Troubleshooting Cross-Grain Scratches from a Wide-Belt Sander
If you do not get the marks when running a new belt, it proves that it is not something in the feed or the hold down system or you would get the marks regardless of belt age. I would guess that it is the belt lap joint and that you need to change suppliers.
From contributor M:
Contributor C hit the nail on the head. Check your belts. You have to be careful with a solid steel contact drum that you don't get overly aggressive with stock removal as they don't dissipate heat as well as a rubber contact drum. This could lead to premature belt wear and cause them to leave marks like you described if you are using a quality belt already.
From contributor R:
Make sure the platen is just below the drum when engaged. That is typically the cause of the problem. Most people don't realize a widebelt will leave ripples unless it has a platen. With the platen you can send parts right to finishing otherwise you need to random orbit sand to remove the ripples.
From contributor J:
One other thing is just make sure you have enough dust collection. Sometimes dust can build up on the lap joint and create problems.
The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).
Comment from contributor C:
For extreme visual purposes, imagine the spice being and eighth inch thicker than the rest of the belt. Every time that spice gets pinched between the contact drum and the wood, the contact drum embosses that spice into the wood and the result is a chatter mark.
Why doesn’t it show up with a new belt? With a new belt, the abrasive is sharp even at the splice. As the belt wears, the splice will load up faster. Just look at a used belt and you’ll be able to spot the spice immediately as there’s a heavy loading in that area. As the splice hits the wood, instead of having the consistent amount of removal the rest of the belt had, there’s the inconsistency due to the irregular amount of wear at the splice.
Here’s how to deal with it - the platen, and more importantly the platen backing. The platen offers a wide surface area to drag that spice out instead of embossing it at the crown of the contact drum. In addition, the backing of a platen is significantly softer than the hardness of the drum (steel, in your case.) As the splice comes between the platen and the wood, instead of being mashed into the wood, it absorbs into the platen. And instead of being in contact with the wood for an eighth inch or so, it’s dragged out for a couple inches. You mention you tried using just the platen alone. What is the backing of the platen? If it’s just a piece of hard felt, substitute that out for a piece of foam rubber (1/4”), then felt (1/8”) and then the graphite. The foam will do a great job to absorb the splice and the felt will help prevent the grain relief that generally occurs with too soft of a platen backing.
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