mortise size?

[sailor]

For mortises, how wide should the mortise be? I have read, 1/3 of stock thickness, and 1/2 of stock thickness. What size do you make them? Does it matter if it is hard or soft wood? Also, how about mortise depth? 1/2 of stock width?

Thanks for any advise.

[Stephen M]

All other things being equal 1/3, but they are usually not. Personally, I make them either 1/4 or 3/8 because those are the (hollow mortiser) chisels that I have that mate with the shimmed dual-blade tennon cutting procedure that I have.

For something like a cabinet door, 1/4" is plenty strong and a 1/4" panel groove makes sense. That is having tennon thickness match pannel groove width simplifies design of frame and panel assemblies.

For carcases I tend to go with 3/8" if I can. That is probably overkill in a lot of cases.

I don't think so.

1.25" for all structural joints ... no particular reason beyond it is more than adequate 98% of my applications and I lake having standards... I have to think a little less.

A table/apron joint is about the only application where I would upsize from there.

YMMV.

-Steve

[Andy]

I usually aim for 1/3 of thickness - if the tenon is much thicker than that, the walls of the mortise become the weak point of the joint. Of course if the stock containing the mortise is thicker, you can make the tenon larger without significantly losing mortise wall strength.

Depends on the project. My bed frame has 1" thick tenons, while the printer stand I'm making now (small free-standing cabinet) has 5/16" tenons, because I wanted to try that size bit in my mortising machine. I'm sure 1/4" would have been more than adequate.

Dunno - I'd guess it would be the same, because any increase in tenon thickness would usually equate to a reduction in mortise wall thickness, regardless of what type of wood you use.

I don't think this is very critical - you can have a through tenon if you want for decoration, or a very shallow, long tenon could offer just as much long-grain glue area as a deep, short one.

Remember it's probably worth about as much as you paid for it! Good luck, Andy

[charlieb]

Here's Ian Kirby's view.

You want half the the joint to be on Part A and half the joint to be on Part B to get the most strength.

So for 3/4" thick stock for the thinnest part you want a 3/8" tenon, leaving 3/16th inches for each side shoulder.

For 1/2" thick stock for the thinnest part you want a 1/4" tenon, leaving 1/8th inches for each side shoulder.

Note that the Halves thing is easy to figure out - mortise width and side shoulders - multiply the denominator of the stock thickness by 2 to get the mortise and tenon size, multiply the denominator of the tenon thickness by 2 to get the shoulder widths

Thickness of thinnest part 3/4 1/2 Mortise width 3/8 1/4 Shoulder width 3/16 1/8

Pretty intuitive, if you start with 3/4 and 1/2 inch stock, give or take 1/16th - round up to the closest 1/4" for the stock thickness for convenience.

charlie b

[Andy]

Interesting - and I guess it makes sense in theory. But I find it hard to believe, if we're talking about pullout strength, or any force applied along the long axis of the tenon. (If we're talking about trying to snap the skinny axis of the tenon, I really don't know, but it seems like force is rarely applied in this direction). I admittedly can't speak from experience as I've never had a M&T joint fail, but in both magazine "joint torture tests" I've seen measuring pullout force, the walls of the mortise have failed before the tenon. I *think* these were set up with the tenon = to 1/3 of stock thickness, but either way, the tenon doesn't seem to be the weak point. Now I want to try my own test, but my current "real" project has a time deadline, so I should finish that first... Andy

[Stephen M]

I'm with Andy.

But first, let me preface by saying that a well executed M&T joint of even poor proportion is going to hold up and the surounding wood will fail before the joint. Just watch those recently posted videos of joint testing. The wood always blows apart under obscene pressure. So the discussion is an academic rather than practical one.

The reason the equal parts appraoch does not optimise joint strength is because the bending strength of wood is not linear with thickness. I beleive that is a function of the square of thickness. If

Hopefully someone with a mechanical engineering background can back me up, or shoot me down :-) on this one.

-Steve

[Lew Hodgett]

Close, but no cigar, the equal parts approach does produce the best results.

Think you may be thinking of the moment of inertia calculation used when computing the stress of a beam as an example.

Picture a rectangle with a base dimension of "B" and a height of "H".

The moment of inertia (A mathematical statement about a shape) for the rectangular section is as follows:

"I", The Moment Of Inertia, is defined as I = (B*H^3)/12

Section Modulus, "Z", is defined as I/(H/2)

Stress, "S", is defined as M/Z.

Based on the above, the strength of of a section varies as the cube of the height of the section.

To make this as simple as possible, tall skinny pieces are stronger than short fat ones when subjected to a bending load.

The fact that the mortise has two (2) walls that are each 3/16, thus a

3/8 total which equals the 3/8 tenon will produce a "balanced" joint.

HTH, didn't want to bore you with the dull stuff, but you asked.

Lew

[sailor]

Thanks to all who gave advise. A mortising I will go.