Strength of Pocket Screws?

[radhak]

Looking for opinions on the strengths of Pocket screws compared with other types of joints. If it matters, I have the Kreg pocket screw jig, but am guessing it should be the same for all?

[gsmittle]

My experience is that pocket screws are plenty strong--the only real issue I've had is when I over-tighten the screw and strip the hole. It usually takes a couple of adjustments of the drill's clutch in some scrap to get the torque just right. Oh, and remember to use coarse screws for softwood and ply, and fine threads for hardwood.

OTOH, I wouldn't expect a pocket screw joint to be as strong as say a mortise and tenon, but it's plenty strong for face frames...

g.

[drumpriest]

As with gsmittle says, the problem isn't with the strength of the screw, it's about getting the joint together correctly. I have not stripped any out, but I've overtightened and misaligned the parts, even when face clamped.

I prefer joinery that is a no brainer during glue-up, it's less hassle. Double dowel is like that, or loose mortise and tenon, which I use most because I'm lucky enough to have a domino. I use the pocket screws to put together case work that'll be hidden.

[leehljp]

I used to use dowels and probably will in the future when a project will look good with well placed dowels. After I purchased a biscuit cutter, I haven't looked back. I also use pocket screws. In a few projects, I have used a combination of biscuits and pocket screws.

On book cases, I make the front frame attached to the side panels by using biscuits and gluing them as one. Top, bottom and center attached non adjustable shelf were attached by pocket screws. This leaves it possible to disassemble into kit form for moving. This one is with my daughter in the States.

I used combinations like this on my router center and midi-lathe table also. I have to get them back to the States in some form, so I plan mine for easy breakdown as well as easy and strong assembly.

[drumpriest]

Hank, the Domino is just like a biscuit cutter on steroids. Same ease of use, much stronger, and better alignment. I agree though, once you can cut effective joinery that easily, it just frees you to do so many things.

I was a biscuit and PS guy for a little while, and router cut M&T for more important things, or double dowels, but I gotta say the domino was worth it for me. Might not be for other people, as it's pretty pricey, but still...

I also hear you on break down designs! I try to keep in mind how I'll move something when I build it, because it's SUCH a pain in the butt to do.

[leehljp]

Hank, the Domino is just like a biscuit cutter on steroids. Same ease of use, much stronger, and better alignment. I agree though, once you can cut effective joinery that easily, it just frees you to do so many things.

I have been watching with intrigue (and jealousy) - your posts on the domino. Since I will be going home in just over 2 years for retirement, I am holding off on most purchases so that I can limit my shipments back - including tools. As it is now, I will have a crate of about 8' X 8' X 8' for tools. What I am worried about is all the stuff that USA customs will plunder through.

Back to the Domino, I am looking at those. My girls (3) and all grown are waiting for me to "build" them beds, book cases, hutches and tables. The domino's alignment would certainly help as I spend considerable time checking alignments before cutting, attaching or glueing up. I have had problems in the past with the biscuit cutter alignments unless everything was perfect.

Thanks for the note!

[radhak]

So y'all don't use glue when using pocket screws? Would glue add substantial strength to the joint?

[drumpriest]

I do use glue with my pocket screws. Funny enough, I mostly use pocket screws now for shop hardware and jigs, and I just really like the screws. They have made their way into projects in unlikely places.

You should watch this video, btw...

http://www.youtube.com/watch?v=DhLfb7m9Fug

Based upon their results, glue can result in an increase in strength, I base this supposition on the failure point of glue only butt joints. They are the lowest, however the still require over 1 hundred pounds of force. Add some screws to that, it gets much stronger. From the video it's no doubt that mortise and tenon is the king, pull apart test at over 4000 lbs of pressure is pretty impressive, and the joint didn't fail, the oak did.

[RayintheUK]

So y'all don't use glue when using pocket screws? Would glue add substantial strength to the joint?

I wouldn't use them in anything I've done so far without glue. I see the pocket screw as an alternative to prolonged clamping rather than a structural element, but I still always use clamps to line up and hold everything (in both planes) before inserting the screw.

Aside from the risk of reaming the screwhole in MDF and softwoods, when inserting the screw without clamping the pieces, there is a tendency for the angle of the screw to drag the alignment of the joint off.

I'd consider pocket screws for use in a knock-down frame only if it was in hardwood, but would prefer to use double dowels as well in that situation - less chance of racking.

As you'll gather, I tend to "over-engineer" my work, but - so far - nothing has ever fell apart!

Ray.

[cabinetman]

I'm not a big fan of pocket screws or biscuits. Before all the fancy jigs, I installed screws and nails in the same manner, except we called 'em just angled, or "skewed screws". They can have their place as a captive method, like Ray described. For an intended KD application, they might be ideal. I just prefer traditional joinery if possible.
.

[gary]

So y'all don't use glue when using pocket screws? Would glue add substantial strength to the joint?

End grain to long grain glue-ups are not very strong.

[leehljp]

IF I were in the States, I would probably be using glue in addition to screws - except for one daughter who prefers to move in a minimalist volume format, and she seems to move about every 3 years or so.

But because I want to have many of my things here shipped back to the States, and volume is a part of the cost, it certainly helps to have the ability to break down the hutch, the book shelves, my router center and my lathe table.

If you will notice that is a wide diverse group of woodworking structures. I haven't been easy on them either and they hold together well. As I said earlier, I would not have done them that way if I were doing it in a place where they wouldn't have to be moved. But having done it, I am VERY surprised at the strength of the pocket screw construction without the use of glue.

[LarryG]

As with gsmittle says, the problem isn't with the strength of the screw, it's about getting the joint together correctly. I have not stripped any out, but I've overtightened and misaligned the parts, even when face clamped.

I solved this problem for good when I learned to leave the Kreg-supplied face clamp in the carrying case and substitute a Bessey K-Body.

[gsmittle]

So y'all don't use glue when using pocket screws? Would glue add substantial strength to the joint?

I DO use glue, but I'm not sure it makes much difference on a butt joint. I've used PS once to join a table top. That turned out pretty well, but I'm not sure I'd do it again. I don't have a Domino or biscuit joiner, and the one time I used dowels it was a pain in the tookus. Guess it's time to learn how to do mortise & tenon..

g.

[Schleeper]

I'm totally sold on pocket screws. (Admittedly, I'm inexperienced enough to have never developed an affinity for other forms of joinery.) I'm using them for the cabinet project I'm working on. I was planning on gluing the joints, also, but I think I'm going to forego that in constructing the face frames. Not only does it not add much in the way of strength (as Gary said, end grain to long grain glue-ups are not very strong,) it may actually make it more difficult to maintain alignment during screw insertion.

When it comes to attaching the wood frames to the plywood boxes, I'll be using pocket screws for the top and sides, and biscuits for the bottom. Those joints will also be glued. Ditto for joining tops and bottoms to side panels of end cabinets, where I obviously can't drive screws through the side to strengthen the dado joint.