Thread: Coffee table - legs to rail attachment

John. Glad you kept it simple. I didn't really know any of that. I've had a look on the net before but didn't search well.

Thanks for the picture but I didn't really need it after your explanation.

Still simplistically - thoughts on what you said to make sure I have it;

So the pressure on the apron means the joint pulls the legs sideward (and down to an angle presumably) and creates tension because there is also downward force on the legs. It will tend to split at the edge of the tenon because past that it's not being pulled sideways - and perhaps the fibres ending due to the mortise means they pull away more easily than the ones that continue to the top.

O.k I think I see the simplicity you mentioned- the force of the apron on the leg isn't directly sideways to start with. Plus the leg is held by the rail perpendicular to it too. That must add a whole lot of extra forces at weird angles that I won't even guess at right now.

This is fascinating.

I see what Arron means about the shoulders. In your example maybe the shoulder at the bottom pushes the leg outward - actually it would create compression in the area aargh!!

It's important not to overthink it. You don't see a lot of broken coffee tables about (unless it's done deliberately).

O.k I think I see the simplicity you mentioned- the force of the apron on the leg isn't directly sideways to start with. Plus the leg is held by the rail perpendicular to it too. That must add a whole lot of extra forces at weird angles that I won't even guess at right now.

This is fascinating.

I see what Arron means about the shoulders. In your example maybe the shoulder at the bottom pushes the leg outward - actually it would create compression in the area aargh!![/QUOTE]
It is fascinating, and we haven't even begun to discuss torsional forces. However, as Arron said, we needn't overthink it. If you bear in mind that a well fitting M&T joint is stronger than the timber, that's all you really need. Arron is also right about leaving the top of the mortice open in the legs. That way you get a wider and stronger joint ... not that you really need it in most cases.

Have fun!

I did actually put the mortises right to the top of the legs based on what the two of you had posted. So thanks. (It may be a reason other people do it.)

You are right that I tend to over think things Arron. I really enjoy the theory. I somehow seem to simplify things when put in practise though.

Take care, Mary

Originally Posted by Mnb

I did actually put the mortises right to the top of the legs based on what the two of you had posted. So thanks. (It may be a reason other people do it.)

You are right that I tend to over think things Arron. I really enjoy the theory. I somehow seem to simplify things when put in practise though.

Take care, Mary

Know exactly how you feel. I tend to be kinky for the theory ... seem to need to understand it. I am the type of bloke who takes pictures of compression arches when in Europe ... I can almost see the forces being transmitted through the stone. However, by the time I have walked into my shop I am thinking KISS ... and nearly always do.

Have a look at this stone bridge. Masonry (stone and concrete) is strong in compression but weak in tension. That's why we put reinforcing steel in concrete ... because steel is strong in tension and prevents the concrete from failing in tension. But when building in stone there is no steel, so we build compression arches. If you can look at the pic and see how the compressive forces are transmitted to the foundations; that the arch ensures that there are almost no tensile forces at play, you understand.
DSC00599.jpg

The Romans were building compression arches over two thousand years ago. It is a very old theory.

I remember learning, at high school probably, about the Romans building arches for strength but I didn't really get the concept. It will sink in more I suspect but yes I see. The force has no where to go except out and down to push against the foundations. (The more you push down from the top of the bridge the more compacted the Center of the arch gets too I think) That is brilliant. I love ideas that seem simple but are brilliant.

Yes - kinky for the theory is a good way to put it. I enjoy it for itself but I think understanding also translates to the practical. Like the mortises yesterday - once I knew the theory it let me keep things simple.

But now I must return to start the tenons

Since I was a kid woodworking has been something that "I'd like to try someday". I'm 43 now but now that I'm actually doing it I feel like a kid again - getting so excited and enthusiastic. It's great fun.

Mary

A couple of years ago I stood under the dome of the Pantheon. It took my breath away. It is the biggest un-reinforced masonry dome in the world, which is about two thousand years old. The Romans sure had the compression arch/dome figured out.

Have fun with the tenons. How do you intend to cut them? I often cut mine with a dado blade (I do love my dado blade).

I'm jealous. I went backpacking through S.E. Asia when I was young but I haven't been to Europe.

The tenons came out well. A dado blade would have been really good but I don't have one. Time to remedy that I think.

For perhaps a Couple of months now I've been using a radial arm saw. It's a Ryobi from the 80s. My dad bought it new back then but it's been sitting in his shed unused for at least ten years. It was the search for the manual that lead me to this forum actually. Anyway, I cleaned it up and replaced the table and fence and squared the blade and it is great. The other side of the blade has a router attachment which is so useful. I suspect that changing from my Ozito table saw to a more powerful and accurate saw has influenced me, but I love it. The blade being overhead has some real advantages but also some disadvantages too. Ripping on it is horrible because I get pelted with sawdust. Still, going back to the table saw once was enough to convince me it's worth it. I get an accurate and beautiful cut.

I ended up just using the blade for the tenons. After a few trial cuts it was the simplest and most consistent way. I used the router attachment for the mortises. I had to use the area behind the table with a bit of mdf across the frame because with the motor up and down I couldn't get the bit high enough. I put a piece of tape on the column for the depth to go to but it wasn't a satisfactory setup.

Thats enough for now

Mary

It's all going pretty well.

Had to add a little shim to the bottom of one tenon because the mortise was too long (too far down the leg so the rail was a little low). But apart from that it seems to be coming together well. I'm having a lot of fun too.

Originally Posted by Mnb

I'm having a lot of fun too.

Excellent. That's why most of us are woodies. It's also the only good reason I can think of to keep doing it.

Originally Posted by Mnb

It's all going pretty well.

Had to add a little shim to the bottom of one tenon because the mortise was too long (too far down the leg so the rail was a little low).

You might try cutting the mortice a tad short next time. Then you can shave the tenon to fit. If there is a shoulder, there is likely no need to shim it, so long as the gap is not visible.

If you get a loose joint, epoxy resin will fill the gaps and give you a strong joint (DAMHIKT).

Definitely not used to the online thing - I had to think a bit before I got 'don't ask me how I know that'.

I figured I didn't really need the shim but it will make the glue up easier which is something I don't have much confidence with. The mortises were the same length as the rails width if that makes sense. Since I wasn't cutting the tenons to fit vertically it didn't occur to me to cut them a little short. Good idea though.

Before next time I need to get my router table back into action. I somehow managed to get a screw stuck in one of the holes of the router when I first attached it to a piece of ply. I made a better table and fence but I can't get the screw out. Any ideas - if anyone else is still reading too?

The epoxy. Quite annoyed at myself for not thinking. I started sanding, briefly. The long rails no longer fit as they did. I could lift them in the air and the legs were held on. There aren't visible gaps but now they slide out instead. I bought some epoxy today but the guy at mitre ten said it won't allow for wood movement. He also suggested nailing though which I am no way going to consider. I think it's fine. Would appreciate reassurance though as I haven't used epoxy. Just use instead of glue yeah?

BTW; thanks for taking the time. I would manage but it helps to have the advice and for some reason being able to communicate about my progress is good too.

Originally Posted by Mnb

Definitely not used to the online thing - I had to think a bit before I got 'don't ask me how I know that'.

I figured I didn't really need the shim but it will make the glue up easier which is something I don't have much confidence with. The mortises were the same length as the rails width if that makes sense. Since I wasn't cutting the tenons to fit vertically it didn't occur to me to cut them a little short. Good idea

The epoxy. Quite annoyed at myself for not thinking. I started sanding, briefly. The long rails no longer fit as they did. I could lift them in the air and the legs were held on. There aren't visible gaps but now they slide out instead. I bought some epoxy today but the guy at mitre ten said it won't allow for wood movement. He also suggested nailing though which I am no way going to consider. I think it's fine. Would appreciate reassurance though as I haven't used epoxy. Just use instead of glue yeah?

There is an old saying that dovetail and m&t joints only go together properly once. Therefore, resist the temptation to trial-fit unless you really need to.

Ordinary pva woodglue should be fine for your glueup. Epoxy is a great glue when it’s needed (I know that because I’ve made three wooden boats myself with no fixings other then epoxy). As you are only trying to get a bond across the wide faces of the tenon then I suggest you save yourself the cost, mess and possible chemical sensitivity and use Pva.

Big glueups are a hassle - I still find myself reduced to a frustrated mess too often. The only thing I can recommend is doing as much preparation as possible - eg, if you are going to use wooden blocks on the faces of your clamps (for protection) then glue them to the clamp first with hot melt. Stick masking tape wherever glue might drop or run. Mark a perfect square out on the bench top and glue up on that so you know when the object is square without having to stop and measure. Consider using a band clamp to bring everything together, then fit your regular clamps. Finally, stand back and check it from all angles when done - very thoroughly.

Cheers
Arron