How deep do mortices/tenons need to be?

[John B]

I have generally made mortices and tenon joints about an inch and a half deep, but I was wating a show on tv where they made them only about 3/4". Is there some rule of thumb?

[Nicky]

I do believe the rule of thumb is the "thirds method."

The tenons length should be about 2/3 the width of the material you're joining.

The size and application drives the descision. Bigger tenons offer more glue surface, however, this is nomally a cross grain joint, and may not be the correct approch.

I'm curious if anyone has some reference sites, or tables.

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[JeffB]

Design considerations will influence this choice - Craftsman style furniture will often use through tenons or bridle joints (open cortices). More glue area is better, but once you get a joint stronger than the surrounding wood, more strength doesn't really add anything. I'm referencing an article that compared the strength of different joints - tenons, biscuits, dowels, etc. in constructing a door. Even a stub tenon (5/8" deep) was surprisingly strong - the joint failed when the stile split apart. The advantage of a deeper tenon is more glue area to distribute the forces over, and a joint that has inherent mechanical strength even if the glue fails.

[lucky4fingers]

"John B" wrote in news:gwEXf.788$ [e-mail]:

My rule of thumb says 3"or 6 ft. whichever is more.

[George]

Deep enough to hold the shoulder of the tenoned piece tightly to the face of the mortised board for framing, deep enough to carry a load if structural.

[Upscale]

Excuse me for saying so, but that doesn't answer his question in the least. It goes without saying that one would want a mortise and tenon to meet those criteria.

I'm hoping there's some study somewhere that correlates the depth of a tenon to the thickness of a rail or some similar variable as well as taking into account the type of wood used and the application that the project is designed to fill. Not having used mortise and tenon joinery yet, I'd be interested in getting specific depth and thickness advise.

[Swingman]

A general "rule-of-thumb": Traditionally the depth of a non-through mortise is about 3/4 the width of the leg or stile.

Depends on a number of things, the size of the workpiece in which the mortise is being cut, whether you are using joinery like mitered tenons, etc.

On a 1 3/4 to 2" " table leg, I generally make my mortises about 1" deep ... maybe a little deeper for thicker table legs, or when I want to miter the ends of the opposing tenons.

In any case, make the mortise about 1/16 - 1/8" deeper than the tenon that is going into it.

[Larry Blanchard]

How do you make the deep mortises? A dedicated mortiser? I looked for a 1/4" router bit for deep mortises in 3/4" stock and the deepest cutting depth I could find was 1". And that was with a 1/4" shank!

[Swingman]

I have a dedicated mortiser, but you might want to consider going to 3/8. I have an Amana (51304) 3/8" that will cut to 1 1/2" deep.

[George]

Sorry, the real answer has been given.

However, for you, the mortise depth should not exceed the total of the depth of wood into which it is made. After that point, it's another joint.

[Peter Huebner]

Without a dedicated mortiser, the procedure goes roughly like this:

-Mark the mortise out with a marking knife or scalpel.

-Set a drill press to the final depth of the mortise, and drill outmost of the waste. I tend to mark a centre line, then use a drill size that is very near the full width of the mortise, overlapping the holes so I start the next hole just into the firm timber next to the last hole. Helps to have wood bits with a centre spur for this, or Fostner bits(sp?).

-Using a sharp firmer chisel, clean out the rest of the waste from the centre.

-Clean up the sides of the mortise with a very sharp flat chisel. The knife cuts will help getting started in exactly the right place, and prevent tear- out. It is a good idea to have the tenon finished by now so you can try the fit from time to time. It should slide in, but be seated firmly, not rattle around. O.t.o.h. you should not have to DRIVE it home.

The strongest joint is achieved by having a shoulder on each face of the tenoned member -- this also lowers the demands on 100% accuracy when cleaning out the mortise ;-)

-P.

[Morris Dovey]

| Swingman wrote: | || On a 1 3/4 to 2" " table leg, I generally make my mortises about 1" || deep ... maybe a little deeper for thicker table legs, or when I || want to miter the ends of the opposing tenons. || || In any case, make the mortise about 1/16 - 1/8" deeper than the || tenon that is going into it. | | How do you make the deep mortises? A dedicated mortiser? I looked | for a 1/4" router bit for deep mortises in 3/4" stock and the | deepest cutting depth I could find was 1". And that was with a | 1/4" shank!

Larry...

I just happened to have my catalog open. I wouldn't suggest using them in a freehand router; but you can get 1/4" x up to 6"OAL solid carbide up-spirals from

(see their catalog page 169).

-- Morris Dovey DeSoto Solar DeSoto, Iowa USA