On Apr 18, 10:15 am, sailor <[email protected]> wrote:
> For mortises, how wide should the mortise be? I have read, 1/3 of
> stock thickness, and 1/2 of stock thickness.
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.
> What size do you make them?
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.
> Does it matter if it is hard or soft wood?
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.
> Also, how about mortise depth? 1/2 of stock width?
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.
> Thanks for any advise.
Remember it's probably worth about as much as you paid for it!
Good luck,
Andy
> 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.
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
> 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?
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.
> Does it matter if it is hard or soft wood?
I don't think so.
> Also, how about mortise depth? 1/2 of stock width?
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
--
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sailor wrote:
>
> 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.
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" <[email protected]> wrote in message
news:[email protected]...
>> 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.
>
> 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'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
--
Posted via a free Usenet account from http://www.teranews.com
Stephen M wrote:
> 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.
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<G>.
Lew