Thread processing method

Thread machining is a process that uses tools to create threads. It employs cutting, turning, milling, grinding, and other processes to machine workpieces. It generally refers to the method of machining threads on workpieces using forming tools or dies. Main methods include turning, milling, tapping, die threading, grinding, honing, and whirlwind cutting. When turning, milling, or grinding threads, for each rotation of the workpiece, the machine's transmission chain ensures that the cutting tool, milling cutter, or grinding wheel moves accurately and uniformly along the workpiece's axial direction by one lead.

When tapping or die threading, the tool (tap or die) and the workpiece perform relative rotational movements, and the already formed thread grooves guide the tool (or workpiece) in axial movement.

Machining Methods

Thread Turning

Thread turning on a lathe can use a forming tool or a thread comb (see thread machining tools). Using a forming tool for thread turning, due to the simple tool structure, is a common method for single-piece and small-batch production of threaded workpieces; using a thread comb for thread turning has high production efficiency, but the tool structure is complex and is only suitable for medium-to-large batch production of short threads with fine teeth. The pitch accuracy of trapezoidal threads turned on an ordinary lathe generally only reaches grade 8-9 (JB2886-81, hereinafter the same); on specialized thread lathes, productivity or accuracy can be significantly improved.

Thread Milling

Milling is performed on a thread milling machine using a disc milling cutter or a comb milling cutter. Disc milling cutters are mainly used for milling trapezoidal external threads on workpieces such as lead screws and worm shafts. Comb milling cutters are used for milling internal and external ordinary threads and tapered threads. Because multiple-blade milling cutters are used for milling, and the length of the working part is greater than the length of the machined thread, the workpiece only needs to rotate 1.25 to 1.5 turns to complete the machining, resulting in high productivity. The pitch accuracy of thread milling generally reaches grade 8-9, and the surface roughness is R5-0.63 μm. This method is suitable for batch production of general-precision threaded workpieces or rough machining before grinding.

Thread Grinding

Mainly used for machining precision threads on hardened workpieces on thread grinding machines. According to the shape of the grinding wheel cross-section, it is divided into single-line grinding wheel and multi-line grinding wheel grinding. Single-line grinding wheel grinding can achieve a pitch accuracy of grade 5-6, and a surface roughness of R1.25-0.08 μm, and the grinding wheel is relatively easy to dress. This method is suitable for grinding precision lead screws, thread gauges, worm shafts, small batches of threaded workpieces, and sharpening precision hobs. Multi-line grinding wheel grinding is further divided into longitudinal grinding and plunge grinding. In longitudinal grinding, the width of the grinding wheel is smaller than the length of the thread being ground, and the grinding wheel moves longitudinally once or several times to grind the thread to the final size. In plunge grinding, the width of the grinding wheel is larger than the length of the thread being ground, and the grinding wheel radially cuts into the workpiece surface. The workpiece rotates about 1.25 turns to complete the grinding, which has higher productivity but slightly lower accuracy, and the grinding wheel dressing is more complex. Plunge grinding is suitable for sharpening large batches of taps and grinding some fastening threads.

Thread Honing

Using a thread honing tool made of softer materials such as cast iron in the shape of a nut or screw, the parts of the already machined thread on the workpiece with pitch errors are honed in the forward and reverse directions to improve pitch accuracy. Hardened internal threads are also usually honed to eliminate deformation and improve accuracy.

Tapping and Die Threading

Tapping is the process of using a certain torque to rotate a tap into a pre-drilled hole in a workpiece to machine an internal thread.

and other methods