On 8/11/2014 10:11 PM, whit3rd wrote:
On Monday, August 11, 2014 9:24:51 AM UTC-7, Leon wrote:

The problem here is very obvious Leon. YOU DON'T HAVE ENOUGH CLAMPS!!

I knowwwwwwww. LOL, The problem is that 8 footers are damn expensive
and I needed 4

The problem with C-shape clamps (including the adjustable-length ones) is
that the rib has to be stiff along its length. The wood handscrew clamps, though,
have one screw in tension and one in compression with no backbone required.
You can make a version of that geometry with a 2x4 in compression and threaded
rod (or a steel cable) in tension, that doesn't rely on the stiff backbone.

Even at 4', your (Bessy?) clamps are bowing a bit.

The besseys and the Cabinet Masters are bowing more than usual because
they are being used at their extreme tips of their clamping surfaces.
If the bars were laying flat on the work and the clamping pressure was
at the bottom of the throats the bow would not be as exaggerated.

The general fix to prevent the clamps from bowing the work is to simply
put clamps on both sides.