Mortise/ Tenon glue ups

[Mark Singer]

As I am working I often think of things that are not that obvious that may help others.... As you know regardless of the glue ....epoxy, pva, or plastic resin...there is a settling of the glue in the joint once it is engaged and clamped. Because each joint has a bit of slop the best position for drying is in the position the piece will be used. For example , for a table a stretcher to a leg should be set with the leg verticle....this insures that the bottom of the joint has good adhesion.. The lower part of the joint is the weakest and is the tension side for most stress. The upper portion is often in tension unless it is moved laterally. So the upper part of a joint is uually pushing against itself....stretcher against leg...
Veritcle Free play is ok on the long grain to long grain side of a loose M and T joint....about 1/16" vertically will allow you to tap the joint flush at the top and reduce any planning after. The long grain adhesion takes most of the stress and develops most of the strength. The lateral free play should not be excessive ...the joint should fit snuggly...I use a small mallet ant tap things together:rolleyes:

[Mark Singer]

It seems very telling to me that there is no response on a thread that discusses how the stresses on a joint act and te best way to orient a piece while the glue is setting, This is a unique subject and hasn't been discussed in this Forum or even woodworking books and should be important to anyone building furniture..... yet , by contrast, there are almost 100 responses and 5000 views on a Jet Sharpener vs a Tormek. Which both essentially do the same thing. Whether you sharpen a chisel with one or the other is there no interest in the structural aspects of the moertise the chisel might make or the tenon it might trim?.....Sorry I don't have pictures....a pretty girl might be just the thing:rolleyes:

http://www.cnn.com/SHOWBIZ/9803/23/oscars.advancer/jack.nicholson.jpg

[Scott Thornton]

I know I appreciate the topic. The irony is, I just came in to "gloat" about a proud moment for me. I just finished my first mortise and tenon joint, two legs with an apron between for a coffee table.

I came in, got online, went to smc, read Mark's post and said..."damn", my legs are laying down in clamps right now...

At least the next set of legs will be right! After that, a kitchen table (not quite as impressive as Mark's Wenge table) and an end table. All cherry, carefully selected boards, the tops look great. I will post pictures in the next week or so when complete.

I will be looking for some advice on finishing.

[Mark Singer]

:confused: Sorry I didn't get Selma's legs in the picture, but the structure is solid as you can imagine:rolleyes:


It is amazing though a pretty girl and a response within 5 minutes....all day no girl...no response

[Scott Thornton]

I need to see the joints where they meet to really tell! :)

If you have a table with 4 legs...aprons and mortise and tenon joints...

Where are the weak points? What prevents movement of the "base"?

Also, Mark, do you have any completed pics of your new Wenge table?

[Mark Singer]

I need to see the joints where they meet to really tell! :)

If you have a table with 4 legs...aprons and mortise and tenon joints...

Where are the weak points? What prevents movement of the "base"?

Also, Mark, do you have any completed pics of your new Wenge table?

Scott still working on it! If you assemble your table without glue and push down as if it had weight on it, you will see the joints begin to open. They will open at the bottom from tension....that is where you want to make sure there is enough glue....which suggests verticle dying

[Jim Becker]

Excellent topic, Mark...and honestly, I missed the first iteration of it earlier today for some reason. Otherwise I might have overcome my shyness and commented then... :D :D :D

[Mark Singer]

Excellent topic, Mark...and honestly, I missed the first iteration of it earlier today for some reason. Otherwise I might have overcome my shyness and commented then... :D :D :D


Selma Hayek isn't bad either....and structurally a good model for the discussion before us:rolleyes:

[Jay Knoll]

Do we have a John Lucas clone in the making? :D

[Mike Cutler]

It is an excellent thread topic Mark.

Quote Mark Singer: "The lower part of the joint is the weakest and is the tension side for most stress. The upper portion is often in tension unless it is moved laterally. So the upper part of a joint is uually pushing against itself....stretcher against leg..."

Is this portion of the paragraph technically correct? I agree that the lower portion of the joint sees the most load, or stress, if you will. How is the upper portion in tension in a vertical plane? Is the word "not" missing from in between "often" and "in" in the second sentence? The sentence seems to imply that unequal lateral force is preloaded into the joint at glueup. I'm either not seeing it, or not reading correctly.

I didn't miss the first posting this afternoon. I was doing a large M&T glueup with multiple, divided through tenons,and I kept thinking about that sentence.
The particular glueup I was doing has split,through tenons that are shouldered into the mortises. Each panel has two sets of these on the top rail and lower stretcher. I was looking at the joints, and trying to place your model into them.
I must not be seeing something though.

In the vertical plane all of the load would be on the tenon shoulders, and the lower edge of each tenon. The stress should be equal. Lateral stresses exerted along the centerline axis of the joint should be distributed along the faces. The uneven amount of mechanical movement should provide for some form of shear load relief.
Forces applied across the joint, or perpindicular to the tenons would have to work against the width of the tenon faces. Once again providing built in shear relief.
When I plan a joint. I always try to plan for the failure of the joint, and look for a way to remove the failure, or mitigate it.

What am I not understanding?

PS. Selma Hayek is obviously a very beautiful woman, but people were looking at your first post. I promise.;)

[Mark Singer]

Mike,
If you assemble a small table and without glue only fitting dry mortice into dry tenon and apply pressure at the top ....move it slightly, the joint will start to open at the bottom. The top is in compression as with any beam or resisting couple. So the shoulders will resist the compression as long as the tension is secure by tension stress resolved at the botton of the joint...try it

[John Stevens]

It seems very telling to me that there is no response on a thread that discusses how the stresses on a joint act and te best way to orient a piece while the glue is setting...

It might not be as bad as it seems, Mark--total views might be a better way to judge whether your message is getting through. Those who value the info you posted may feel no need to post a response, and those who differ probably don't want to appear to be disagreeable, especially with a moderator who is very popular.

[Mark Singer]

John,
Activity has really picked up since I added my Latina assistant:rolleyes:

[Kirk Poore]

As I am working I often think of things that are not that obvious that may help others.... As you know regardless of the glue ....epoxy, pva, or plastic resin...there is a settling of the glue in the joint once it is engaged and clamped. Because each joint has a bit of slop the best position for drying is in the position the piece will be used.
...
The lateral free play should not be excessive ...the joint should fit snuggly...I use a small mallet ant tap things together:rolleyes:

Mark:
I think you will find that in a snug joint, there will be no "settling" of the glue. This implies that the glue is being pulled down by gravity, and in snug joint the glue layer will be too thin for gravity to overcome the surface adhesion of even wet glue. Thus you should not look for settling, but (as you pointed out) rely on coating the tenon with a proper layer of glue over it's entire face. If the glue is flowing, I would suggest your tenon is too thin for your mortise and you need to shim it to the proper thickness.
Your analysis of the tension & compression on different parts of a horizontal M-T join is right. But I don't care to trust just glue in these circumstances. When ever practical, I will pin my M-T joints.

Kirk

[tod evans]

mark, givin the design of a m/t joint i believe it would depend on which direction pressure was applied to the leg/apron assembly whether the top or bottom opened.....if a person where to slide a table on carpet then one side would be under compression at the top while the leg at the other side would be placed under extention..so i really am not getting the "jest" of the thread( although i did get the "chest" of the thread;) ).....02 tod

[Mark Singer]

Kirk,
The glue often runs down the leg ....gravity is doing its thing!

[Kirk Poore]

Kirk,
The glue often runs down the leg ....gravity is doing its thing!

Ah, of course--squeeze out. I was meaning in the areas which should be in contact.

Kirk

[Mark Singer]

Ah, of course--squeeze out. I was meaning in the areas which should be in contact.

Kirk

Yes but the same thing happens within the joint because there are always small gaps and voids

[Mark Singer]

Structural design of concrete recognizes that the bottom face is in tension....the upper face is in compression. Concrete is strong in compression and weak in tension so the reinforsing steel , which is good in tension is placed in the bottom section to balance the stress resistance of the slab. If you isolate a joint the same is true....a lot is direct shear forces, but as deflection of the joint begins the bottom of the joint is in tension and that is why allowing it to dry in the same orientation adds strength...by eliminating potential voids and insuring good adhesion.

[Mark Singer]

Just to keep the activity alive here is Cameron Diaz.:rolleyes:


http://disney.go.com/disneyrecords/Song-Albums/mousercise/images/minnie_mouse.gif

[Cecil Arnold]

Okay Mark, you lost me on that one, back to Selma.

[Mike Henderson]

Mark, I don't know a lot about structural stuff, but let me ask a question.

If you have a beam, and put weight on middle of it, it will deflect downwards in the middle. The top of the beam will be in compression and the bottom of the beam will be in tension. However, because of the deflection, it seems that at the beam supports, the top of the beam will move inward, while the bottom of the beam will move outwards. Is that not correct?

Seems that an apron on a table would do the same thing. If you deflected the center of the apron downwards, at the legs the top of the apron would move inwards while the bottom would move outwards, causing the legs to splay outwards.

But you're much more of an expert on structural matters so I must be wrong in this analysis somewhere.

Mike

[Steve Clardy]

Okay Mark, you lost me on that one, back to Selma.


Yea. Forget the mortise/tenon stuff;)

[Mark Singer]

Mark, I don't know a lot about structural stuff, but let me ask a question.

If you have a beam, and put weight on middle of it, it will deflect downwards in the middle. The top of the beam will be in compression and the bottom of the beam will be in tension. However, because of the deflection, it seems that at the beam supports, the top of the beam will move inward, while the bottom of the beam will move outwards. Is that not correct?

Seems that an apron on a table would do the same thing. If you deflected the center of the apron downwards, at the legs the top of the apron would move inwards while the bottom would move outwards, causing the legs to splay outwards.

But you're much more of an expert on structural matters so I must be wrong in this analysis somewhere.

Mike

Mike,
I agree with you . I think that would happen under significant load. It basically supports my idea about the stress as well. What I may do is make a joint and assemble it with glue....then saw it in half and test the realive strength of the top verus te bottom....glue would migrate to the bottom and strenghen that area which is most important...

[Mark Singer]

Okay Mark, you lost me on that one, back to Selma.

Someone sent me a PM suggesting that Selma was not appropiate for stimulating activity in my thread....I think he was correct...I do have some boring subjects though....I would rather watch guys arguing over which sharpener or saw is better than think about how something is constructed....BTW that was the dress she wore on the Academy Awards airing at 5;00 on nartional TV....we must follow a higher standard:rolleyes:

[Mike Henderson]

Mike,
I agree with you . I think that would happen under significant load. It basically supports my idea about the stress as well. What I may do is make a joint and assemble it with glue....then saw it in half and test the realive strength of the top verus te bottom....glue would migrate to the bottom and strenghen that area which is most important...
Oh, I see. You were concerned with the stress when you move the table. In that case, the trailing leg will drag putting a stress on the bottom of the M&T joint.

Sorry, I though you were thinking of weight on the top of the table.

Mike