Ok ,i pulled a file jig out of the wrapper for a demo for the op ,this one is made by oregon ,they should be 10 bucks or less ,it rests on the cutter and the raker to hold the file for a good all purpose sharpen that is not too aggressive on the angles ,similar to stock
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Here is what the top of the cutter looks like after i swiped the cutter a few times ,notice the corner is real pointed ,not bent over like it hit something
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Here are the side angles it produces ,notice the laser etched mark stihl puts on the cutter ,the side profile lines up pretty close .
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Here is what happens if hold the file too low ,it makes more hook in the side ,it cuts like this ,but ends up very grabby and takes a saw with a lot of power to make it work right and a light hand ,the more hook will also dull quicker ,notice the laser etch is not curved like this .
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the main goal is to file away till you get the point back in the outer corner ,if hit a nail or the dirt or a rock ,it may require a lot of the cutter to be filed back ,sometimes it takes 1/3 of the cutter away to get to a good corner again which looks like that chain will need ,may be good to find someone with a grinder to save that chain ,going to be a while with a file to get it serviceable again .