Fastener threading is a method of using tools to produce various internal and external threads. Today we will explain several threading methods in detail. Let’s take a look!
What Are the Thread Cutting Processes?
Thread cutting refers to the process of producing threads on a workpiece using forming tools or grinding tools. This includes turning, milling, tapping, threading, grinding, lapping, and whirling. During turning, milling, and grinding, the machine tool’s drive chain ensures that the turning tool, milling cutter, or grinding wheel accurately and evenly moves one lead along the workpiece’s axis with each revolution of the workpiece. During tapping or threading, the tool (tap or die) and the workpiece rotate relative to each other, and the tool (or workpiece) moves axially guided by the previously formed thread groove.
Thread Turning
Thread turning on a lathe can be done with either a form turning tool or a thread combing tool. The principle is that a rotating workpiece interacts with a single-point threading tool that moves precisely along its axis to “cut” the thread profile. Thread turning with a form turning tool, due to its simple tool structure, is a common method for single-piece and small-batch production of threaded workpieces. Thread turning with a thread combing tool offers high productivity, but due to its complex tool structure, it is only suitable for turning short, fine-pitch threads in medium- to large-scale production.
The pitch accuracy of trapezoidal threads turned on ordinary lathes can generally only reach level 8 to 9; processing complex threads on specialized CNC thread lathes can significantly improve productivity or accuracy (e.g., ISO 4H/6g).
Thread Milling
Thread milling is performed on a thread milling machine using a disc milling cutter or a comb milling cutter. Disc milling cutters are primarily used for milling trapezoidal external threads on workpieces such as lead screws and worms. Comb milling cutters are used for milling internal and external standard threads and tapered threads. Because they use a multi-edge milling cutter and the working length of the cutter is longer than the thread being machined, the workpiece only needs 1.25 to 1.5 revolutions to complete the process, resulting in high productivity.
The pitch accuracy of thread milling can generally reach level 8 to 9, and the surface roughness is R5 to 0.63 microns. This method is suitable for batch production of threaded workpieces with general precision or rough processing before grinding.
Thread Grinding
Thread grinding is primarily used for machining precision threads on hardened workpieces on thread grinders. Depending on the cross-sectional shape of the grinding wheel, it can be categorized into single-thread and multi-thread grinding wheels. Single-thread grinding can achieve a pitch accuracy of 5-6 and a surface roughness of R1.25-0.08 microns. The grinding wheel is also easier to dress. This method is suitable for grinding precision screws, thread gauges, worms, small batches of threaded workpieces and relief grinding precision hobs.
Multi-wire grinding wheel grinding is divided into two types: longitudinal grinding and plunge grinding. In longitudinal grinding, the grinding wheel width is smaller than the length of the thread being ground, and the wheel moves longitudinally in one or several strokes to grind the thread to its final dimension. In plunge grinding, the grinding wheel width is larger than the length of the thread being ground, and the wheel radially cuts into the workpiece surface. The workpiece is ground in approximately 1.25 revolutions. This method offers higher productivity, but slightly lower precision and more complex wheel dressing. The plunge grinding method is suitable for grinding large batches of taps and grinding certain fastening threads.
A nut-shaped or screw-shaped thread lapping tool made of a softer material such as cast iron is used to grind the workpiece in both forward and reverse directions where pitch errors exist, thereby improving pitch accuracy. Hardened internal threads are also often ground to eliminate distortion and improve accuracy.
Tapping and Threading
Tapping is the process of producing internal threads by screwing a tap into a pre-drilled hole in a workpiece using a specific torque.
Threading is the process of cutting external threads on a bar (or tube) workpiece using a die. The accuracy of tapping or threading depends on the precision of the tap or die. Although there are many methods for processing internal and external threads, small-diameter internal threads can only be processed by taps. Tapping and threading can be performed manually or using a lathe, drilling machine, tapping machine, or threading machine.
Thread Rolling
This is a non-cutting process that uses hardened rolling or flat-pressing dies to plastically deform the workpiece to form threads. Thread rolling is typically performed on a thread rolling machine or an automatic lathe equipped with an automatically opening and closing thread rolling head. It is suitable for mass-producing external threads for standard fasteners and other threaded fasteners.
The outer diameter of rolled threads generally does not exceed 25 mm, and the length does not exceed 100 mm. The thread accuracy can reach Class 2 (GB197-63). The diameter of the blank used is roughly equal to the pitch diameter of the thread being machined. Rolling generally cannot produce internal threads, but for softer materials, a fluted extrusion tap can be used to cold-extrude internal threads (maximum diameter of approximately 30 mm). The operating principle is similar to tapping. The torque required for cold-extruded internal threads is approximately twice that of tapping, resulting in slightly higher precision and surface quality.
▌The advantages of thread rolling are:
- Surface roughness is less than that achieved by turning, milling, and grinding;
- The thread surface after rolling is stronger and harder due to cold work hardening;
- High material utilization;
- Productivity is exponentially higher than that of cutting, and automation is easily achieved;
- Rolling dies have a long service life. However, thread rolling requires a workpiece material hardness no greater than HRC 40; the blank requires high dimensional accuracy; the rolling die also requires high precision and hardness, making die manufacturing difficult; and it is not suitable for rolling threads with asymmetrical tooth shapes.
Thread Forming and Rolling Thread
Thread forming is two pieces of thread forming with thread plate staggered 1/2 pitch relative arrangement, the static plate is fixed, the dynamic plate for parallel to the static plate of the reciprocating linear movement. When the workpiece is fed into the two boards, the dynamic plate forward rolling pressure on the workpiece, so that its surface plastic deformation into the thread.
There are 3 kinds of thread rolling: radial thread rolling, tangential thread rolling and rolling head thread rolling.
Radial thread rolling: 2 (or 3) with a threaded wheel mounted on the shaft parallel to each other, the workpiece is placed on the support between the two wheels, the two wheels rotate in the same direction and at the same speed, one of the wheels are also for radial feed movement. The workpiece is rotated by the thread rolling wheel, and the surface is extruded radially to form a thread. For some of the precision requirements are not high screw, can also be used in a similar way rolling molding.
Tangential thread rolling: also known as planetary thread rolling, the rolling tool consists of a rotating central roller and three fixed arc-shaped silk plate. Thread rolling, the workpiece can be fed continuously, so the productivity is higher than thread rolling and radial rolling.
Thread rolling head: It is carried out on automatic lathes and is generally used for processing short threads on workpieces. There are 3 to 4 thread rolling wheels evenly distributed around the workpiece in the rolling head. When thread rolling, the workpiece rotates and the rolling head feeds axially to roll the workpiece out of the thread.
