This article is part of the DFM for CNC Machining: Design Rules and Tolerance Checklist on CNX Precision.
Threaded inserts design gives CNC machined parts strong, reusable threads in materials that are too soft, thin, or brittle to tap directly. An insert is a fastener that sits in a drilled or molded hole and provides a durable internal thread. At CNX Precision, we help customers select and install helical coil, press-in, ultrasonic, and self-tapping inserts for plastic and metal parts across electronics, automotive, medical, and industrial products. This guide explains the main insert types, when to use them instead of tapped holes, hole preparation, installation methods, and the design rules that keep joints strong and serviceable.
What Are Threaded Inserts?
A threaded insert is a cylindrical fastener with internal threads on one side and an outer feature that grips the host material on the other. It turns a weak or thin threaded section into a robust connection. Inserts spread load over a larger area and resist pull-out, stripping, and cross-threading. They let you assemble and disassemble a joint many times without wearing out the base material.
A directly tapped thread in soft aluminum or plastic may hold on the first assembly, but the threads deform a little each time the fastener turns. After several cycles the hole strips, and the whole part may need repair or scrap. An insert replaces that fragile thread with a hard, wear-resistant one, so the joint keeps its clamp load and torque values over the service life of the product.
Inserts come in many forms for plastic and metal. The right choice depends on the host material, wall thickness, expected load, and how often the joint opens. Thoughtful threaded inserts design weighs all of these factors before part selection, because changing an insert later usually means changing the hole size and the surrounding geometry.
Common Threaded Insert Types
Helical coil inserts, often sold under the Heli-Coil name, are wound wire coils that screw into a tapped hole. They rebuild stripped threads and add strong, wear-resistant threads in soft metals such as aluminum and magnesium. The coil spreads load along the full thread length, which reduces stress concentration. Coil inserts are also the standard repair choice when a thread is damaged in the field. An installation tool threads the coil in like a screw, and the tang is broken off afterward with a simple punch.
Press-in inserts are pushed or pressed into a drilled hole. Their knurled, barbed, or grooved exterior grips the host material mechanically. They suit plastics and soft metals and install quickly with an arbor press or a simple fixture. Some press-in designs include a locking feature that resists rotation under torque.
Ultrasonic inserts are installed with high-frequency vibration that melts the surrounding plastic so it flows into the insert knurl. They create a strong, low-stress fit in thermoplastics and are popular for machined and molded plastic housings. The process is fast and repeatable, which suits automated assembly cells.
Self-tapping inserts cut their own mating thread as they are driven in. They need no pre-tapped hole and work well in castings, foamed parts, and soft materials where tapping is impractical. They are often used in die-cast housings and composite panels. Compare strength, cost, and installation needs for each type against your application.
Threaded Inserts Design Guidelines
Sound threaded inserts design starts with hole preparation. Drill the pilot hole to the diameter and depth the insert manufacturer recommends. A clean, round, burr-free hole is essential for pull-out strength and for keeping the insert square. These checks are part of solid threaded inserts design.
Match the insert to the host material. Use ultrasonic or press-in inserts for plastics. Use helical coil or self-tapping inserts for metals. Confirm that the surrounding wall or boss is thick enough to resist expansion, cracking, or deformation during installation. Thin bosses can split, so add material or choose a lower-stress insert style.
Also match the insert to the fastener. The insert internal thread must match the screw size, thread class, and fastener material you plan to use. Verify that screw engagement reaches at least one full diameter for full strength, and add a counterbore or chamfer where the fastener enters so it self-aligns and does not cross-thread during assembly.
Placement matters too. Keep inserts far enough from part edges to avoid breakout, and space multiple inserts so their stress zones do not overlap. Provide a flat, perpendicular seating surface so each insert goes in straight.
Control tolerances end to end. The pilot hole diameter, the insert outer diameter, and the installation depth all interact, so hold each to the manufacturer printed tolerances. Verify torque-out and pull-out performance on first-article parts, and spot-check production parts. Documenting these values in your threaded inserts design file makes repeat builds consistent.
Installation quality matters as much as part selection. Press inserts in slowly and squarely to avoid tilting. For ultrasonic inserts, control the energy, force, and depth so the plastic flows without burning. For helical coil inserts, use the proper installation tool and break off the tang. Inspect every insert for flush seating, correct depth, and free-running threads before final assembly.
When to Use Inserts Instead of Tapped Holes
Choose inserts when the base material is too soft to hold a tapped thread, such as aluminum, magnesium, or plastic. Use them in thin walls where a cut thread would strip under load. Use them in joints that see repeated assembly and disassembly, because the insert protects the base thread from wear and extends service life.
Inserts also let you use a lighter or less expensive host material while still achieving a strong, serviceable joint. They are common in electronics enclosures, automotive brackets, medical device housings, and consumer products. Applying good threaded inserts design in these situations prevents stripping, reduces warranty failures, and keeps assemblies easy to open and reassemble.
There is a cost tradeoff. Inserts add part cost and an installation step, but they usually save money when they prevent stripped holes, rework, and field failures. In high-volume production, dedicated ultrasonic or press fixtures pay for themselves quickly. Weigh the fastener torque the joint needs against what a plain tapped hole can survive before you decide.
Frequently Asked Questions
What is the best threaded insert for plastic?
Ultrasonic inserts give a strong, low-stress fit in thermoplastics. Press-in inserts are a simpler, lower-cost option. Choose based on the plastic type, wall thickness, and expected assembly volume. For heavily loaded bosses, a through-hole with a shoulder can outperform a blind hole.
Can threaded inserts be removed and reused?
Helical coil inserts can be removed with an extraction tool and replaced. Press-in and ultrasonic inserts are harder to remove without damaging the hole, so plan for serviceability in your threaded inserts design from the start.
How deep should the pilot hole be?
Drill the pilot hole at least as deep as the insert, plus clearance for chips or a bottom land. Blind holes also need relief for displaced material so the insert can seat fully. Follow the manufacturer specifications for the exact diameter and depth to reach full pull-out strength.
For related information, see our guide to cnc machining service and 5-axis cnc machining and cnc milling vs turning, and cnc machining tolerances.