sizing clamping culls

[W CHAN]

Hi,

Sorry for the newbie question. I was reading the recent issue of Fine Woodworking and it suggested that I use culls in order to distribute the pressure when clamping a wide area. This all makes sense in theory, but my question is how much of a convex do you make? I'm using 1.5"x1.5" lumber at both 24" or 32" in length. Would an arc where about 1/4" is taken off each end of the lumber bee too much? Or not enough? Also, is this too thick of a piece of wood for a clamping cull?

Sorry for the dumb question, but just trying to get an idea from the experienced people on the newgroup. Is it helps, I'm clamping up a plywood cabinet that's 26" deep, hence the 32" long clamping cull (3" on each end for the clamps to grab onto).

Thanks! Winthrop

[joeljcarver]

clamping a wide area. ... but my > question is how much of a convex do you make?

I don't intend to be picky, but you mean "cauls". A "cull" is something you reject and discard.

Try glueing a wooden biscuit at the middle of the caul. That will give you enough pressure.

Joel Jacobson

[Seeker]

I used some 1" X 1" square steel tubing and put a 1/8" bend in the middle this worked much better than wooden cauls as it resists bending better. And to distribute the pressure and prevent marking the wood I installed strips of 1/4" x 1 1/2" MDF with slotted holes so as the tube bends it moves independent of the MDF strips.

[Morris Dovey]

| Sorry for the newbie question. I was reading the recent issue of | Fine Woodworking and it suggested that I use culls in order to | distribute the pressure when clamping a wide area. This all makes | sense in theory, but my question is how much of a convex do you | make? I'm using 1.5"x1.5" lumber at both 24" or 32" in length. | Would an arc where about 1/4" is taken off each end of the lumber | bee too much? Or not enough? Also, is this too thick of a piece of | wood for a clamping cull? | | Sorry for the dumb question, but just trying to get an idea from the | experienced people on the newgroup. Is it helps, I'm clamping up a | plywood cabinet that's 26" deep, hence the 32" long clamping cull | (3" on each end for the clamps to grab onto).

Winthrop...

It's not a dumb question - but I think you're trying to make the problem more difficult than it needs to be.

Instead of depending on the springiness of the caul (which isn't practical to consider controlling because there are just too many variables), consider using more clamps. It's almost a given that there's no such thing as "too many clamps" - but it's more practical to add clamps than to engineer a non-linear caul for each length you want to clamp.

I use straight cauls and space clamps anywhere from 3" to 16" apart, depending on what it is that I'm gluing and what kind of glue.

Keep in mind that clamps aren't terribly difficult to make and that it's not necessary to rush out and spend big bux to have the clamps you need. There's a photo of one of my better (simple, inexpensive, reliable) box clamps at that might provide an idea or two.

-- Morris Dovey DeSoto Solar DeSoto, Iowa USA

[Jeff Gorman]

"W CHAN" wrote

Perhaps the best way would be trial and error?

Try a gentle curve on each caul, cramp them with the curved faces in opposition and check the quality of the interface. You could get an idea of the applied pressure from the force needed to operate the cramps.

Jeff G

[bridger]

a bit late to this thread, but here's a quick way to generate the curve of a clamping caul: first figure out how long the caul needs to be and get a piece of wood that length and just a bit thicker than it needs to be. experience will quickly teach you what that thickness will be- like after the second caul... now set it up on blocks on your workbench- the one with the nice thick rigid top. the blocks don't need to be much- 3/4" scraps will be fine. place them under the ends of the caul-to-be and set up a clamp at the midpoint clamping the caul to the benchtop. now apply as much pressure to the clamp as you will need to pull the assembly that the caul will be used on together, minus a little bit. how much pressure that will require is another of those experience-taught things... now take a third chunk of whatever scrap it was that you blocked the caul up off of the table with and use it to scribe a straight line along the now curved belly of the caul. unclamp and cut or plane to the now curved line you just drew. a second round of clamp pressure testing may be in order at this point. once you are happy with this curve, glue pads to it or sand it smooth and wax it or whatever treatment you like on your assembly aids.

take an hour, set up and make a dozen or so- they're very handy to have around.

[Morris Dovey]

| a bit late to this thread, but here's a quick way to generate the | curve of a clamping caul: | first figure out how long the caul needs to be and get a piece of | wood that length and just a bit thicker than it needs to be. | experience will quickly teach you what that thickness will be- like | after the second caul... now set it up on blocks on your workbench- | the one with the nice thick rigid top. the blocks don't need to be | much- 3/4" scraps will be fine. place them under the ends of the | caul-to-be and set up a clamp at the midpoint clamping the caul to | the benchtop. now apply as much pressure to the clamp as you will | need to pull the assembly that the caul will be used on together, | minus a little bit. how much pressure that will require is another | of those experience-taught things... now take a third chunk of | whatever scrap it was that you blocked the caul up off of the table | with and use it to scribe a straight line along the now curved | belly of the caul. unclamp and cut or plane to the now curved line | you just drew. a second round of clamp pressure testing may be in | order at this point. once you are happy with this curve, glue pads | to it or sand it smooth and wax it or whatever treatment you like | on your assembly aids.

How did I not think of that? It's simple. It's elegant. It's beautiful and I don't have the heart to snip even a single word.

A perfect case of better late than never!

-- Morris Dovey DeSoto Solar DeSoto, Iowa USA

[CW]

Just screw it to the edge of a board, ends only. Shim in the middle for the amount of curve wanted. Run it through the tablesaw. Unscrew. Instant curve. Faster.

[bridger]

yes, but how much curve is that? using the clamp to curve the board gives you direct feedback about the amount of pressure *that* clamp will take to bend *that* board. once you have that data, your method of making the cut is pretty good, except that the caul will move as you cut off some of the wood that is under tension. depending on the sizes of everything it might not be a problem, though.

I like the bandsaw and OSS for cutting the curve, buy that's me....