Q: We have a wide-belt sander, but the thickness of the sanded stock varies within a piece. Also, the thickness readout does not agree with the actual thickness. I am new to this machine and so could use some suggestions about troubleshooting.

A: It is not uncommon to find that the readout that the machine provides for thickness is not accurate. One reason is that different belts have different thicknesses, so each setting must accommodate the belt thickness. Even devices with belt thicknesses stored in memory are often incorrect. Further, it is possible, especially in older machines, that the device was not properly calibrated in the first place. So, you need to recalibrate and maybe change a few internal settings. I do see a lot of people who use the present settings and then have a small correction chart alongside the machine, rather than spending time recalibrating.

Regarding thickness variation, there are several machine factors which can cause this. (I am assuming that the machine did run perfectly when it was first delivered, so we do not have a design or construction problem.) They include:

  • improper head alignment so things are not parallel when they are suppose to be;
  • a head that bends slightly, especially in the center, when cutting heavy loads (high stock removal, using the machine like a planer instead of a sander);
  • a head that is out of balance so when operating at full speed it bends;
  • a worn head so that it is no longer uniform in size and shape;
  • bearing run-out;
  • improperly aligned hold down mechanism;
  • worn bed plate (with a single-head sander especially) which often results because the full width of the sander was not used, but pieces were predominately sent through the center of the machine;
  • inadequate dust removal system so that dust accumulates on the lumber and prevents smooth surfacing; and
  • worn or loaded belts, especially a problem if the pieces are sent through the center of the machine most of the time.

Perhaps the first step is for you to measure the thickness of your sanded stock every half inch across the width. Take these measurements near both the entering and the tailing end of the piece. Measure ten pieces or so. Plot the results. Next, change the sanding belts; then, set the machine to the same settings as before, run some pieces through and then repeat the measurements.

If you are having rather severe problems, I suggest that you hire a consultant to analyze the machine and get it working correctly again. Then, put in a good employee training program on proper use of the machine (including feed speed, digital readouts, stock removal or depth of cut), care of the belts, and also put in a good maintenance program.

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