How flat is flat?

Hi all,

I'm a long time lurker and 1st time poster a builder.
I have a question for experienced builders.

When you prepare wood for jointing ... for example for a laminated neck. How flat you need yhe pieces to glue them together and get a nice invisible join?

I'd like some numbers, I know about the light tests .. but my cheapo steel rule seems not flat enough :-( ...

As the subject says .. how flat is flat?

Not to sound like an idiot, but... an invisible glueline requires parallel pieces. Not necessarily "flat," just completely and totally parallel. Of course, getting each piece dead flat is the easiest way to make them parallel, so...

On a piece as short as a guitar neck, I'd go for both surfaces being completely flat (no light at all underneath a straightedge) and absolutely smooth (handplaned, not sanded) prior to gluing. Longer pieces, like tabletops and the like, can be "sprung" together by intentionally leaving an infinitesimally slight gap in the middle, but doing this with a guitar neck makes no sense, IMHO.

Long-time lurker here, but rather than match your pieces to a steel rule and then gluing them together, I would match the pieces together. If you can see light, use a block plane to level down the high areas. Once you can't see light holding the joint together, you should be good to glue. Keep in mind that wood glue re-introduces moisture to the wood and may cause the pieces being jointed to move a little anyway, so clamping IMO is just as important as the jointing.

Lots of ways to accomplish it, but it kinda depends on what tools you have available.

Jointer is best, IMO (but I'm completely a hack, so maybe the real woodworkers have better ideas). If you're going without powered shop tools (like a jointer), you can clamp the two boards on top of each other and sand the edges with a long flat block to get them parallel.

I'd say an invisible joint means that the mating surfaces need to be smooth and straight in relation to each other. Jointer and planer surfaces leave more of a glue line because the mill marks are actually more like hills and valleys between them. I've found that my jointed pieces glue up with less of a line if I use a cabinet scraper on them before gluing. A spring joint is actually made where the center of the joint shows a tiny bit of light and the clamp pressure in the center puts compression on the ends of the boards.
This little extra compression is supposedly going to keep the ends from opening up. I've never experienced wood joint failure, so I don't know if that is valid or not.

I also think a drum sander of the performax variety also produces a better glue joint in my opinion than a cheaper model planer does.

clamping IMO is just as important as the jointing.

This is very good advice, as well; for me, the most complicated part of any glue-up isn't the joinery, it's figuring out how to secure the piece while the glue dries.