Replies

1. 
   RickL | Dec 13, 2009 05:24pm | #1

   2 x4's work fine or maple or poplar a bit scaled down work well too. I use an 1/8" crown over a 48" length and have a template to duplicate more to be consistent. Makes a difference I find when gluing up panels to have them all the same. It's not rocket science and although few seem to know of or understand the use of cauls they go way back in time.
   Softer cauls of MDF would work better with a larger arc. MDF is heavy and I prefer a lighter weight caul myself but whatever works for you. There's no one right answer...

1. 
   DonStephan | Dec 13, 2009 11:02pm | #2

   That's an excellent point you raise.  If I understand you (and it makes perfect sense) the amount of taper per foot ideally will depend on the stiffness of the caul itself.  If the caul flattens too easily its going to provide little clamping pressure, and if the caul flattens too hard then too much clamp pressure is needed to flatten the caul.

   That approach suggests just about any wood or built-up sheet good can be used.  Start with a relatively shallow taper from midpoint to each end and see how much clamping pressure is needed to flatten the caul.  Too easy, increase the amount of taper; too hard, decrease the amount of taper.  A taper in the range of 1/16" to maybe 3/32" per foot?  If more taper needed, make the caul wider?

   I like a wider caul so there's less chance it will fall over under clamp pressure, and I like the cost of left-over MDF. 

   Any other thoughts out there?

1. 
   RickL | Dec 14, 2009 01:12am | #3

   I think your original crown of 1/8" per foot for a was too extreme. You wouldn't want to squeeze the glue from the center. I find an even pressure spread out is best. If you wanted to measure the pressure there are films available...
   http://www.tekscan.com/pressureindicatingfilm.html?gclid=CLnjtaq01J4CFag65QodD20lqg
   A low tech version would be leafs of stiff paper under your caul and see if they pull out or tear when you try to remove them. The earliest use of cauls I've seen is in old veneer books where the cauls are used to press veneer. The veneer is applied to the substrate and paper is put over the veneer. Plywood is applied to the top and bottom of the panel and cauls or curved battens are applied in pairs with the curve facing the ply to spread pressure evenly across the panels. The cauls are curved battens and can be used to align boards for panel glue ups or to spread the pressure across cabinet sides to help during the gluing of carcase work.

1. 
   DonStephan | Dec 14, 2009 01:43am | #4

   Great suggestion for determining when enough clamping pressure has been applied to the cauls.  In this application, they are used to bring into better alignment boards that are being edge glued to make a tabletop, so it's not as much of a problem if the pressure initially is all at the high center point until the caul's clamps are tightened.  Without the cauls the edges are within a strong  thirty-second of each other, just using the cauls to try to improve on that so less time with smoother and card scraper.

   By the very nature of tapered cauls, though, isn't the pressure always going to be focused at the high center point until clamping pressure brings the surface of the cauls down evenly?  I wouldn't use them without the necessary clamping pressure to bring them down evenly.  If clamping veneer, torsion boxes, several layers of 3/4" sheet good, or some such likely would be used to spread out the effect of the cauls over a larger area of the veneer.

2. 
   YesMaam27577 | Dec 14, 2009 03:19pm | #5

   Last time I needed some that long, I went to the home center and bought a few of the boards that they call straight construction lumber.

   Used the obvious crown that was already there in the boards.

   I won't be laughing at the lies when I'm gone,
   And I can't question how or when or why when I'm gone;
   I can't live proud enough to die when I'm gone,
   So I guess I'll have to do it while I'm here. (Phil Ochs)