Cut Tapping, Form Tapping, and Thread Milling: Which Method Should Be Used for CNC Internal Threads?

Internal threads are common in housings, plates, fixtures, manifolds, bushings, and general machine components. Three widely used CNC methods are cut tapping, form tapping, and thread milling. Each can produce an acceptable thread when correctly applied, but they differ in how the profile is generated, how chips are managed, torque demand, suitable materials, and dimensional adjustment. The nominal diameter alone is not enough to select a process. The thread standard and class, material, blind or through condition, effective thread length, post-machining finish, quantity, and inspection method must be reviewed together.

A forming tap displaces ductile material instead of cutting it. Because it does not need large chip flutes, the tool core can be more rigid, and chip entanglement is eliminated. The method is generally suited to materials with sufficient ductility, including appropriate aluminum alloys, low-carbon steels, and selected stainless steels.

Thread milling combines a thread mill with helical interpolation. The cutter orbits the hole axis while moving in Z to generate the pitch. Since the cutter is smaller than the thread diameter, chips have an evacuation path, and a tool failure is less likely to block the entire hole cross-section than a broken solid tap.

This table is a selection guide, not a process guarantee. The final method must reflect the actual material, part geometry, machine, holder, cutting tool, and acceptance criteria.

Before threading, the hole diameter, roundness, straightness, depth, and position must be controlled. Burrs or a sharp entrance can interfere with assembly. A suitable chamfer supports tool entry, but it must not reduce the specified effective thread length. There is no universal pre-hole diameter for every material and method. It must be selected from the thread standard, tolerance class, specific tool, and toolmaker’s recommendation. If plating or anodizing follows machining, the drawing must state whether the acceptance size applies before or after finishing and whether the thread is masked.

A mating bolt can reveal an obvious problem, but it does not replace a specified gauge. The GO plug gauge must enter the required thread length under the applicable inspection rules. The NO-GO member must not exceed the permitted engagement. Gauges must match the thread system, pitch, class, and calibration requirements. Special applications may require pitch-diameter measurement, profile verification, or a first-article report. Chip removal, preservation oil, and coating condition should be agreed before final inspection.

VietTech machines internal threads on CNC-milled and CNC-turned components and inspects them against the agreed drawing requirements. The method is evaluated from material, dimensions, quantity, and access; one tool or process is not assumed for every component.

• Complete designation, such as M6 × 1–6H or 1/4-20 UNC-2B.
• Blind or through hole, effective thread length, and drilled depth.
• Material, hardness, and heat-treatment condition.
• Number of threaded holes per part and order quantity.
• Surface finish, masking, and the required acceptance condition.
• Mandatory thread-making method when controlled by design or specification.
• GO/NO-GO gauging, inspection report, and traceability requirements.