For a perfectly cut tenon and mortise, how much space should there be between the sides of the mortise/tenon?
When chopping a mortise, it seem the sides are not smooth at all and there is less glue surface area than if cut by a mortiser. Does this effect strenght?
Replies
Hmmm... the first part of your question is fairly subjective, in a sense. I don't measure the actual gap, but I know it when I "feel it"... for me, that means that the joint requires some pressure to assemble and that the tenon doesn't wobble or move in the mortise at all. If the tenon does move in the mortise, I'll either slip a planer shaving or two, flat-wise, alongside the side of the tenon, or -- if using floating tenons -- remake some tenons that are slightly thicker.
If you need to use a mallet and a tap or two to get the joint together, that's OK, too... you just shouldn't have to pound it in place.
As to the second question, yes, you DO need smooth walls in order to get the most long-grain to long-grain contact in the joint, the source of much of the joint's strength (outside of the mechanical strength). Some people can (or will take the time) to chop mortises that have such smooth walls; it's in the realm of possibility. Mortisers and routers can also do the job, but -- as always -- it's really the skill of the operator that matters, no matter what the tool.
David
Look, I made a hat -- Where there never was a hat!
Edited 12/9/2002 3:54:10 PM ET by davamoore
I HAVE A QUESTION ABOUT THE SMOOTHNESS OF THE MORTISE. I USED TO USE A ROUTER AND MY MORTISES WERE ALWAYS VERY SMOOTH, YOU CAN IMAGINE, AT 25,000 RPM. THEN I PURCHASED A MORTISER AND NOW I NOTICED THE INSIDES OF MY MORTISES ARE NOT AS SMOOTH, AS A MATTER OF FACT THEY ARE ROUGH, CAUSED BY THE CUTTING OF THE CHISEL TEARING UP THE WOOD. I HAVE THOUGHT OF GOING BACK TO THE ROUTER AND SELLING THE MORTISER. AM I JUST NIT PICKING OR IS THERE TRUTH TO WHAT I AM SAYING?????
RAY
I HAVE A QUESTION ABOUT THE SMOOTHNESS OF THE MORTISE. I USED TO USE A ROUTER AND MY MORTISES WERE ALWAYS VERY SMOOTH, YOU CAN IMAGINE, AT 25,000 RPM. THEN I PURCHASED A MORTISER AND NOW I NOTICED THE INSIDES OF MY MORTISES ARE NOT AS SMOOTH, AS A MATTER OF FACT THEY ARE ROUGH, CAUSED BY THE CUTTING OF THE CHISEL TEARING UP THE WOOD. I HAVE THOUGHT OF GOING BACK TO THE ROUTER AND SELLING THE MORTISER. AM I JUST NIT PICKING OR IS THERE TRUTH TO WHAT I AM SAYING?????
RAY
You're right, the sides of the mortise should be as smooth as possible. While there is some mechanical strength in the joint itself, much of the joint's strength comes from the glue bond between long-grain woodfibers. A smooth-cheeked tenon sliding up snugly against, and being glued to, a smooth-walled (cheeked) mortise provides the maximum amount of contact between these fibers.
When the cheeks of the mortise or tenon are rough, you've effectively lost that key contact between the two halves of the joint, wherever there's a "valley," or low spot. Multiply that throught the joint and you've potentially lost a great deal of strength.
Now, I wouldn't say that you must have absolute, 100% smoothness... but whatever you can do to approach an unbroken surface, the better. I don't use a mortising machine, so I can't comment on what you might do to improve its performance... but if you want to continue using the mortiser (maybe set up is much faster than when you use the router, for example), you might consider cleaning up the mortises with a hand-chisel after they've been cut by the machine. This would probably work better if you cut the mortises slightly thin up front, so you wouldn't make them too wide for the tenon during clean-up. Or, you could cut your tenons slightly thick and then, after cleaning up the mortises, trim the tenons to size.
DavidLook, I made a hat -- Where there never was a hat!
.003 inches per glue line. I saw it some years ago in an FWW article about gluing.
.003 inch = thickness of ordinary paper.
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