Thread processing refers to the methods of machining various internal and external threads using threading tools.
I. Thread Cutting
Generally refers to methods of machining threads on a workpiece using forming tools or abrasives, mainly including turning, milling, tapping, die threading, grinding, lapping, and whirlwind cutting. During turning, milling, and grinding, the machine's drive chain ensures that the tool (turning tool, milling cutter, or grinding wheel) moves one lead accurately and uniformly along the axial direction for every revolution of the workpiece. During tapping or die threading, the tool and workpiece rotate relative to each other, and the tool is guided axially by the pre-formed thread grooves.
II. Thread Turning
Thread turning on a lathe can use forming tools or thread chasers. Using forming tools is common for single-piece and small-batch production due to simple tool structure. Thread chasers offer high efficiency but have complex structures, making them suitable for medium to large-batch production of fine-pitch short threads. Pitch accuracy for trapezoidal threads on ordinary lathes is generally Grade 8–9.
III. Thread Milling
Performed on thread milling machines using disc or comb cutters. Disc cutters are used for trapezoidal external threads on lead screws or worms. Comb cutters are used for internal/external common threads and tapered threads. Since it uses a multi-blade cutter longer than the thread length, the workpiece only needs to rotate 1.25–1.5 turns to complete, offering high productivity. Accuracy is Grade 8–9, with surface roughness Ra 5–0.63 μm.
IV. Thread Grinding
Mainly used for precision threads on hardened workpieces. It is divided into single-line and multi-line wheel grinding. Single-line grinding achieves Grade 5–6 accuracy and Ra 1.25–0.08 μm roughness; dressing is easy. It is suitable for precision lead screws and gauges. Multi-line grinding includes longitudinal and plunge grinding. Plunge grinding is faster but less accurate and involves complex dressing.
V. Thread Lapping
Using nut-type or screw-type lapping tools made of soft materials (like cast iron), the parts of the thread with pitch errors are ground in forward and reverse rotations to improve pitch accuracy. It is also used to eliminate deformation in hardened internal threads.
VI. Tapping and Die Threading
Tapping uses a tap with a specific torque to process internal threads in a pre-drilled hole. Die threading uses a die to cut external threads on bar or pipe stock. Accuracy depends on the tool's precision. Small-diameter internal threads can only be processed by tapping.
VII. Thread Rolling
A processing method that produces plastic deformation using forming rollers. It is suitable for mass-producing fasteners. The outer diameter generally does not exceed 25mm, and length is under 100mm.
Advantages: 1. Lower surface roughness; 2. Higher strength/hardness due to cold work hardening; 3. High material utilization; 4. Multiplied productivity; 5. Long die life.
Methods: Flat die rolling (moving/stationary plates) and cylindrical die rolling (radial, tangential, or rolling heads).
