Drum sander tips
In the shared shop environment you have to assume every tool is slightly out of adjustment unless you adjust it as you are starting your job, and assume it will be out of adjustment the next day you need to use that tool.
The drum sander should be adjusted slightly tighter on the motor side. On a project desiring high precision and parallel you should make the adjustment for your job and hopefully put it back when you are done.
For jobs narrower than 16” not requiring enough precision to adjust the machine perfectly, assume the drum is not parallel, but hopefully relatively close, and keep the same side of the board toward the motor from start to finish. We should only be taking about thousandths difference between ends of the drum. The sanding wrap folding from clamp to outside of the drum might make as much difference if adjusted optimally. By keeping one side toward the motor all 4 sides are essentially flat, not perfectly parallel, but only detectable in a situation where you line up opposite sides or more than a couple rows. For most projects it’s good enough and not worth the time to bother trying for perfection.
When you sand from both sides including when you followed the instructions from the manual, the drum is not parallel to the conveyor and placing a long precision straight edge across the 30” direction will show if you are measuring precisely enough. Without the precise comparison hopefully it appears flat.
If the drum is parallel or tighter away from the motor and a very wide piece is sanded from both sides, or the entire board doesn’t stay in the plane of the conveyor, a sharp edge may be ground near the middle much like what sometimes happens if resawing a 6” wide plank with the table saw from both sides. More typically the drum is more than a couple thousandths different and the crown will be more evident.
If any width board is sanded from both sides and the last pass is not aggressive enough to sand the entire width a crown will be created and it won’t fit flat against another board, but this mis-adjustment is generally easily hidden by keeping one side toward the motor. But that is a short cut that is unacceptable on a project where great precision is required.
Therefore, if you want the tool to be adjusted precisely use appropriate material and make the adjustments you need. All of us in a shared space should expect that others will not change the tool adjustments back and that our projects may suffer if we assume how a tool is adjusted without at least checking if the adjustments are correct for our usage.