In terms of shear strength, which is stronger:
Glue joint that is also screwed?
Glue joint that is pinned with dowels?
Assume that both joints are glued up in ash, with Titebond III. Also assume equal numbers of screws or dowel pins. Say, every three inches.
This is not an idle question - I am about to fab up a new carry yoke attachment system. Did I mention that SHEAR strength is whats important?
In terms of shear strength, which is stronger:
not a wood worker, but
my thoughts are:
screws pull the parts together, are not as useful for lateral force
if the glue is perfectly waterproof, and the bond is as strong as the wood, and the dowels are as strong as the wood, the joint might be nearly as strong as solid wood?
Can you use both dowels for lateral strength and screws(stainless?) for holding power?
Let us know what you think.
Build a test model and try it
Sorry to offer such a simple solution, but try screwing the gluing and see which test pieces “Crack” under pressure. See if you can use a vice or Bessy clamp to pull the joint across and see which one holds better. I cannot visualize what you are building, but would think the more glue the better. This meaning the dowels have more gluing surface, which should be stronger than the wood.
Wood expands and contracts as it absorbs and looses moisture. In some situations, especially where glued dowels are placed across the grain they can actually caused joint failure - such a placement can hinder expansion and contraction. Dowels generally serve only as alignment points - they add very little strength, if any. Screws on the other hand are not glued in place, are not generally a hindrance to wood movement and serve as tensioning devices. …hope that makes some sense… - Randall
It would be nice to see the project
but Arkay is right. I use dowel rods generally as alignment tools but they do add some strength in certain applications such as “T” joints in seat frames for canoe seats. Machine screws with washers and nuts work well for situations where you are overlapping or joining cross grain parts together but you need to use proper size drill bits and predrill. When using wood screws it’s also important it predrill hardwood with the correct size bit to prevent splitting.
A good old lap joint or laminated joint that is glued up with some good glue alone can be far stronger than you think.
Use pocket hole screws.
Get a Kreg jig, drill pocket holes and use the Kreg brand pocket screws appropriate for the wood you’re using (fine threaded for hardwood, course for soft). Wood working mags have done tests and proper joints glued and pocket hole screwed were nearly as strong as mortise & tenon joints (considered the strongest there is). You also don’t need to use clamps, the jig lines the pieces up very flush and there’s no need to wait for the glue to dry to complete assembly.
If you have a router, you can easily make “loose” tenon joints, where instead of one tenon being part of one of the joinery pieces, you simply mortise both pieces and use a separate tenon, much like a dowel. I have never used dowels, but they are not considered the strongest joint around, probably just above a biscuit joint.
One thing which you may already know, or not if you’ve never done any woodworking: Be sure you line up your joints long-grain to long-grain. End grain joints are about the weakest glue joints there are, regardless of the method. If you absolutely have to use an end grain joint (ie, a butt joint at a 90 deg angle), the only way to get a strong joint is through mortise & tenon, or secondarily a loose tenon. A box joint is equally as strong, or a dove tail joint is the strongest for 90 angle joinery. But they’re also pains in the arse, and way above the average woodworker, even using an expensive jig.
Do some searches for woodworking tips and jigs and you’ll find way more info than you probably even want. Good luck, “and remember, there’s no more important rule than to where these, safety glasses.”
Assuming the dowel you use has similar characteristics (strength and moisture content) to the wood being glued, the dowel wins hands down because the stress is distributed over a wider area.
This is especially true when you consider that the diameter of dowels is usually at least twice that of the inside diameter of any screw that might be used.
On the other hand, a properly fabricated and glued joint will be stronger than the wood you are gluing together so that no reinforcement should be necessary.
dowels or biscuits
Tests indicated that the screws
have little to do with pocket hole joints’ strength. The strength comes from the glue joint after the enormous pressure exerted on the joint from the pocket hole screws “clamping” action. I’ve actually removed screws before and left them out.
Of course, the screws do help with forces against the joint. Breaking strength was, again, slightly less than M&T, but higher than dowels or biscuits.
Sorry I can’t remember the mag, but I’ll try to find a study online.
Ok, found one that indicated PH joints are 50% stronger than M&T.
“Why is it stronger?
Pocket Hole joints are remarkably strong for a couple of simple reasons. First, a mechanical screw has physical properties that make it much stronger than a biscuit, dowel or tenon. Secondly, the amount of constant clamping force placed on the joint line by driving the screw combined with today’s glue technology makes for a sensationally strong wood to wood bond. An independent lab conducted a test of shear strength between a Pocket Hole joint and a mortise and tenon joint - result the Pocket Hole joint tested approximately 50% stronger!”
Another good link about joinery here: http://sawdustmaking.com/About%20Joints/about_joints.htm
Maximize glue surface
To a very large degree the glue is what holds the boards together. Screws, dowels, bisquits, nails, etc are predominately there to align and hold the work together until the glue dries. Two boards properly jointed and glued will break in the wood beside the glued joint because the joint is the strongest part. Following this logic - you want a fairly precise joint where the wood is well enough machined to give uniform contact and a joint structure that maximizes the contact area and thus the amount of wood glued together. A box joint is fairly simple and does this well. Dovetails do it too but are substantially more difficult. No amount of hardware will make a butt joint strong where to pieces of wood are just glued together at 90 degrees…
I planned on running a couple of tests. Screwed & glued, doweled & glued, Titebond III, Gorilla, and glued only. Maybe even through -bolted.
Interesting observations about dowels and moisture.
This evening I will post a link to my Grovestreet album and a pic of my current setup. (cannot access here at work)
Here is the situation. I am a big guy. I was concerned with the way my weight distorted teh inwales of my Magic after a full day of paddling. I added a seat hanger spacer immediately under the inwale, running from 8" in front of the front seat hanger bolts to the rear thwart.Bolted through the inwale at front, attached under the rear thwart it distributed my weight over four points per side rather than two.
There is a protruding section towards teh front that acts as a cleat for mounting the carry yoke. Whole thing was fabbed out of red oak (all I could get).
I recently bought some lovely ash sections from Ed’s canoe, milled at 3/4"x3/4". The seat spacer is an easy replacement, but the protruding cleat will need a section glued on to allow yoke attachment. The current red oak board I cut and contoured from a 3/4x1-1/2 piece and reinforced the yoke attachment point with screws (red oak & splitting being on my mind). Everything has worked so far, but I want to match teh ash inwale and I want to chamge my yoke attachment from bolt-in to clamp-on (to allow me to adjust the balance pointdepending on what I might have lashed into the boat during a portage).
Phew. Pictures will make it clearer. And the SS inchworm is where I attach the snap-clip for my seat-pad system.
You know the 100 year old joints in furniture that still hold?
It’s called a rubbed joint- you sand the pieces until light won’t show between, use a good quality pva (white waterproof glue) on BOTH pieces, rub them together along the grain until they start to stick and then clamp for 24 hrs.
It works by forcing the glue into the pores and heating the joint by friction.
With a good quality glue the wood will fail before the glue joint. Unfortunately this joint is rarely used now because it’s too time consuming and is almost a lost art
With a jointer jig for a router or an
actual jointer, this isn’t necessary today. One can get a much more perfect mating surface this way than "sanding until no light shines through).
I guess one could still perform the rubbing after jointing the boards, but clamping pressure with either clamps or pocket hole screws should do the same thing (force the glue into the pores). My impression is that any “heat” produced by the friction could be detrimental, not beneficial.
But I’m not a glue chemist, so that’s not an educated opinion.
The jointer will cause some deflection in thin (under 4/4) stock due to the upwards pressure from the blades(remember, they spin UP from the back of the head)also sanding helps to slightly roughen the grain, and the whole idea of rubbing is to provide enough heat and friction to force the water in the pva to be absorbed into the material, which then allows the actual adhesive to follow it from capillary action (warm water absorbs faster than cold)
This doesn’t work with epoxy or cyanoacrylates as they contain no water, but pva is about 40% water hence the longer dry time as the water has to evaporate before the adhesive can cure. water (or alcohol) is only the transport agent.
Here is current setup.
easier and will work for sure
just get some hard foam and cut it into a wedge with about 6 - 12 inches wide and with a height that starts about a half inch below your seat and increases to about an inch above your seat (while resting on the floor of the canoe under the seat). Wedge it under your seat. Sit. No stress on the rails. Works for me and I’m a big gal. Mine is removable so that I can stow stuff under the seat for a long trip but you could just as easily use something like contact cement to secure it to the canoe.
I LIKE woodworking a lot and it’s still easier this way.
Leave it to a woman . . . .
Nice idea. Really, good solution.
You forget ONE thing, though. Men are LOOKING for excuses to use our powertools, duh!
This, BTW, is the ONE area men do not seek shortcuts, where a woman would . . . .
Nice change of pace. Wish I could add something else, but it seems well covered.
that both joints will give you good service and work about equally. I think the thing to consider here is that when the joints start to fail which one will take longer to fail and I think that would be the screwed and glue joint. I have seen dowels come out from failed glue joints, I have seen dowels break out due to failed glue joints. Too tight a fit squeezes out the glue, too loose a fit puts extra strain on the glue connection.
With all the labor and effort you are putting in, you should bite the bullet and get a more sutable wood! Cherry, ash, walnut or SYP would all be better choices.