Diehl Machinery (straight line ripsaws, or in this case a lumber jointer)
use a "chain" to feed and control the stock as it travels past the blade
or cutterhead.  The chain must have a face consisting of sharp points to 
keep the stock from shifting. Diehl manual states that when the points are 
reduced to flats of .025" square, straight line ripping is no longer 
predictable.  When the flats increase to .040" square, Diehl states the 
chain is worn out.  This is a photo essay on sharpening a Diehl 660 lumber 
jointer chain by means of purpose ground tooling on a heavy metal shaper.