What are Self Tapping Threaded Inserts?

Self tapping threaded inserts, also known as self cutting thread inserts, are internally threaded metal bushes from Accu's Threaded Inserts range that cut their own mating thread as they are driven into a plain pilot hole in plastic, wood or non-ferrous metal, so the part needs no tap, no heat and no press. Each is a short cylinder with a metric internal thread and a larger external thread on its body, with cutting edges at the leading end. Some also carry a flange that seats on the surface of the part. Once fitted, the screw wears against metal rather than against the part, so the joint can be taken apart and refitted again and again.

Self cutting thread inserts suit parts that are already machined, moulded or printed, from aluminium enclosures and marine fittings to furniture frames, 3D printed housings and moulded electronics cases.

How Self Tapping Threaded Inserts Work

A self tapping threaded insert goes into a plain pilot hole drilled, moulded or printed to the Recommended Hole Size (HS). It is driven in square, and as it turns, the cutting edges at its leading end form a thread in the wall of the hole for the external thread to follow, locking the insert into the host along its length. That larger external diameter spreads the screw's load over far more material than a tapped thread of the same size, which is what gives a soft host its resistance to pull-out and stripping. The host material decides the type: Stainless Steel - (A2) Self Tapping inserts and Trisert-3 are rated for wood, plastic and non-ferrous metal, while brass Trisert and Thread Fix are rated for thermoplastics.

Take, as a working example, the die-cast aluminium housing of an outdoor CCTV camera, where M4 screws hold a rear cover that comes off at every maintenance visit. Tapped straight into the aluminium, the thread wears a little more each time the cover is refitted until a screw spins in a stripped hole and the housing is scrap. This is the job self tapping threaded inserts for aluminium are made for: a Stainless Steel - Marine (A4) Trisert-3, driven into a plain pilot hole after casting, gives those screws a stainless thread that copes with rain and salt-laden coastal air. The same logic holds for an M10 Stainless Steel - (A2) Self Tapping insert in a plywood equipment case or a brass Trisert in a moulded ABS enclosure.

Materials Self Tapping Threaded Inserts Are Made From

Each insert type is made from metals chosen to suit the host material and the service environment:

• Brass: Used for Trisert and Thread Fix, a free-cutting alloy (CW614N on Trisert) that forms a clean thread in thermoplastics.
• Stainless Steel - (A2): The material of the Self Tapping range from M3 to M20, pairing good general corrosion resistance with the strength to cut its own thread in wood, plastic and non-ferrous metal.
• Stainless Steel - (303): A free-machining austenitic grade used for Trisert-3, holding an accurate thread for general industrial use.
• Stainless Steel - Marine (A4): Molybdenum-bearing stainless steel for Trisert-3 inserts exposed to chlorides, salt spray and washdown chemicals.
• Steel: A strong, plated alternative to stainless for Trisert-3, relying on its zinc nickel plating rather than the base metal for corrosion resistance.
• Titanium - (Grade 5): Ti-6Al-4V, a high strength, corrosion resistant alloy at little more than half the density of steel, used for Trisert-3.

Four finishes are offered:

• Natural: The uncoated standard for every brass insert and for Stainless Steel - (A2).
• Clear Passivated: A chemical treatment on the stainless steel and titanium Trisert-3 inserts that supports their protective oxide layer.
• Matte Black: An option on Stainless Steel - (A2) Self Tapping inserts at every size from M3 to M20, giving visible fixings a low-reflectivity look.
• Zinc Nickel plated and Trivalent Passivated: Used on steel Trisert-3, where the zinc nickel protects sacrificially and the trivalent passivate seals it without hexavalent chromium.

Sizes & Types of Self Tapping Threaded Inserts Available

Accu's range spans a wide spread of metric sizes across four insert types:

• Insert Type: Self Tapping, Thread Fix, Trisert and Trisert-3.
• For Use With: Thermoplastics, or Wood, Plastic & Non-Ferrous Metal (Requires Pilot Hole).
• Thread Size: M2 (2 mm) to M20 (20 mm).
• Length (L): 4 mm to 27 mm.
• Outer Diameter (OD): 3.5 mm to 8.69 mm.
• External Diameter (OD): 5 mm to 26 mm.
• Recommended Hole Size (HS): 3.1 mm to 25.4 mm.
• Flange Diameter (FD): 4.7 mm to 12 mm.
• Minimum Wall Thickness: 1.35 mm to 3.35 mm.
• Minimum Boss Diameter (BD): 5.8 mm to 14.7 mm.

Not every combination exists. M10 to M20 is Stainless Steel - (A2) Self Tapping only, M8 adds a single brass Thread Fix, M3.5 is Thread Fix only and M2 and M2.5 come only as brass Trisert. Stainless Steel - (303), Stainless Steel - Marine (A4), Steel and Titanium - (Grade 5) are limited to flanged Trisert-3 from M3 to M6, and neither brass type is rated for wood or metal. Outer Diameter (OD), Flange Diameter (FD), Minimum Wall Thickness and Minimum Boss Diameter (BD) are listed for Trisert and Trisert-3, while Self Tapping and Thread Fix inserts give an External Diameter (OD) instead.

Where a cold-fitted insert is not the right answer, the alternatives sit alongside this range: Brass Threaded Inserts offer heat set and press-in options for thermoplastics, Threaded Screw Inserts bring every installation method together and Threaded Inserts For Wood cover furniture and board, while Rivet Nuts suit sheet and tube too thin to take any insert. Whichever insert fits, Machine Screws complete the joint.

FAQ

Q: What size hole does a self tapping threaded insert need?

A: Each insert lists its Recommended Hole Size (HS) in the tech spec, and the figure depends on the insert type as well as the thread size. At M3, Trisert and Trisert-3 need 4.1 mm to 4.4 mm, Thread Fix needs 4.5 mm and Self Tapping needs 4.6 mm to 4.8 mm. At M6 the figures are 8 mm to 8.4 mm, 8.6 mm and 8.9 mm to 9.4 mm respectively, while at M8 Thread Fix needs 10.6 mm and Self Tapping 10.9 mm to 11.4 mm. Where a range is given, trial the hole in the actual part; Accu's engineers also offer free technical support when the host material is unusual.

Q: Which metals do self tapping threaded inserts for metal suit?

A: Aluminium, brass and other non-ferrous metals, whether the part is a casting, an extrusion or a machined block. Of the four types, only Stainless Steel - (A2) Self Tapping inserts and Trisert-3 are rated for non-ferrous metal; brass Trisert and Thread Fix are for thermoplastics. Steel and cast iron fall outside the specification of every type, so for thin steel sheet or tube, Rivet Nuts are the usual route.

Q: How do I choose self tapping threaded inserts for plastic?

A: All four types can go into plastic, so the choice comes down to thread size and the part itself. Brass Trisert and Thread Fix are rated for thermoplastics specifically, and brass Trisert is the only type at M2 and M2.5 for small moulded and 3D printed parts. At the other end, the Stainless Steel - (A2) Self Tapping range continues to M20. Before a boss is moulded or printed, check the Minimum Wall Thickness and Minimum Boss Diameter (BD) listed for Trisert and Trisert-3, since a thinner wall risks splitting as the insert cuts its thread.

Q: Do self tapping threaded inserts for wood need a particular type?

A: Yes. Only Stainless Steel - (A2) Self Tapping inserts and Trisert-3 are rated for wood, and the Self Tapping range runs up to M20 for heavier fixings in hardwood frames, plywood cases and timber structures. For furniture and board work at M6 or M8, Threaded Inserts For Wood offer a zinc alloy alternative.

Q: Should I use self tapping or heat set threaded inserts?

A: It depends on the host material and the equipment available. Heat set inserts need a heat press or ultrasonic tool and only suit thermoplastics, because they rely on the plastic softening and flowing around the insert. A self tapping threaded insert goes in cold, so it can be fitted to a finished part at any stage, and the Stainless Steel - (A2) Self Tapping and Trisert-3 types also suit wood and non-ferrous metal. Where a thermoplastic part already passes through heat setting equipment, Brass Threaded Inserts cover the heat set types.

Bespoke Self Tapping Threaded Inserts Manufacture.

High precision, bespoke manufacture of Self Tapping Threaded Inserts to customer specification. State of the art facilities specializing in both small batch prototyping and large scale manufacture.