Thread Length

Thread length is the axial length of a screw thread on a part, often specified as effective thread engagement length in CNC machining—the portion that carries load and resists pull-out or stripping. In shop practice, it must be distinguished from total drilled or tapped depth in blind holes because drill point and tap runout reduce usable threads.

On the shop floor, thread length is set less by the drawing dimension and more by the joint’s load case, material, and process method. In steel, 1.0–1.5× diameter engagement is often enough for ordinary loads, while aluminum usually needs about 2.0–2.5× diameter because the softer material strips sooner. For blind holes, an unthreaded relief at the bottom—at least half the hole diameter—keeps the tap from crashing and ensures the last few threads are full form. Programmers must separate usable thread length from total drill depth, since drill point geometry and tap runout eat into the bottom. Longer threads require special taps, longer thread mills, more passes, or deeper tapping cycles, all of which add cycle time and tool wear. A 3× diameter cap is common guidance; beyond that, extra threads rarely improve joint strength enough to justify the cost.