Pocket hole jig

The screw comes off a table, not a formula

There is no equation for how deep a self-tapping washer-head screw should seat in a given thickness before it risks breaking out of the far face. It is tested by the people who make the jig, and what they publish is a table of material thickness against screw length.

Two thicknesses cover most of what a home shop builds, and getting them the wrong way round is how a screw tip appears through the show face of a cabinet side.

Coarse thread, unless it is hardwood

Coarse threads hold better in softer, more open material — pine, poplar, plywood and MDF — because there is more wood between the threads for them to grip. Fine threads are for dense hardwood, where a coarse thread acts like a wedge and splits along the grain.

That is the whole rule, and the jig's own packaging usually repeats it. Getting it wrong in hardwood announces itself as a crack running out of the pocket.

Spacing along a joint

Along an edge-to-edge glue-up, the holes go roughly every six to eight inches — close enough that the joint is pulled tight along its whole length, far enough apart that you are not drilling all afternoon. The arithmetic is the same "fewest points that keep every gap under a maximum" question a fence run asks, which is why the calculator answers it with the same function.

Where it does not belong

A pocket screw is a clamp that stays in the joint. That makes it superb for a face frame, a cabinet carcase, a jig, a shop fixture — anything where the joint is in compression or shear and the holes end up hidden.

It is a poor choice where the joint will be racked repeatedly, like a chair or a table base, because everything is resting on two screws in end grain rather than on a mechanical interlock; that is a mortise and tenon's job. It is also the wrong answer wherever the pockets will show and cannot be plugged convincingly.

What this page does not tell you

• —The screw table is the jig maker's published figures for their own screws. A different maker's screw of the same nominal length can have a different thread form and head, so check the chart that came in the box.
• —Pocket screws and glue are not alternatives. Glue the joint as well wherever the grain direction lets it hold — the screws then act as the clamp while it cures.
• —Drill a test hole in scrap of the same thickness after changing the jig's collar setting. A stop collar that has crept is the usual cause of a screw appearing where it should not.