Self-tapping and thread-forming screws are widely used to join parts without pre-tapped holes. These fasteners are categorized by their point and thread geometry, which determines how they engage the material (either cutting/chipping or displacing material). Common types include Type A (coarse thread, pointed), Type B (fine thread, blunt), Type AB (combination of A and B), and Type 25/BT (chip-cutting point for plastics), as well as advanced thread-forming designs like Taptite, Plastite, Hi-Lo (double-lead), and EJOT PT® screws. This report explains how each type works, compares their performance (torque-to-strip ratios, thread engagement, cold-working effects), and gives pilot‐hole and material‐selection guidelines for procurement and design.

Chip-Forming vs. Chipless Engagement

Self-tapping screws form threads in the workpiece via two fundamental mechanisms:

• Chip-forming (thread-cutting) screws have cutting flutes or notches that remove material (chips) as the screw is driven. This produces clear thread cavities with relatively low insertion torque. Examples include Type 23 (Type T) and Type 25 (BT) points, and specialty “Taptite®” cutting variants. MonsterBolts explains that thread-cutting screws “have flutes cut into their threads that remove material as they drive, creating mating threads in wood, plastic, or metal”[1]. Because they cut material, they require looser pilot holes (typically ~75–85% of screw diameter) to allow chip clearance[2]. Chip-forming screws are useful when chips can be tolerated (e.g. through holes or ductile metals) or when very low drive torque is needed.
• Chipless (thread-forming or thread-rolling) screws plastically displace material rather than cutting it. They form internal threads by cold‐working the hole, which work-hardens the material and creates a tight-fit thread with no chips. Examples include Type A, B, AB (which have pointed tips and no cutting flutes), Taptite® and DUO-Taptite® (trilobular/tri-lobed thread-rolling screws), Plastite® (trilobular screws for plastic), Hi-Lo® (alternating high/low double threads), and PT®/Delta PT® As MonsterBolts states, “Thread-forming screws don’t cut material — they displace it. Their lobed thread forms mating threads in plastics or thin metals with minimal debris, creating a tight fit and high pull-out strength”[3]. Likewise, “thread-rolling screws (e.g., Taptite®-style trilobular screws) combine forming and rolling action to create machine-quality threads in untapped steel or aluminum. They reduce drive torque while improving resistance to loosening”[4]. Because chipless screws generate internal stresses, they often require slightly larger pilot holes (typically ~85–95% of the screw’s major diameter) to achieve the desired thread engagement[2].

Chip-forming screws typically have lower drive torque and produce chips, but they create weaker threads (strips more easily) than chipless screws. Chipless (forming/rolling) screws need higher insertion torque but “typically create the strongest reusable threads”, especially in metal[5]. In plastics, thread-formers also give stronger joints and self-locking action. In practice, chipless screws like Taptite and Plastite are preferred in high-volume metal assembly for their high strip-out resistance and built-in locking, while chip-forming screws (like Type 25/BT) may be used in soft plastics or brittle materials to minimize stress and cracking.

Tapping Screw Types (Types A, AB, B, BT)

Type A, B, and AB Screws (Sheet-Metal Tapping)

Type A screws have widely-spaced, coarse threads and a sharp gimlet tip. They can start in thin sheet metal or soft materials without a pilot hole. They are generally used in thin galvanized steel, aluminum, plywood or hardboard[6][7]. Type A screws “drill or nest a hole in thin sheet metal, resin plywood, composite boards, etc.”[8]. Because they displace material (chipless), they require higher drive torque than cutting screws, but they form fairly deep threads in thin material. Typical use is HVAC duct, enclosures, or fixtures with thin panels[7]. Type A screws often do not need a pilot hole in thin sheet; if used in thicker or harder metal, a pilot of ~70–80% diameter may be drilled[9].

Type B screws have finer threads and a blunter tip. They are used in heavier or non-ferrous sheet metal, plastics, and hardboards[10][11]. Because of the blunt tip, a small pilot hole is usually required (especially in steel). Type B has no cutting flutes, so it also forms threads by displacement. In practice, fine threads give greater engagement in thicker metal, but drive torque is higher than Type A. For example, fastener guides note that Type B is typically used with medium to heavy gauge metal and “requires pilot hole”[11][12].

Type AB is a hybrid: it has the pointed tip of Type A but the fine thread pitch of Type B[13]. This makes Type AB very versatile for sheet metal: it taps easily (like A) yet has good thread engagement (like B). Type AB is a thread-forming (chipless) screw commonly used in thin-to-medium sheet steel, aluminum, and composite panels[13][7]. It is often used in place of either A or B when a range of panel thicknesses or materials must be fastened. Because of its fine threads, Type AB holds better than Type A and can be used in slightly thicker gauges (with a pilot if needed). For plastics, Type AB is sometimes specified (with thread cutting considered for brittle plastics)[14].

Type 25/BT (Thread-Cutting for Plastics)

Type 25 (also called Type BT) has coarse threads and a special cutting point with a chip-clearing flute. These screws are specifically designed for plastics and other soft materials[15][14]. Unlike Type A/B, Type BT cuts the mating thread as it is driven, evacuating chips. McMaster notes that Type BT “cuts threads into drilled holes as they’re turned, so they require less driving torque and cause less stress on material than thread-forming screws. … Also known as Type BT (Type 25), they have coarse threads that grip soft plastic”[15]. In practice, one drills a clearance hole in the plastic or light alloy, and the Type BT screw cuts its own thread. These screws offer very low installation torque (good for brittle plastics) at the expense of producing chips and somewhat lower pull-out strength. Typical failure mode in plastic is thread stripping or cracking if over-torqued. These are the old-fashioned “self-tapping plastic screw” (e.g. Polydrive) used in enclosures.

Summary of Tapping Screws

• Chipless (Type A, AB, B): Form threads by displacement. Good for sheet metal and ductile material. Higher drive torque, but no chips, and generate “cold-formed” threads that are strong. If used correctly they avoid cracks, but overtightening can split thin sheet or strip threads. Pilot holes are optional in thin gauge (Type A/AB) but recommended (~70–80% dia) in thicker metal[9].
• Chip-forming (Type BT): Cuts threads in plastic or soft metal. Requires predrilled hole, but insertion torque is low and it avoids bursting thin plastic bosses. Chips must be accommodated. Pilot holes are relatively larger (to allow chip clearance).

Thread-Forming/Thread-Rolling Screw Types

Beyond the generic tapping screws, specialized thread-forming screws have profiles engineered for high-performance joints. These are all chipless designs (they displace material):

• Taptite® (trilobular/thread-rolling) screws – A family of tri-lobed thread-forming screws (from REMINC/Stanley/PEM). The Taptite shank is slightly triangular, with three lobes. As WCL explains, “The unique ‘trilobular’ shape of the Taptite screw roll forms a mating thread in an unthreaded pilot hole. The threads are work hardened as they are formed, providing a strong joint or assembly. By elastic recovery, the metal fills in behind the triangular lobes to provide maximum contact and a secure assembly. The tri-lobular shape … assures low driving torque, generates a prevailing torque that exceeds the level of locking screw; an interference fit provides resistance to vibrational loosening.”[16]. In short, Taptite screws form high-quality internal threads in steel or aluminum by cold-rolling, work-hardening the mating threads and yielding very high strip-out torque. They drive easier than a solid round screw of equal size, yet develop a strong clamping tension. DuO-Taptite (four-lobe) further improves locking. Tests show very high drive-to-strip ratios and built-in locking torque: “The higher, more uniform, strip-to-drive torque ratio of DUO-TAPTITE® screws provides a built-in safety factor against over-driving. Eliminates broken screws, damaged mating threads and inferior fastenings.”[17]. In practice, Taptite (and variants like Taptite II, Taptite 2000) are used for medium-to-heavy steel and aluminum bosses (cast or sheet). They always require a drilled pilot hole (no flute). Pilot-hole diameters are chosen to achieve roughly 65–75% thread engagement for best shear strength[18]. (Typical guidance is ~90–95% of screw major dia. in steel, slightly larger in aluminum[19][2].) Over-torquing usually causes screw breakage rather than thread stripping, since the screw is harder than the parent metal. Taptite screws also produce a substantial residual “prevailing” torque (self-locking), reducing vibration loosening.
• Plastite® screws (Trilobular for plastic) – These are tri-lobed thread-forming screws specifically for thermoplastics. Like Taptite, Plastite has a three-lobed profile, but the thread geometry is optimized for plastic bosses. The Plastite 45° variant has very wide thread pitch and a narrow thread angle to minimize hoop stress in the plastic[20]. Semblex notes that Plastite threads “roll form high-quality internal threads with no damage to the molecular structure of the plastic, significantly reducing the danger of material failure.” The wide lobes allow plastic to cold-flow and fill the spaces between, giving exceptionally high pull-out resistance and vibration resistance[21]. Key features of Plastite include very high drive-to-strip torque ratio (so strip-out of plastic is rare) and a built-in locking effect. Semblex states, “The unusually high drive-to-strip ratio of PLASTITE® screws permits a wider span of torque settings… Strip-out is virtually eliminated.”[22]. In design, if over-torqued the plastic boss will strip (not the screw), typically in a cylindrical burst pattern. The wide thread spacing in Plastite allows more plastic to remain between threads, strengthening the boss against bursting[20][23]. In summary, Plastite screws are the workhorse for engineering plastics: they displace plastic (no chips), require moderate drive torque, and give extremely high strip-out resistance with self-locking.
• Hi-Lo® screws (double-lead thread-formers) – Hi-Lo screws (Lisi fasteners) have two threads on the shank: alternating high and low threads. The high threads are sharper (30° included angle) and the low threads are shorter (only ~40–50% height at 60° angle)[24]. This innovative profile means the high threads “cut” deeply into the material (for strong shear engagement), while the low threads displace less material. The result is greatly reduced radial stress and increased pull-out strength. WCL describes that “The 30° included angle of the high thread form reduces radial or bursting pressure to one-half of that generated by the conventional 60° thread form. Boss cracking in plastic is dramatically reduced”[25]. Because Hi-Lo has a smaller minor diameter, more base material remains between threads. This increases axial shear area, so Hi-Lo screws have much higher pull-out force than a conventional screw of the same size[26]. Importantly, the 30° high-threads displace less material, so drive torque is lower; meanwhile, with more material left between threads, strip-out torque is higher. In effect “Low driving torque and high stripping torque provide maximum protection against stripping problems”[27]. Hi-Lo screws are often used in tough plastics, particle board, or wood composites (and even in some light metal). They typically require pre-drilled clearance holes. In summary: double-lead Hi-Lo geometry yields very deep thread engagement with low insertion force and excellent pull-out/strip safety.
• EJOT PT® / Delta PT® screws – EJOT’s PT screws (and their licensed versions like Delta PT) are specialized self-tapping screws for plastics. They have a steep thread angle (~30°) and profiled root, specifically to reduce radial stress in thermoplastics. Apex Fasteners explains: “The PT® screw features a 30° thread angle which reduces radial stresses, provides increased thread depth, lowers installation torque, and improves material flow due to high axial component.”[28]. The deep thread profile balances load between the screw and plastic, and resists relaxation of the clamped joint. PT screws displace plastic (no cutting edges) and are essentially self-locking by design. They are ideal for direct fastening into rigid or ductile plastics without inserts. Pilot holes are required and sized for the particular plastic grade; EJOT recommends ~85–90% of diameter depending on plastic hardness[2].

Torque-Strip Behavior and Cold-Working Effects

The insertion (drive) torque vs. strip-out (failure) torque is a key metric for screw performance. Chip-forming screws typically have lower insertion torque but also lower strip torque. Chipless, thread-forming screws often exhibit a high drive-to-strip torque ratio, meaning they have a wide safety margin before threads strip. For instance, tests show DUO-Taptite has a much higher strip torque than a comparable straight-thread screw[17]. Similarly, Plastite’s drive-to-strip ratio is very high[22], so it’s forgiving of over-torque. The tri-lobed designs (Taptite, Plastite) inherently generate a prevailing torque (like a lock screw): WCL notes Taptite’s lobes “generate a prevailing torque that exceeds the level of [ANSI] locking screws”[16], due to elastic recovery of the material filling behind the lobes. In contrast, conventional Type A/B screws have no such locking feature and rely solely on friction for locking.

Cold-working (work-hardening) is another effect of thread-forming screws. As the screw displaces material, the mating threads in the part become cold-worked and stronger than if they were cut. WCL explicitly says Taptite “threads are work hardened as they are formed, providing a strong joint”[16]. This increases the strength of the joint – typically the screw yields or breaks before the formed threads strip. In plastics, the relationship is reversed: the metal screw is much harder than the plastic, so plastic strip-out is the limiting failure. Semblex notes for Plastite that “hardened screws are significantly stronger than plastic, so if the assembly becomes over-torqued, usually the plastic will give way and strip out”[23]. In designing plastic joints, wider thread pitch (as in Plastite 45) spreads stress and leaves more material between lobes, reducing bursting pressure[20].

In summary, thread-forming (chipless) screws generally have: – Higher strip-out torque (built-in safety factor)[17], – Prevailing torque (locking) due to interference fit[16], – Work-hardened threads in the material[16], – and require higher drive torque than cutting screws.

By contrast, thread-cutting screws have easier insertion (low drive torque) but produce weaker threads and no self-locking (so lower strip torque). MonsterBolts’ FAQ summarizes: “Thread-rolling screws typically create the strongest reusable threads… followed by thread-forming in plastic, then thread-cutting”[5].

Pilot Hole Recommendations

Proper pilot (clearance) hole sizing is critical for consistent performance. Recommended hole sizes vary by screw type and material. In general:

– Thread-cutting screws (Type 23/25/BT) need smaller percentage holes (~75–85% of screw diameter) to allow chip clearance[2].
– Thread-forming screws (PT, Plastite, Hi-Lo in plastic) often use intermediate holes (~85–90%) suited to the plastic’s flexibility[2].
– Thread-rolling screws (Taptite in steel/aluminum) require the largest fractional hole (tight fit). MonsterBolts notes ~90–95% of diameter for Taptite in metal[2]. (REMINC data suggests aiming for ~65–75% thread engagement – roughly 90–95% clearance in steel)[18][2]. WCL’s Taptite table, for example, shows steel pilots about 80–90% of major dia for sizes 4–1/4″[29][30], and aluminum holes slightly larger (to account for extrusion effects).

More specifically, aircraft and fastener catalogs recommend:

– Type A/AB screws often need no pilot in thin gauge (they cut their own entry). If used in thicker or hard steel, a pilot roughly 70–75% of the screw diameter is typical[9]. For example, Fastener charts suggest a #4 Type A hole of ~0.086″[31] (about 75% of #4 major ~0.112″).
– Type B screws always use a pilot hole (~70–75% dia).
– Type BT (Type 25) in plastic use clearance holes sized per plastic (usually quite snug for rigidity). Often manufacturers specify holes slightly below major dia so the plastic is deformed into thread.
– PT and Plastite screws: EJOT and REMINC data indicate pilot diameters ~85–90% of major diameter, depending on plastic hardness.
– Taptite screws: a drilled hole is always used. For steel, WCL’s guidelines (above) generally give pilots around 0.75–0.85×diameter, achieving roughly 65–75% thread engagement[18][2]. In aluminum bosses, a slightly larger drill (with extruding) can be used to extend engagement length.
– Hi-Lo screws: Preformed holes are required. Because Hi-Lo displaces less material, its pilot can be fairly close to the nominal thread size (often ~90% dia). “Hi-low screws have a reduced minor diameter and therefore displace less plastic material”[32], so small holes are acceptable. (Exact pilot tables depend on manufacturer.)

As a rule, it is best to consult supplier data sheets for each screw. In all cases, tests should be done to verify correct hole size for sufficient thread engagement without excessive stress.

Comparative Properties Table

Screw Type / Point	Thread Profile / Action	Typical Materials & Uses	Drive/Stripping & Locking	Thread Engagement	Typical Failure Modes
Type A (Gimlet point)	Coarse triangular thread; sharp point; thread-forming (no flutes)	Thin metal, plywood, composites, softboard[8][7]	Moderate insertion torque; low self-locking. Thread forms by displacement.	In thin sheets, can engage through thickness; typical local engagement ≈ sheet thickness.	Splitting of thin sheet or cracking brittle materials if overdriven; thread strip in very hard material.
Type AB (Hybrid)	Fine thread (like Type B) with pointed tip (Type A)[13]; chipless forming	Thin-to-medium metal, composites, plastics; general-purpose sheet-metal	Similar torque to Type A; better pull-out than Type A. No flutes, forms threads.	Similar to Type B (deeper threads, more turns engaged) in thin material.	As with Type A – risk of strip or material creep if overtightened. Suitable for reuse.
Type B	Fine triangular thread; blunt/tapered point; chipless forming	Medium/heavy sheet metal, plastics[33][11]	Higher drive torque than A/AB; requires pilot hole.	Many threads (fine pitch) engaged; robust in thicker metal.	Cracking in brittle materials; thread strip if boss too thin or screw over-torqued.
Type BT (Type 25)	Coarse thread; cutting flute (chip-forming)[15]	Plastics, soft alloys[15][14]	Very low insertion torque (cuts threads); no prevailing torque. Chips must be cleared.	Limited by boss thickness (threads cut into plastic).	Plastic split or strip-out if overtightened; poor reuse (threads degrade).
Taptite® (tri-lobe thread-rolling)	Triangular trilobular cross-section; round crest; chipless rolling[16]	Unthreaded holes in steel, stainless, aluminum, zinc diecast. High-volume assemblies, vibration-sensitive joints.	Low driving torque (tri-lobe), high strip-out torque, strong prevailing torque (self-locking)[16]. Work-hardens threads.	Designed for ~65–75% engagement in steel (usually 2× diameter length)[18]. Very high shear strength.	Screw breakage (screw stronger than material); very rarely strips threads. Good vibration resistance.
Plastite® (tri-lobe for plastics)	Tri-lobed thread; wide pitch (single or double-lead)[34][35]	Engineering thermoplastics (nylon, ABS, PC, thermosets); plastic housings.	Moderate drive torque; extremely high strip-out torque[22]; inherent self-locking.	Deep engagement – large thread depth. Plastite 45: very deep and wide threads[20].	Plastic boss strip-out (boss failure) under extreme torque; boss bursting if poorly designed.
Hi-Lo® (double lead)	Two-start thread: alternating high (30° angle) and low (60° angle) threads[36]	Plastics, particle board, wood, masonite. Often in composite panels.	Low drive torque (shallow cut); high stripping torque[27]. No chips.	Equivalent to two fine threads; more shear area. Small minor dia means >material between threads[26].	Wood/plastic splitting dramatically reduced (small boss edge stresses)[25]. Screw pull-out under extreme load.
PT® / Delta PT	Single-start, high-angle (30°) thread with profiled root[28]; chipless	Rigid thermoplastics (PE, PC, PVC, Nylon, fiber-reinforced plastic).	Lower drive torque (steep thread), built-in self-locking.	Very high thread depth in plastic; pilot often ~85–90% diameter[2].	Plastic boss strip or screw cam-out if mis-sized hole or excess torque.
Thread-Cutting (Type 23/17/F)	(For reference) Cutting flutes, various pitches and angles.	Hard materials (Type 17 for cast iron; Type 23/BT for die-cast, brittle plastic)[37].	Low insertion torque (cutting action); no prevailing torque.	Machine-thread quality (since it cuts threads).	Lower strip torque; chip evacuation may cause imperfections.

Table: Comparison of self-tapping screw types[16][36][28][20].

Selection Guidelines (Material, Thickness, Assembly)

Choosing the right screw depends on material, thickness, and service conditions:

• Steel sheet (thin, ~24–16 ga): Use pointed, coarse threads (Type A or AB, or self-drilling Tek screws). Coarse threads bite well with minimal pilot. For example, fastener guides recommend Type A/AB for light gauge[7]. Type A/AB will form threads in 18–22ga without pilot (or use minimal pilot). Avoid Type B in very thin material as its fine threads may over-form or split the steel edge.
• Steel plate (thicker, >18 ga): Use fine-thread or thread-cutting screws (Type B, or Tek 2/3 for heavy gauge). Type B screws require a drilled hole (pilot) at ~75% diameter[12]. For very thick or high-strength steel, consider thread-rolling screws (Taptite/Taptite II) with adequate clearance holes, which give higher clamp loads and are easier to drive with power tools. Type 17/23 are used for structural steel as cutting screws.
• Aluminum and soft alloys: Coarse threads grip soft material better[38]. Type A or AB in aluminum, or thread-rolling screws, are common. For extruded or cast parts, tri-lobe screws (Taptite or newer Delta Series) can form stronger threads than cut screw. Avoid mixing dissimilar metals without proper plating to prevent galvanic corrosion (e.g. stainless screws in aluminum).
• Plastics: Generally use thread-forming screws to avoid cracking and to maximize thread engagement. EJOT PT/Delta PT are preferred for nylon and rigid plastics; Plastite screws are excellent for polycarbonate, ABS, or filled plastics. The Hi-Lo pattern is popular in hard plastics and composites (especially for large bosses or wood/plastic combinations). Only use Type 25/BT (cutting screw) in softer or brittle plastics where stress relief is needed (the coarse-cut thread minimizes boss pressure[14]). Always pilot-hole as specified – many plastics require precise hole sizing (over-sizing can cause premature strip-out).
• Assembly considerations: For high vibration or repeated use, use screws with locking/thread-forming features (e.g. Taptite, Plastite, hi-lo) which generate prevailing torque. For permanent joints, self-drilling or self-tapping screws with flutes can be used quickly, but they should be tightened to the correct torque to avoid stripping (which is more critical with cutting screws).
• Boss design: Ensure enough boss length (typically 2× screw diameter or more for thread-rolling) and diameter. REMINC guidelines suggest ~65–75% of the hole per thread engagement to reach full shear strength[18]. For thin bosses, consider using extruded holes or design inserts.

In summary, choose chipless, thread-forming screws (Type A/AB/B or trilobular designs) for strongest joints when rework or vibration is expected. Use chip-forming screws (Type 23/25/BT) when minimal insertion torque and chip clearance are priorities, or in brittle materials. The below table and discussions provide a quick guide for selection based on material and application:

• Steel (thin): Type A/AB or self-drilling (no pilot).
• Steel (thick): Type B or thread-rolling (pilot hole).
• Aluminum: Coarse Type A/AB, or Taptite (if high strength needed).
• Plastic (tough): Plastite 45/48-2, EJOT PT, Hi-Lo.
• Plastic (brittle): Type BT (Type 25) / thread-cutting.

All recommendations above should be validated with tests. Proper pilot hole selection and controlled torque are essential to achieve the expected performance.

Sources: Authoritative fastener guides and manufacturer data were used. Key references include fastener technical guides[16][17], engineering datasheets[22][36][28], and fastener supplier catalogs[6][7][2], which were cited inline for specific data and guidance.

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