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Mortise and Tenon Layout

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Forum topic by I_love_lamp posted 08-10-2018 02:54 PM 307 views 0 times favorited 9 replies Add to Favorites Watch
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I_love_lamp

4 posts in 24 days


08-10-2018 02:54 PM

Topic tags/keywords: joint mortise tenon question tip

Hi all, I’m fairly new to woodworking and found myself confused with the layout of a through mortise and Tenon for a workbench, specifically the width. I have a 3.5×3.5 leg (mortise) and a 1.5×5.5 rail (Tenon). I’ve seen the rule of thumb for Tenon thickness to be 1/3 the stock making mine 0.5 in and bc this will be a through m&t the length of the Tenon is already determined for me by the size of the leg. Where I’m confused is with the width of the Tenon. I was planning on making it 4.5 in but read something about how there could be an issue with wood movement of the Tenon of this size damaging the mortise eventually. should I make the width shorter? Any advise will be greatly appreciated. Thanks!


9 replies so far

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John Smith

1247 posts in 246 days


#1 posted 08-10-2018 03:07 PM

personally – I like to see a small shoulder on each side of the tennon
to hide any potential issues with the mortise cavity.
does anyone recommend two tenons ??

-- Graduated Valedictorian from the University of HardKnocks --

View Sludgeguy's profile

Sludgeguy

33 posts in 206 days


#2 posted 08-10-2018 03:26 PM

I agree with John Smith. I would provide a 1/4” shoulder on top, bottom, and sides. This results in tenons that are 1 1/4” wide by 5 1/4” tall. I just built a large table with nearly identical aprons.

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Rich

3342 posts in 673 days


#3 posted 08-10-2018 03:34 PM

You’re fine. My rule of thumb is 6” before I split them. You can see in the third photo for my entry door project that the top rail tenon is one piece and the lock rail and kick rail are split. Naturally you only apply glue to one when they’re split so the other can float.

-- Half of what we read or hear about finishing is right. We just don’t know which half! — Bob Flexner

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Rich

3342 posts in 673 days


#4 posted 08-10-2018 03:37 PM


I agree with John Smith. I would provide a 1/4” shoulder on top, bottom, and sides. This results in tenons that are 1 1/4” wide by 5 1/4” tall. I just built a large table with nearly identical aprons.

- Sludgeguy

The OP’s dimensions are correct. He’s simply asking about wood movement with the tenon glued cross grain into the vertical grain of the mortise. If that’s too big, you risk either splitting of the rail, or failure of the joint.

-- Half of what we read or hear about finishing is right. We just don’t know which half! — Bob Flexner

View clin's profile

clin

912 posts in 1080 days


#5 posted 08-10-2018 03:48 PM

I’m no expert, but I think somewhere in the range of 3 to 3.5” is considered maximum for tenon width to avoid issues with wood expansion. As with anything, wood type and the environment it is used in matter. I don’t think two smaller tenons makes this go away. The entire board is going to expand. Two tenons likely would avoid the tenon splitting, but not the board itself.

Now, I think I’ve seen two tenons used where say the top is glued normally and the bottom is just pinned with an elongated hole in the tenon. Much like some breadboard ends are done.

In the OP’s case, I’d just make the tenon smaller. A 3” wide tenon is still pretty big.

Something to consider is to make it a drawbore M&T joint. If unfamiliar, this is where you drill a hole through the joint and pound a peg or two to hold things together. The trick is you offset the holes in the tenon slightly in a way that as you pound the peg in, it pulls the tenon in. This make an EXTREMELY tight joint. Glue really isn’t even needed, but I’d still use it.

Here’s a few links on using drawbores on a workbench.

https://www.mortiseandtenonmag.com/blogs/blog/drawboring-the-workbenches

https://www.popularwoodworking.com/workbenches/schwarz-workbenches/the-lazy-man%E2%80%99s-drawboring-delight

-- Clin

View Rich's profile

Rich

3342 posts in 673 days


#6 posted 08-10-2018 04:26 PM


I m no expert, but I think somewhere in the range of 3 to 3.5” is considered maximum for tenon width to avoid issues with wood expansion. As with anything, wood type and the environment it is used in matter. I don t think two smaller tenons makes this go away. The entire board is going to expand. Two tenons likely would avoid the tenon splitting, but not the board itself.

- clin

Having done hundreds of them, I can assure you that even 6” is safe. I only split them after that to play it safe, but I’ve never had a failure. It also makes a difference that the OP’s rail is 1-1/2” thick.

Also, splitting them does make a difference since you only put glue on one and the other floats with the movement of the rail (pinning is optional — I never bother). It’s similar to other techniques like only gluing the front part of a sliding dovetail, so the rear can move.

These are fundamental woodworking concepts.

-- Half of what we read or hear about finishing is right. We just don’t know which half! — Bob Flexner

View Woodknack's profile

Woodknack

12234 posts in 2464 days


#7 posted 08-10-2018 09:02 PM

I would provide a 1/4” shoulder on top, bottom, and sides. This results in tenons that are 1 1/4” wide by 5 1/4” tall.
- Sludgeguy

The shoulders of a tenon are at least as important than the tenon itself as they make the table rigid by resisting racking. It’s why the rule of thumb for tenons is 1/3 the thickness, or you could say the shoulders should be 2/3 the thickness.

I was planning on making it 4.5 in but read something about how there could be an issue with wood movement of the Tenon of this size damaging the mortise eventually.
- Ilovelamp

You’ll be fine. Wood movement over 4 – 5 inches is negligible.

-- Rick M, http://thewoodknack.blogspot.com/

View I_love_lamp's profile

I_love_lamp

4 posts in 24 days


#8 posted 08-10-2018 09:18 PM

Thanks everyone for the feedback. Sounds like I should be fine with my current plan of making the width 4.5”. You guys are awesome.

View clin's profile

clin

912 posts in 1080 days


#9 posted 08-10-2018 09:34 PM


You ll be fine. Wood movement over 4 – 5 inches is negligible.

- Woodknack

This is certainly not true in all cases. Tightly fasten a board over that range and it might split. It may be fine if the wood stays in a relatively narrow range of humidity. A 5% change in dimension is not out of the question with large moisture changes. That would be 1/4” out of 5”. You might see this in outdoor furniture and perhaps in an unconditioned shop space.

And a 2% change 0.1” out of 5” is very possible and is not negligible. Now for interior applications in air conditioned spaces typical of many furniture applications, you likely won’t see this much. But without knowing the specifics you run a risk.

Other factors include type of wood and whether it is flat saw (worse) versus quarter sawn.


Also, splitting them does make a difference since you only put glue on one and the other floats with the movement of the rail (pinning is optional — I never bother).

- Rich

I agree with that and that was my point in commenting on it. I think some people have the mistaken belief that splitting the tenon into two and then gluing in both tenons somehow addresses this issue of wood movement. As you point out, that’s not the way to apply spit tenons.

-- Clin

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