I might be mistaken, but I don't think you can use a drum sander to flatten
boards, just like you can't use a planer to flatten boards. The rollers
will compress the board flat, sand it, then the board springs back to it's
original shape.

First thing to do is confirm that the board was indeed dead flat before you
put it through the sander. If not, try taking several very, very light
passes with the concave side down. Then flip the board over and repeat.
With patience, you can pretty much flatten a board with this technique, and
enough extra wood thickness...

Bob


What model sander is it? I have the Delta 18-36 and do not have this
problem.
wrote in message
...
Hi woodworkers,
I have a 18" drum sander, standard type of
a unit. Problem is that every piece of timber I feed thru it turns out
to be concave shaped. Not all that noticeable if I just want to smooth
a surface, but when I join two pieces together the hollow is
noticeable. (like putting a straight edge along the timber)
I have tried everything I can think of but an answer illudes me.
Any suggestions appreciated.