Engineering Insight

Mollart Engineering created a manufacturing system that makes use of deep-hole gundrilling to machine titanium orthopaedic components such as bone screws.

In recent years, there has been a growth in demand for surgical-grade titanium for trauma-related components. Compared with 316 stainless steel, biocompatible titanium offers superior corrosion resistance and a favourable strength-to-weight ratio. Titanium, which can flex when implanted in the human body, also performs better than steel at resisting material fatigue that can lead to breakage. 

In particular, the Ti-6-4 ELI specialised grade of titanium has become popular among medical device companies in recent years. At present, an estimated 80% of the US medical device industry is using that alloy. Recent figures show some 200 000 titanium trauma screws are now produced annually and this number is predicted to grow as medical device companies across the globe begin to specify titanium in their products.

Surgical-grade titanium is a difficult material to machine, and this may lead to supply chain issues as demand for the material surges. Drilling deep holes in the material, for instance, can be challenging. The tensile strength of titanium reduces the speed with which the metal can be machined using conventional tooling. In addition, titanium is a poor conductor and is thus slow to dissipate heat generated during the cutting process, explains Guy Mollart, Managing Director of Mollart Engineering (Chessington, UK). In addition, its low modulus of elasticity can cause slender workpieces to deflect if care is not taken during machining. “These factors reduce the viability of using traditional twist drills to make longer holes,” Mollart says.

To facilitate the machining of titanium parts for orthopaedic applications, Mollart Engineering, working with tubing supplier Botek, developed a deep hole gundrilling system for the fabrication of trauma nails and similar applications. Mollart’s application engineering team developed the process for the production of small bone screws. Like other trauma products, these are increasingly being produced from titanium instead of 316 stainless steel.

Traditionally, stainless-steel bone screws have been made on sliding head lathes that manufacture the complex thread form, screw head and through hole in a single cycle. Similar to the production of titanium femoral nails, the drilling of the central hole tends to govern the output of the machine. Problems with tool life, part geometry and surface finish also exist. 

The “hollow bar” method employed in Mollart’s system involves the creation of 1000-mm bar lengths with the potential stresses removed. The method neutralises the flexing potential of the material. The material goes through several destressing passes as the bar lengths are created, after which it is gundrilled to create 18-mm-diam through holes. Afterwards, the bore is finish honed to prevent surface cracking and the material is mounted on a mandrel and drawn to produce the femur or tibia tube to the specified diameter. At that stage, it is ready for further processing. 

The constitutive elements of the gundrilling process—tool geometry, drill point support, carbide grades, feed, speed and coolant pressures—enable manufacturers to overcome the limitations of conventional techniques. Holes in tibia or femur nails can be drilled in a single pass to 18 mm diameter with a penetration rate of 12 mm per min. “The development of gundrilling will offer the capability to lift productivity and ensure quality through geometric, size and surface finish,” says Mollart. “Long holes required for trauma components as small as 6 mm can be successfully drilled in 13-mm-diam titanium with a 3.5-mm wall thickness,” he adds.