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
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
I'm curious if anyone has some reference sites, or tables.
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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.
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
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.
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,
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!
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 ;-)
firstname dot lastname at gmail fullstop com
Larry Blanchard (in email@example.com) said:
| 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!
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 www.kbctools.com (see their catalog page 169).
DeSoto, Iowa USA
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