Tenon Width 1/3 or 1/2?

[Preston Baxter]

Any MEs or Wood Material Scientists out there? I've noticed two schools of thought on Mortise and Tenon joint width. All this is assuming both parts of the joint's stock are the same width. Some folks/texts say the tenon should be 1/3 the thickness of the stock, and others say 1/2 the thickness. Supposedly the 1/3 rule keeps either cheek and the tenon with the same probability of breaking out, and the 1/2 rule says that if you have equal amounts of material in the cheeks and in the tenon that the joint will be stronger. What are the advantages or disadvantages to both methods? My gut instinct says the 1/2 is stronger, if the joint is properly glued, but I could see where the 1/3 method could be stronger if it were a dry, drawbored joint.

[Bill Houghton]

This came up as a discussion on another forum about a year ago, after Popular Woodworking, in two articles in one issue, recommended both 1/3 and 1/2. Chris Schwartz (sp?), the editor, suggested 1/3 for hand tools (less likely to blow out the cheeks of the piece with the mortise) and 1/2 for power tools, for the reasons you cite in your post.

No direct knowledge myself, but I suspect that the strength of the tenon itself is probably most important - if it's thick enough to be adequately strong, you're probably OK.

[John Thompson]

I use the 1/2 on pieces that are prone to be racked by outside sources... dining and kitchen talbes.. beds.. etc. I also have a tendency to compensate somewhat by designing using a bit thicker stock.

I find the 1/3 sufficient for chest of drawers.. book-cases (depending on size and how much weight it will potentially be subjected too).. coffee tables.. etc., etc.

BTW.. if you have teen-agers or there is one within a 20 mile radius, I would fore-go the thought of delicacy usually associated with what I mentioned as 1/3 items and go for the 1/2.. I am currently replacing a lot of items house-wide for the "First Lady" with more delicate items as the "boom-bang-crash-opps-"sorry mom" crowd had made their long awaited exit. Experience is a wise teacher indeed. ha.. ha...

Sarge..

[Raney Nelson]

Maybe I'm misunderstanding here, but the 1/3 rule is based on the stock that the mortise is in, not the tenon stock. The tenon is just sized to fit the mortise, which is usually the more critical part of the joint, and the more likely to fail. For instance, in a frame & panel with 3/4-inch stock, if you use the 1/2 rule, then the 3/8-inch tenon may be stronger than a 1/4 would be, but the thin (3/16") mortise walls are what is going to fail every time. The tenon is never going to be the weakest link in that case.

edit to add: in cases where the mortise stock is significantly larger (such as stretcher-to-leg joint on a kitchen table) I don't worry too much about it. I usually make the tenons a convenient size to fit my tooling (mortise chisels) and still leave shoulder(s) where I want them. 1/2 is pretty common in those circumstances for me.

[Jesse Cloud]

All I know is that if you look at the joint strength tests that appear in the woodworking mags every now and then, its almost always the cheeks that break... so I lean toward 1/3.

[Chuck Nickerson]

If the stresses are going to be vertical, thicker tenon gives a stromger joint. If the stresses are lateral or racking, mortise wall thickness is important. A child's bed probably needs more lateral and racking strength than an adult's bed. A massive wing-back lounging chair probably needs less racking strength than a dining chair. YMMV

[Richard Niemiec]

What Jesse said. I've consistently used 1/3 and never had a problem. RN

[andy brown]

Hi,
Probably a bit late in the day for a response, but I'm just reading the softcover book ' Fine Woodworking' on Joinery and the interesting article by Ian J Kirby.
I was brought up on the 1/3rds' system and never thought more of it, save that sometimes I thought the tenons a bit too thin!!

But Ian explains and shows very simply that the need is really for a bigger tenon and he proposes 1/4 shoulder, 2x 1/4 tenon, and 1/4 shoulder again. You then have good solid tenon and sensible sized shoulders for neatness.

Simple and brilliant.

Thank you Ian.

Andy.

[harry strasil]

probably not relivent, but the original framing square was intended for timber framing thus the 1.5 inch tongue and the 2 inch body as those were the most used tenon sizes. A Bridge builders square was 2 inches on the tongue and 3 inches on the body with a slot down the middle.

[Jesse Khangura]

I have read the 1/3 for handtools and 1/2 for machine as well. As I remember it, 1/3 is better (stronger) however using a 1/4" mortising bit doesn't work as well in regards to to chip clearing as a 3/8" mortising bit. I think with handtools 1/3 is the way to go.

[Wilbur Pan]

If the stresses are going to be vertical, thicker tenon gives a stromger joint. If the stresses are lateral or racking, mortise wall thickness is important. A child's bed probably needs more lateral and racking strength than an adult's bed.

One would hope that an adult's bed would need more resistance to vertical stresses. ;)

[Chuck Nickerson]

One would hope that an adult's bed would need more resistance to vertical stresses. ;)

But I'm getting married Dec 28th, and all my friends assure me the squeaking will sto soon enough. :-)

[Lee Hingle]

Chuck,

What's the one food that will put an absolute stop to all of that?



Wedding cake;)