Wide Belt Sander Tracking Marks

Troubleshooting wavy, snakelike sanding marks left by a wide belt sander. December 21, 2010

We have a large hard maple trim job and are having some issues with the quality of finish as it comes off the wide belt. We are currently using a SAC 37" sander with Klingspor abrasives. After the part comes out of the moulder, we are sending it through the sander with one pass at 120 and then a finish pass at 150. The sander has an electric eye tracking system that keeps the belt tracking, but the side to side movement shows up in the finished product. We are running with a platen on both passes.

Is there a good way to eliminate or minimize the lines? They show up in the stain/finish application big time. Should we be sanding a different way, or should I find someone with another type of machine to finish sand? All the profiles are basically S4S with eased edges.

Forum Responses
(Architectural Woodworking Forum)
From contributor L:
The correct answer is to random orbit sand to get rid of the straight line scratch marks. The other answer is to adjust the tracking so it is more balanced and doesn't move across the roller. Then it goes much slower side to side. On some machines it is not possible. The machine uses the back and forth action to eliminate the need for tracking. Or ramp up the conveyor belt speed for the last pass(es).

From contributor B:
It could be an issue with the abrasive on the Klingspor belts. Our edge sander belt supplier changed their raw belt stock source and we had huge problems with the new belts leaving scratches. We had to go from 80 grit to 100 grit to solve the problem. In the past the 80 had done what we needed with this process.

From contributor J:
I have this same problem - a wavy S-shaped pattern. I tried different brands of belts, adjusted speed of tracking, nothing works. Lots of finish sanding with a random orbit.

From contributor K:
Just had the same problem. Turns out the air pressure was too high on the belt. When we tapped on the belt, it sounded like a drum skin - way too tight. We dropped the pressure down to 4psi and all snake marks are gone. Also do a visual inspection of the drum and make sure there are no imperfections on it.

From contributor C:
Typical tracking of a wide belt sander. We use many of these machines. Try using a finer finish grit, lower air pressure on belt tension, and many of these machines have a manual adjustment rod that will allow you to set the belt travel. Maple will show these wavy lines more due to the density of the material. RO sanders will break up the defined scratch, but could also cause divots if uneven pressure is applied.