Fundamentals of thread whirling

Whirling is used all over the world to manufacture ball-screw and pump spindles; extruder, steering and EPS worm gears, ball-spindle nuts and special screws. The high levels of precision and short throughput times are decisive to the economic use of whirling machines. But what is whirling and how does it work? As suppliers and strategic planners approach the market in the Middle East, it is important to understand the advantages of this process. 

Whirling is a particular type of milling that is position rotates at high speeds around a slowly turning workpiece. The cutting plane on the tools' cutting edges is adapted to the respective thread pitch by swivelling the thread-whirling head around its vertical X-axis. The tool's large swivelling range enables users to realize high thread pitches and to cut right-handed and left-handed threads. The radial setting of the whirling head determines the thread depth. The tool's design permits down-cut, conventional and up-cut milling.

This approach delivers two major benefits: First, the workpiece's slow rotation at a speed of around 20min-1 prevents any imbalances from creating negative effects in the work piece. Secondly, bending or knocking – as may occur in work pieces rotating at high speeds – are impossible at such slow speeds. An added benefit lies in the even and favourable formation of shavings. This helps to perfectly conduct the heat into the chips thus preventing the work piece from becoming hot.

Whirling process
During the whirling process, the changeable disks produce short moon-shaped chips. Compared with thread milling the softer cut requires a relatively low cutting power. This means that greater cutting rates can be achieved, which in turn means that faster cutting and feed speeds are possible than those realized with milling. The quality of the finish may also be compared with that produced by polishing. The surface topology realised with the thread-whirling process also results in better protection against wear than that produced by polishing and grinding.

The chip formation particular to thread-whirling also means that in spite of uninterrupted cutting tools made from ceramics and brittle hard metals may be used. These offer the benefit of long tool life due to their high content of carbides and nitrides. Also, the fact that between 80 and 90% of the heat is removed with the chips means that most thread-whirling processes do not require liquid coolants, which reduces the corresponding costs, is environmentally-friendly and keeps operations clean.

Complete machining has today become essential to achieving economic throughput times when working with rotation-symmetrical components. Whirling is increasingly replacing such conventional manufacturing methods as milling, turning and grinding. These applications mean that whirling machines must fulfil the highest demands on rigidity and precision without, however, excluding complete machining.

LEISTRITZ manufactures whirling machines in different sizes and with a variety of features. While the more compact versions come along with five axes (A-,C-, X- and Z-axes with an NCcontrolled tail stock), the larger machines are available with additional support rests (NC axes), loading supports and automatic charging and unloading systems where the number of possible axes may range up to 15. The demands made on the control and drive concepts are consequently very high.

Machines from LEISTRITZ are equipped with direct measuring systems and special highly precise whirling units with torque drives, which may also be equipped with CBN tools, which allows spindles of up to 3.5 tons to be hard-machined and which makes the finishing process of grinding and polishing superfluous.

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