Self-Tapping Screw

Self-tapping screws are designed to create their own mating threads as they are driven into materials such as plastic, metal, aluminum, and sheet metal, eliminating the need for pre-tapped holes or separate nuts. They are broadly classified into two types: thread forming screws and thread cutting screws, each suited for different material properties and application requirements.

How They Work

Type	Mechanism	Ideal Applications	Key Characteristics
Thread Forming Screws	Displace material around the pilot hole, causing it to flow around the screw threads . No material is removed; instead, it is compressed to create a tight, zero-clearance fit.	Plastics, light sheet metal, non-ferrous castings	Higher driving torque required; excellent vibration resistance; often eliminates need for locking devices; can be reused if installed properly
Thread Cutting Screws	Remove material via cutting edges and chip cavities, creating a true machined thread in the substrate.	Sheet metal, aluminum, die castings, cast iron, brass, plastics, resin-impregnated plywood	Lower driving torque; creates chips during installation; ideal for materials where disruptive internal stresses are not desired; may strip if frequently removed

Thread Forming Screw Point Types

Type	Description	Typical Applications
Type A	Sharp, gimlet point with widely spaced threads; for thin sheet metal (0.015″–0.050″ thick). Not recommended for new design.	Thin sheet metal, resin-impregnated plywood, asbestos combinations
Type AB	Combines locating point of Type A with thread size and pitch of Type B; versatile thread-forming screw.	Light sheet metal, non-ferrous castings, plastics
Type B	Blunt point with finer threads; for heavier sheet metal (0.050″–0.200″ thick).	Heavy sheet metal, non-ferrous castings, plastics, impregnated plywood
PT Thread (Taptite Trilobular)	High-performance thread-rolling design with trilobular cross-section; creates clean internal threads without chips.	Plastics, die castings, high-stress applications
Plastite Thread	Optimized for plastic materials; reduces hoop stress and minimizes boss cracking.	Thermoplastics, engineered plastics
High-Low Thread	Alternating high and low thread heights; provides superior pullout resistance in plastics and particle board.	Plastics, particle board, composite materials
Type C / CA	Coarse or fine pitch machine screw thread with blunt tapered point (Type C) or gimlet point (Type CA). Eliminates chips; permits field replacement with standard screws.	Heavy sheet metal, die castings

Thread Cutting Screw Point Types

Type	Description	Typical Applications
Type 1	Single flute cutting edge; produces fine standard machine screw thread. Ideal for field replacement applications.	General purpose; field repair and maintenance
Type 17	Long, sharp point with fluted chip cavity; coarse tapping screw thread.	Wood, particle board, composite lumber
Type 23	Fine thread series with maximum thread cutting area and excellent chip clearing; minimum tightening torque.	Hard materials requiring precision threads
Type 25	Coarse Type B thread with cutting edges; large chip clearing capacity.	Plastics, soft materials
Type F	Machine screw thread with blunt tapered point; multiple cutting edges and chip cavities.	Heavy gauge sheet metal, die castings, cast iron, brass, plastic

Applicable Standards

Standard	Scope
ASME B18.6.4	Thread forming and thread cutting tapping screws – inch series
ASME B18.6.5M	Metric thread forming and thread cutting tapping screws – metric series (historical; dimensions remain widely used)
ISO/TC 2/SC 13	Fasteners with non-metric thread – includes thread-forming screws, tapping screws, drilling screws

Available Head Styles

Flat Head (with or without undercut) – countersunk for flush mounting
Pan Head – general purpose with wide bearing surface
Truss Head – extra-wide bearing surface for soft materials
Unslotted Hex Head – external drive for high torque
Slotted Hex Washer Head with Serrations – integrated washer with locking serrations

Materials

Steel with Zinc Plating – general purpose, moderate corrosion resistance
Steel with Black Oxide – mild corrosion protection, matte finish
18-8 (300 Series) Stainless Steel – Type 302, 304; good corrosion resistance
410 Stainless Steel – hardenable stainless; higher strength, suitable for drilling through harder materials

Surface Finishes