Troubleshooting Wavy Lines with a Widebelt Sander

Question
We have a two head 42 inch wide Butfering sander. We bought it new about eight years ago. We use it regularly but we are not a high production shop. It has lately been producing wavy lines on the product. This seems to happen whether or not we also engage the platen. The machine has good dust extraction and we are using top of the line 3M purple belts. The belts are paperbacked and are uni-directional. They have a serpentine stitch rather than a continuous seam.

We had a machinery repairman here today. He is a former Stiles mechanic and presumably reasonably qualified. His determination was that the steel rollers and rubber rollers were in very good condition. He could not find any problem with them. He could also not identify the cause of the wavy lines. He took a sample board with him to consult with others in his field. To try to get in front of this a little bit I thought I would ask here if anybody else has seen a problem like this, and if anybody has a suggestion where to look?

Forum Responses
(Solid Wood Machining Forum)
From contributor G:
I’m not exactly sure what type of wavy lines you’re getting, but sometimes lines are seen on pieces as chatter marks. Most sander operators have seen this, coming from the belts' splice. Test for this by smearing craylon (like bright blue) onto the belt splice, which will give color to the chatter marks as the belt rotates and the splice strikes the piece (for a few turns). Also if you turn the machine conveyor speed up and if you have marks from the belt splice those marks will get wider apart. Slow it down, they get closer.

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