Re: Grinding a scrub blade
Russell Adkins
>There is a neat little trick for controlling the grinding angle on scrub plane and similarly cambered blades that does not require any purpose made hardware. Just snap a circular rare earth magnet onto the bevel side of the blade so that its center is coincident with the center of curvature of the cutting edge. So, if your blade has a 3-inch radius, place the center of the magnet 3 inches from the cutting edge and laterally in the middle. Place one of those cups offered with the magnets over the magnet as it will greatly increase the holding force to the blade and reduce unwanted attraction to other metallic objects. Nonskid pads are also offered for the magnets but I have not found these necessary for this application.
The side of the magnet now provides the perfect surface to rotate the blade around while grinding the edge. Some method of controlling the distance from the edge of the magnet to the grinding surface is needed for adjusting the bevel angle. For the Tormek grinder all that is needed is the standard universal support (the bar upon which the other jigs are mounted). Just lay the blade across the support, slide it up until the magnet grabs the bar and rotate the blade about the support until the bevel contacts the stone. Use the Tormek bevel adjusting tool (the Pro AngleMaster) and adjust the height of the support until the desired bevel angle is achieved. This procedure is exactly as described in the Tormek instruction book.
Keep finger pressure near the cutting edge while grinding and rotate about the magnet as necessary, periodically monitoring the developing edge. The magnet conveniently keeps itself and the blade in correct contact with the support bar unless undue sideways pressure is applied.
The actual size of the magnet is not critical as long as its radius is considerably less than the radius of curvature of the cutting edge. For a Stanley No. 40 scrub plane I use a 1-inch diameter magnet with a 1-1/8-inch diameter cup. For a 3-inch radius cutting edge this provides 2-7/16-inches of blade length between the magnet and the cutting edge, which is plenty long to reach from the back of the support bar to the stone. This distance, whether too short to allow a proper bevel or too long (longer than the blade), is the fundamental limitation of this technique.
I use a cheap plastic dial caliper to determine where to place the magnet on the blade, although a simple jig could be used for a given desired camber. When grinding an existing blade of unknown radius one can use the technique that Tormek describes in their manual. Blacken the bevel with a magic marker and try a sample magnet position. Rotate the stone by hand while rotating the blade about the magnet. Check where the bevel is being ground and make adjustments as necessary.
After grinding I create a secondary bevel by stroking the blade freehand on bench stones and then stropping on the Tormek leather wheel. If the magnet has not been removed the universal support may be set up for the leather wheel and the angle adjusted for the microbevel. This is also the standard resharpening procedure. One could set up a support bar for the bench stones that would use the magnet support, but I have found freehand honing satisfactory as only a few strokes are required. When the microbevel gets out of hand regrinding is in order.
One can imagine extensions to this method for elliptical or other non-circular cutting edges, but I have not tried these. This magnet trick provides a very repeatable grinding angle which will maximize the life of the blade.
