Inre: pinning mortice and stenon joints
I fully understand the concept of pinning M&T joints so that the pins draw the joint tight as they are driven home, however; I have one question on the finer points of execution. Do any of you folks out there have any guidance for how to determine the right amount of offset to put on the pinholes in the morticed timber versus the tenoned timber? What’s not enough? What’s too much? Can one induce joint failure by offsetting too much?
Replies
1/16" to 1/8". You'd want it just enough to drive it home w/o bashing any wood fibers. Generally, in softwoods the larger offset works well and for hardwoods the smaller one works well.
Now I have some information to ponder! Thank you all who responded.
My applications are frame and panel closet doors and, maybe if I dream on it long enough, a nice maple or beech woodworker's bench with purpleheart accents here and there...
I don't believe the pinned m&t joint was ever meant to "strengthen" a mortise and tenon. Rather, it acts solely as a lock to prevent the joint from coming apart.
While this helps, it's the broad glue surface area from a well-fitting joint that will last.
I take your point, but isn't a "lock that keep something from coming apart" by definition strengthening the joint. Strength in this instance = ability to resist stresses (e.g., stresses trying to pull the joint apart).
What is the article that concluded the pinned joint is weakened? I would like to see their methodology. A single article is not a necessary authority on the subject. There could be serious flaws in their method or they may have identified a real issue.
Rich
I don't think you meant to direct this to me, as my response to reports on the article was similar to yours. I mention this just in case you wanted redirect so as the proper person gets a notification of your response/question.
Sorry!
As I understand a pinned M&T, you are right it really isn't to "add strength" as much as it is there to keep the shoulders pre-loaded to avoid racking when the glue fails (if there is even glue in the joint). The strength of a M&T joint comes from the shoulders resisting racking forces. The tenon and pin lock and preload this interface.I'm not sure fine 18th C. furniture was, as a rule, drawbored. I tend to agree with those who feel the joint was clamped, glued, drilled, pinned and clamps released.
-Dean
And keep the location of the pin close to the shoulder of the tenon, but not too close to the edge of the mortise piece. If it's too close to the end of the tenon, the end grain will pop out - nand the pinning will have no effect. If it's too close to the edge of the mortise piece, the side tissue will fracture. Rich
willingt,
Adam will probably jump in with an authoritative answer, but in the meantime...For furniture sized joints, 1/16" draw will be plenty. 1/32" will do the job. Hardwoods will need less draw than softwood, and be less forgiving (more brittle) of excessive offset. No-one has mentioned the species of the pin. It should be something that will readily flex as it wiggles thru the offset. Hickory, ash, or white oak would be good choices.
It isn't necessary for pinned joints to be draw bored. Just boring thru the glued joint after the clamps are drawn up will be very satisfactory, and eliminates the guesswork, and chance of splitting the mortise, or fracturing the tenon. Then, you can make the pins out of the same wood as the rest of the project. The piece can be pulled out of clamps and set aside immediately after pinning, which can be a time saver, if assembling a number of doors or chairs, with a limited number of clamps.
The best argument for drawboring that I can summon up would be where no glue is available, or where no clamp long enough can be had. In looking at old work, I've seen many instances where the joint has failed due to the pin breaking at the tenon/mortise interface, or split out the sides of the mortise, allowing the joint to rack. Of course, one only sees the failures, in repair work.
For drawboring, the pins should be whittled nearly round, and pointed, or nearly so, so as to help it find its way thru the offset holes. If the joint is drilled and pinned after clamping, just knock the corners back octagonal for 1/2" or so, and drive them home. As the square section enters the wood, it will deform the hole to its shape (occasionally vice versa). It is important that the size of the square of the pin be no larger than the dia of the hole, to prevent splitting.
Regards,
Ray Pine
Ray,
I think you are right on two counts:
Most furniture M&T with pins doesn't need an offset hole, as long as the dowel is a good fit all the way through. I like to drill and pin the joint after it is made, glued up and dried. It is just insurance against a joint in which the glue comes undone, for whatever reason.
If there's room, I prefer two dowels, set one above the other and close to the joint line. You can make the dowel out of contrasting wood and it is then a feature; or (if there is sufficient depth through the M&T) cap a dowel with a decorative plug of the same diameter, if the dowel-timber used was selected for strength.
Offset pins are really a green woodworking technique, used particularly in large M&T joints employed in framing, where the whole assembly will shrink from its green condition with the offet pin keeping the shoulder of the tenon tight.
The timber species used for the pin is important: it needs to be strong and stiff for furniture (with no offset) such as the oak et al you mention. But the pin needs to be fibrous and bendy for green woodworking, so it will actually bend (rather than snap)in the pinned joint as it dries out and shrinks. Willow, or sometimes ash, are used by those in the green woodworking circles I am familiar with.
Any bend in the pin and its path through the M&T, so-introduced by the shrinkage of the joint, makes the pin very secure as it cannot then drop out of the hole should it become slighty loose over time.
Perhaps those who make furniture that will spend its days in a climate with large humidity changes would benefit from offset pins through the M&Ts but it seems overkill for most furniture, unless made from green wood.
Lataxe
There was an article in Woodoworking Magazine either the #3 or #4 edition about Drawboring. It was very interesting and gave you instructions to make the drawboring pins, how to use them and how to consturct the joint.
I hate to bring this up, but I think I just saw an article in a competing wood magazine that seemed to indicate that pinning the M&T joints actually weakened them. I just glanced at the article and didn't give it my full attention. Did anyone else happen to see the article?
I saw the article as well. It said that (in spite of intuition to the contrary) pinning the joint does weaken it.
Mike D
As you say, that seems counter intuitive. How exactly does pinning the joint make in more prone to failure? I'm really curious, because it seems equivalent to saying wearing belts and suspenders makes it more likely your pants will fall down.
After I got home I looked at the article (WOOD mag, Nov.2006) more closely and the pins don't weaken the joint BY MUCH - but it definitely doesn't strength it. In their words "Though decorative, dowel pins added no strength... If you want dowels for appearance, lengthen the tenon or cut a through-tenon to surround dowels with more wood for added strength." The M&T joints tested were 1.75"wide by 1.5" long.
The tests were conducted at Iowa State University's Structural Materials Testing Facility.
Les
Thanks, Les. I suppose the question is what one means by "strengthen it." What sort of stress was being applied in the test? Perhaps pins weakened slightly as far as resisting that particular stress, but strengthened it greatly with respect to another type of likely stress.
I generally think of pinning as another poster above said, as insurance to prevent any sudden failure should the glue deteriorate/fail over time. It can aslo help prevent stressed from seasonal movement to the extent the pins might hold the joint more steady mechanically.
At the end of the day, most woodworking joints are over-engineered for their applications and thus slight weakening becomes academic.
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