Thread: Mortise and Tenon - 3/4 or 1/2 Standards?

I've also found this to be quite interesting:

http://www.newwoodworker.com/setupmort.html

Barry,

I generally make the tenons 1/3 the thickness. I think most of the good books suggest 1/3 for a single tenon. I am not sure of the ratios when making double tenons.


- Wood Borer

Hi wb,

Since the area of cheeks are the important area, 1/3 seems a very little glueing/contact area to me.

Where the sholders don't really come into play other than to show a clean join.


Anyone else care to comment?

Barry, the rule of thumb is for the tenon to be 1/3 the thickness of the stock.

Understood.

I'm curious what you think about the length of the tenon.

How far / deep should it fit into the mortised workpiece.

The length of the tenon will depend on the forces the joint is likely to be subjected and on the length of your router bit!

For example a gate I recently made, the tenons went all the way through (70mm long) however on a frame for a light diffuser the tenons were only 20mm. The tenons were also wedged.

The quip about the bit seems to determine the maximum depth on jobs where I am too slack to use the mortise chisels.

- Wood Borer

SilentC,

I agree with your 1/3 rule where, as is the case in Barry's example, the stock in which the mortice is being cut is the same thickness as that in which the tenon is cut. However, where the mortised stock is thicker, as, for example, where a table's leg is joined to its apron with a non-flush joint, the tenon can with advantage be thicker than 1/3 of the apron's thickness. If I were using 20 mm stock for the apron, and 40 mm stock for the legs, I would use a 3/8" (9.5 mm) tenon.

Rocker

Rough rule of thumb for thickness is 1/3, as already mentioned. This must be guided by other factors, such as the width of the nearest mortice chisel or router bit. Bearing in mind that if the timber gets thicker than 50mm you might want to think of a double tenon.

While the width rule is well known, I have not come across rules for the other dimensions. I eagerly await learning something new. My rules of thumb are:

The width of the tenon depends on the use of the joint. For example, a mortice and tenon joint for a heavy duty door is going to have to contend with strong forces of twisting. So it needs to be as wide as possible (full width). This would be even more necessary with a soft timber such as Oregon. Otherwise the width is just a cosmetic factor, dictated by the desire to hide the mortice.

The length of a tenon is similarly dependent on the nature of the stressed/load that it must endure. I tend to make the depth of a tenon about 1/3 the width of the timber into which it connects, increasing this according to my perception of the load required, decreasing this if the stresses are light.

Reading what I have written I feel the need for someone who knows to say something intelligent!

Regards from Perth

Derek

Rule of Thumb:

This term for "a simple principle having wide application but not
intended to be strictly accurate" dates from 1692. A frequently
repeated story is that "rule of thumb" comes from an old law
regulating wife-beating: "if a stick were used, it should not be
thicker than a man's thumb."

I was taught the old 1/3 rule many years ago and it was explained that as well as tenon thickness, the width of the cheeks has a bearing on the overall strength of the joint. probably as the cheeks tend to absorb skewing loads thus preventing overloading the glueline between the sides of the tenon and mortice.

Originally posted by silentC
Rule of Thumb:

This term for "a simple principle having wide application but not
intended to be strictly accurate" dates from 1692. A frequently
repeated story is that "rule of thumb" comes from an old law
regulating wife-beating: "if a stick were used, it should not be
thicker than a man's thumb."

Wrong forum!!!!!!!!!!!!!
This belongs in the B&D forum.

I thought "Rule of thumb" derived from the old expression "nipples like a brickie's thumb".

Ooops, wrong forum?

Is that why I keep sucking my thumbs?


Cheers, Al the sucker

The straight dope on 'rule of thumb' (if you can be bothered):

http://www.straightdope.com/columns/000512.html

It was the phrase "a simple principle having wide application but not intended to be strictly accurate" that I wished to bring to people's attention, not the wife-beating story. I left that in there for the general amusement of all. Excepting my wife. She does not find it amusing at all.