Threaded Hole in CNC Machining

A threaded hole has a spiral-shaped thread along its inner wall that fits the external thread of a matching screw or bolt. This allows a fastener to tighten directly into the part. For example, a screw can pass through a cover plate and engage a threaded hole in the housing beneath it, securing the cover without a separate nut.

What Matters in Design and Machining

For designers, a threaded hole is part of the fastening system. Its dimensions must match the screw, while the surrounding material and engaged threads must support the connection’s loads.

For machinists, the challenge is producing the required thread within the available space. Tapping uses a tool that cuts or forms the thread. Thread milling uses a rotating cutter that follows a spiral path. The choice depends on the material, thread dimensions, hole depth, and production requirements.

Deep holes and blind holes—holes that stop inside a part—can make chip removal more difficult during cutting. Trapped chips may damage the thread or break the tool. Form tapping produces threads by reshaping suitable material rather than cutting it, but requires a different starting-hole size from cutting taps.

Thread depth, tool access, and tolerances also affect machining time and inspection. Specifying more depth or tighter tolerances than the connection needs can increase cost without improving its function.

Details Designers Can Overlook

Specify the thread completely. For example, M6 × 1 identifies a metric thread with a 6 mm nominal diameter and a 1 mm pitch, the distance between neighboring threads measured along the hole’s axis. The nominal diameter is not the starting-hole diameter. Include the applicable tolerance class—the permitted dimensional variation—so the machinist can select suitable tooling and inspection gauges.

Choose engagement for the connection. Thread engagement is the length over which the screw and hole threads actually engage. Determine it from the material, fastener strength, and expected loads. Making the threaded hole deeper does not help if the screw never reaches those additional threads.

Separate hole depth from thread depth. A drilled hole may extend beyond the depth of its complete threads, and the installed screw may engage only part of that threaded length. For blind holes, allow bottom clearance appropriate to the machining method. Check that the screw clamps the assembly before its tip reaches the bottom or encounters incomplete threads. For through holes, state whether threading must also extend through the part.

Check supporting material and access. Review wall thickness, distance to edges, and nearby features so the hole has adequate support without breaking into an adjacent pocket. Leave room for the threading tool to reach the feature. Standard thread sizes, practical depths, and function-based tolerances help keep production straightforward.