We are having problems when we sand our assembled face frames. We use a random orbital sander after assembly, and we always seem to get a rounded over edge, especially on the corners. What process do most people use to sand face frames?
I also use a random orbit sander on face frames, door rails and stiles, and the raised portions of raised panels with no problem. All edges remain nice and crisp. One thing to check is that the sander pad is still flat. I replace mine when the edges begin to get lower than the center. Also, is it hook and loop or stick-on? I recommend stick-on since the backing is harder and won't droop over the edges. Technique could also be a contributing factor. Don't press down too hard - the weight of the sander should be enough pressure.
In general, a firmer pad would be the way to go. With good technique, it can be done with a soft pad, but it's tricky. With a hard pad, just always keep the sander flat and moving. Oh, and make sure the paper is tight against the pad - frayed sandpaper edges round corners off very nicely.
1. Use a carpenter's pencil to scribble pencil lines across the corner joint (and all along the frame if needed). Start sanding and watch carefully as the pencil lines begin to disappear. They should sand away pretty much at the same time along the entire area being sanded. If they sand off to one side, you will be clued in that you are applying uneven pressure to the sander and must make an adjustment in "feel" as you sand. If the pencil lines are coming off on both edges but remain in the center, the pressure may be even but it is too hard, effectively causing the sanding pad (and disk) to sand deeper along the edges. This is where PSA pads (which are not cushioned like H&L pads and paper) have the edge in flat sanding.
2. Sand the backs of the frames reasonably flat to 100 - 120 grit. Attach the frames to the boxes and screw adjoining cabinets together, mating the frames tightly like they will be when installed. Now sand the face of the frames flush at the center stiles and all the rest of the frame area except the outside stiles and about 8-10" of the rails on the side where another cabinet will butt against it. Unscrew these cabinets and put the next one up against the unsanded side and keep moving through your set of cabinets, sanding right across the 8-10” of unsanded frame that was left from the previous cabinet. Even if you do an uneven job of sanding, the cabinets will line up flush in the front when they are installed. I use this technique when I do a set of paint grade cabinets that will be installed and then painted on site. The edges between cabinet stiles remain crisp and flat, one against another.