Thread whirling has become a popular process for sliding-head machines, especially among bone-screw manufacturers. Bone screws differ from other industrial screws in that there are no female threads to mate.

They are screwed into human or animal bones for medical repair applications and must not loosen once fixed in place; they therefore feature a large pitch size, as well as a large ‘thread depth’ and length.

Thread-whirling tools have their inserts set on the round cutter body that is driven and tilted at the appropriate helix angle. The spindle revolves at about 3,000rev/min, while the bar stock revolves in the same direction but at a much slower speed — some 10-30rev/min.

The inserts shave the bar stock at the centre of the bar, as well as the upper or the lower side of the bar stock.

To shorten surgery time, bone screws with double-lead threads were introduced recently. However, producing double-lead bone screws ‘conventionally’ takes longer than single-lead threads.

NTK Cutting Tools, Hemel Hempstead (www.ntkcuttingtools.co.uk), has developed tooling that allows the single-pass machining of double-lead screws.

The inserts have a different geometry for the first and second threads, even though both threads have an identical form.