Strength of Pocket Screws?

[LinuxRandal]

Oh, and remember to use coarse screws for softwood and ply, and fine threads for hardwood.


g.

One thing you forgot to mention there, coarse of fine going from one type to the other. (oak into plywood, or pine into oak)

Make the threads match the wood it STOPS in.

[BobSch]

One thing you forgot to mention there, coarse of fine going from one type to the other. (oak into plywood, or pine into oak)

Make the threads match the wood it STOPS in.

Good point. And an easy one to get wrong.

[poolhound]

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

I nearly always use glue unless the PS joint is on some temporary fixture.

I also have a biscuit joiner and choose one or the other depending on the application. PS are quick and easy and are my choice for face frames or other joinery when it will be hidden and doesnt need to be "uber" strong.

I have also used them on occasion for cupboard doors and either left the holes when its a utility door (shop use) or filled them with matching plugs.

In the right place using PS and contrasting plugs can make a nice decorative feature.

[JimD]

I dug out the November 2006 Wood issue with the joint strength testing of the video referenced above. The testing that they did that included pocket screws was of simulated face frames and picture frames. They used 2 inch wide pieces of 3/4 oak. That limited the biscuit size to a #0 so the penetration of the cut was about 1/4 inch. For shear, the pocket screws about doubled the strength of the pure butt joint. Pull apart strength was not quite as much of a gain but still substantial. They were gluing the joints.

The other joints tested were pure butt, butt plus two dowels, and biscuits (as I mention above). The pure butt was the weakest, then the biscuit, then the pocket screws, then the dowels. I do not think the article says the diameter of the dowels but I think there was mention in a later issue that they were 3/8.

I do not think this says a lot about strength in different applications but this test (as opposed to the drawer joint test where dovetails were tested as weaker than a nailed butt joint) seems to have been appropriately conducted. In other words, for fairly narrow joints, biscuits so not provide as much strength improvement as the other joints. If the pieces had been wide enough for a #20, I think the tests would have had different results.

I remember a test in FWW or FHB where they tested door joints including mortise and tenon, biscuits, and dowels. Biscuits were better than dowels in that test. Logically the size of the dowels, number of dowels, size of biscuits, number of biscuits and possibly other factors would influence results.

I think you have to be a bit careful applying these results to other circumstances but I think pocket screws are a pretty decent joint for face frames where they can be hidden easily and not a whole lot of strength is necessary. I usually assemble them using the quick clamp vise grip type clamp that came with my jig (from Penn State industries).

I use mortise and tenon, dovetail, biscuit, lap joints, glued and nailed butt joints, glued and screwed butt joints, and pocket screws depending on the application. They will all work fine if you build them right and apply them appropriately.

Jim