Processing panels with an automatic wide belt sander is a great way to prepare them for finishing, whether you want to smooth off their surface or even fine-tune their dimensions. However, when unsightly blemishes such as “chatter marks” occur on panels, some forensic work is required to discover just what the culprit is in your machine setup.
Where to start?
In order to highlight sanding defects, chalking a sanded panel is an excellent procedure to use. It will readily magnify such sander defects as machine chatter, belt splice marks, streaks and hesitation marks.
The procedure can be accomplished in three straight foreword steps. Start by grabbing an MDF panel and rip it into 7 mm wide strips. MDF is the preferred test board material due to its overall tight thickness tolerance.
Next, mark the trailing end of three strips “MS” (motor side); “C” (centre); and “OS” (operator side). Finally, start your sanding machine.
With all heads running, feed the boards into the machine (one on the operator side, one down the centre and one down the motor side), but with the leading ends all in line. Once the trailing end of the boards reaches the beginning of the machine, press the emergency stop for the machine. This will stop the conveyor and heads immediately — it should not damage the machine or the abrasive belts. When safe to continue, open the machine to allow the boards to be removed from the machine.
A very useful tool
At this point the boards now become a very useful tool that will indicate quite a few things about how the sanding machine it actually operating. For example, the boards will show how much material each head is actually removing and show the operators an example of the scratch marks, streaks, chatter marks or belt splice marks from each head — and which head assembly provided the marks.
The boards will also show if each sanding head is cutting evenly across the width.
Next, using a large crayon or chalk as a marking tool, position the marking tool flat on the test board/panel (use the long face not the tip), and using a large sweeping motion, colour the test board/panel. This procedure should be used on each sanded surface created by each sanding head.
You will now have a test board/boards that will give you two things. First is a sample of the sanded finish that is being created by each sanding head and second, a chalked surface for each sanding head that will magnify chatter, belt splices and streaks. This is so you can identify the sanding head that is the source of these problems.
Other solutions:
Check belt tension first – this is often the culprit. Insufficient tension allows the belt to vibrate and bounce, creating those repeating marks. Make sure you’re within the manufacturer’s specifications.
Examine your platen and contact shoe – wear, damage, or improper adjustment here will telegraph through to your workpiece. The platen should be flat and smooth, and the contact shoe should be properly positioned with even pressure distribution.
Verify feed speed and belt speed relationship – if your feed rate is too fast relative to belt speed, you can get chatter. Slowing the feed or increasing belt speed often helps. The ratio between these needs to be appropriate for your material and grit.
Inspect the sanding belt itself – worn belts, improper joint alignment, or belts that aren’t tracking properly will cause repeating marks. Make sure the belt is seated correctly and the splice is good.
Look at the backup pad or drum – any out-of-round condition, bearing wear, or buildup on the drum will create periodic defects. Clean the drums and check for worn bearings.
Consider your abrasive grit progression – sometimes chatter marks from earlier grits become more visible rather than being removed. Make sure you’re not skipping grits and that each pass is actually removing the previous marks.
