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I do my chain by both looks and feel. It's amazing how sensitive the human touch to surfaces and texture. An index finger touch to the cutter of a chain instantly tells when a chain needs sharpened. I will do four strokes with a file and guide. After these initial strokes, touch isn't much good in my opinion because now the leading edge's dull roundness is almost gone--this does not mean the cutter is sharp! If the leading edge of a cutter reflects light, that is to say, looking directly down onto the top of a cutter, a dull edge will appear shinier than the rest of the cutter. This is due to the leading edge being lower and slightly rounded down than the remainder of the cutter's angled top profile and is acting like a sled runner. This dull lower edge gets polished during the cut because it tends to ride over the wood. I file one cutter until it no longer reflects light, then I file the rest of the cutters using the same amount of strokes used to get the first one right. I will then do another visual and file all again if not up to par. Happiness is a good, sharp and clean file. Liberal doses of WD-40 on file during sharpening keeps the filings from piling in the gullets. WD-40 is used on the file after filing to keep it from corroding, too. I have several flat mill files new 25 years ago that are almost as good as new. A file card is a good tool, too. Note: the PC software used on this forum kicked out my original message when I tried to use a common term for a common type of file. The word is synonymous with "illigetimate offspring" (ba$tard).
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