Mortice and Tenon query

[sCORCH]

Hi All,

I was hoping you could help with what, hopefully, is a fairly simple question.

The "rule of thumb" (I hate rules...) as I understand it is that the tenon/mortice should be 1/3 the thickness of the timber.

So far as I can tell, this is primarily because you don't want to increase the mortice size to the extent that it will weaken the timber that will have the mortice chiselled out of it.

What I haven't been able to find an answer to is whether this rule applies to circumstances where the tenon is coming out of a piece of timber significantly less wide than the timber out of which the mortice is coming.

In this case, I am doing a fairly small dining table (say 1200 x 810). The posts are roughly 65mm x 65mm. The rails are likely to be only 19mm thick, since it is a smallish table and doesn't need a huge amount of support (or weight, for that matter)

Is there any reason that the tenons out of the rails can't be 15mm thick (or more)? I can't see any structural reason why the width of the tenon itself should make any real difference when in this case the width of the mortise will not be greater than the 1/3 anyway (which isn't ideal, but there you have it), but maybe someone can illuminate me...

Thanks,
sCORCH

[Lumber Bunker]

You can have the tenon any size you want, but there are a few things to think about which when considered lead you back to your rules.

The shoulders of the tenon work to help stop racking and keep things square (or out of square) and they don't need to be that big, just big enough to cover your crappy mortice edges :) so it also tidies up the joint.
As your mortice gets smaller and smaller, (therefore the timber on the sides of the mortice get stronger and stronger) the tenon its self gets smaller and smaller (read weaker). With legs 65mm you have no problem with making a large mortice. and with 19mm tenons you have the perfect 1/3 size tenon of 6mm (1/4 inch) which is also the same size as your smallest chisel i bet? funny how it all works out.

saying that you don't need to go that small and I probably wouldn't. What is the next size chisel you have 12mm? that will determine how easy you can clean out the mortice to match your tenon. as long as you have enough shoulder left on the tenon to cover your digging out of the mortice.

As far as weight on the table, no it doesn't really matter how thick the tenons are. I'm yet to see a table fall straight down, they all ways tilt (rack) and then fall. As people get up form the table they lean on it, not straight down but the push themselves away too causing sideways pressure on the joints. which is kept in check by the should of the tenons and the snugness of the mortice too.

Good luck..

oh and look up tool slaving

[echnidna]

The shoulders of the tenon give the joint strength as they resist twisting/bending forces.

But 1/3rd rule is only a guideline.

Personally I'd go with 3mm shoulders giving you 13mm tenon thicknesses, which would be very strong.

[sCORCH]

Thanks Guys,

all good suggestions and exactly the information I was looking for. I think 3mm shoulders sounds like a good plan - fortunately the mortice will be done primarily with a router, as my chisel abilities are truly, truly, ordinary.

Much appreciated.

sCORCH

[silentC]

You should size the mortice to your router bit. I'd be using a 1/2" bit and 1/2" chisel to square it up. That leaves 1/8" shoulders on your tenon.

You also need to think about what happens inside the leg where the tenons from adjacent rails meet.

[sCORCH]

You should size the mortice to your router bit. I'd be using a 1/2" bit and 1/2" chisel to square it up. That leaves 1/8" shoulders on your tenon.

You also need to think about what happens inside the leg where the tenons from adjacent rails meet.

Yeah that's a good point - I was thinking that I'd mitre the edges of the tenons so that they joined up in the middle.

Alternatively I was going to move the tenons towards the outside of the legs on each side, so they simply didn't meet - this leaves a little concern about the possibility of the joint tearing out on the outside, but in a table I'm not sure how much lateral force there would be on the joint - I'm undecided on that as yet, so I was probably going to do a few test runs to see what works.

Your advice is welcome on that point, however - since either way I'm going to practice first...

sCORCH

[Pusser]

SCORCH, I would not appologise for using a router for your mortices. Only the true darksiders would exclusively use a chisel. Most probably use a router and would use a morticer if they had one!

Pusser

[silentC]

If you keep at least 1/4" of meat on the outside wall of the mortice, you should be OK. Mitring the ends of the tenons is a common solution. The other way is to check out opposing corners in the tenon ends so that they interlock/overlap. Even if you move the tenons to outside of the leg, it's a good idea to have them as long as they can be. Most of the force on a table is in racking of the rail to leg joint, so nice long tenons are good.

People usually put a haunched tenon on table rails so that the resistance to twisting extends to the top of the leg.

[sCORCH]

People usually put a haunched tenon on table rails so that the resistance to twisting extends to the top of the leg.

As I understand it, a "haunched tenon" is where the tenon has no top and bottom shoulders in the strict sense, but there is a "step" cut in the tenon so that the tenon in fact runs to the full height of the post for the first Xmm.

Does that sound about right?

If so, any suggestions for the length that should be left before the tenon is stepped in that fashion?

Thanks again,

[silentC]

It can still have a bottom shoulder, but otherwise yes.

According to Gary Rogowski, the haunch should be square when viewed from above and 1/3 the width of the full tenon (including the haunch). So the depth of the mortice for the haunched part of the tenon would be equal to the thickness of the tenon (1/2") and the full-depth part of the mortice would start half the width of the full part of the tenon from the top of the leg.

Hard without pictures.

[silentC]

This is what he is suggesting. I'm not sure about making the haunch so deep, but the idea is to keep plenty of meat in the top of the leg above the tenon without losing the resistance to twisting.

[sCORCH]

Thanks Silent - I appreciate you going to the trouble.

[AlexS]

You should size the mortice to your router bit. I'd be using a 1/2" bit and 1/2" chisel to square it up. That leaves 1/8" shoulders on your tenon.

You also need to think about what happens inside the leg where the tenons from adjacent rails meet.

Nothing wrong with this, but I'd probably round the corners of the tenon with a rasp, rather than square the corners of the mortise. Only because it's a bit easier & quicker. Possibly, in theory, it may also be stronger, but it ain't gunna fail either way.