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Adjustable Dado Jig

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Using the completed jig

I have a walk-in closet which recently underwent a rapid unplanned disassembly of sorts:

To avoid future problems, I am going to replace the wire closet shelves with some sturdier shelves constructed mostly from 3/4″ plywood. My plan for construction involves joining the sheets of plywood at right angles using dados.

Dados can be cut in wood in a variety of ways. One method is to use a set of special dado blades on a tablesaw to cut a groove of the correct width. This can be unweildy however for making dados across the width of a long sheet. Another option is to use a router guided by a straight edge to machine the groove into the board. 3/4″ plywood is often actually 22/32″ thick. Specially sized router bits are sold that can in theory make the correctly sized dado in one pass. This method requires lots of careful measurement and setup of the router guide and can lead to cutting mistakes if the router drifts away from the straight edge for any reason.

In order to make my task easier, I decided to build a simple router jig to cut dados across a 2′ wide sheet. I began by ripping some scrap pieces of plywood on the tablesaw.
Ripping router guides

I made two guide pieces by screwing the thin strips of plywood to the top of wider base pieces. The thin strips will act as a guide for the base of the router when cutting dados.
Guides complete

Next, I put a 1/2″ straight router bit into the router and ran it along each of the guide strips, trimming the edge off of the wide base pieces.
Trimming the router guides

A perpendicular cleat was attached to each end of one of the guides with some screws.
Attaching the cleats

I drilled a set of holes in each of the cleats to form a slot. The other router guide was connected to this slot with a T-bolt and hand knob. This allows the second router guide to slide along the cleat to adjust the width of the dado.

To use the jig, you can place your board in between the guides, push them together, and tighten the knobs to lock the dado width in place. Then, you can place the jig on your workpiece and align the opening to exactly where you want the dado to be and clamp it in place.
Using the completed jig

By running the router with the 1/2″ straight bit up one guide and down the other, it will cut a dado to exactly the right width and in exactly the right spot with minimal measuring and with less chance of error than the other dado cutting methods. Hopefully this jig will save me some time and effort throughout the rest of my closet building project. Stay tuned for updates!