I read a couple of articles on pocket screw joint strength.The Kreg site says that a pocket hole screw joint with glue (which would be an end grain glue-up) is stronger that a mortise and tenon. This is what Kreg states;
“A independent lab completed testing a few years back that showed that a pocket hole joint failed at 707 pounds when subjected to a shear load while a mortise and tenon joint failed at 453 pounds (approximately 35% stronger). Pocket hole joints are tremendously strong for a couple of reasons. 1. The use of a mechanical fastener (screw) is significantly stronger than the material around it (wood), and 2. The amount of direct clamping force placed on the joint by driving the screw combined with today’s glue technology makes for a sensationally strong bond.”
Woodworking Magazine says essentially the same thing. Although they say the joints failed with about the same force, but the PHS joint shattered/splintered beyond repair and the M&T could have been fixed (at least the one in the test. I thought there was also an article in FWW that says the same thing, but can’t seem to find it.
For face frames, etc that are under a heavy load, would a pocket hole joint be as effective as a mortise and tenon?
Denny….
Replies
Hi Denny ,
My take on face frames , the pocket screws with glue hold the joints well , if you use glue and clamps and an occasional fastener if applicable the face is glued to the case front , this is where the real strength comes from imo .
I would not rate an M & T to a dowel or any other different joint but I will tell you that a properly done PS joint has more holding power then dowels or biscuits and the tensile strength of the steel screw is huge compared to the others. So you get pulling / holding power and strength without having to wait on clamps .
The PS are far more accurate as well for face layout and such .
dusty
Within the last year or so, Wood magazine has run a series of tests at one of the college labs. Their results showed the M&T to be the strongest joint. Will try to look it up later & post the actual ratings.
The pocket screws may work just fine in the life time of a kitchen cabinet--perhaps 20-30 years, but still not be a good choice for fine furntiture expected to last for generations. Wood shrinks or swells, the metal screw doesn't. That means that eventually the screw loosens.
Denny-
The quote says the test was conducted with shear loading. I've been trying to think of a situation in which either a pocket screw butt joint or a mortise and tenon joint would be subjected predominantly to shear loading.
I can conjure up an unrealistic case of a face frame with a heavy door fixed to the stile; if the rails are attached to the cabinet and the stiles are free to move, then there would be shear loading. But we don't attach face frames that way, do we?
Can you think of some cases of shear loading?
Don
Don, this is my thought as well. I don't see where there is much shear in most furniture. I went back and reread the FAQ on this and they do say;
"Although pocket hole joints are exceptionally strong, the rule of thumb is to never use a pocket hole joint in place of a structural bolt, hanger or fastening device."
So Kreg says that PHS are not substitutes for hangers (such as for 2X's), etc where the joint is subjected to shear forces.
Most furniture or base cabinets would be more subjected to lateral forces, maybe a poor example, but like leaning on an kitchen island cabinet. I just don't get the feeling that a PHS would stand up to that and a M&T would do better. But then when the face frame is glued to the cabinet frame, that should give it the strength it needs, maybe????
Not an engineer (or experienced woodworker) and making my assessment on what would seem to make logical sense.
Denny , You got take into consideration the entire face assembly when glued to the box assembly becomes less about a pocket screw joint and it's strength comes from the whole system .
When that face is glued to the box it's all good , neither can move .
For shear forces think about the base cabinets that get 1 1/4" Granite tops , some of the weight is indeed absorbed by the PS joints in the top rails and such . I have never had a failure other then a few stripped screws per year .
regards dusty
Only on the stiles :-)Work Safe, Count to 10 when your done for the day !!
Bruce S.
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