Length of a Tenon

[Coop]

At what point is the length of a tenon become negligible? 

[JohnG]

Once it exits the other side of the piece 

[Chestnut]

Once wood movement becomes large enough that it's an issue?

[Ronn W]

Interesting question,  I think that you are asking - at what length of tenon is there no point in making the tendon longer?  I tend to make my tenons shorter than other wood workers.  I will, on lighter (smaller) projects, use a 1 or 1 1/4"" tenon when others would use 1 1/2".    My quess is that once the glue surface on each side of the tenon develops enough strength to be stronger that the tenon itself adding more length to the tenon gains nothing.  But that really only applies to joints that will see a lot of force.  In most cases I would think that you only need enough tenon length to allow the glue strength to resists whatever forces the joint may be subject to.  Good luck in calculating that number.

[Chet]

About a million years ago my shop teacher would preach that the length of a tenon should be between 1/3 and 1/2 the width of the apron or rail if it was a cabinet door.  I have more or less followed this on most projects.

When I did my bed I went with longer ones, if I remember they were 2 1/2 inches into the legs and there were four on each side, the stiles were 3 inches wide.

[Coop]

On 10/3/2023 at 10:49 AM, Ronn W said:

Interesting question,  I think that you are asking - at what length of tenon is there no point in making the tendon longer?  

Correct.

I understand that the longer the length, the more wood to glue surface and the harder it will be to pull the joint straight apart. I’m wondering at what point does the length no longer play a part in the shear and tensile strength? 

[G Ragatz]

On 10/3/2023 at 10:28 PM, Coop said:

I’m wondering at what point does the length no longer play a part in the shear and tensile strength? 

I would guess that strictly from a joint-strength perspective, longer is always better right up until you have a through-tenon, at which point a longer tenon doesn't add any gluing surface (not considering the strength of a wedged tenon here).

That said, I've always understood the rule of thumb to be a tenon length of 1/2 - 2/3 the width of the stock - a leg or stile - being mortised. But that may be overkill, depending on the application. Certainly, plenty of well-made rail-and-stile cabinet doors have much shorter tenons - not that much shear pressure to deal with. But on a bed frame, it might be a different story (see comment from @Chet above).

[Coop]

And a bed frame I will be making and I always take @Chet’s advice into consideration! Thank you for your response. 

[Chestnut]

I get the  1/2 to 1/3 rule but it also doesn't make sense. If you have a 9" rail the shorter tenon would be 3" which is longer than needed Also who has a 3" long bit to cut said tenon?. Going to cut it by hand? On the other side a 1"x1" divider would only get a 0.33-0.5" tenon which isn't long enough. Now i can hear people thinking "but those are extremes most parts are 2.5-3.5" wide!" I agree and given the typical part size you'd see a 1"-1.75" long tenon. I'd shoot for that range regardless of part size. After 1.5" the wood around the tenon is most likely weekend enough that the mortise board would split or break.

For a bed I'd go as long as you have a bit to cut the mortise.

[wtnhighlander]

When making decisions about joinery, I try to consider how the joint will hold best without glue, then proceed from there.