Pocket Screw Questions

[Jon Farley]

This is another of many questions that I am sure to ask here at SMC regarding a major project I have recently undertaken. The project is 'freestanding altar' for my church, which is in reality is just another name for a fancy table. Based on advice I've gotten here, I have decided to construct the top from MDF with 3/16" oak veneer covering both sides.

That brings me to my next question. Since I have decided to forego a solid top in favor of the MDF/veneer top, I now am looking at using pocket screws to attach this top to the solid oak apron. I originally would not have considered this but it since I'm no longer using a solid top, and seemingly have removed any wood movement issues from the equation, it appears to me as though pocket screws would be ok. Am I correct?

If so will pocket screws hold ok in MDF? Can I also use glue? Any suggestions are more than welcome. Thanks...Jon

[Brian Jarnell]

I would use the metal Z clips.
I am about to use them on a solid oak table,but would equally use them on your job.
Mind you there seem to be many ways to skin a cat.

[Bill Fields]

Jon--nothing wrong w/ MDF as long as one understands its limitations.

Just make sure that:

-You pre-drill full clearance holes for thru applications, and drill closely to screw-thread inner dimensions for the under piece.

-Double apply glue--1st coat to soak the fibers for a few minutes, then a liberal coat for final glue-up/clamping.

MDF is great stuff and a lot cheaper than equivalent ply. HEAVY though--perhaps 50% heavier than ply!

BILL

[Jim Becker]

If your base is still made of solid stock, you still have wood movement in the equation. That said, it's likely to be less of an issue. You can deal with it by slightly enlarging the pilot holes in the sides of the base that the pockets screws ascend up to the top through, leaving a slight amount of "wiggle room". Just a hair is fine.

[Byron Trantham]

I'm with Brian, Z clips. Easy to install and do a great job of holding a top down.

[Charlie Plesums]

This sounded like a good plan until I caught that your oak "veneer" was 3/16 inch thick.

If you have solid wood you have movement, especially across the grain.

If you have veneer, you ignore the movement. (Actually the movement is still there, but the wood is so thin that the expansion of the wood fibers can make the piece "thicker" rather than "wider". If you buy the veneer, a fairly standard veneer thickness is 1/42 inch, "thick" veneer is 1/24 inch.

Where does the wood change from being a solid wood (where movement has to be considered) to being a veneer (where wood movement can be ignored)? I have not done any great studies, but I have listened to lots of raging debates (or raging woodworkers whose projects were spoiled by a layer pulling off a base after a year or two). Based on what I heard from many separate discussions, I concluded that veneer needed to be under 1/8 inch thick - preferably well under 1/8 inch. Or maybe sawn to 1/8 inch then sanded. Therefore I strongly recommend that you make your oak thinner.

To your other question...The overwhelming majority of the wood expansion and contraction is across the grain, with virtually no movement the length of the grain. Therefore when attaching a normal apron to a normal table top, the wood movement tricks are to accommodate the movement of the top, not especially for any movement in the aprons. Assuming you solve your veneer problem, you won't have movement in the top, so pocket screws are fine. But a there is a trick (I think it was from Fine Woodworking) if you want to be sure. Cut a biscuit slot in the top of the apron, where the pocket screw comes through the apron. This allows the pocket screw to pivot along the apron to handle wood movement the length of the apron. (Wood movement across the apron will just make the apron flex). I use this often, even with solid wood tops - fast and works like a charm.

[Jon Farley]

Thanks for all the advice on the pocket screws. Now based on some things Charlie said, I have even more questions about other aspects of this project, but that is a good thing as I want to make sure I get everything right on this one. Stand by for a new thread..Jon