Chris Foschaar  is application/sales engineer with  Horn USA Inc.Click image to enlargeby Chris Foschaar

Thread whirling may be the better option over single point threading


As many know, thread whirling is a version of thread milling–with an exception: the inserts are mounted on the internal diameter of the cutter body instead of being situated on the periphery of the cutter body.

Thread whirling units provide the proper side clearance for inserts by pivoting on an inclination angle relative to the component. This allows for minimal stock to be removed when grinding side clearance on the inserts, thus creating stronger cutting edges for a stable machining process.

The process
A whirling ring rotates at high speed around a slowly rotating workpiece (C axis). The rotation of that workpiece when combined with the Z axis or sliding head stock, correspond to the thread pitch that’s required for the application. Cutting tools in thread whirling are mounted in the tool ring and move relative to the workpiece. By implementing cutting inserts in the whirling ring with properly designed thread form, edge preparation, and coating selection for your specific application, you will be able to increase the cutting parameters. Additionally, this will allow for increased throughput on the machine and the potential for extending insert tool life.

Thread whirling is a good choice for machining parts in the medical and dental industry (e.g. bone screws, spinal screws and dental implants), equipment manufacturing (e.g. lead and ball screws), and automotive (e.g. worm shafts).

The process is recommended to be used with Swiss CNC lathes, sliding headstock style machines, of which there are many builders, including Citizen, Star, Tsugami, Nomura and Hanwha. Thread whirling can also be performed on application specific machines, such as the Leistritz whirling machines.

To select the right machine for thread whirling, you need to consider several factors:

  • applications and target markets
  • whether the machine will be dedicated or diversified
  • complexity of your applications (e.g. material types, tolerances and thread form types as multiple whirling units may be required)

The benefits
Thread whirling may be a better option than single point threading for most applications and here’s why. If you consider the example of bone screws being machined using a single point threading process, it requires multiple threading passes for roughing and finishing as well as special support for longer screws. Unique thread forms during this machining process can cause deflection and taper that will impact the final quality of your thread. Lastly, this process is typically associated with long cycle times.

Using the machining process of bone screws, thread whirling is up to four times faster than single point threading. You will also achieve greater tool life and higher part quality. The improvement in surface finish is the result of the tangential cutter path and the increased rigidity. And unlike single point threading, it gives you the ability to machine dual lead thread forms in one pass.

In summary, you get the accuracy of single point threading with the benefits of thread milling.

If you’re considering this process for your threading needs, remember to consult your tooling supplier. There are multiple types of insert cutters and holders, and different whirling rings (e.g. modular) for different types of machine tools. SMT

Chris Foschaar is application/sales engineer with Horn USA Inc.


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