Flying by: Surfaces during thread whirling with indexable carbide inserts are comparable to grinding

Thread manufacturing processes can initially be divided into three main groups: Primary forming. Forming, cutting. Primary forming is mainly used for plastic screws and nuts by means of injection molding.

Threads that are exposed to high stresses are usually hot or cold formed. The most important process here is certainly the thread rolling of external threads. Because a high output is achieved with the best fatigue strength.

Due to the high accuracies to be achieved, the machining manufacturing process plays a special role. Most motion threads, such as trapezoidal spindles. ball screws and measuring spindles, where high accuracy is required. are manufactured by machining.

When it comes to machining production methods, various manufacturing processes are known, such as thread turning, thread chasing, thread cutting, thread tapping, thread milling, thread whirling and thread grinding. Which process is ultimately used in production depends on several factors: Total number of pieces, surface quality, thread profile, tolerance, material, production equipment, acquisition costs, running costs (such as tools), piece output, susceptibility of the system to faults.

The long-chipping processes, such as thread turning, thread cutting, tapping and thread chasing, are sufficiently well known with regard to their advantages, but also their considerable disadvantages, especially in the case of long-chipping materials with regard to the susceptibility to faults in the machine area.

The three short-chipping processes are shown as examples, whereby the less well-known thread whirling process is largely presented.

In thread milling, a distinction is first made between two processes: short thread milling and long thread milling. Short internal and external threads, usually pointed profile threads, are produced with a cylindrical thread milling cutter that is provided with the entire thread profile along its length. The adjacent thread profile rows of the milling cutter have no pitch. The profile spacing corresponds exactly to the thread pitch to be produced; the profile teeth are relief-ground. HSS milling cutters are mainly used, whereby only threads of the same pitch can be produced with one milling cutter. In most cases, counter-rotation is used.

High cutting forces during thread milling

The short production time and the comma-shaped, short chips can be seen as an advantage. Disadvantages are the high cutting forces, which have a negative effect on accuracy and surface quality, especially with longer lengths. Furthermore, the milling cutter diameter also changes during regrinding, which is very cost-intensive, so that a correction of the adjustment paths or the path radius is necessary, for example on machining centers.

Because a milling cutter is required for each thread pitch, this method is mainly used for mass production. The surface geometry created during chip formation is also not ideal, especially at the bottom of the thread. Long thread milling is used for long thread lengths. The disk-shaped profile cutters are often made of HSS with relief-ground teeth or, in the case of larger profile widths, are fitted with indexable inserts. The milling axis is inclined according to the thread pitch or pitch angle. For larger profile widths that are to be produced in one cut or larger pitch angles, a profile correction is required on the tool.

Synchronous milling is preferred for quality threads. The simple production of long threads, usually in one cut to full thread depth, is particularly advantageous. The resulting short chips can be easily removed. Furthermore, face milling is also advantageously used for large profile widths such as in the extruder screw area. With long lengths, small diameters and larger profile widths, greater pitch deviations and poorer surface qualities are to be expected. Similar to milling, the rotating tool performs the cutting movement during grinding. The corners of the abrasive grains remove comma-shaped chips from the workpiece. Hardened workpieces such as ball screws with profiled wheels are usually ground. The parts are often pre-profiled using other processes so that only around 0.15 mm is removed from each thread flank. Full profile deep grinding is also very common, in which the entire profile is removed in a single cut. The grinding wheel is continuously profiled, for example using a dressing diamond roller.

The achievable surface qualities, the production on hardened materials and the profile accuracies are to be mentioned as advantages. Disadvantages are certainly the high acquisition costs of the machines and the complex separation of cooling lubricants, chips and grinding corundum.

Favorable chip formation during thread whirling

The relatively slow feed (nR) of the workpiece (main spindle of the machine) always has the same direction of rotation as the tool ring during economical synchronized whirling. The carbide-tipped knives rotate at high peripheral speed (speed nw) without vibration and uniformly.

These increases are considerably smaller compared to milling and relate to the thread core. On the flanks, this value is only about ¼ Y, depending on the flank angle of the thread.

The high cutting speeds used and the favorable enveloping circle cut result in high feed rates and short cutting times with the highest quality. In conjunction with a user-friendly and economical tool system, precision threads are produced with high productivity. Due to the short comma-shaped chips, downtimes as a result of long chip formation, as with turning, are eliminated.

Internal threads of long lengths are produced using the so-called reciprocating whirling process. The rotating and oscillating tool is supported in the core hole by means of a stationary guide hand. The tools are equipped with a multi-bladed indexable insert.

Internal fastening threads on heavy parts are produced, for example, on large boring mills using special whirling devices. This process has been introduced in turbine construction. For smaller workpieces on machining centers, the threads are produced using the circular process with appropriate thread whirling tools. The tools have four-edged inserts that do not need to be adjusted.