Last year I bought a 20 year old Butferring 52" combi head sander. It’s a really fine 2-ton machine and overall very reliable. This is pre-computer programmable, so not a lot of fuss. However, frequently I get cross-grain marks similar to planer marks, except they're harder to sand out than planer marks would be. I can run the same board with each of the three settings (calibration with steel contact head only, graphite platen only, or combi mode) and I still get the marks. It seems the only time I don't get them is with a brand new sanding belt. The head is solid steel with spiral serrations/relief grooves. Any diagnostic thoughts would be appreciated.
(Solid Wood Machining Forum)
From contributor C:
I am not a wide belt man, but you have answered your own question. If you get the same marks regardless of which head you use (apparently a 3 head machine), it is not one of the heads because contact rolls and platens are entirely different.
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.
Comment from contributor C:
I’ve been repairing widebelt sanders for over 18 years and your problem is the most common problem I encounter. Your chatter marks are coming from the abrasive belt. As you mentioned, they disappear with a new belt. Some abrasive belts are better than others, but they all have a splice and the area of the splice is never the same thickness as the rest of the belt.
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.