Thread Content
This post was last edited by Dimension Hardware Technology on 2018-2-24 at 13:54. Threads are one of the common geometric features in mechanical engineering and are widely used. There are various processes for manufacturing threads, such as thread rolling and thread drawing based on plastic deformation, turning, milling, tapping and threading based on cutting operations, thread grinding, and thread lapping. I. Types of threads: Based on the tooth profile, they can be divided into triangular, trapezoidal, rectangular, serrated, and circular arc threads ； Based on the thread direction, they can be divided into left-hand and right-hand threads ； It can be divided into single-wire and multi-wire types based on the number of spiral lines ； Based on the shape of the thread shank, they are classified into cylindrical and conical types, etc. II. Elements of threads Threads consist of five elements: thread profile, nominal diameter, number of starts, pitch (or lead), and rotation direction. 1. Tooth profile: In the cross-sectional area passing through the thread axis, the contour shape of the thread is called the tooth profile. There are tooth profiles such as triangular, trapezoidal, serrated, circular arc, and rectangular. 2. Diameter: Threads have a major diameter (d, D), a minor diameter (d2, D2), and an pitch diameter (d1, D1). When specifying a thread, the nominal diameter is used, and this is the diameter that represents the size of the thread. The nominal diameter of a regular thread is its major diameter. 3. Number of threads: A thread formed along a single helix is called a single-threaded thread, while a thread formed by two or more helices arranged at equal intervals along the axis is called a multi-threaded thread. 4. Pitch and lead The pitch (p) is the axial distance between corresponding points on the middle diameter of two adjacent teeth. Lead (ph) is the axial distance between the corresponding two points on the middle diameter of two adjacent teeth on the same helix. For single-start threads, lead = pitch ； For multi-start threads, the lead = pitch × number of starts. 5. Rotation direction: Threads that tighten when rotated clockwise are called right-hand threads ； A thread that tightens when rotated counterclockwise is called a left-hand thread. III. Thread machining: The method of using thread machining tools to create various internal and external threads. 1. Thread cutting generally refers to the method of machining threads on a workpiece using forming tools or abrasives, including turning, milling, tapping, threading, grinding, and cyclone cutting. When turning, milling, and grinding threads, with each rotation of the workpiece, the machine’s transmission mechanism ensures that the turning tool, milling tool, or grinding wheel moves a precise and uniform distance equal to one pitch along the axis of the workpiece. During tapping or threading, the tool (tap or die) rotates relative to the workpiece, and the pre-formed thread groove guides the tool (or the workpiece) in its axial movement. 2. Thread turning: Threads can be turned on a lathe using forming cutters or thread formers (see thread machining tools). Turning threads with a forming turning tool is a common method for producing threaded parts in single-piece and small-batch production, thanks to the simple structure of the tool ； Turning threads with a threaded comb cutter results in high production efficiency, but the tool has a complex structure and is suitable only for turning short threads with fine pitches in medium- to large-scale production. The pitch accuracy of trapezoidal threads turned on ordinary lathes generally only reaches grade 8–9 (JB2886-81, the same below); when the threads are machined on specialized thread turning machines, productivity or accuracy can be significantly improved. 3. Thread milling is primarily used to machine precise threads on hardened workpieces on thread grinding machines. It is divided into two types based on the shape of the grinder wheel: single-line wheel grinding and multi-line wheel grinding. Single-line grinding with a grinding wheel can achieve a pitch accuracy of grade 5 to 6, and a surface roughness of R1.25 to 0.08 micrometers; the grinding wheel is also easy to dress. This method is suitable for grinding precision lead screws, thread gauges, worms, small batches of threaded workpieces, and dressing precision hob cutters. Multi-line grinding of wheels is further divided into longitudinal grinding and plunge grinding. In the longitudinal grinding method, the width of the grinding wheel is less than the length of the thread to be ground; one or several movements of the wheel in a longitudinal direction are sufficient to grind the thread to its final size. In the plunge grinding method, the width of the grinding wheel is greater than the length of the thread to be ground; the wheel cuts radially into the workpiece surface, and the workpiece needs to rotate about 1.25 times to be fully ground. This method offers high productivity, but lower precision, and the grinding wheel requires complex truing. The plunge grinding method is suitable for grinding large quantities of taps and for grinding certain threads used for fastening.