# Tenon & Mortis to work piece ratio



## Sawtooth (Dec 31, 2008)

Hey, any thoughts about a tenon and mortis ration?

What I'm talking about is the relationship between the thickness of the pieces and the length of the tenon. I have a sitting bench I'm working on that has legs that are 1 and 3/4 thick and the plans call out for a 1 and 1/2" mortis and tenon.

I'm thinking of changing this to a 1 and 1/4" mortis and tenon.


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## dovetail_65 (Jan 22, 2008)

Go here it has everything you need to know and more about M & T sizing:

M & T Size relationships

From the link above:

The proportions of the tenon should be the 'thirds rule'. 

The *tenon *should be two thirds with the haunch being about one third. 
The *thickness* you will remember should be approximately one third of the thickness of the material adjusted to the nearest chisel size whether machine or hand. 
The *'tongue'* or depth of the haunch should be about the same as the thickness of the tenon but should not be less that 10 mm. This tongue should either fit into the groove formed or step into the rebate.

*Historical Notes: * Traditionally in wood trades and mostly joinery, M & T’s were set out as in the following:

*Thickness of tenon* = 1/3 of (t) thickness (to nearest chisel width)

*Width (w) or depth (d) of tenon *= less than 5 x (t) thickness (and no more than 125 mm)

*Depth of haunch tongue* = same as thickness, but no less than 10 mm


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## Sawtooth (Dec 31, 2008)

What is the Width (w) or depth (d) of tenon = less than 5 x (t) thickness (and no more than 125 mm)

5 x the thickness? doesnt seem right.


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## dovetail_65 (Jan 22, 2008)

It just means the tenon be no more than about 5 inches in any direction(L or W). In this authors opinion anything more is not required no matter the size of the work pieces. I think the 1/3 rule is most important.


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## bobj3 (Jan 17, 2006)

Hi 
I agree with the 1/3 rule but it can be anything you want it to be, it comes down to the load it needs to take on  and how it will take the load..

Sometimes shorter is better than long  it's hard to break a short stick..

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nickao65 said:


> It just means the tenon be no more than about 5 inches in any direction(L or W). In this authors opinion anything more is not required no matter the size of the work pieces. I think the 1/3 rule is most important.


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## Sawtooth (Dec 31, 2008)

So are we talking length? To have a mortise thats 1 and 1/2 in a 1 and 3/4 inch lwide leg would be too long.

I'm thinking more like 1 and 1/4 but the 1/3 rule would put me at about 3/8. That doesn't seem long enough?


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## bobj3 (Jan 17, 2006)

HI sawtooth

width, so to say 3/4" wide stock 3/8 wide , length = 3/8" x 3 = 1 1/4" the norm, 1/4" wide ,,,1/4" x 3 = 3/4" and so on...

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Saw Tooth said:


> So are we talking length? To have a mortise thats 1 and 1/2 in a 1 and 3/4 inch lwide leg would be too long.
> 
> I'm thinking more like 1 and 1/4 but the 1/3 rule would put me at about 3/8. That doesn't seem long enough?


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## dovetail_65 (Jan 22, 2008)

It is long enough. You can see below it is the thickness of the tenon that really matters NOT the length. As long as you do not go to a really short "stub tenon" you are fine.

Here is a list of recent testing results for some typical joints. Look at the strength in a regular miter joint! Not as big a weak link as many believe!

This testing uses RACKING forces.

HALF LAP 1,603 lb.
BRIDLE 1,560 lb.
SPLINED MITER 1,498 lb.
*3⁄8-IN. MORTISE & TENON 1,444 lb.*
3⁄8-IN. FLOATING M&T 1,396 lb.
*MITER 1,374 lb.*
3⁄8-IN. WEDGED M&T 1,210 lb.
3⁄8-IN. PINNED M&T 1,162 lb.
5⁄16-IN. M&T 988 lb.
BEADLOCK 836 lb.
DOWELMAX 759 lb.
*¼ -IN. M&T 717 lb.*
POCKET SCREW 698 lb.
DOMINO 597 lb.
BISCUIT 545 lb.
BUTT 473 lb.
COPE & STICK 313 lb.
*STUB TENON 200 lb.*

You can see those new devices like the expensive Festool Domino may be fast(maybe not), but nowhere near the strength the old methods have. The old ways are still the best ways as far as joint connections and that includes using the new adhesives with the joints. New adhesives were used on all the joints in this testing.

Joinery Shootout at FWW


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## Sawtooth (Dec 31, 2008)

Got it! Thanks for the help. So I'm going to shorten up the length. I'm not comfortable with leaving only 1/4" on the back side of each mortise. I did find this artical that kina laid out the idea that the the length was not the critical factor.
http://www.lowes.com/lowes/lkn?action=howTo&p=Build/MortTen.html


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## Mike Gager (Jan 14, 2009)

the longer (or deeper) the tenon the more glue area it will have so in turn the stronger it will be


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