Threaded Insert Design for Aluminum and Plastic Parts

A threaded hole in 6061 is not a thread. It is a promise that the first three threads will carry the load and the rest will strip quietly at 60 percent of the bolt's rated strength. Every aluminum or plastic housing assembled more than twice needs a decision about inserts, and that decision is usually made on the drawing as an afterthought — "M3 x 6 deep, 4 places" — with no boss, no wall and no insert callout.

Why aluminum and plastic threads fail early

Aluminum has roughly one third the shear strength of steel, and a cut thread in 6061-T6 engages only two to three full turns before the hole bottoms out. Those turns see the entire preload, so the first assembly is fine and the third is not. The failure is not gradual: the thread shears, the fastener loses clamp load, and the joint starts to fret.

Plastics are worse: a cut thread in ABS creeps under sustained load, so a screw tight in January is loose in June, and glass-filled grades chip at the crest with the chips ending up inside the enclosure.

Helicoil: cheap, proven and often enough

A helical insert is a diamond-section stainless coil wound into a tapped hole. It creates a hardened female thread, spreads load over the full engaged length, and costs $0.10 to $0.45 per piece installed. Installation is a standard tap plus a driving tool, so a shop already running your part can add inserts without new equipment.

For M3, the free-running coil needs a 3.2 mm tap drill and an insert outside diameter of about 4.0 mm, so a boss of 6.0 to 6.5 mm outside diameter with 1.2 mm of wall is enough in 6061.

The limits are real. A helicoil does not seal, so it is unsuitable for a fluid path, and it can back out if the fastener is removed often, because the coil is held by friction and a small tang.

See also: Deep Pocket Machining: Depth-to-Diameter Limits, Corners and Deflection and O-Ring Groove Machining: Tolerances That Decide Whether the Joint Seals.

Keensert and solid inserts: when the joint must come apart

A key-locking insert is a solid bushing with internal and external threads, secured by two or four keys driven down after installation. The external thread is much larger than the internal one, so the load spreads into a bigger cylinder of parent metal. For M3 the external thread is typically M6 x 1, which means a boss of 8.0 mm minimum and an installed cost of $0.60 to $1.80 per piece plus a tap and key tool.

Pull-out and strip resistance run 40 to 70 percent above a helicoil of the same size in 6061, and the joint survives dozens of assembly cycles without the insert moving. That makes key-locking inserts the right answer for serviceable covers, hydraulic manifolds and any joint with a torque specification above 1.2 N·m on M3.

Molded and press-fit inserts in plastics

Brass inserts for plastics come in three flavours: heat-set, ultrasonic and molded-in. Heat-set is the default for low and medium volume because a thermal press pushes the insert into a molded hole in two to four seconds with no mold change. Molded-in inserts require a pin in the tool and add 3 to 8 percent to cycle time.

Boss diameter follows the insert, not the screw. For an M3 brass insert with a 4.6 mm outside diameter, the boss should be 7.0 to 8.0 mm outside diameter, giving 1.2 to 1.7 mm of wall. Go below 1.0 mm and the boss splits on installation or stress-cracks three weeks later in the field. Boss height should be at least the insert length plus 0.5 mm.

Boss geometry and engagement depth

Consider a 1.5 mm aluminum wall with an M3 thread cut directly into it. There is no boss, so the thread is a partial thread in a 1.5 mm sheet backed by air, and two threads engage at best. The strip load is roughly 0.6 kN, well under the 2.0 kN a normal M3 preload can generate, so the thread strips during assembly rather than in service. Adding a helicoil does not rescue the geometry: the coil needs 4.0 mm of diameter and the wall offers 1.5 mm.

One diameter of engagement carries about 70 percent of the achievable load in aluminum, 1.5 diameters carries about 95 percent, and beyond 2 diameters you gain almost nothing because the load concentrates at the first threads anyway. Specify 1.5 D as the target and 1.0 D as the floor for M3 and above.

Hole depth must exceed insert length plus the tap chamfer plus chip clearance, so add 0.8 to 1.2 mm in blind holes and specify that the hole is blown out before installation. A helicoil sitting on a chip at the bottom is 0.3 mm high, and the cover no longer sits flush. For plastic bosses, the hole is undersized by 0.1 to 0.2 mm relative to the insert's minor diameter: too tight and the boss splits, too loose and the insert spins.

What to specify on the drawing

Put the insert on the drawing as a line item with a part number or a full specification: thread size, insert type, material, installed length, boss diameter with tolerance, and a note to install after anodizing, because anodize builds 5 to 15 µm per surface and will bind a coil on first assembly.

State the removal torque requirement if the joint is serviceable. A helicoil in 6061 should hold 0.6 N·m of removal torque on M3; a key-locking insert should hold 1.0 N·m. Without a number, the shop installs to the insert maker's standard and neither of you knows whether the joint is adequate.

If you are still choosing between a boss and a thicker pad, send the section through the contact page and ask for a DFM note on the wall thickness. We quote insert installation as a line item so you can see what it adds per part, and the materials page covers which aluminum grades hold threads better than 6061.

Frequently asked questions

Is a helicoil strong enough for an M3 screw in aluminum?

Yes, for most static joints. A helicoil with 1.5 D engagement in 6061 holds roughly 2.5 to 3.5 kN pull-out, which exceeds the preload a normal M3 screw develops at 1.0 N·m. It is not sufficient for repeated disassembly or for fluid sealing.

How thick must a boss be around an M3 insert?

At least 1.2 mm of wall around the insert body, which puts a helicoil boss at 6.0 to 6.5 mm outside diameter and a key-locking boss at 8.0 mm. For plastics, use 1.2 to 1.7 mm of wall and never below 1.0 mm.

Can I put an insert into a 1.5 mm wall without a boss?

No. The insert needs 4.0 mm of diameter for M3 and the wall offers 1.5 mm, so tapping removes the wall completely. Add a local boss, thicken the pad, or switch to a press-fit nut designed for sheet.

Should inserts be installed before or after anodizing?

After. Anodizing builds 5 to 15 µm per surface and will close a small thread or bind the coil. Mask the holes if the insert must go in first, and specify the masking on the purchase order.

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