Many textbooks and vocational schools instruct mechanics to grind down the tip of the blade, which, due to the taper, increases its thickness and consequently allows more precise engagement with the slot in the screw. This approach creates a set of graduated slotted screwdrivers that fit a particular screw for a tighter engagement and reduce screw head deformation. However, many better-quality screwdriver blades are already induction-hardened (surface heat-treated), and tip grinding after manufacture compromises their durability. Thus, it is best to select a tip made to fit precisely to begin with, and avoid weakening the factory heat-treatment.