mortices again

[engineerone]

having read joyce and so on it is recommended that when you are cutting tenons in a piece of wood about 4 inches deep or so, you actually have a gap in the middle to help stop twisting.

if in an apron therefore, you have two m/t joints on each corner,
is it worth offsetting the tenons on either piece to make them fit more easily???

also on this size of wood, i am wondering (in this lightweight case) how tall to make each tenon. i was thinking about a spacing of 3/4,3/4,3/4,3/4, and 1 inch, but any better or more experienced ideas please??

paul

[sainty]

Paul

I always thought that the purpose of the gap in a large tenon was to prevent any movement issues as a result of a large cross grain situation. I dont see how having a gap on the tenon would prevent twisting more than using one tenon.

Unless of course you are talking about the depth being thickness (in my mind) rather than the height? In that case I could be completely confused. Very possible on a Friday evening!


Rgds

Sainty

[engineerone]

joyce calls them double tenons and says they are to protect against the inevitable shrinkage.

he also says they are rarely used in modern furniture

i do understand a haunch is really the preventor of the turning moment, but i am just interested in any better ways of doing it.

and of course it is the friday night before christmas

paul

[mrgrimsdale]

engineerone said:

joyce calls them double tenons and says they are to protect against the inevitable shrinkage.

A wide tenon, say the whole 4" of a 4" rail, can loosen due to cross grain shrinkage. Making it into 2 means less shrinkage each, and the inner edges will actually pinch tighter if there is any shrinkage. The middle gap has a haunch to close the gap and to add more glued area to the joint. So that'd make 2 tenons and 1 gap - 3 roughly equal spaces.

he also says they are rarely used in modern furniture
i do understand a haunch is really the preventor of the turning moment, but i am just interested in any better ways of doing it.

There aren't better ways to do these things other than traditional with variations as descibed by Joyce - unless you count pseudo tools and devices such as Steve Maskery's thingy wossitcalled ;D
A rail at the top of a stile, or apron at top of a table leg, has top of tenon reduced and haunched. This is to reduce the extent of the mortice at the end where it could ease open. So that'd make 2 (smaller) tenons and 2 gaps 4 roughly equal spaces.

[engineerone]

thanks jacob i was waiting for you to come along and offer the right advice

paul

[jfc]

Your Auntie Joyce seems to know her stuff ;D