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. snipped-for-privacy@yahoo.com
How wide are the pieces?
I assume you have checked that the drum is flat (hard to think it wouldn't be) so the only thing I can suggest is that you are trying to remove too much material at once and that the paper isn't removing equally everywhere. Even that is a stretch of the imagination but try running the same piece through (at the same setting) 3- 4 times and see if things improve.
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I assume we are not talking about taper due to the drum not adjusted parallel? In other words, is the concavity due to putting the edge of two boards together where each went through the sander and each side that went through the same side of the sander are joined? If that was the lower side of the drum, that would cause concavity.
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
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