John Dotday, Business Development Manager, Sandvik Coromant, Fair Lawn, NJ.

CoroMill 325 from Sandvik Coromant

The increasing demand for bone screws, implants and other microcomponents creates a need for new techniques that produce specific thread forms quickly and with high tolerances. Human bone has a hard outer shell and a softer, almost sponge-like core. As a result, bone screws need to be hard and sharp enough to penetrate the outer shell, but the thread form must resist from separating from the soft core. In the competitive market of bone screw design, manufacturers change their thread profiles every two to three years, and with each profile might come 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 orthopedic surgeon for his/her patient.

Bone Screws & Threading

Bone screws come in any number of lengths and diameters, in standard and custom designs. Generally, they tend to have a high aspect ratio (screws are much longer than their diameter), making their manufacture with a traditional single-point threading tool problematic because the screw tends to bend during cut. With single-point threaders, special support devices are needed for the screws. In addition, bone screws have deep threads, so many short cuts must be made, which is a slow process that leads to shorter tool life. Finally, finishing steps are typically required.

The other fundamental limitation associated with using a single threading cutter for machining bone screws is the limited helix angle of only 7°. Modern screws are designed with helix angles up to 20°.

As a result of these drawbacks, a few years ago the medical industry began making the 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 headmachine. Other steps in the screw manufacturing process — 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.

Thread Whirling

A thread whirling attachment consists of the ring (holder) and inserts. It can quickly be attached over the spindle screw outside of the machine in a clean environment.

Most machinists use a CNC Swiss-style lathe to perform thread whirling. The Swiss-type turning centers are well suited for this task because the cutting point is close to the guide bushing for increased support for long length to diameter screws. The shorter distance between the cutter and guide bush also minimizes vibration during the cutting operation. Swiss-type lathes have the precision and repeatability needed for a variety of medical tooling applications including thread whirling. The thread whirling ring is simply mounted onto the spindle. For very high-volume bone screw production, an alternative to using a thread whirling attachment on Swiss-style lathe is a dedicated thread-whirling tool.

Sandvik Coromant’s thread whirling ring, the CoroMill 325, uses high-performance GC1105 inserts, which are moutned at a differential pitch on the ring. With differential spacing, one of the biggest potential problems in thread whirling is addressed: chatter. 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 (Figure 2), with the spefic dimensions and tolerances spelled out in ISO 5835-1991 for medical screws.

During whirling, the workpiece rotates slowly while the tool rotates at very high speed in the same direction. The thread pitch dictates the tool rotation rate and the bar feed rate. The ring is positioned slightly off-center from the part so that a single insert engages the workpiece per rotation. The helix angle (up to 25°) is set by the angle of the cutter ring relative to the workpiece. Each insert is designed specifically to achieve a desired thread profile.

Machine and Bone Screw Maintenance

Thread whirling uses a tangential cutter path and its increased rigidity means that surface finish is improved and burrs are minimized due to reduced chip load, for increased tool life. With thread whirling, finished surface quality equals or surpasses that of a grinding process.

Thread whirling creates well-defined chips on materials that are normally difficult to machine, such as 316 LVN stainless steel, titanium or cobalt chrome, common materials used in today’s bone screws. Generally speaking, titanium, with its properties of very light weight, high strength and biocompatibility, is used in bone screws that will remain in the body, while stainless steel is used in screws that the surgeon will later remove.

Originally published in Society of Manufacturing Engineers Medical Manufacturing Yearbook, a manufacturing magazine published by Manufacturing Engineering Magazine.