Split Tenon Dimensions

[ngabriel]

Hi all, I'd like to join an 7 1/4" x 3/4" piece of stock to a 2.5" x 2.5" post/stile. I'm new to this, so I'm at the mercy of Google. I've read that split tenons are essential for cases like mine and I'm asking for tips and pointers on the actual dimensioning. Specifically on the thickness and width.

Thickness:

I read that anywhere from 1/3 to 1/2 the stock's thickness is desired. Since I'll be making my tenons on the table-saw, 1/2 will work (3/8").

Width:

This is where I find information to be less consistent. One rule of thumb I've seen is that the width should never exceed 2.5". Another is the width should not exceed 5x the tenon's thickness. If following the latter, I'll go no wider than 1 7/8". The issue I have with this is that I was hoping to only split the tenon once (two tenons). Having a width of 1 7/8" seems like a big waste in potential gluing surface on a 7 1/4" board. I think I'd have to split the tenon twice instead to create 3 tenons. That would be a more work, which is fine, but is it necessary? Overkill? Plus, that seems like it wouldn't leave me with a lot of space for the shoulders (1.625"/3).

Length:

As far as the length is concerned, I pretty consistently see that it should be 1/2 of the width of the stile (so in my case 1 1/4").

So here is what I'm thinking, I'll split the tenon once such that I two tenons with the following dimensions (again on 3/4" stock):

Thickness: 3/8"

Width: 2 1/2"

Length: 1 1/4"

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Side-Shoulder Width: 3/16" (3/16" + 3/8" + 3/16" = 3/4")

Top/Middle/Bottom Shoulder Width: 1" (3/4" + 2 1/2" + 3/4" + 2 1/2" + 3/4" = 7 1/4")

Anyone have any tips or rules of thumb that I should be taking into consideration?

[thewoodwhisperer]

Hey Neil. The reason you are finding conflicting info and ranges of recommendations instead of exact numbers is simply because it all depends. Wood species, wood orientation, and climate will all have an impact on whether something like this is even necessary. And in some cases, you may hear people making the joint improperly, and negating their original intent.

So it's always a good idea to investigate the wood species you're using and get an idea of what the maximum amount of movement will be. I recommend using the Wood Shop Widget to do this. Once you do this, you'll have a much clearer idea of how much movement your tenon will have to endure.

Now I definitely would rule out the three tenon option. And although 2.5" is the rule of thumb, I might just go with 3" tenons. Something about leaving so much shoulder room that I don't like, lol.

Also consider that if you glue both tenons in place and both of your mortises are perfectly sized, you aren't really allowing for much in the way of expansion. So you might consider gluing only one tenon and letting the other one float in a slightly oversized mortise. Depending on the season and your climate, you may need to build for mostly expansion, or mostly contraction. This will vary by region.

So in my opinion, i wouldn't over-think it too much. Either go with your dimensions or bump the tenons to 3" wide, glue one tenon and give the other one some room to move. Then grab the beverage of your choice and enjoy a hard day's work.

[ngabriel]

Hey thanks Marc, that was useful. This project is a small twin-sized bed-frame. Since I plan on painting it, and because I'm inexperienced, I'm making it from Poplar. My wife already has her eyes set on my SketchUp plans though so I'm sure it wont belong before I'm making something similar for our bedroom (not from Poplar I'm sure). So I think this little be of practice will be helpful.

Thanks for the gluing tip also. I did read that somewhere else too but I'm glad to see you validating the practice.