I recently made cove molding on my tablesaw. But after the job was done, I noticed a dark stain in the cast iron on the outfeed side of the cove cut. What caused this?

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Q:

I recently made cove molding on my tablesaw, cutting at an angle across the blade. But after the job was done, I noticed a dark stain in the cast iron on the outfeed side of the cove cut. What caused this? And how can I get rid of the stain?
—Michael Patterson, Athens, Ga.

A:

The same thing happened to the saw in our shop, Michael. We talked to several manufacturers about this, and none could help us pinpoint the exact cause. The best we can figure out, the culprit is the heat generated by making this trapped cut. The sawdust that isn't sucked away by the dust collector gets heated by the friction of cutting and dragged along in the groove by the board as you cut. This warm debris apparently causes a reaction with either the cast iron and/or any coatings applied to the top. This results in the dark stain you see above.

To remove the stain, hand-sand the top with 400- or 600-grit wet/dry sandpaper. (But don't use water!) Sand in straight lines front to back to minimize visible sanding scratches. You might not remove all the stain, but should get most of it, depending on how deep the stain has penetrated the cast iron.

To help prevent this from happening again, use a throat insert with a wider opening that will allow more debris to fall through. A zero-clearance insert, although providing tear-out protection, limits the amount of debris slipping through the slot. Also, before making cove cuts again, try cleaning the saw top with a solvent, such as mineral spirits, that evaporates quickly. This should remove any residue from previously applied surface lubricants or protectants. (When done with your cuts, clean the top and reapply those products as needed.)

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