Strength Of Mortise & Tenon Joint

[Wiley's Woodworks]

I'm building a massive woodworker's workbench. I'm taking design elements from 3 different benches in Christopher Schwarz's books and a set of plans from Benchcrafted. My preferred joinery to attach the top to the under frames is to cut tenons on the tops of the legs, insert them into mortises cut into the bottom of the bench top, glue and draw bore the tenons tightly into the mortises.

The dimensions of the legs from 3 different plans are all within 1/2" of each other on width and thickness--~5"W x 4"T. Meeting the thickness with the lumber I have on hand will require laminating up 3 separate boards. One set of plans specifies this, and utilizes a hack I would like to use. The outside boards are cut ~2" shorter, so when a leg is glued up the longer center board automatically becomes the tenon. Seems like a smart way to do this step, so I would like to do my joinery this way.

I can mill down all 3 boards equally, which will make the tenon thinner but will adhere to the standard M & T formula of the tenon thickness is 1/3 of the total thickness of the workpiece, making the shoulders' thickness equal to the tenon thickness, or I can glue up the entire leg extra large and then cut down the shoulders to get to the final thickness of the leg. If I do it this way the tenon thickness will be about 1/2" greater than the thickness of each shoulder.

My big ?: What gives the mortise and tenon its strength? Is it the thickness of the tenon inserted into the mortise pocket, or is it the width of the shoulders, cut square and glued under pressure to the mating piece? If someone can say with certainty I am overthinking this, I would appreciate it.

 

[Mike Davis]

The one third rule is used when the tenoned member and mortised member are equally sized beams so that they each have similar strength. If I understand correctly you are mortising the bench top. You should have no worries about the strength of the bench top. Make the leg tenon any way that is convenient.

 

[FredP]

I would not do that. I did it on my bench. The top is going to move. Mine did and I had to remove the top and widen the mortise to allow for it. The top tried to move and the base didn't. After enlarging the mortise the. Top settled down and all was well. The top is a couple hundred pounds so it doesn't move around.

 

[Wiley's Woodworks]

I would not do that. I did it on my bench. The top is going to move. Mine did and I had to remove the top and widen the mortise to allow for it. The top tried to move and the base didn't. After enlarging the mortise the. Top settled down and all was well. The top is a couple hundred pounds so it doesn't move around.

One of the plans calls for making the rear mortises 1/8" thicker to allow for movement, and I will do that. All 4 legs will be flush with the edge of the top; I will be installing a leg vise, so I might as well make all 4 legs uniform. Because the leg tenons will be close to the outside edge it will be easy to draw bore peg the tenons. Should I still use glue or leave it dry so the top can move with the seasons?

 

[Wiley's Woodworks]

The one third rule is used when the tenoned member and mortised member are equally sized beams so that they each have similar strength. If I understand correctly you are mortising the bench top. You should have no worries about the strength of the bench top. Make the leg tenon any way that is convenient.

Mike--I don't have any worries about the strength of the top. My concern is that if the joint fails, the weakness will be in the leg tenon. I'm leaning toward making the tenon > 1/3 the thickness of the leg, leaving each shoulder < 1/3 the thickness. Hope this clarifies. Give this design your seal of approval if you would do it this way.

Referring to Fred's post, how would you allow for the top to move with the seasons. If I use glue in the mortises, it seems to me this will lock the structure in place as one rigid piece, making the under frame "fight" the bench top with the inevitable in the top. Two 3/8" white oak dowels in draw bore placement in each joint will hold the two together, glue or not. To glue or not to glue, that is the ?.

 

[Mike Davis]

I did not glue my top to the legs. My top is 3” thick and weighs about 120 pounds. I used round tenons on the legs, closely fitting holes in the front side where my side vise is. I want this side to be stable, aligned, non moving. The two holes at the back side of the top are double drilled about 1/2” to elongate the floating mortise and allow for the top to expand and contract. The weight of the top is plenty to hold everything together. Yet, I am able to disassemble and move my bench in 15-20 minutes. It all fits in the back of my Honda CRV. I had it set up at my gallery showing and moved it by myself.

 

[mdbuntyn]

If you aren't planning to start before the weekend, you can ask Schwarz directly. He's started doing an open Q&A on Saturdays from 7am-5pm on the Lost Art Press blog. If you've never commented on any of his blog posts before, find one to comment on, or else your question will get stuck in the moderation que. Lost Art Press

 

[Mike Davis]

Mike--I don't have any worries about the strength of the top. My concern is that if the joint fails, the weakness will be in the leg tenon. I'm leaning toward making the tenon > 1/3 the thickness of the leg, leaving each shoulder < 1/3 the thickness. Hope this clarifies. Give this design your seal of approval if you would do it this way.

Since the top is very large compared to the leg, there is no concern about the strength of the mortise or rather the wood around the mortise. The mortise is not the weak link.

Therefor, the strength of the tenon is the focal point. The rule of 1/3s does not apply since the strength of the mortise is not a factor.

So, make the tenon as large as possible or practical. 3/4 of the leg thickness would not be out of bounds. The shoulders need only be strong enough to resist shearing which is doubtful if only 1/8” wide. Yes, 1/3 will work fine. Any amount larger than that will also work fine.

 

[NcFaering]

So, I just made a workbench from the anarchist workbench plans. It feels absolutely bombproof. I could feel all the joints tighten and draw in when hammering the pegs in. I haven't had it through a full range of seasons yet, so can't really tell you about movement. But I love it to pieces.

About the glue question. From the schwarz book.
"ASSEMBLY
I assemble the bench using liquid hide glue because it has a long
open time, it is easily cleaned with hot water (even after it has set up)
and it’s reversible if things go incredibly awry. I don’t use bar clamps.And the glue is just extra insurance because the drawbore pegs provide
all the pressure you need.
Assemble the bench upside down on sawhorses. First glue the ends
of the bench base together and knock the drawbore pegs in a little to
hold things together. Don’t drive any pegs home until the entire bench
is together"


So he suggests using glue. after building it and going through the whole process, I would trust whatever Chris Schwarz says. I am so happy with my bench so far.

 

[Dee2]

So, I just made a workbench from the anarchist workbench plans. It feels absolutely bombproof. I could feel all the joints tighten and draw in when hammering the pegs in. I haven't had it through a full range of seasons yet, so can't really tell you about movement. But I love it to pieces.
[snip]
So he suggests using glue. after building it and going through the whole process, I would trust whatever Chris Schwarz says. I am so happy with my bench so far.

pictures?

 

[NcFaering]

I knew I was forgetting something.... The only big change I made to the plans were to increase the height. Mine is 40inches tall because I can't stand a low workbench. Also followed the bench crafted instructions for adding a wagon vise. Chose jatoba/black limba for the endcap because, why not? I kinda regret the jatoba as it was an absolute pain to cut through, I ruined 3 router bits cutting the mortise channel

 

[Howie]

I'm building a massive woodworker's workbench. I'm taking design elements from 3 different benches in Christopher Schwarz's books and a set of plans from Benchcrafted. My preferred joinery to attach the top to the under frames is to cut tenons on the tops of the legs, insert them into mortises cut into the bottom of the bench top, glue and draw bore the tenons tightly into the mortises.

The dimensions of the legs from 3 different plans are all within 1/2" of each other on width and thickness--~5"W x 4"T. Meeting the thickness with the lumber I have on hand will require laminating up 3 separate boards. One set of plans specifies this, and utilizes a hack I would like to use. The outside boards are cut ~2" shorter, so when a leg is glued up the longer center board automatically becomes the tenon. Seems like a smart way to do this step, so I would like to do my joinery this way.

I can mill down all 3 boards equally, which will make the tenon thinner but will adhere to the standard M & T formula of the tenon thickness is 1/3 of the total thickness of the workpiece, making the shoulders' thickness equal to the tenon thickness, or I can glue up the entire leg extra large and then cut down the shoulders to get to the final thickness of the leg. If I do it this way the tenon thickness will be about 1/2" greater than the thickness of each shoulder.

My big ?: What gives the mortise and tenon its strength? Is it the thickness of the tenon inserted into the mortise pocket, or is it the width of the shoulders, cut square and glued under pressure to the mating piece? If someone can say with certainty I am overthinking this, I would appreciate it.

Glue surface area + shoulder.

 

[Rwe2156]

I wouldn't mortise the legs either. A through mortise in a 3-4" top is not an easy feat, unless you do the "ghost" mortise method. It's a ton for work for very little gain. This is something that started with the Roubomania following Chris Schwarz's book.

Why not just make trestles? Mortise a stretcher in between the front and back legs and add a base. Then simply bolt the top to the stretchers. Over size the holes if you're worried about movement. You could park a truck on this thing~

 

[tvrgeek]

Modern glue kinds of overshadows a lot of conventional wisdom. Fancy joints were necessary when the best we had was hide or fish glue.

 

[Wiley's Woodworks]

I wouldn't mortise the legs either. A through mortise in a 3-4" top is not an easy feat, unless you do the "ghost" mortise method. It's a ton for work for very little gain. This is something that started with the Roubomania following Chris Schwarz's book.

Why not just make trestles? Mortise a stretcher in between the front and back legs and add a base. Then simply bolt the top to the stretchers. Over size the holes if you're worried about movement. You could park a truck on this thing~

View attachment 218650



View attachment 218651

Dr. Bob--My under frame design incorporates trestle style leg frames connected by lengthwise upper and lower stretchers and a lower shelf. The top will be 3 1/2"+ thick, and the mortise pockets will be 2" deep. I don't want to run bolts through the top from the top because the furniture maker in me can't stand exposed bolt heads. I want an all wood top with no countersink pockets, just like the ones in your pictures.

The leg trestles are fixed and rigid from front to back. I still need to deal with the wood movement in the top. There will be 2 pieces made by laminating 4" x 1 1/2" sugar maple face-t0-face. The 2 pieces will be about 12" wide, resulting in 26"+ of bench top that will be subject to seasonal movement. The top will be Roubo style, split top, with a 2" W insert down the middle, ala the Benchcrafted design. I'm confident it will be ok to use glue in the front leg M & T joints. My concern is the back half of the top. I can't come up with a design other than making the rear mortises 1/8" thicker (front to back) and just relying on the weight of the 1/2 of the top to hold it in place. Seems like heavy pressure front to back, like cross planing or hammering on a chisel, will cause the back top piece to shift around. End to end the mortise and tenon joints will be tight, so shifting lengthwise under pressure won't be an issue. Again, maybe I'm obsessing over 1/8" of front to back movement; somebody please tell me if I am. I just don't want any detectable looseness or wobble in my future pride-and-joy workbench.

 

[Wiley's Woodworks]

I knew I was forgetting something.... The only big change I made to the plans were to increase the height. Mine is 40inches tall because I can't stand a low workbench. Also followed the bench crafted instructions for adding a wagon vise. Chose jatoba/black limba for the endcap because, why not? I kinda regret the jatoba as it was an absolute pain to cut through, I ruined 3 router bits cutting the mortise channelView attachment 218645View attachment 218646View attachment 218647

Lee--Your design of both benches is really close to how I am going to make my bench:

• thick laminated top made by glueing up boards face-to-face so the edges (thickness) of the boards form the top.
• a rigid, trestle style under frame with an upper and lower stretcher on all four sides using M & T joints that are glued and draw bore pegged.

How did you account for the inevitable wood movement in the top, or did you just assume (hope?) the under frame is so strong nothing is going to blow it apart. This is what I would like to do with glued and pegged M & T joints connecting the top to the under frame. I'm just not 100% confident this design will work.

 

[pop-pop]

If I understand, your plan has top halves that are completely separate. Suppose that you joined the top sections together for say 6” at either end. This would yield a 2” wide slot for the bench-length minus one foot.

Now you could eliminate the top stretcher in your trestle and then mortise the trestle legs directly into the benchtop. Seasonal movement would be taken up by slight bending in the legs and lower trestle leg-stretcher joints.

Just a thought.

 

[Mike Davis]

Wait, it’s a split top? Why are you thinking about wood movement at all? Two separate pieces, not one solid full width piece. No problem.

 

[Rwe2156]

Dr. Bob--My under frame design incorporates trestle style leg frames connected by lengthwise upper and lower stretchers and a lower shelf. The top will be 3 1/2"+ thick, and the mortise pockets will be 2" deep. I don't want to run bolts through the top from the top because the furniture maker in me can't stand exposed bolt heads. I want an all wood top with no countersink pockets, just like the ones in your pictures.

The leg trestles are fixed and rigid from front to back. I still need to deal with the wood movement in the top. There will be 2 pieces made by laminating 4" x 1 1/2" sugar maple face-t0-face. The 2 pieces will be about 12" wide, resulting in 26"+ of bench top that will be subject to seasonal movement. The top will be Roubo style, split top, with a 2" W insert down the middle, ala the Benchcrafted design. I'm confident it will be ok to use glue in the front leg M & T joints. My concern is the back half of the top. I can't come up with a design other than making the rear mortises 1/8" thicker (front to back) and just relying on the weight of the 1/2 of the top to hold it in place. Seems like heavy pressure front to back, like cross planing or hammering on a chisel, will cause the back top piece to shift around. End to end the mortise and tenon joints will be tight, so shifting lengthwise under pressure won't be an issue. Again, maybe I'm obsessing over 1/8" of front to back movement; somebody please tell me if I am. I just don't want any detectable looseness or wobble in my future pride-and-joy workbench.

1)You‘re not going to get that much movement in 12”
2)If I’m understanding you have a cross brace you can fasten the top to, right?

As I mentioned with the Roubo popularity came the idea the legs HAVE to be mortised into the top.

Can you supply a pic or drawing of the trestle design

 

[NcFaering]

I just went by the plans and trusted in the design and plans. My thought was that the author has built a bunch of benches and knows what works and what doesn't. If it breaks, at least I learned a lot building it.