Thread: A mortice and tenon question

As I read it, you have 26 M&Ts to fix up. That's a lot of faffing around with paddle pop sticks, and it will still look like a patch-up job.
I'd look at cutting off the existing tenons, and make up router guides so you can a) route round ended mortices in the ends of the rungs, and b) round the ends of the mortices in the rails. These mortices should be the same size. You can then make a single length of wood to fit the mortices and cut it into short lengths to make floating tenons.

I thought end grain glueups are weak. All the strength comes from glue on the long grain.

longer tenons would benefit from providing more long grain surface area and reduce any racking that could be applied on the joint.

Adding the paddle pop sticks would have minimal benefit.

Please correct me if I'm wrong on any of these points.

Regards

Adam.

Sent from my SM-G950F using Tapatalk

My understanding of the question is a bit different: the tenons are 7mm long and the mortices 12mm deep, leaving a gap between the end of the tenon and the back of the mortice. This is not seen and filling it will not give any strength improvement.

If the tenons are still a sliding fit into the mortices then I would just reassemble them with PVA (or glue of choice). If they are a bit loose then you can thicken the tenons by glueing a shaving or 2 to 1 side. Given that the table has lasted this long, including a certain amount of punishment from children, there should be no need to redesign the joints.

By the way, the photo in the other thread only shows 9 rungs - is it a pic of a similar table for reference only, or do I need to go back and learn how to count?

Endgrain doesn't glue well, so filling the mortice to touch the end of the tenon will give minimal, if any, added strength. Waste of time IMO.

Why not squeeze a little polyurethane glue in the bottom of the mortices ?
When it expands it will fill the gap amd glue the end grain on the tenon to the bottom of the mortise.
Dont use to much or it will come out everywhere and make a mess and probably even push the joints apart if not clamped effectively.
Ive just been having so much fun with this glue and am keen for others to enjoy having it cut off their arm hairs before being allowed to sit down at dinner.
H

Originally Posted by clear out

Why not squeeze a little polyurethane glue in the bottom of the mortices ?

Because, even though PU expands as it sets, the foam is NOT gap filling like epoxy. The maximum gap PU will generally bridge while retaining its strength is about 0.5 mm, a 5mm gap may as well be filled with styrofoam for all the good it will do.

When you factor in the mess and potential to push the joins apart, PU will likely do more harm than good.

Correct me if I'm wrong, but as I read it, the rungs themselves are the correct length, and their ends fit against the rails. It is the tenons that are shorter than the depths of the through mortices. As they stand, when the table is assembled, there will be a 5mm deep hole in each mortice, and these will be visible from the outside of the table. I don't see where there will be any end-grain joints.
If this is correct, each rung will have 7mm of tenon supporting it at each end. This will provide adequate support for most expected vertical loads, but probably not much against any racking loads that may occur. If there is some other means of providing support for lateral loads, eg skirts or end rails, you could simply assemble the table as is, and glue strips of matching veneer along the side rails to cover up the holes.

No, there's no visible hole, the mortises are just deeper than the tenons.

The question is does it matter if the ends of the tenons don't touch the bottom of the mortises. The end grain joins come in if the OP is to fill the mortises so that the tenons touch the bottom.

Originally Posted by elanjacobs

No, there's no visible hole, the mortises are just deeper than the tenons.

The question is does it matter if the ends of the tenons don't touch the bottom of the mortises. The end grain joins come in if the OP is to fill the mortises so that the tenons touch the bottom.

OK, in that case, as you say, no point in filling the mortises.

Are the mortice-and-tenon joints structural, or part of the aesthetic design? I suspect that they form part of the latter. In which case, the joints are created for assembly and not strength. Someone just made the mortices deeper without much other in mind than making a hole.

Regards from Perth

Derek

Thanks for all the advice gentlemen.
The mortices don’t go through. Just 5mm deeper than the tenons.
Looks like I can glue back up without filling the gap.
And yes labr there are only 9 rungs. 😀😀
I made 6 ladder back chairs in the style of Charles Rennie Macintosh that had 13 rungs. That was 26 M & T per chair times 6 equals 156 M & T’s. So I think the number 13 may be burned into my subconscious.