Need some advice on how to back out screws to allow for movement of table top

[Shift]

I'm about to start build a harvest table using 2" (1.5") pine ...

Lots of people suggest backing out the screws a bit when joining the table top to avoid cupping allow for seasonal movement.

My question is just how much do you back them out, and on which joints?

I'm assuming that it's only in the apron to table top joints, and not the edge joints between the actual table top pieces or any of the support structure. Do you back out the screws on the breadboards?

thanks

(corrected the title to avoid confusion for future readers)

[Pierre Vallee]

hm! interesting... what is 'backing out a screw'? and what does it actually do? pardon my ignorance. On the otherhand, if a board will cup, what will stop it? depends on the grain, how it't oriented, and how you alternate each board (i.e. grain curve up and down). I've had boards twist along the length, so that is another issue to think about. 'all boars are not created eaqual'.

[Shift]

As far as i understood, the idea is to drive the screw all the way in and then back it out just a tiny bit, so that there's some space/play in the joint to allow for expansion/contraction.

[Rick]

Shift, there are reasons not to do this.  The screws act like clamps to hold the boards together while the glue hardens.  If the screws are not snug, the glue may be useless.  There really should not be any expansion/contraction at the joints. 

Shift said:

As far as i understood, the idea is to drive the screw all the way in and then back it out just a tiny bit, so that there's some space/play in the joint to allow for expansion/contraction.

[Shift]

thanks rick.

[Pierre Vallee]

I believe that 'Shifty' is correct, I didn't understand why one would want to back-out screws. Is there one? At any rate, they are self tapping screws and are meant to pull the pieces together and hold. Is there any reason to back a screw out? anyone?

[Jeff]

First I've ever heard of it.  When my grandfather taught me to join a table top, there weren't any Kreg jigs.  His tops were always just glued.  For that to work, they have to be clamped until the glue dries.  Plane the pieces to the desired thickness, alternate ring orientation up-down, use pipe clamps across the joints and some pieces of angle-iron clamped on opposite sides of the faces to keep the faces aligned.   I've done Kreg-based tops the same way, but without the clamps--leave 'em screwed tight to dry.  I can't think of any reason that backing off the screws would do anything useful.

[Rick R]

My first thought would be that the edges of the boards should be planed. Once they are flat, there should be no issue with cupping. I just built a tabletop using 2" thick cherry and had to dowel it and use pocket screws. I had problems wirh cupping until I planed the edges. I still had some gaps but overall it looks good and the wife is happy.

 

RR

[Jeff]

Rick, I was thinking that maybe the advice to back off screws resulted from using non-square material.  I just thought it goes without saying that pieces for a tabletop must be planed and joined before they're glued up.  In case anyone's wondering, a joiner is like a planer, except it's extremely precise, and designed to cut the edges of boards so that they are precisely square to the face.

If your material isn't properly squared, you'll have a devil of a time trying to lay up a flat tabletop, and the joints will indeed pull the material out of planar alignment (is this what was meant by "cupping" in the original post?) as they are tightened.

I guess that'll teach me to assume...

[Pierre Vallee]

Soulds like Jeff's advice is spot on. But, cupping is due to the grain of the wood and what part of the log it was cut from

[Jeff]

Exactly, Pierre.  I think that's where the confusion in the discussion came from.

In case anyone's interested, actual cupping (i.e., distortion of the finished piece over time) can be prevented by properly drying materials before planing and joining.

Pierre Vallee said:

Soulds like Jeff's advice is spot on. But, cupping is due to the grain of the wood and what part of the log it was cut from

[Pierre Vallee]

as well, look at the end of the board and alternate the face of grain, i.e. cup up and cup down

[Shift]

Thanks for the schooling everyone.

Cupping was definitely the wrong word to use.