Increasing demand for bone screws, implants and other micro-components creates a need for new techniques that produce specific thread forms quickly and with high tolerances.
The hard outer shell and a softer, almost sponge-like core of the human bone requires bone screws to be hard and sharp enough to penetrate the outer shell, without the thread form separating from the soft core, according to John Dotday, Business Development Manager for Sandvik Coromant.
The competitive market of bone screw design sees manufacturers change their thread profiles every two to three years, with each profile coming in four different screw designs. A bone screw can be as unique as the human bone it penetrates, and in some cases is designed personally by an orthopaedic surgeon for their patient.
Available in any number of lengths and diameters in standard and custom designs, bone screws tend to have a high aspect ratio, requiring the need for special support devices during manufacture when using the traditional single-point threading tool. Bone screws also have deep threads, requiring many short cuts to be made, which is a slow process that leads to shorter tool life.
Another fundamental limitation associated with using a single threading cutter for machining bone screws is the limited helix angle of only seven degrees. Modern screws are designed with helix angles up to 20 degrees.
These drawbacks led the medical industry to shift from single-point threading tools to thread-whirling for the production of bone screws. Thread-whirling is a type of thread-milling process in which the cutter edges of the tool reside on the inside of the cutter ring rather than the outside. The entire threading operation is performed in a single pass, typically using a thread-whirling attachment on a sliding head machine.
Other steps in the bone screw manufacturing process including turning of the front and back of the screw head, drilling of the head, and clearance turning of the hole bottom are also performed using the thread-whirling attachment.
Sandvik Coromant’s thread-whirling ring, the CoroMill 325 uses high-performance GC1105 inserts, which are mounted at a differential pitch on the ring. Chatter, one of the biggest potential problems in thread whirling is addressed with differential spacing. Inserts with differential pitch allow chatter to become a non-issue, significantly extending tool life. For standard threads, inserts are designed to create HA and HB thread forms, with the specific dimensions and tolerances spelled out in ISO 5835-1991for medical screws.
Thread-whirling uses a tangential cutter path and its increased rigidity enables improved surface finish with the burrs minimised due to reduced chip load, giving increased tool life. Thread-whirling delivers a finished surface quality that equals or surpasses that of a grinding process.
Thread-whirling is ideal for difficult to machine materials such as 316LVN stainless steel, titanium or cobalt chrome, which are commonly used in today’s bone screws.
The full article is featured in the latest issue of the AMT magazine by AMTIL