Bookcase Joinery, Simple Method, Secure Joints

[Sam Murdoch]

Recent discussions by folks pretty new to cabinet making about how to make tenon/dado joints to attach shelves to bookcase or cabinet sides inspire me to create this post. I hope I can be clear and provide a useful post.

My objective is to describe an easy to build, no money down, accurate and user friendly router sled for making the dados into cabinet sides ready to receive the tenoned shelf ends. This is ONLY ONE OF MANY TECHNIQUES, but is a good place to start as it requires only material on hand and a router of any make. The sled will be useful for many other applications too and is easily modified.

In the case of attaching fixed shelves to cabinet sides I prefer rabbeting the end of the shelf rather than fully seating the entire thickness of the shelf into a dado in the cabinet side. You can see what I mean in the first sketch.

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This is better for at least 3 reasons: 1) You get a shouldered joint which allows for 1 more glue surface. 2) The shoulder helps to resist racking better than the full shelf as a tenon. 3) If you are using plywood shelves the rabbet is especially useful as it allows you to control the thickness of you tenon regardless of any irregularity in the thickness of the ply - (plywood is never perfect in its thickness).

Here is the sled -

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I ALAYS RUN MY DADOS FIRST then make the shelf tenons to fit.

The key features of the sled are: (I HAVE NO IDEA WHY THE FONT HAS RESIZED HERE - OH WELL...)

1) 1/2” plywood base with two side rails attached that are set apart a distance that allows the base of your router to slide easily but with no side to side play.

*** See note # 9 about this below.

2) The sled is longer than the width of the 2 sides clamped together.

3) At the entry end, under the 1/2” panel is a cross piece that is wide enough to allow you to clamp across your work and also down to your work table. The out end cross piece does not need to be longer than the width of the sled. I usually make the front and back cross pieces the thickness of the cabinet side material so that I can screw down through them into my work table without deflecting the sled table or needing to support the cabinet sides. Since I work off a big table rather than a bench I often screw the sled down to my table rather than using clamps, especially at the outboard end.

4) I typically assemble the sled keeping the cross pieces perpendicular to the sled table. Making the sled as a “square” ensures that your set up for each dado is easier and more accurate.

5) I bore a hole comfortably bigger than the router bit I intend to use down through the sled table and the front cross piece. This hole should be at least 3/8” away from the start edge of your cabinet side so that you can start the router and plunge to depth BEFORE ENGAGING YOUR FINISHED WORK

6) A stop on the top of the sled at the out board end. Your sled will remain more accurate (parallel) if you don’t plow right through the end with a full depth slot.

7) I usually use a 3/8” or 1/2” straight router bit for making a dado for a shelf thickness of 3/4” to 7/8”.

8) OFF OF THE FINISH WORK I run the router through the fully assembled plywood sled. This establishes my cut line.

9) ••• You can keep your rails wider apart than the router base for the purposes of a controlled rout that is wider than your router bit. For example if you want to make a 3/4” slot but you only have a 1/2” bit - keep your guides apart a 1/4” wider than the router base. Run up the left rail and back to the front following the right rail for a perfect 3/4” dado.

PREPPING THE SIDE PANELS

Now, before I use the sled. I prepare the 2 sides of the cabinet. As you can see from the sketch, I clamp the left and right panels together - front edge to front edge. This insures that no matter how you come out the back ends - a little crooked or with a little blow out - your front dados will be perfectly mated.

A note about blow out - the back cross piece serves the purpose of providing a blow out preventer. This is why I attach the 2 cross pieces to be pretty tight to the width of the combined sides. Now you will need to move the sled along the sides to make the slots so it can’t be too tight. Leave the braces a 1/16” or less further apart than the sides but when you set up for the cut pull the back brace tight. The front end will not blow out so you can have a tiny gap there.

The sides should be perfectly aligned at the top and the bottom. Think about it.

I mark all my shelf locations - usually the top line and be especially careful to MARK THE CUT SIDE WITH AN X. If you don’t see the X when you set the sled on the line you know you are on the wrong side. Ask me how I know this.

CUTTING THE DADO

The sled already has the slot cut so you need only align the edge of the slot to the line on your panel - with the X showing.

Establish the depth you will need to plunge, factoring in the 1/2” thickness (or whatever) of the sled table. Some woods and ply will allow a full plunge and rout with one setting. Some harder woods should be dadoed in 2 or 3 steps. The router should operate smoothly. Set the depth accordingly. Always wise to do some trial runs on sample boards.

Check your set up. Clamp the sled. Run the dado. Move to the next line.

A WORD ABOUT TENONING THE SHELVES.

With some care and a thoughtful set up you can use the sled to tenon the shelves too. I prefer to use the table saw for this. I don’t use a dado set. I simply do 2 operations. I always do trial runs with scrap boards that are precisely the thickness of the shelves.

You can also do an easy job of this with a router table or shaper - but this is post is about doing good work with fewer tools.

1) run all the shelf ends flat over the saw with the fence set at the needed depth. Use a feather board on the fence to hold your shelves flat to the table. This is normally a pretty shallow cut and so a pretty safe and fast operation.

2) Run the shelves vertically along the fence to finish the tenon. ALSO HERE use a feather board to keep your work tight to the fence. Try the fit in the previously routed dados until you find the perfect fit (with your trial pieces before your finished pieces).

I also use this TS technique for rabbeting the sides and top and bottom for receiving the back.

OK? If I need to be more clear just ask. Again, there are many other ways to achieve the same results. This is just one system that has served me well for many years.

[Ken Matthews]

Sam, Thanks for posting this. I really appreciate it. I built a built-in bookcase in a knee wall in my bonus room this weekend and decided to use the method you illustrate. I built the sled out of laminated shelving and plywood and it works great. I don't have a table saw so I used the router for the tenons also, it was a little tricky but the sled made it easier. Thanks again for taking the time to post it and for the great illustrations. Mark

[glenn bradley]

A great thread with good detail Sam. Thanks for posting. Just a variation (as you say, so many ways of doing things); I'll add that shelves can be oriented so that the shoulder hides any inconsistencies in the plywood surface. As shown here for shelves below eye level and as you show for shelves above eye level.

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The dado is usually very straight, the plywood surface often is not, leaving small gaps.

[Justin Ludwig]

Glenn, you bring up a good point. One thing to consider when mounting the shelf this way: if the shoulder rabbet is made with a router, as I do, plywood is bad to splinter and those will show. Shallow passes or scoring the end of the rabbet will prevent that. If dado on a TS, its no worry at all.

[Lee Schierer]

A great thread with good detail Sam. Thanks for posting. Just a variation (as you say, so many ways of doing things); I'll add that shelves can be oriented so that the shoulder hides any inconsistencies in the plywood surface. As shown here for shelves below eye level and as you show for shelves above eye level.

Capture.JPG

The dado is usually very straight, the plywood surface often is not, leaving small gaps.

This way also produces a much stronger shelf in terms of load carrying ability, particularly with plywood shelves where the plys are likely to split under heavy loading.

Tear out can largely be eliminated by covering the cut line with masking tape prior to making the cut. The tape will stop most of the grain lifting. Just remember to pull the tape off toward the cut edge when removing it.

[Sam Murdoch]

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This way also produces a much stronger shelf in terms of load carrying ability, particularly with plywood shelves where the plys are likely to split under heavy loading.

I don't know that this is a valid point, as you lose the benefit of the shoulder bearing on the cabinet side and are more likely to have the joint at the top of the shelf open/pull away from the cabinet side in that orientation. Of course the tenon should be at least or more than 1/2 the thickness of the ply for best strength. In any event probably a moot point. How much load will one place on a dadoed plywood shelf? A really heavy anticipated load would require cleats, different materials, or a better design altogether. Still for general purposes, add a face frame to this construction and a back that overlays the shelf/sides joint and this is a very strong cabinet.

As for the up or down version and the tear out factor yup, there is a chance of tear out either way. With the router sled arrangement I describe I don't get tear out as the sled bearing on the plywood seems to prevent any lift. The way I add the tenon to the shelf on the TS eliminates tear out there. If I do the shelf tenon with the router I typically prescore the shoulder cut with a knife and/or again, make certain that the sled carrying the router is very firmly pressing against the cut line. In any case if there is a bit of tear out on the shelf tenon it will be on the underside of the shelf. Yes, the visible part of the joint changes orientation as the cabinet gets taller but I guess if this is an issue you can build the cabinet with shelves up or down as they relate to eye level. I haven't had any problems with the method I described done with care. One method among many for one to consider. Glad it worked OK for you Ken .

[Larry Edgerton]

I do mine the same as you Sam, except now I have a SSC dado machine. I do like to spoil myself......