We have a 54" SCM 2 head wide belt sander that is creating chatter marks on all our panels. We have tried all kinds of different paper and have even recently replaced the second head. We have adjusted speeds also. Is there anywhere else I should look or adjust?
(Solid Wood Machining Forum)
From contributor C:
You may want to consider getting Howard Grivna's manual on widebelts.
What you want to do is look for consistency vs. inconsistency. Example: a belt splice chatter will give consistent, equally spaced marks and you may take a crayon and color the splice heavy - then run a piece through and see if the chatter marks are now highlighted with crayon or not. Also, if it is the belt splice, the marks will widen in distance if you speed the feed up or come closer if you slow the feed speed down. Hold down slippage will usually give inconsistent chatter marks and will usually concentrate themselves on the front and trailing ends of piece being sanded. Drive slippage, which can be very consistent due to a broken tooth on a cog or mechanical drive, will usually be wide spaced, in unison with the turning of the drive.
Probably the worst thing to do is start spending big money replacing parts hoping to fix it. This chatter is most often developed at the drum (which by the way can also give consistent marks due to a damage or defect in the drum - but is rare). The crayon will show the difference here. Sometimes this chatter developed at the drum can be eliminated with the platen, simply sanding out the chatter at that trailing stage. First!, learn what is generating the problem, then specifically address that adjustment, repair, cleaning or change in belt.
Second, don't use skive joint belts; use butt joint belts with tape backing - they run smoother. Just don't flex the joint when storing them - they are plenty strong.
Third, use closed coat resin bond aluminum oxide "film back" plastic backing on belts of 100 grit and finer. If you use coarser grits, heat generated may melt the film backing. Silicon carbide is crystalline and retains a sharp cutting edge, however, it is not as durable as aluminum oxide. These belts were 10% cheaper than cotton cloth belts.
Fourth, on coarser grits, use zirconium alumina grit belts with polyester cloth backing. The coarser the grit, the heaver the backing, with butt joint tape back splicing. Zirconium alumina is crystalline zircon in an aluminum oxide matrix and is almost bulletproof.
Abrasive Beltmasters in Colorado makes a cleaning solution you mix with water that works great cleaning these belts and the water doesn't affect the joint - no belt chatter! I was using a Sandingmaster 2 belt 52x75 sander. Proper adjustment of the sander is necessary, however, the correct belts are paramount, otherwise you'll always be fighting it. Zirconium alumina belts are available through Norton under the name "Norzon." The film back belts are available through 3M. For any given grit size, aluminum oxide does not leave quite as much of a scratch pattern as silicon carbide, and it is more durable.
Comment from contributor H:
Chatter mark complaints are probably the most common complaint regarding widebelt sanders. Most often it is caused from the splice of the abrasive belt being a little (it doesn't take much) thicker or thinner than the rest of the belt. For example, the belt is going around and sanding .005" inch and the splice tries to sand .006" inch, the result is chatter. Rarely is there a manufacturer with a perfect splice (consistently anyway). The key is to try to eliminate or minimize the splice mark causing chatter.
Harder contact drums maximize the splice because the surface area of contacting the wood is small and the hard rubber "pounds" the splice mark into the wood. Softer drums don't "pound" the splice into the wood so much (a little absorption of the splice into the rubber drum happens.)
Platens are on machines to eliminate splice-mark chatter. First, the platen provides a longer path for the splice to contact the wood meaning instead of hitting the wood and then leaving it, the splice is dragged a bit which helps hide the variation of the belt's thickness at the splice. Second, the platen backing (behind the graphite) is soft, which lets the splice (and abrasive mineral also) be absorbed into the platen rather than pushed into the wood.
The rule of thumb for the amount of platen pressure is to feel the wood exiting the sander. If it is lukewarm from the platen, it should be fine. Too much pressure will burnish the wood causing stain to not absorb. Also, too much pressure is "pre-compressing" the platen backing which minimizes the amount of absorption of the belt splice.
Although belt splice causes the most amount of chatter problems, there are other sources as well. The drum could have a flat spot on it or there could be a balance issue with the drum, brake disk and/or top idler roller.