Another Mortise and Tenon question...

[koslonc]

I'm planning a coffee table project which will use 2" square legs and 3/4" thick aprons with 3/8" thick tenons. My question is about the mortise depth. Is is better/stronger to stop each mortise short of intersecting each other in the corners or should the mortise meet and use 45 degree miters on the ends of the tenons of the aprons?

Thanks in advance for the collective wisdom and experience on this site!

 

[Gofor]

Either way will work fine as long as the tenons are a good fit on the wider side. The strength in a glued tenon joint is in the wide flat surface area: the more that fits well, the stronger the joining. Theoretically, that means running them together gives a stronger joint. However, each tenon must be trimmed shorter than the depth of the intersection to prevent interfering with each other, pretty much negating any extra surface area.

Running the mortises together can cause a couple problems that can easily be avoided, but you should be aware of:

1. Tenons too long causing interference with each other. (When doing this, I miter mine so that there is at least a good 1/8 between the ends and double check the shoulder fit with both pieces clamped into place)

2. Glue/chips pushed into the first joint can run into the intersecting mortise, causing problems when gluing the second one. Not a big problem if the mortise is well cleaned and both sides are glued at the same time. If not gluing both sides at the same time, the dried glue can really mess up your day trying to clean it out to fit the second one. This can be a major headache if using expanding glues like the urethane Gorilla Glue, etc.

If you are going to glue up the sides first, and then put in the end aprons (or vice versa), you may have less problems going with a mortise depth just shy of intersecting. If hand chiseling the mortises like I do, that can often be close to impossible, so I make sure to thoroughly clean out any chips/splinters from the bottom of the mortises and I have enough clamps and a large enough work area to glue all at once.

Hope this helps.

Go

 

[Bas]

What Gofor said. Another consideration is how much strength you need, i.e. is it necessary to get the maximum length tenon or can you go a little shorter and not have to worry about mitering/ interference. For a coffee table, a tenon of 1" would be plenty and fit without issues. For a heavy dining table, you'd want to squeeze out the extra 1/4"

 

[Bill Clemmons]

Good advice from both Mark and Bas. Generally speaking, the more "cheek" area you have, the stronger the glue joint is going to be. But you probably don't need maximum strength for a coffee table, since they typically don't receive a lot of abuse (unless you have my grandkids over :gar-Bi). If you were building something like a harvest table, where maximum strength would be more important, I would opt to reinforce the M&T joint by pegging it.

Bill

 

[koslonc]

Thanks for the advise guys.

FYI, here is what my design looks like. I actually was planning to pin the tenons with a contrasting wood - but mostly for aesthetic reasons. Because I wanted the pins centered on the leg width, it forced me to extend the tenons to the point where they intersected each other.



And here is a x-ray close-up of the leg joinery. Hopefully you can make out the details here.



So I think I'm leaning toward using intersected mitered tenons, but I will leave each tenon short by about 1/16" to allow for the potential problems mentioned by Mark.

It's my first real mortise and tenon project so if I'm heading down the wrong path, feel free to straighten my out!

Thanks.

 

[Gofor]

What you propose will work to a point, but there are a couple of caveats. I suggest you draw it a top view to scale to see exactly how your joint will look inside.

If the aprons are inset 1/4" from the show face of the leg, and you use a 3/8" dowel on the leg centerline, the edge of the dowel hole will line up exactly with the inside edge of the existing mortise (if your machining is exact).

That means you will get little extra support by doweling past the tenon thickness (which you can't do anyway as you will hit the dowel from the other face) and the dowel will also be right at the edge of the miter on the tenon end.

You can gain extra strength several ways:

. Moving the apron to 1/8" from the leg face will gain an extra 1/8".
. Using a 1/4" dowel will gain another 1/16".
. moving the dowel holes closer to the apron shoulder (common in a lot of doweled furniture designs)

You can add 1/16" clearance by reducing the tenon thickness to 1/4". This will result in some loss of strength against diagonal forces on the leg/apron frame, so would depend on how much support the top attachment method would give against a corner blow to one of the legs.

You can also gain dowel hole depth by offsetting the location of the dowels on each face (so the holes do not intersect, also common).

For a balanced look, you may want to consider using 2 dowels on the front face, and only one (centered height-wise) on the side face. The side will be subject to less torque from the shorter length of its apron, so does not need as much reinforcement.



You didn't mention if you are going to attempt to draw-bore the dowels/tenons. Do not do this if you are keeping the dowels on the centerline of the leg, as you will blow-out the meager amount of the end of the tenon. This being your first tenon project, I would not advise attempting draw-boring it anyway.

I am not trying to make this complicated, but doing a scaled drawing of the joint can greatly aid visualizing how close the pieces are and help see alternatives.

Good to see you expanding your talents and expertise.

Go

I apologize for not including an illustration, but am pressed for time right now. Will do so later if you need me too.

 

[gfernandez]

I actually was planning to pin the tenons with a contrasting wood - but mostly for aesthetic reasons.

This may be a silly comment, but if the pins are for aesthetic reasons only, does it really matter if you go through the tenon or not? Why not just use a 1/2" dowel so it looks like the tenon is pinned?

 

[koslonc]

This may be a silly comment, but if the pins are for aesthetic reasons only, does it really matter if you go through the tenon or not? Why not just use a 1/2" dowel so it looks like the tenon is pinned?

I like the look first and foremost, but if I'm going to go through the effort of making the pins and drilling holes for them, I figured I may as well try to do it 'for real' and learn something in the process.

 

[Bill Clemmons]

I like the look first and foremost, but if I'm going to go through the effort of making the pins and drilling holes for them, I figured I may as well try to do it 'for real' and learn something in the process.

If you're going to do it "for real" and expect to get any real strength from the pegs, then take Mark's suggestion and move the dowels closer to the apron. Lining the dowels up in the center of the leg may seem like the right place, but it is not in keeping w/ traditional pegged M&T joints. The peg is typically placed in the center of the tenon. (i.e. if your tenon is 1 1/2" long, the peg will be 3/4" from the edge of the leg where the apron attaches.)

Bill

 

[Jeff]

How about offsetting the mortises and tenons on the adjacent faces of the legs which will eliminate the intersection/interference issues? They could be haunched to add a little extra strength if desired. Single pins could be inserted as well if you like the look of that corner to corner exterior offset. A second pin on each face is simply a "dummy" for looks if you like that too. :dontknow:

You get maximum tenon length, but laying out and keeping track of the parts is a bit more complicated and time consuming. The suggestion depends on the width of your aprons and how much wiggle room you have across that width for the layout.

IMHO a coffee table is not usually subjected to a lot of stress and racking forces in common use unless Bill's grandkids come to visit.

+/- $0.02, but we're all continuing to learn and trying new ideas.