December 7, 2020
Whirling Machine Series – Part Four
Yesterday’s Grinding is Today’s Whirling
New possibilities with whirling technology from Leistritz
The high surface quality of screws and ball screws can be achieved by whirling. Leistritz has made advancements to what was traditionally a roughing method into a high-precision, high quality surface finish process that made grinding unnecessary. Leistritz, the industry leader in whirling machine technology has set a new standard.
In this four-part series, we examine the economy, technical possibilities, applications, market development, innovation and the environmental impact around the whirling process. And the advantages to manufacturers of parts with helical geometries such as EPS screws, ball screws and others could be produced quickly and without cooling lubricant in a single step? What can be technically achieved with modern whirling machines is the content of this fourth and final article in the series.
Whirling, like grinding, is a cutting process. However, with grinding the focus is on the final profile shape or a particularly precise surface is an inherently a slow process and adds additional steps in the manufacturing process.
Milling on the other hand, which includes whirling, demonstrates its strength by removing material very quickly. And is able to consolidate what was a multi-step process and move to a more economical whirling process while achieving the required part quality is of utmost importance.
Whirling technology has improved tremendously over the years in terms of quality and accuracy. Through the combination of geometrically defined cutter profiles coupled with high-speed cutting (HSC), whirling is able to produce exceptional part quality in a wide range of standard and hardened materials.
Whirling also scores high with its lower heat generation cutting action and heat dissipation abilities over traditional milling. Heat which is largely removed with the chip allows the whirling process to be performed dry, eliminating the need for coolant or lubricants.
Let’s review two applications.
Ball screws of various sizes and lengths are widely used today. They are heavily relied upon and take on the task of dynamic positioning to an exact location. Ball screws are used in such as in aircraft, missiles to move control surfaces, electric fly by wire controls, automobile power steering units that translate rotary motion from an electric motor to axial motion of the steering rack. Also, they are used in manufacturing equipment, such as machine tools, robots and precision assembly equipment just to name a few.
When whirling a ball screw verses grinding a ball screw are compared side-by-side, whirling produces the same part 70% faster than that of grinding. Whirling is not limited to any particular material type or hardness. Everything from metal forging blanks, hollow shafts to solid bars can be machined.
Another benefit by whirling, is the elimination of possible burns on the part. Burn marks are always a risk when grinding and are produced by significant pressure placed on the workpiece surface by the grinding wheel.
Internal whirling compared to grinding and turning
When internal whirling, a cutting tool head moves into the part cavity, as the cutting tool rotates it carves the desired geometry into the inner wall of a part. This is how the internal thread of ball screw nut or inverted planetary nut is created.
Internal whirling is twice as fast as grinding and turning. With the Leistritz LWN 100, internal threads with a length-diameter ratio of up to 20:1 can be manufactured. With external whirling, grinding burns and surface tension don’t occur due to the intermittent cutting pressure.
Mechanical benefits of whirling
In contrast to grinding and turning, the production part rotates slowly in the c-axis during the whirling operation. The part rotation does not aid in the actual cutting itself but advances the workpiece in the linear z-axis of the whirling head to create the helix. The tilt angle of the head determines the pitch of the workpiece, the plate geometry, and the workpiece profile.
Due whirling’s low angular speed the radial forces placed on the workpiece are also lower than during grinding. This contributes significantly to the fact that the inner profiles of workpieces can be manufactured with the length-diameter ratio of up to 20:1 already mentioned. Inside diameters down to 14mm (.055”) are possible, and this with a simultaneous length of up to 220mm (8.66”). A Leistritz LWN 100 with a module of 0.8 to 3 and a swivel angle of up to 42 degrees can implement the current worm gears for electric power steering (EPS).
When whirling, short comma shaped chips are formed carrying away heat that from the finished part. Comma shaped chips indicate the cutting tools and machine are performing optimally and are free long, unbroken chips. This also provides for easy chip removal and more importantly a good surface finish and protection against workpiece damage.
Precision and surface quality are important for transmission components. For automotive components such as electric power steering (EPS) screws, It’s a necessary requirement for the service life of the vehicle and for the smooth operation under today’s demanding driving conditions. Just whirling alone, on a Leistritz LWN 100 is able to reach a DIN class 6 (as honed) gear quality finish with a surface roughness of Rz = 2‒4 µm. This meets the requirements of most applications and this finish class can be achieved in most materials.
The range of machinable materials is wide and identical to that for grinding, milling and turning. In the case of whirling unalloyed tempering and structural steels, these are, for example, steel C45 (material 1.0503) or machine steels. 42CroMoS4V (material 1.7227) can be processed for highly stressed components such as gear shafts. Stainless steel is the rust proof austenitic X10CrNiMoTi18 10 (material 1.4571), which is suitable for use in natural fresh water.
A vast majority of materials can be whirled without using cooling lubricants. This is an enormous process and cost saving advantage. However, minimal lubrication may be necessary for very tough materials.
Shapes and product examples
Whirling is the most economical process when it comes to producing parts with helical geometries. Components of these geometries include high-precision planetary rollers, ball screws, plasticizing screws, eccentric screw rotors, modular screws as well as a wide variety of drill types. Components with asymmetrical and symmetrical curve and rotation surfaces can also be manufactured. Eccentric screw rotors used in the pump industry are a typical representative of this type. Internal whirling can be used to produce nuts for inverted planetary drives or ball screws and trapezoidal or special threads.
Start a discussion today how Leistritz can help improve your applications with whirling, check out “How metalworking works today: whirling as a high-precision process” or contact us directly.