Tapping screws type 1 and 2, and trilobular thread-forming screws — point, use and how to choose

A tapping screw forms its own mating thread as it is driven. A machine screw needs a tapped hole or a nut; a tapping screw goes into a pilot hole or thin sheet and makes the thread itself by displacing material or partly cutting it, which removes tapping and nuts from the assembly line.

The most common mix-up on the shop floor is ordering type 1 when type 2 was needed, or the reverse. Head and size look identical on a drawing, but the point and thread differ, and so does the material they suit. Trilobular thread-forming screws, often called by the trade name TAPTITE, belong broadly to the same family but differ in cross-section and in how they form the thread.

Type 1, type 2 and trilobular screws at a glance

Type 1 and type 2 are classes of the KS B 1032 annex. The main body of KS B 1032 follows ISO, with ST2.9, ST3.5, ST4.2 designations and different dimensions; type 1, type 2 and M3/M4 sizes sit in the annex (the former main body). Most tapping screws made in Korea follow the annex, and our type 1 and type 2 follow the annex. If your drawing specifies ST sizes or an overseas type (Type A, B etc.), dimensions and thread form may differ, so tell us when requesting a quote.

Type 1 has a sharp point and starts easily. It is sometimes used without a pilot hole in thin sheet metal, wood or soft plastic, but that does not mean it can go into steel sheet without one. To skip hole-making in metal, consider a dedicated product such as a self-drilling screw suited to the material and thickness.

Type 2 has a blunt point, so it relies on an existing hole. The closer thread grips well in thicker material, and the blunt point is safer where it protrudes through the part.

Type 2 slotted and unslotted

A type 2 screw with a small slot at the point is called slotted (cutting); one without is unslotted. The slot cuts part of the material and lowers driving torque; the unslotted type mainly forms the thread by deforming the material.

All 196 type 2 items in our catalogue are slotted — shown as "Type 2 (cutting)" in the Point row of each product page. Unslotted type 2 is made to order.

How do general tapping screws and trilobular screws differ?

A trilobular thread-forming screw is, broadly, a kind of tapping screw, but it differs from the round type 1 and type 2 in shape and in how it forms the mating thread. Its rounded-triangular (trilobular) cross-section lets the lobes plastically deform the material to form the thread. In Korea the product family is customarily called "TAPTITE", but TAPTITE® is a registered trademark managed by REMINC/CONTI. Because it forms rather than cuts, it produces few chips and tends to resist loosening, depending on conditions; actual performance depends on the mating part, pilot hole and tightening conditions.

• Type B — pitch and use similar to type 2 tapping screws. Aluminium die-casting, non-ferrous metals, electrical parts sensitive to loosening.
• Type C — same pitch as a metric machine screw. If the product is designed for metric compatibility, a standard machine screw of the same size can be used in the thread formed at first assembly. Actual compatibility and retention of clamp force after repeated assembly must be confirmed by the product specification and testing. Used in electrical housings and battery packs where chips are not acceptable.

We stock case-hardened carbon steel trilobular thread-forming screws in M3 and M4, both type B and type C (pan head; zinc, coloured or nickel plated). The last letter of the part number, B or C, shows the type.

Material decides the base it suits

Case hardening gives carbon steel tapping screws a hard skin, so their own thread survives while forming one in steel sheet. Heat treatment is shown per part number in the "Heat treatment" row of each product page.

Austenitic stainless steel cannot be hardened by quenching. It is the choice where rust is not acceptable, but in thick or hard mating material its thread may fail first, so it should be confirmed against the actual part.

Why we don't publish a pilot-hole table

The question we hear most is "what pilot hole diameter should I use?" We deliberately do not give a single table. For the same M4 screw the right hole depends on whether the part is cold-rolled sheet or aluminium die-casting, on its thickness, and on whether the hole is punched or drilled. Without knowing the mating material and hole condition, one number presented as the answer does more harm than good.

The reliable way is a trial on the actual part. A hole that is too small can push thread-forming torque too high; one that is too large leaves too little engagement and stripping resistance, so the screw may spin or strip. A stripped thread is therefore not by itself a reason to enlarge the hole. Do not just check that the screw goes in; look at:

• Thread-forming torque — the torque needed to form the mating thread
• Tightening torque — the torque used in assembly
• Stripping torque — the limit at which the mating thread fails
• where relevant, behaviour on re-assembly and whether chips or burrs form

Test items and acceptance criteria are set to suit the application and customer requirements. Before production, request samples through Quote & samples and test them.

How to choose

1. Base material — consider type 1 first for relatively thin sheet, and type 2 for sheet with pre-made holes or non-ferrous metal. Material, thickness, hardness and the required clamp force all affect the choice, so do not decide by material alone; wood and plastic may be better served by dedicated screw forms.
2. Pilot hole — for general tapping screws, first provide a pilot hole suited to the mating material. Type 1 starts easily with its sharp point, but that does not mean it can be used in steel sheet without a hole; to skip hole-making, consider a dedicated product.
3. Chips — where chip control matters (electrical, battery), consider trilobular thread-forming screws first, as they can produce fewer chips than cutting types. Burrs at the pilot hole, flaking of the finish and damage to the mating part can still create particles, so confirm by a trial assembly.
4. Disassembly and re-assembly — if the design needs a standard metric screw to replace the screw after first assembly, review the compatibility of trilobular type C. Confirm damage to the formed thread and clamp-force retention over repeated assembly by actual testing.
5. Corrosion and mating strength — review corrosion resistance for the service environment (moisture, salt, etc.) together with the hardness of the mating part, and choose between stainless and case-hardened plated carbon steel. Where needed, confirm by trial assembly and corrosion testing.
6. Confirm by trial — pilot hole and driving torque are set on the actual part.

For driver bits see cross-recess driver bits; for thread engagement see engagement depth.

Tapping screws in our catalogue

Every item has a 3D model and drawing, so fit and engagement length can be checked at the design stage. Type 1 and type 2 follow the KS B 1032 annex.

TAPTITE® is a registered trademark managed by REMINC (Research Engineering & Manufacturing Inc.)/CONTI. Our trilobular thread-forming screws are our own manufacture and are not genuine or licensed TAPTITE® products.