I've had my Dewalt since they first came out about 15 years and I had a similar problem. Tightening the lower blade too tight will cause the blade halder to stretch and requires replacement. I even made a wooden wrench to add torque which made matters worse.

 

The solution I found was to drill a hole into a piece of 1/4 thick plexiglas about 2 inches wide and 4 inches long. The hole needs to be drilled with a drill press so it is at 90 degrees to the plexiglas. The hole diameter is slighly smaller than the threads on the lower thumb screw. Use the lower thumb screw to tap threads into the plexiglas.

 

Now screw the lower thumb screw into the plexiglas until the tip just protrudes past the plexiglas.Color the metal tip with a black sharpie pen.Move the jig back and forth over a flat abrasive surface until the metal is flat and the ink removed. Now it will hold a blade with light finger tight pressure for several months.

 

Doug Turner