Mortise & Tenons with >=2" Stock

[MisterDrow]

When joining stock that is 2" or greater in thickness, is a single mortise & tenon sufficient or is there a marked increase in strength in using a double mortise & tenon?

EDIT: To offer more information, this will be for some poplar cantilever legs in this particular instance.

[Brendon_t]

A double m&t essentially doubles the glue area of contact of long grain to long grain.  I would call that stronger. 

[wdwerker]

Is it a timber-frame or a piece of furniture ? Double tenons will have a larger glue surface but what sort of stress the joint be exposed to?

[Tpt life]

This was in a study somewhere. It challenged the strength argument. I believe it had to do with the strength of the tight fit and the likelihood that a double tenon will compromise a percentage of the fit. Marc addressed it, but I don't remember when. He ended up using or teaching the double, so he may have dismissed the nay sayers. 

[MisterDrow]

12 minutes ago, wdwerker said:

Is it a timber-frame or a piece of furniture ? Double tenons will have a larger glue surface but what sort of stress the joint be exposed to?

Furniture... a desk, to be exact. The top will be 1" solid walnut and will be holding a couple flat-panel monitors and some high-end digital art equipment (roughly 80lbs on it's own).

22 minutes ago, Brendon_t said:

A double m&t essentially doubles the glue area of contact of long grain to long grain.  I would call that stronger. 

That was my thinking but I wanted to ask the experts

[BonPacific]

6 minutes ago, MisterDrow said:

Furniture... a desk, to be exact. The top will be 1" solid walnut and will be holding a couple flat-panel monitors and some high-end digital art equipment (roughly 80lbs on it's own).

That was my thinking but I wanted to ask the experts

I'm certainly not an expert, but it doesn't sound like you'll be stressing your joints that much. Your table will be holding what, 150lbs at the most? Think of a step stool that needs to hold 200+lbs, or a two-person bench. Neither require (though some might feature) a double m&t. What about using a drawbored tenon? That gives you physical strength in addition to the glue.

I don't know what your particular shop setup is. If you're using a Domino, there's almost no additional work involved, so no reason not to add more tenons. If you're doing it by hand, a single tenon is going to be a lot more accurate.

[MisterDrow]

It's just a home garage shop... very beginner-level tools. I'm making a tenoning jig for my table saw and a mortising jig for my plunge router this weekend. I had planned on a single tenon but I saw some things online about double tenons and wondered if it would really make that much difference.

[RichardA]

A single tenon  will do the job, a double is twice the work and not necessary for that amount of weight!

[MisterDrow]

I like the notion of keeping it simple. I am intrigued by @BonPacific's mention of a drawbore M&T, though... might be fun to try with a single tenon.

[gee-dub]

Drawbore and pegged tenons are weaker than standard M&T for a given size.  The difference in strength probably doesn't come into play for most use and has to do with the interruption of the tenon's material (the hole).  Larger stock can either have larger single tenons or double tenons.  Here's Marc's take and here's another.

[Andy Wright]

Single M&T will be more than strong enough, but why not mix it up and do half of the joints single and the other half with double. No one will ever know what the joint is unless you tell them, but it will give you an opportunity to experiment and try something new

[Eric.]

12 hours ago, gee-dub said:

Drawbore and pegged tenons are weaker than standard M&T for a given size.

Huh?

[MisterDrow]

47 minutes ago, Eric. said:

Huh?

Yeah, that threw me, too... from what I have read (and simply by understanding the mechanics of them) drawbore tenons are incredibly strong.

[Chestnut]

A failure plant is created in the tenon that weakens the joint over all. Draw bore joints prestress the joint basically causing a continual load, reducing the amount of external load that can be applied. In pinned and draw bore M&T moving the pin further back would increase the strength compared to closer to the edge but the result is always the same.

Think of it as being similar to scoring and snapping shims. The score isn't very deep but has a large impact.

[Eric.]

6 minutes ago, Chestnut said:

A failure plant is created in the tenon that weakens the joint over all. Draw bore joints prestress the joint basically causing a continual load, reducing the amount of external load that can be applied. In pinned and draw bore M&T moving the pin further back would increase the strength compared to closer to the edge but the result is always the same.

Think of it as being similar to scoring and snapping shims. The score isn't very deep but has a large impact.

[wdwerker]

I have seen failed draw bored tenons where the hole was too close to the end. As to comparative strength I have no opinion . 

[MisterDrow]

11 minutes ago, Chestnut said:

A failure plant is created in the tenon that weakens the joint over all. Draw bore joints prestress the joint basically causing a continual load, reducing the amount of external load that can be applied. In pinned and draw bore M&T moving the pin further back would increase the strength compared to closer to the edge but the result is always the same.

Think of it as being similar to scoring and snapping shims. The score isn't very deep but has a large impact.

But it's not a score across the whole tenon... it's a hole drilled near the shoulder of the tenon and once the glue dries, that joint is solid... there's no continual load. Marc even did a video on drawbore tenons and talked about how they will last long after the glue fails (if it ever does).

2 minutes ago, wdwerker said:

I have seen failed draw bored tenons where the hole was too close to the end. As to comparative strength I have no opinion . 

Yeah, I would think that you'd want to put it as close to the shoulder as you can without compromising either piece in the joinery.

[Chestnut]

It's an argument of relatives. Is a 5% strength decrease weaker? Technically yes. Does the longevity make it better overall? Yeah probably but the statement was that the ultimate strength of equal size M&T joints being stronger then equal size pinned and draw bore joints. Argue mechanics of materials all you want but when you remove material you make something weaker, that is a strait fact.

1 minute ago, MisterDrow said:

But it's not a score across the whole tenon... it's a hole drilled near the shoulder of the tenon and once the glue dries, that joint is solid... there's no continual load. Marc even did a video on drawbore tenons and talked about how they will last long after the glue fails (if it ever does).

This is assuming the glue fails over time. Is PVA glue flexible enough on a 1" tenon to avoiding seasonal movement failure? 2'? Your right it's not a scribe but it's similar, hence why i said similar.

[drzaius]

Draw bore joints have another advantage in that you don't need clamps for the glue up. Sometimes that is a big deal.

[AceHoleInOne]

Just pin the damn thing and move on. 

 

-Ace-

[BonPacific]

Just now, AceHoleInOne said:

Just pin the damn thing and move on. 

 

-Ace-

But we haven't hit two pages of discussion yet. 

[MisterDrow]

12 minutes ago, AceHoleInOne said:

Just pin the damn thing and move on. 

 

-Ace-

But it's so fun to rehash the same thing over and over and argue with each other! LOL

I think I will do drawbore on these joints for the cantilever legs. The ability to avoid clamps on something with odd angles and the longevity are very appealing. Plus, they just sound damn cool to make and I didn't know anything about them before this thread.