How will I know if my dadoes fit well enough?

[Tyler Bancroft]

I've started cutting dadoes by hand for the first time to fit shelves for a small pine bookcase. To my eye, they're far from perfect (occasional deviations from squareness in the walls and small visible gaps in the joint fit on the ends, for example), but I can fit the shelves by hand (or with some light mallet taps), and the fit is tight enough that I can pick up the shelf and the joint stays intact. I'm going to go out on a limb and guess that this is tight enough to glue well, but is there a good rule of thumb for testing the fit of a dado? I'm planning on building some bookcases out of less forgiving wood than pine, and I'd rather not have them self-destruct when full of books.

[William Fretwell]

Tyler, firstly pine is not a forgiving wood, it’s so soft more care is needed. Do your shelves have a shoulder for the dadoe?
It is far easier to get a good look with a shoulder. As for working as a shelf I see little risk of failure, the book case carcass joints provide structural strength, the shelves will stay put if they don’t bend ridiculously!
If you glue them the area is considerable so they won’t go anywhere.
I would use 1/3 of the width of the side for my dadoe. The shelf shoulder can be very small to cover up the dadoe edge, a shoulder plane would work for that.

[Jim Matthews]

If you need a persuader to assemble the shelves, it should be tight enough for glue.

The dado housing the shelves should be 1/3 the thickness of the sides, roughly speaking.

Regarding stiffness under load:


Apply a "stretcher" support on at least one of the shelves in the middle of your case.

A stretcher is a full width run made from the same material the shelves, rotated 90 degrees to change the grain orientation. An excellent (and strong) alternative is to build stretchers from plywood - perhaps the same plywood from the back of your bookcase - as a stretcher.

Put the largest, heaviest books on the bottom shelf.

Use a "French" cleat to affix the case to the wall.

My first bookcase failure came when an overloaded free standing case "racked" on an uneven floor.

[Jim Koepke]

You might also want to consider using stopped dados, at least on the front where the shelves will be seen.

Here are a couple of images for comparison:

410413

A through dado is the easiest and in some situations works fine. This is especially true if there will be molding around the face of the carcass.

Without molding a stopped dado and cutting a curve at the front corners of the shelves looks better imo:

410414

Sometimes opening up a view to the joinery hides it well.

jtk

[Robert Engel]

Sounds like they are tight enough.

Of course it all starts with accurate layout and proper scribe lines. A paring block and a wide 1 1/2 - 2" chisel are a good aid to ensure square walls. Tuning up can be done either by addressing the end of the shelf or the dado with a side rabbet plane.

One issue in using dados in carcase is poor glue strength (virtually an end grain glue up). It is very common to see antiques with bowed ends and gaps in the dados due to glue failure. There are two basic options: a fastener or dovetailed ends. Fasteners can consist of screw/plug, dowels, Dominoes or biscuits. (Obviously with the latter two you need to adjust the depth of the dado to accomodate). Another (often attractive) option is wedged through tenons.

410421

Dovetailing the shelves will prevent the carcase spreading apart.

410422

[Tom Bender]

Hi Tyler
What William said "Pine is not a forgiving wood" Plywood id=s not great either. Maple and Walnut and many others are good.

A shoulder is a big improvement.

Start your dado with a knife cut made with a guide clamped to the piece to guide your knife.

An option to the stopped dado is a thru dado with the shelf projecting out past the side.