I built an adjustable fixture that allows me to make very accurate loose
mortise and tenon joints. I have been using tenons fabricated from hardwood,
either Oak or Poplar. Needless to say, it requires some work to achieve the
1/4 inch thickness required for a tight joint.
I am now considering using 1/4 inch tempered hardboard. It is stable, strong
and has a uniform thickness. Any have experience with this technique??
Hardboard has no strength. It will help with alignment but can easily
snap under stress.
If this is something you do a lot you should plane or wide belt or
drum sand a bunch of real stock to the proper thickness. The other
much more common option is to use plywood. Maybe baltic birch will
have consistent enough size or you can adjust the mortise to fit the
Very different physical characteristics there. Much better IMHO to make the
tenons from the same wood they will be used to join, so that the tenon expands
and contracts at the same rate as the mortises.
"Needless to say"?? Apparently you don't have a thickness planer available to
you; with a good thickness planer and a modest amount of practice, achieving
any particular thickness is quite easy. You don't even need precision tools to
measure very small variations from the desired thickness. If the desired
thickness is 1/4", plane four boards to the same thickness, then stack them
and measure the combined thickness. If the stack measures 1-1/32", you know
each board is 1/128" oversize. If a stack of eight is anywhere between
1-31/32" and 2-1/32" then you know each board is 1/4" +/- about 0.004".
I wouldn't even dream of using hardboard, tempered or otherwise, for floating
tenons. The whole point of a mortise-and-tenon joint is its strength, which
would be significantly compromised by the substitution of hardboard for solid
wood. Additionally, the very fact that it *is* stable is another reason it's
unsuitable for this purpose. The wood that you're joining will expand and
contract with changing humidity, while the hardboard will not, thus
compromising the glue joint. Best practice is to make the loose tenons from
the same wood that they are used to join.
I think it would depend on what you are building. If you are building
heavy furniture or something similar, listen to all of the other
posters. If you are making something light like picture frames, then
try the hardboard. I have been using the Lee Valley hardboard biscuits
for years to make picture frames and have never had a failure.
I also have a very quick system for cutting tenon slots. I use a router
with a slot cutting blade, a router table and two fences. The main
fence is positioned so the blade is correctly positioned for the joint.
The second fence is positioned loosely on the other side of the piece
to be cut (this prevents kickback). ie so the piece being cut is
sandwiched between the two fence. The left end slot goes into the blade
from the right, the right end goes into the blade from the left. Using
this system you can accurately cut the eight slots for a picture frame
in about a minute.
Buy a 6mm bit, then buy these,
Then buy this,
Biesemeyer fence, featherboards to run the stock straight.
Vernier calipers to mike the test thickness, dial indicator
with magnetic base to help tap the fence over as needed.
I shoot for an easy 0.005" max slack. Most glues have
ample shear strength to make the joint indestructible.
Birch veneer ply will be uniform and a heckuvalot stronger.
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