Many grinders, but not all, require Self-Aligning Spherical Washers in order to run true enough for sharpening. Spherical washers are made to correct misalignment due to seating problems against the the shaft shoulder or an improperly machined shaft nut, both of which cause wheel wobble. The Spherical Washers act like a ball and socket joint to compensate for any slight misalignment. Sold as a set, with a pair of matching ball and socket washers, plus a flat washer. Made in the USA.