Based on the material to be fastened and reusability criteria, Self-Tapping screws are classified into Thread cutting and Thread forming screws. Forming screws are used on softer materials and can provide a firm hold. Care should be taken for over fastening (often results in breaking of the material). Cutting screws are used for hard materials but are often less reusable after dismantling. Get self tapping screws free UK delivery from tappex.co.uk.