Sliding Dovetails in Bookcases

      A furnituremaker tries dovetail joinery for bookcase shelves, and likes the result. February 3, 2011

Question
I have been commissioned to build a custom home library, and I am using sliding dovetails to join the shelves to the bookcase sides. The plan is to use HW ply for the sides, and 13/16 solid for the shelves. Wood choice is either mahagony or walnut, the client hasnít made a choice as of this moment. The question is regarding the size/angle/depth of the dovetail.

A few more details:

-The shelf depth will be about 10Ē.
-I am using face frames, so I can use through dovetails.
-I am leaving a 3/16Ē expansion gap at the back of the shelves, and gluing just the front edge of the joint to allow for movement.

Having not done this type of joint before, and knowing I have about 320 of these joints to do, I am relying on others advice (other than donít do it) to do it right. I am hoping for some advice on router bit selection, use of checking jigs to ensure consistency, etc. Also, I originally planned to use HHG for the joints, but am considering using CA to soak the front edges of the joint to lock into place. I use this method for binding flat-top guitars, and expect my instruments to last 100 years or more (so far no evidence to prove otherwise). The main reason for considering this method is because eight of the cases are curved, so it will be much easier to get everything fitted perfectly that way, without fighting work time.

Question
(Furniture Making Forum)
From contributor O:
I wonder if a sliding dovetail is the right joint to use. The plywood will be very prone to chipping out and will not have a lot of strength. The sliding dove tail joint would be a better choice if the sides of the case were also made of solid wood. For plywood a simple dado would work better.



From the original questioner:
I did consider that, and havenít totally disregarded going to solid. I guess testing is necessary before proceeding in the wrong direction. Part of the attraction I felt, was ease of assembly. Though it seems like a lot of joinery, I have the ability to automate much of it, so the tradeoff was worth it. Now, I need to come up with a recipe that might work for this joint. I have spoken to a few who felt it wasnít a problem, but they arenít the ones who will be standing by the work I am, and I need to be sure.


From contributor J:
I agree that it would be better if your sides were solid. As for ease of assembly, a sliding dovetail is never easy to assemble unless it is really sloppy, and that defeats the purpose doesn't it? In my opinion, epoxy is the adhesive of choice for a sliding dovetail - lots of open time, and extremely slippery.

Machining the sides I would do in two steps, first a straight bit, followed up with the dovetail bit, probably around 7-9 degrees. I might sub that to a CNC shop with that many to do. I'd probably set up a horizontal router table to do the ends of the shelves if it was too wide for the multi-router. Sounds like a nice job, have fun. I wouldn't rule out the dado till after you try the dovetails either.



From the original questioner:
I hadnít thought of epoxy. I can leave slop enough for easy assembly. With easy shelves in each case, even a sloppy fit should make for a sturdy construction. I can get the machining done once I have a good spec. so I will do some testing starting out at your recommendation.


From contributor A:
I don't know how tall the shelves will be but you could use a half sliding dovetail on the middle shelf. The top part of the joint is a dado and the bottom has the dovetail. Dado or use adjustable pins for the remaining shelves. A through tenon in the middle shelf would also work. Sliding dovetails are a tough joint to assemble and it sounds like you have a lot of them to do. The force on a bookshelf is down. The dado gives more strength because it has more meat.


From contributor F:
I built some ten foot tall shelves with dado joints and used yellow glue. I was able to get the whole thing put together and squared up without difficulty. Since the shelves were 4' long I used 1" thick stock and nailed plywood on the back to support them. They are going on 15 years old and still don't sag under a full load of books. I think the dovetails may be overkill.


From contributor W:

1. Why dovetails? Did the client ask for fixed shelves? If so, dadoes would be sufficient and a lot less trouble.

2. In general, a case should be structurally sound without the shelves - the back holds the sides in position. No need to dovetail the shelves at all, they could be adjustable.

3. Curved cases? If the sides aren't parallel to each other, the sliding dovetail won't work.

4. You are setting up a finishing nightmare. If you aren't doing the finishing, then don't worry about it. If you are, have you accounted for the extra time to finish fixed shelves?

Please think this through again. You might spend some time looking for/at standard library construction - there's a good reason that most people do things the same way.



From the original questioner:
I am considering everything before I cut one single dovetail/dado.

1. Why dovetails? Did the client ask for fixed shelves? If so, dadoes would be sufficient and a lot less trouble.

It was a selling point (valid or not) based on the clients past experience with bad construction. I know it sounds like a lot of work for little gain, but part of the attraction for me is ease of assembly. If the machining is within tolerance, and it will, there is no reason I canít slide everything together with 90min epoxy and the thing will be basically assembled. Yes that is perfect world thinking, but many factors can be controlled with careful planning.

2. In general, a case should be structurally sound without the shelves - the back holds the sides in position.

No need to dovetail the shelves at all, they could be adjustable. Definitely a consideration

3. Curved cases? If the sides aren't parallel to each other, the sliding dovetail won't work.

Actually, I can join one side to all the shelves, and then bring the other side into place.

4. You are setting up a finishing nightmare. If you aren't doing the finishing, then don't worry about it. If you are, have you accounted for the extra time to finish fixed shelves?

The shelves will be pre-finished, along with the sides, and the back.

Please think this through again. You might spend some time looking for/at standard library construction - there's a good reason the most people do things the same way.

Will do, I havenít cut a single piece of lumber yet, so until then. I have heard a lot of responses about the difficulty of doing this, but nothing to say impossible. I will weigh it out before proceeding.



From contributor F:
I agree that you can attach shelves to one side, then attach the other side, regardless of the shape of the shelves. The sides need not be parallel.


From contributor M:
It sounds like I'm in the minority here, but a sliding dovetail joint seems like a good choice to me. The alterations to your stated plan that I would make are:

1. Make the sides solid. Building in expansion stress is not the way to sell quality joinery.

2. Use a stopped sliding dovetail. I prefer well fitting joints assembled with yellow glue, where the stopped joint allows you to drive it home a bit at the end. (If you make it sloppy, use epoxy, and the stopped joint will help to keep your expensive, slippery, slow-setting glue from augmenting your finish to the neglect of the joint.) Depending on your design, stopping the dovetail might even allow you to attach face frames before finishing and assembly.



From the original questioner:
After all considerations, and hearing many opinions I decided to use the ply sides and solid shelves - a good choice considering the cost difference to use solid for the sides. I started with epoxy, but the joints ended up so nice and tight. Titebond original became the better choice. Once the shelf gets into the side more than three inches or so, it squares itself up, and just slides in with no binding at all. Itís way better that I expected. The dovetailing became the smallest part of the job, compared to the edge joining for the shelves. I am halfway through the project, and yet to do the curved bookcases (a new challenge I am sure). I have a video below showing the dovetailing of the shelf ends, and case sides, using a 5/8", 7 degree bit.

Here are some images.


Click here for higher quality, full size image

Shelves in the drying room.


Click here for higher quality, full size image



From contributor J:
Looks like it came together well. You only glued part of that joint right? You're as fond of 80/20 as I am, nice looking setup.


From the original questioner:
Yep - just an inch or so. A couple taps with a rubber mallet to flush with the side. Not much time to work, so adjustments must be made quickly.


Click here for higher quality, full size image



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