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Exact-Width Dado Jig

Cut dadoes to the desired width with the help of this handy shop aid.

Exact-Width Dado Jig

Exact-Width Dado Jig

Here's a jig for routing bookcase or cabinet-side dadoes that exactly match the thickness of your shelf stock. Better yet, no special bits are needed. Just use an ordinary straight bit and a guide bushing. (We used a 1" guide bushing and a 1/2" straight bit.) To start, cut a 3/8" rabbet 3/8" deep along the inside edge of both guide rails (A). Then complete the jig as shown in the drawing below. To customize the rails for a different bushing and bit, install them in your router, and trim the protruding lip of the rabbet in the guide rails, as shown in the Dadoing Detail. The remaining lip will now match your bushing/bit combo.

To adjust the jig for the exact width of your shelf stock, slip the jig over the shelf stock as shown in the Adjusting the Jig drawing. Pull the guide rails (A) tightly against the stock, and tighten the wing nuts. Slip the jig off the stock, and clamp the jig onto the piece being dadoed, centering the opening between the rails (A) over the marked dado on the side panel. Adjust the depth of cut with your router sitting on top of the rails. Start the router and make one pass with the guide bushing riding against one of the rabbeted rails. Make a second pass riding the bushing against the opposite rabbeted rail.

If you like this project, please check out more than 1,000 shop-proven paper and downloadable woodworking project plans in the WOOD Store.

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Comments (10)
johnw50 wrote:

I built it and it does'nt work with the guide bushings the dado is still undersized I ended up using a pattern bit. maybe I'm doing something wrong with the guides & can't figure it out. Any Help?

6/11/2014 04:44:46 PM Report Abuse
liam.c.murray wrote:

I built a jig like this to use with a short (1/2" long, 5/8" diam) pattern cutting bit. Then I realized I needed to cut dados for 1/2" and 5/8" bb plywood (i.e., actually slightly narrower than those measurements). I'll keep my other jig for 3/4"+ dados and will make this one to use a collar for the smaller ones. It is hard to find pattern cutting bits smaller than 1/2".

11/30/2013 10:43:54 PM Report Abuse
Cyber-Wil wrote:

I have a jig very similar to the above, but have mine configured so that the router base runs against the fence and I simply use a 1/2" straight bit. Same sizing process applies in that you use the shelf to set the width of the cut. No collar required and once the jig is built to rough size, one pass with the router against the fence with the 1/2" bit and the jig is ready for use. Works great.

10/11/2013 08:46:27 AM Report Abuse
ptomnitz wrote:

Seems to me it will be a one time use as the router bit will cut away 1/8" of the rabbit.

9/20/2012 10:51:13 AM Report Abuse
lesterborja wrote:

My router guide collars are longer (1/2"+) than the depth of the 3/8 x 3/8 dado cut in the guide pieces and if I cut the dado deeper, say 9/16", not much is left for sizing the jig to a workpiece. it works, but only marginally. Guess I could cut down the collar length, but hate to make a change that cannot be reversed.

3/22/2012 12:26:34 PM Report Abuse
raybarker05 wrote:

I tried a pattern cutting bit. The dado was too wide. Seems the bearing wasn't quite the same size as the bit. Maybe a bad bit?? Don't know.

1/26/2012 09:15:44 AM Report Abuse
bjwisdom123 wrote:

Won't the bit cut into the side rail?

2/26/2011 03:36:54 PM Report Abuse
Eric Brigham wrote:

Good design. I might make one of these, but slightly tweaked: basically with a detachable base plate/lip on the guides (instead of that 3/8 reabbet) so I could use different size bits on the same jig.

12/4/2010 02:46:52 PM Report Abuse
jesse.roscoe wrote:

@harry_rutland: i liked your idea of using a pattern cutting bit instead, but then i thought of possible problems. suppose your 1/2 inch diameter bit has a 1 inch cutting length. if you want to make a shallow dado, you might end up with your bearing riding above your straight edge. and if you try to compensate by using thicker hardwood for your straight edges, then you'll be limited on how deep your dadoes can be(of course that part depends on your router's capabilities). just my 2 cents.

1/31/2010 11:06:11 AM Report Abuse
harry_rutland wrote:

Good idea, but why not use a pattern cutting bit? No offset calculations required.

11/2/2009 05:13:49 PM Report Abuse

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