Heavy-duty whirling attachment with motor

A heavy-duty whirling attachment driven by its own, high power motor, to thread-whirl components like self-tapping bone screws and surgical implants, has been introduced by Geo Kingsbury.

If a sliding-head lathe user wants to thread-whirl components like self-tapping bone screws and surgical implants, the machine supplier will normally provide an attachment that fits into a live position in either a tool turret or the front end-working station.

The German lathe manufacturer, Traub, in addition to offering that solution, has now introduced a far more robust alternative in the form of a heavy-duty whirling attachment driven by its own, high power motor.

Availability in the UK is through sole agent, Geo Kingsbury Machine Tools.

Among the first British users is Eastleigh-based contract machinist, Ottaway Engineering, which retrofitted the unit in mid 2006 to its Traub TNL26 sliding-headstock mill-turning centre.

Established in 1964 and currently employing 30, the firm specialises in close tolerance work in difficult-to-machine materials such as obscure grades of stainless steel, titanium, high nickel content alloys, and plastics that tend to move after they have been cut; generally the jobs that nobody else wants.

Industries served include aerospace, defence, medical, instrumentation and optronics.

Having extra thread whirling power and rigidity allows the subcontractor to produce up to 4.5 mm deep threads at high helix angles in a single pass on its bar-fed Traub lathe.

The independent whirling unit is too heavy for mounting in a turret and has to be secured, but not driven, by two of the five stations in the front endworking slide.

This rigid construction provides for vibration free machining under extreme conditions.

Standard materials can be machined in typically half the time and hard or tough metals can be tackled with comparative ease right up to the available 200 mm length, according to managing director, Luke Ottaway.

In the case of multi-start threads (three-start for example), the headstock feeds the bar out a short distance through the whirling attachment, which remains stationary as the first thread is being cut.

As it is normally not possible to feed the entire thread length back through the guide bush to start the next cut in a similar way, the spindle indexes through 120 degrees and the bar is clamped while the whirling head is traversed onto the short length of unsupported bar to cut the second thread.

This is repeated for the third thread.

Mr Ottaway remarked that there is no discernible witness mark at the many points where machining recommences.

Surface integrity on the flanks of the thread is therefore maintained, which is essential for medical implants to insure against ingress of bacteria.

Good surface finish and absence of burrs makes whirling the method of choice for threading medical components.

To obtain parallel results from conventional methods, thread milling or hobbing will take up to four times longer and in the case of single-point chasing, around 10 times longer - if the thread can be cut at all.

The Traub TNL26 was installed at the Eastleigh works in 2003.

It is the only sliding-head lathe on site, so Mr Ottaway wanted a machine of 26 mm bar capacity that could tackle the widest possible range of jobs.

The rigidity and high power spindle of the German-built machine enable full-contact gear hobbing and broaching of stainless steel, while the availability of a Y-axis was unusual at the time, as was the ability to have four tools in-cut simultaneously.

Unpainted steel surfaces in the machining area and good quality seals around the machine allow water-based coolant to be used, rather than neat oil, which is the norm for sliding-head turning.

Mr Ottaway was keen to avoid neat oil, as while it is good for a machine it is less so for an operator.

With batch sizes at the Eastleigh works typically in the 50 - 500 range, there is frequent manual intervention for set up and hence a lot of operator contact with coolant-covered surfaces.

A notable feature of Traub's self-powered thread whirling attachment is the clamping of six, rather than three or five, inserts around the inner diameter of the cutting head.

Moreover, in place of conventional circular inserts that are difficult to set, often require special gauges and do not allow tight geometry to be held, Ottaway Engineering has scored another UK first by using new, indexable, carbide-coated inserts from Horn.

The three-cornered, profiled inserts are stronger than similar types on the market, allow greater cutting depth and can be set quickly to a consistent height using a positive location system.

This prolongs tool life and allows tolerances of a few microns to be held, with excellent surface finish.

Once the inserts have been located, the cutting head has been secured in the whirling head and the unit inclined to the required helix angle, the head rotates at typically 3,000 rpm around a slowly rotating (10 - 20 rpm) workpiece, which ensures better concentricity than if the bar were not turning.

Speed of axial bar feed determines the thread pitch.

All parameters are set centrally from the machine's Traub TX8i control.

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