If you look at some of the American how-to videos referenced on this site you'll be instructed to sharpen the blades with an angle grinder without removing them from the mower - but that is for one-piece, mower-width blades. As far as I am aware it is normal to sharpen swing-back blades, either on a grinder or with a file, but it is important to balance the cutter disk including the blades when you finish sharpening. You can use a jig for this - I believe there is at least one type in the on-line store. I prefer not to remove the cutter disk if I can help it, so I remove the blades, sharpen them, then equalize their weights by grinding a bit more off the heavy one(s). You have to use some sense, and experience, when doing this: for basic safety you must end up with blades that have structural integrity at high rpm and under impact conditions. When you find you've ground them back a significant amount or they have any kind of crack or weakness, it is time to replace them. Of course you have to do this as a set: resharpened blades weigh less than new blades, and you must have accurate balance so they all have to be the same. Remember to weigh each blade complete with its entire set of mounting hardware, and keep them together as a set.
Last edited by grumpy; 01/01/10 05:35 AM.
