Joinery Rules of Thumb

I will admit that recently I have been so focused on boat building that I sometimes forget what I have learned about fine woodworking. It was a good idea for me to stop for a second and refresh my thoughts by reviewing these tips from the Port Townsend School of Woodworking.

Traditional Rules of Thumb for Joinery

Mortise and Tenon Sizing:

• Generally the thickness of the tenon is sized one-third the width of the stock (with frame-type constructions--it can be much thicker in apron-to-post type joints). This would also be the width of the groove if there is one.
• The depth of the mortise should be at least three times its width. Thus a 5/16-in. wide mortise should be about an inch (15/16 to be exact) deep at minimum. Five times its width (1 9/16-in. in this case) is the most you would ever need to go (reference: Moxon). The tenon is sized to the depth of the finished mortise, less about ⅛-in. to provide clearance for the mortise’s inevitably rough bottom.

• The length of the tenon, at least in the common rail-to-stile connection joint, should be two-thirds the width of the rail. (Moxon)
• The maximum length of the tenon shouldn’t be more than six times its thickness. If the board is so wide it needs more than this length (as in the bottom rail of a large door, or the breadboard end of a table for examples), make multiple tenons that don’t exceed this 6X length-to-thickness ratio.
• The haunch is usually a cube--defined by the width/thickness of the tenon.

Rabbets and Dadoes:

• Dadoes rarely need to be very deep: one-fourth the thickness of the stock receiving the dado is usually more than enough; rarely if ever more than one-third.
• Rabbets generally follow the same rule.
• The width of the dado is equal to the chisel (plus a scant for clearance) that will clean out the dado. Choose chisel about five-sixths of the stock thickness.
• Rabbets are usually made equal to the stock width, unless they are “fitting rabbets” for dados--in which case they are simply made deep enough to allow the stock to fit in the chisel-sized dado. The fitting rabbet shouldn’t be more than ¼ the thickness of the stock.

Dovetails:

• Half dovetails or half pins at the edges of the board are generally sized to one-half the thickness of the pin board at the shoulder line. (If sized less than that, they may cause a split-out when the joint is assembled).
• The tails are generally sized less than three times the width of the intervening pins.
• Half-blind dovetails (which typically show on the sides of a drawer box, but not its face.) are “capped” on the face by one-sixth to one-fourth the thickness of the face stock.
• Tail angles vary from 1:6 to 1:8. Choose steeper angles in soft wood to avoid compression failure. Note that dovetails serve not only to resist tension stress, but to maximize the amount of end grain coverage at the corner of the structure.