There's a really good
wiki-how that sums up what you need to know about sharpening your
chainsaw with a hand file.
It seems the experts suggest
a machine grinding at a shop after every 5 hand sharpening episodes.
You can buy small attachments
for a Dremel to make the job easier, but these little hand files are a
lot cheaper.
Machine sharpening of the blade seems to harden the metal of the blade to the point that I can never seem to be able to truely sharpen it again with a file.
Extra Tip: If you can't seem to keep your chain tight, try flipping your bar over. You should really rotate this regularly to even out wear.
That would explain a lot --- it does seem like sometimes sharpening a chain with a file does little to nothing. I'd just assumed that was when the chain was really worn out.
On the other hand, Mark thinks that some machine sharpening might be better on the chain than others --- the most recent time we got our chains sharpened, they were still very sharpenable with the file afterwards.