I started doing some research on open-end drum sanders and wondering if drum sanders are similar to a planer. Meaning a planer reduces thickness but does not flatten. Are drum sanders similar to planers or do they actual flatten surfaces while reducing thickness.
Yes, drum sanders are similar to planers. Both register the board on the bottom surface, which is pressed down by the feed rollers, so the top surface will end up following the profile of the bottom. By using a sled of flat wood (i.e. plywood), and shimming under the high spots on the bottom so that the feed rollers cannot deform the wood you can get a flat surface on the top. When flipped over and run through the sander/planer, this flat surface on the bottom will result in the other surface being flat as well. Lots of info on the web about making/using a sled.
While you can 'plane's wood on a sander, it's a long, tedious process. A planer can take up to an 1/8" in one pass although usually take less than a 16th. The feed speed on a sander is 4-8 times slower and no more than 1/32 per pass. Plus if you do a lot of rough lumber, the sandpaper gets pretty expensive. Sanders excell at finish sanding especially tough grain such as birds eye.