Why does my pocket hole joinery ?

[Jon Nuckles]

I used pocket screws to build boxes for my kitchen cabinets out of 3/4” plywood. While I had a bit of a learning curve at first driving the screws without pulling the joint out of perfect alignment, the screws definitely provided a solid connection. Based on your description, the only thing that comes to mind is to be sure you use the set up block to get the collar on the drill bit set at the proper depth for the length of screw you are using. There is not a lot of wiggle room to work with. (I have the K5, but assume the K4 works the same way.)

[Larry Foster]

kent, it's working so far keeping my goats out.

Thanks, Reinis and Jon.

Should I compensate slightly since 3/4 plywood is less than 3/4?

How little torque do you use when driving screws?

[glenn bradley]

1. Carefully square the parts
2. Set up Kreg jig correctly and clamp parts firmly before screw
3. Softwoods and plywood are usually very fragile. Avoid to use impact driver. Use drill set up at minimum torque. Make final adjust by hand
4. It is a good idea to combine pocket screws and glue at some cases
5. Practice. As most skills it comes with actual use

I think Osvaldo outlines the items required for success very well. I wish I could have done it as thoroughly. I'll add that there is an expectation with pocket hole joinery. I think of it about the same as nailing or screwing butt joints. If the screw thread is you only source of strength, the stress the joint will take is decent but, limited.

[Jim Foster]

Are you creating the pocket hole on the "Right Side" of the joint. The pocket should be created along end grain, so the screw is fastened into grain that can hold the threads, and not end grain. Also, once or twice, I've used a hand driven screwdriver when the material was soft so the created threads were not stripped by power drill.

See photo below, borrowed from a Rockler Web page

Can you provide a photo of one of the failed joints?

pocket-hole-detail.jpg

[Matthew Hills]

Pocket screws have generally been a pretty good choice for a butt joint, but you do have to look at overall design for the expected loads.

How did the joints fail? Is the head pulling through, or the threads pulling out of the wood?
What type of loads are causing the failure? Shear should be pretty good (limited by the material dimensions); tension and torsion will be weaker.

Over-driving the screws will significantly weaken the joint.
Glue is a good idea, particularly for plywood.

Matt

[David Eisenhauer]

I used to assemble lots of face frames using pocket screws and found that (as mentioned above by others) I had best success by doing the final screw tightening with a hand screwdriver rather than a powered driver to get the last 1/2 - 3/4 turn or so on the screw. I could feel the screw bringing the two parts into firm-solid-tight contact without overtightening when doing the last bit by hand. This was in a commercial environment where extra steps need to pay for themselves to be effective. The failure rate went to just about zero when doing this. Also, using two screws rather than one per joint makes a world of difference and that was the limiting factor for me when designing the width of my face frame material.

[Lee Schierer]

As others have noted, firring strips are about the poorest choice of wood for pocket hole joint strength. Firring strips are made from wood that is too poor quality to make much of anything else except maybe wood pulp. I use primarily hardwoods in the pocket hole screw joints that I have made. I have only rarely had a screw strip out and those occurred using soft wood. I generally use 1-1/4" screws with the fine threads. I tried some generic brand screws once and they were terrible, so be careful which brand screws you buy. I bought a box of 1,000 Kreg screws and they last a good long while.

[Larry Foster]

Thanks for all of the replies.
I'm in the middle of a 48 hour work/on call weekend.

If I get a break before dark I'll see if I can get a pic of the gate which is where the last failure was.

I talk about the firring strips and the gate but, I've had as much trouble with plywood.

I'll try the last little bit of tightening by hand and see if that makes a difference

[Andrew Pitonyak]

Larry, in the unlikely event that you live near Columbus Ohio, we can do one together.... I have built numerous things (such as cabinets) using pocket holes, and that have worked very well.

[Reinis Kanders]

I also find that his style of clamp helps to align joints better than flat clamp.

https://www.amazon.com/KHCRA-Automax.../dp/B01H1WFPWM

[Nick Decker]

I also find that his style of clamp helps to align joints better than flat clamp.

https://www.amazon.com/KHCRA-Automax.../dp/B01H1WFPWM

Agreed, those are very handy. I only have one, so I'll use it on one of the pocket holes to get things lined up and clamped down, then add another parallel or pipe clamp somewhere else along the line for insurance.

[Art Mann]

I have an entire shop filled with storage cabinets, tables and benches that were assembled with plywood and pocket screws. These were built around the same time about 15 years ago and they have been heavily used/abused. I don't have a hint of failure anywhere. They work great on plywood as well. You have to install them correctly and use enough of them to do the job. I think it is the best assembly method yet devised for cabinetry where appearance is not an issue.

I only use pocket hole joinery in hardwoods and they work great. Can’t see them working well in plywood.
Would only use in softwoods where there is no stress.

[Larry Foster]

Andrew, that is a generous offer that I wish were practical.
I'm due east about 225 miles.

Unless I get a real urge for Skyline and White Castles.

Reinis, I have that clamp and not real adept with it yet.

[Richard Coers]

I consider a pocket hole screw to just be a clamp. It's not a structural member, it's not a structural joint if the glue wouldn't hold it if you clamped it instead. Most often it's a tiny bit of end grain, butt glued to long grain. That is not a structural joint.

[Larry Foster]

Ok.
I was able to sneak out and get some pics

The first is of the stiles that pulled out

The second is is the gate with patches to replace the stiles that fell out.
The top and bottom 2 are the only ones that stayed

The last is a 3/4" plywood cabinet that had to be glued.
Not sure if the screws are holding much or not

One other thing.
The screws seat outside the holes and don't go in the pocket hole.
Like in the right hand photo