I have a Ryobi 8" dia. 5/8" arbor grinder rated for fixed 3600 RPM. Is there a way to convert this to a variable speed grinder? Looking to change to CBN grinder wheels and it appears the slower speed (1800 rpm) is the preferred speed for sharpening tools. Does the RPM make a major difference when sharpening plane irons and chisels other than to minimize burning? What difference does changing to a smaller diameter wheel make? Thanks