Detailed Explanation Of Seven Thread Processing Methods –– Material Removal Methods

In mechanical manufacturing, thread processing is a crucial step. Based on the forming principle, thread processing can be divided into "Material Removal Methods" and "Material Forming Methods". This article will explain in detail four core methods that form threads by removing material through cutting and grinding.

1. Thread Cutting (Overview)

Thread cutting is a broad concept referring to methods that process threads on a workpiece by removing material using formed cutting tools or grinding tools. It mainly includes turning, milling, tapping, threading, grinding, and lapping. Their common characteristic is that the machine tool's kinematic chain must ensure precise helical movement between the tool and the workpiece.

2. Thread Turning

This is the most fundamental and widely used thread processing method.

Working Principle: On a lathe, the workpiece rotates, and a form tool or thread chaser moves uniformly along the workpiece axis to cut the thread groove.

Tool Types:

Form Tool: Simple structure, suitable for single-piece and small-batch production.

Thread Chaser: Multi-tooth design, high production efficiency, suitable for medium and large-batch production of fine-pitch short threads.

Precision and Application: Accuracy on ordinary lathes generally reaches Grade 8~9. Processing on specialized thread lathes can significantly improve productivity and precision.

3. Thread Milling

An efficient semi-finishing or roughing method.

Working Principle: Uses a disc-type milling cutter or comb-type milling cutter on a thread milling machine. The cutter rotation is the primary motion, while the workpiece rotates slowly and coordinates with the axial movement of the cutter to form the thread.

Tool Types:

Disc-Type Milling Cutter: Mainly used for milling trapezoidal external threads on components like lead screws and worms.

Comb-Type Milling Cutter: Used for machining internal and external general threads and taper threads. The workpiece only needs 1.25~1.5 rotations to complete the process, making it highly efficient.

Precision and Application: Pitch accuracy generally reaches Grade 8~9 with good surface roughness. Suitable for batch production of general-precision threads or as roughing before grinding.

4. Thread Grinding

The preferred method for processing high-precision, high-hardness threads.

Working Principle: Uses a formed grinding wheel on a thread grinding machine for precision machining of hardened workpieces.

Processing Methods:

Single-Rib Grinding Wheel Grinding: The grinding wheel is easy to dress, offers extremely high precision (up to Grade 5~6), and provides good surface quality. Suitable for precision lead screws, thread gauges, worms, and small batches of precision threaded workpieces.

Multi-Rib Grinding Wheel Grinding: Higher efficiency, divided into traverse grinding and plunge grinding. In plunge grinding, the workpiece only needs about 1.25 rotations to complete, suitable for batch relief grinding of taps and fastening threads.

Application: Mainly used for precision thread processing of hardened workpieces.

5. Thread Lapping

A finishing method aimed at improving pitch accuracy.

Working Principle: A threaded lapping tool made of softer material (like cast iron) is used to lap pre-machined threads through forward and reverse rotation, correcting minor pitch errors.

Application: Often used to enhance the pitch accuracy of precision threads or to eliminate deformation in hardened internal threads caused by heat treatment.

6. Tapping and Threading

Essential methods for processing small-sized threads, especially internal threads.

Tapping: Uses a tap to process internal threads in a pre-drilled hole. Small-diameter internal threads almost exclusively rely on this method.

Threading: Uses a die to cut external threads on bar or pipe stock.

Characteristics: Processing accuracy directly depends on the accuracy of the tap or die. It can be performed manually or on machines like lathes, drilling machines, and tapping machines.