjusting tightness along the length of the jig.  If your jig ends up being 100% perfectly square and parallel along its entire length and you don’t care about your work piece then do what you want….. if otherwise then use the tape.

It is also possible to clamp your workpiece to the jig using traditional clamps providing the work piece is as long as the jig. Some times I use a clamp on each end and a few go-bars in the middle. Very flexible jig, that is why I like it.

The Modification

Frid’s jig had router stops that were different than what I ended up with. His had 2 stops which were placed on each end. They were pieces of hardwood with a slot cut-through in which a screw and washer were used to hold it in place. Mine aren’t necessarily better but I wanted to try using t-track so that I could put my router stops anywhere along the jig. Turns out that the original design would have worked just as well in most cases because the stops and router are so big. I still like the t-track but because I set it and it’s support 1/16″ lower than the jig side it causes my stop to rise up on the opposite end. Not really a problem, just kind of weird. Putting a couple of washers between the t-track and the stop piece made it sit better. If you click on the picture at top of this post you can see a small gap between the stop (dark brown piece on top) and the jig top.

Mortise jig without the router.

Jig Components

• Jig box, u-shaped with 2 sides and a 3-piece laminated bottom
• Stop blocks for setting mortise length
• Shims of various thickness’, for stacking up inside jig to set work piece height
• Go-bars, used to hold work piece in jig
• Clamp pads – for clamping jig to my workbench

Size:

• Length: 30″
• Distance between sides walls: 3.25″
• Inside wall height: 4″

Materials:

• 3/4″ Birch plywood – finished both sides
• 30″ length of t-track
• Plywood scrap for stop blocks
• Various materials for build-up blocks
• 3/16″ hard wood scrap for go-bars
• Hardwood scraps for router guides

Router Stuff

My mortising router is a 2- 1/4 hp Porter Cable router that I use for mortising.  I also have the Porter Cable premium edge guide which I highly recommend because it allows you to make controlled micro adjustments of the router bit location relative to where you want to cut the mortise.

The standard edge guide is one where you have to put the connecting rods into the router and tighten them down with a couple of screws.  Holds great….. but loosening them and moving them a 64th of an inch is next to impossible.  This isn’t a problem with the router guide accessory.

I found it necessary to make a custom base plate for the router with an additional edge guide so that the router moved along the length of the jig without moving side to side. Without the extra guides the router can move away from the intended line and make some pretty rough looking mortises.

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