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Simultaneous Whirling and Turning – Synchronized Whirling for Simultaneous Turning and Whirling to Increase Effi ciency in Thread Production

Synchronized whirling is a type of whirling with increased rotational speed of the workpiece. Exploiting this altera-tion on, a new process was developed for simultaneous whirling and turning in order to enhance productivity of machining of bone screws, see Fig. 1. Compared to con-ventional whirling, process time is reduced signifi cantly by parallelization of whirling and turning.

Conventional whirling is a machining process related to form milling. The whirling head is a ring which rotates at high speed with its cutting tools facing inwards. The whirl-ing head envelopes the slowly rotating workpiece, see Fig. 2. The envelopment of the workpiece results in a longpath of engagement between cutting tool and work-piece. This leads to favorable cutting conditions and often renders a burr-free workpiece. The good cutting conditions

also allow for machining of diffi cult-to-machine materials. For this reason, whirling is used for production of threads with high surface requirements, such as spindles for lead-screws and ball screws, pump screws, extruder screws, and bone screws.

Division III – Mechanical and Electrical Engineering

wbk Institute of Production Science

Figure 1: Simultaneous thread whirling and longitudinal turning in the production of bone screws.

Figure 2: Process kinematics of thread whirling

Karlsruhe © KIT 2017

In many cases, a portion of the material to be removed is located above the outer diameter of the thread, see Fig. 3.Cutting the material above the thread by turning allows increasing the feed rate for whirling, while reducing wear of the cutting tools. However, due to the low rotation speed of the workpiece during whirling, turning is not possible at the same time and requires an additional, preceding process step.

New Simultaneous Process

The new, simultaneous process was developed within the framework of the joint research project “GeWinDe“. Unlike conventional whirling, rotation speeds of tool and work-piece are synchronized according to the ratio of threads and number of cutting tools in the whirling head. In between the engagements of two consecutive cutting tools, the work-piece completes a whole revolution around its axis. This allows rotating the workpiece fast enough for a turning operation at the same time. The whirling tool only cuts the thread, while the material beyond the outer diameter of the thread is removed by turning. The removed material, process forces, and, hence, wear of the cutting tools on the whirling

head are reduced considerably. The thread is machined from the blank in a single integrated process step.

Increased Feed Rates through Improved Cutting Conditions

The kinematics adapted by synchronization result in a change of the engagement path of the cutting tools with the workpiece. Compared to conventional whirling, the synchronized rotations of workpiece and tool enhance the envelopment of the workpiece, see Fig. 4. The already favorable cutting conditions of conventional whirling are further improved. Kinematic roughness and cutting thick-ness decrease. Therefore, feed rates in rotary whirling can be higher without increasing the load of the cutting tools or the kinematic roughness. The integrated process of simulta-neous whirling and turning signifi cantly reduces production time. The change in cutting conditions allows increasing the feed rate, while the same surface quality is maintained and tool load reduced.

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Figure 4: Path of cutting tool engagement with the workpiece for conventional whirling (a) and synchronized whirling (b).

Figure 3: Material removal at the thread of a bone screw.

Joint Project:GeWinDe – Effi cient whirling of threads through simultaneous turning

Project coordinator:

Project partners:

Karlsruhe Institute of TechnologyKaiserstraße 1276131 Karlsruhe, Germany

Jan Klosewbk Institute of Production Science Phone: +49 721 608-47865Email: jan.klose@kit.edu

This research and development project is

funded by the German Federal Ministry

of Education and Research (BMBF)

within the Framework Concept

“Research for Tomorrow’s Production”

and managed by the Project Manage-

ment Agency Karlsruhe (PTKA). The

author is responsible for the contents of

this publication.