Troubleshooting Wide-Belt Sander Chatter

      Track down the cause of chatter marks in the wood before trying to fix the problem. Defects in the belts, the drums, or the drive are the usual suspects. April 30, 2006

Question
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?

Forum Responses
(Solid Wood Machining Forum)
From contributor C:
You may want to consider getting Howard Grivna's manual on widebelts.



From contributor D:
Wide belt chatter can come from a few different things: 1) belt splice, either the type or its change (swelling due to wet cleaning). 2) slippage of the part due to improper hold down pressure or defective or dirty holddowns. 3) slippage in the conveyor drive unit.

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.



From contributor G:
I think you might also consider bearing runout and ineffective dust exhaust system. Does this occur with very light stock removal? High stock removal with a sander can cause problems.


From the original questioner:
We have been testing out new belts, some paper back and some cloth back and not reusing the washed belts and the chatter is decreasing. Thanks for all the info!


From contributor D:
Sounds like the belts are giving you the problem? 3m makes a resin backed belt designed to run wet or dry - no cloth to swell and with a splice that's actually thinner than the belt itself. They helped our problem with chatter. Later we stopped wet cleaning all belts and instead used abrasive belt cleaner sticks (large ones special made up) and cleaned the belts right on the machine. A little hairy but saved time, lowered the belt inventory and did not affect the splices. Be sure to let new belts hang a while before use. Chatter can be a real pain! Sometimes the drum type and its hardness can make chatter more likely.


From the original questioner:
Definitely going to be making a hanging rack for our belts.


From contributor D:
While the rack we made was tube metal and rod, we slipped over the rods 6" PVC sections. They are approximately the same diameter as the idle rollers in the machine. Rack was engineered to hold several belts and take up as little room as possible and on wheels. When we stopped washing and instead cleaned on machine, the rack spaces were seldom full (needed far less inventory). But was very useful for us while we washed belts to dry them on. Use a waterproof marker to write grit on inside of belt and each time washed, put a mark on inside to indicate. Helps find grit needed immediately, and gives history of use at a glance. Mark on the PVC tubes the grits to help further organize and keep like grits together.


From contributor M:
That's funny, I farmed out some wide belt sanding to a shop with a 36" SCMI and there were these horrible wide lines perpendicular to the grain which took me about an hour to sand out. How's that for a timesaver? The guy doing the sanding seemed to be taking tons of light passes and the lines were still apparent. As someone who plans to purchase a widebelt in the near future, this really freaks me out. A $16,000 sander that creates more work? That's not good for business.


From contributor D:
It's not the brand of sander but the setup, adjustment and proper function of the various components involved. These days, widebelts are pretty much perfected - all brands. And all of them, if not properly adjusted and having properly functioning components, are capable of giving what you saw. And yes, the lines can be tedious to deal with. The best cure is stop sanding on the wide belt, repair, replace, adjust, and coordinate its components - (including the type of belts being used and how they are cleaned), then run the parts back through. When it's properly adjusted, it will remove the lines quickly, and won't cause the problem anymore. There is a level of knowledge that must be reached to have any widebelt function properly. And the customer service of any brand is of utmost importance.


From the original questioner:
The only lines that I can think of that would go perpendicular to the grain would be chatter. This was the problem we had and after all the testing we have done, we have found out that it is belts, belts, belts! The machine is working great now and we are managing our belts better. Thanks for everyone's input.


From contributor M:
Did you end up going with a different brand of belt or did the cleaning process do the trick?


From the original questioner:
We are still using the same brand but we are only washing the belts a couple of times. We now mark our belts by numbering the times washed and we are also storing them better and are currently having a rack built to hang them.


From contributor L:
First, you can cut your belt costs in half if you use the right kind of belts and wash them. Forget paper back and cotton cloth back belts. They absorb moisture (washing or not), which is the No.1 cause of belt slap (chatter).

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.



The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

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.



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