CNC sliding head autos perform thread whirling

CNC sliding head automatic lathes can perform thread whirling to produce deep and course thread pitches, taper threads and special forms with often burr-free sharp edges on short and long parts.

The ability of the thread whirling process to generate very difficult to machine deep and course thread pitches, taper threads, special forms with often burr-free sharp edges has found a new lease of life in the application of the process through CNC sliding head autos and particularly in the production of medical components.

This has largely come about from the rapid rise in the demands of the medical sector for implants such as bone screws and bridges with their high surface finish and tolerance requirements in difficult materials such as titanium and high grade stainless steels.

As a result application development on threads as small as 1.2mm diameter has taken advantage of the construction and configuration of the sliding head automatic lathe that provides an ideal platform for medical type components by combining a multitude of processes into a high precision, single 'one-hit', cost effective turn/mill cycle.

And, following an increase in awareness of the process, new applications are spinning-off almost daily for thread whirling with the same advantages being applied to non-medical parts such as multi-start worms, scrolls and course pitch spirals across a wide range of different sector applications.

The thread whirling head such as those supplied by Floyd Automatic Tooling of Baldock in Hertfordshire, UK, can also be retrofitted to any CNC sliding head machine providing there is space to mount the unit and a drive to the tool head is available.

This capability also further improves the adoption of the process.

In order to whirl a thread, a multi-tooth cutter is rotated around the outside diameter of a part to be threaded to generate the thread form in a single pass, which is considerably quicker than other methods of creating threads.

And here the advantage of the sliding head auto is outlined by Citizen's UK agent NC Engineering of Watford that the complete cutting action is performed very close to the guide bush of the machine creating maximum bearing support adjacent to the cutting tool insert and thus providing an extremely rigid set up.

Says managing director Geoff Bryant: "This machining scenario, similar to a steady used on fixed head lathes, enables the thread whirling process to be exploited providing precise control of the thread size and surface finish on the flanks of the thread while maximising the material removal with better than average tool life.

Also, with this configuration, long lengths of consistently high accuracy thread, up to the capacity of the machine can be produced in a single pass.

The attachment to create the thread whirling process comprises a tool holder body, that in the case of a Citizen machine, can be mounted on the turret or a tool slide using the normal rotating tool drive which is run at around 3,000 rev/min.

The toolholder can contain either three or six indexable carbide inserts mounted radially in the holder that are ground to the full form of the thread.

The holder is then inclined and set at the required helix angle of the thread.

The bar material used to produce the part is rotated in the spindle at around 60 rev/min in the same direction as the whirling head which helps maintain roundness and concentricity.

At the start of the cut, the whirling head is fed direct to finish depth using the programmable X-axis enabling the complete thread form to be generated in a single cut.

Within each revolution of the spindle, each insert is in contact with the material with the Z-axis feed of the machine programmed to create the pitch of the thread.

When the required length of thread is machined, the whirling head is retracted radially then axially to clear the workpiece.

The application team at NC Engineering has carried out considerable development work in conjunction with Floyd Automatic Tooling where director Eric Chamberlain maintains threads as small as 1.2mm in diameter, developed for the dental industry can now be whirled with the maximum diameter and the length of thread or part governed by the capacity of the machine.

This length for the Citizen sliding head machine is 32mm diameter by 320mm long.

Explains Chamberlain, compared to a conventional fixed head lathe, which requires a tailstock or steady to support the material when producing longer threads, problems mount when difficult materials are involved.

"There is absolutely no comparison between the two machine processes when a CNC sliding head machine is used," he insists.

However, Bryant maintains, specifying the process with the latest sliding head machine developments takes advantage of easier programming, and the generation of even higher productivity gains with the added reduction in non-cutting or idle times and faster data processing at the control.

In addition, the recent addition of a programmable unloading device from Citizen eliminates possible damage to a finished component by controlling its transport out of the working area of the machine.

Parts up to 200mm long can be automatically unloaded to an in-built conveyor with the cycle performed while the machine is continuing with production of the next part.

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