When sharpening the blades for your mower, is it best to sharpen them on equipment that has high RPM's of low RPM's? Have been sharpening with a bench grinder. Just purcahsed a sharpener form lowes however, it only runs at 120 RPM, seems awful slow. also, should the wheel be soft or hard? The cost of some of the sharpening machines ($900 or more) is just way out of my price range. I am going to buy another blade balancer though. Have the little $5.00 cone job.