Advantage titanium

In response to the growth in demand for the use of surgical grade titanium in preference to 316 stainless steel for trauma related components, Mollart Engineering, in conjunction with its tooling partner Botek and a specialist supplier of cannulated tube to the medical industry, has developed a drilling process for producing holes in the difficult to machine material that creates a totally new series of options for component supply. 

Through the Chessington-based operation of Mollart Engineering the medical industry can obtain a fully developed deep hole gundrilling system for creating the holes in trauma nails, for which some 30 machines have already been delivered worldwide.

Companies also have the option to have components produced complete within the subcontract machining service provided at Mollart's Chessington, Surrey and Resolven in South Wales facilities or obtain precision machined thin wall drawn tubing from the material supplier. In this form it has the bore finished to size and is able to meet the stringent geometric tolerances and surface finish requirements demanded by the industry.

The change to the highly specialised grade of titanium Ti-6-4 ELI (6% aluminium, 4% vanadium) in favour of the traditional 316 stainless steel has already led to 80% of the US medical industry changing to the new material due to the advantages obtained. Indeed, latest figures are showing some 200,000 trauma screws are now being produced a year and this number is predicted to achieve high levels of growth as the global medical industry progressively adopts the new material.

Ups and downs

High on the priority list for surgeons to use titanium are the greater levels of purity, biocompatibility, high strength to weight ratio and resistance to corrosion. There is a far lower risk from material fatigue against stainless steel that can lead to breakage and the composition of titanium means it has the ability to flex in the human body. The material also eliminates harmful toxins being released when exposed to body fluids for long periods of time.

However, the downside in meeting the growing demands for surgical grade titanium are the problems related to machining and especially the drilling of deep holes. For trauma components depth to diameter ratios of 40:1 are common as parts such as femur and tibia nails can require through holes to be produced in excess of 400mm in depth. While the tensile strength of titanium is ideal for its intended application in the human body, this forces down the speed the material can be machined using conventional tooling.

“As a result, machining times become long and expensive and tool costs escalate due to the demands of the process and shortened life in service,” says Guy Mollart, managing director of Mollart Engineering. He describes how titanium is a very poor conductor which means it is slow to dissipate any heat generated in the cutting process causing welding at the edge of the tool. In addition, its low modulus of elasticity can cause slender workpieces to deflect if care is not taken in the machining process.

Mr Mollart continues: “These factors reduce the viability of traditional style twist drills on longer holes, not only due to the restrictions imposed because of very low penetration rates in the material but more important, in achieving, let alone maintaining the high precision demands for accuracy, geometry and surface finish.”

He outlines how very detailed development of the gundrilling process involving tool geometry, drill point support, carbide grades, feed, speed and coolant pressures overcome these limitations and allow a component such as a tibia or femur nail to be drilled in a single pass to 18mm diameter with a penetration rate of 12mm per minute. “Such is the technology that a gundrill will maintain a concentricity and straightness within 0.015mm TIR over a depth of 400mm,” he explains. Other long holes required for trauma components as small as 6mm can be successfully drilled in 13mm diameter titanium with just a very thin 3.5mm wall thickness.”

Collective thinking

Central to Mollart's development programme to meet the meticulous demands on the titanium products by the medical industry was the very close association with the specialist tubing supplier and its drilling tool partner, Botek. Between them they created a process whereby the central bore of the trauma components is gundrilled and can be supplied ‘as machined' to customers to add further value processes as part of their component supply to surgeons.

The hollow bar method involves the creation of bar lengths of 1,000mm with all the potential stresses removed, any flexing potential of the material that can build up eliminated and the bore totally finished ready for any final special processes that may be required. In creating the bar length, the material is initially turned involving several passes over its length to de-stress the material which is then gundrilled on a Mollart machine to create an 18mm diameter through hole. This bore is then finish honed in order to remove any traces of pick up or machining marks that could be amplified and lead to surface cracking and eventual failure.

The material is then mounted on a mandrel by the tubing supplier and drawn to produce the high quality femur or tibia tube to the specified diameter ready for further processing. As Mr Mollart states: “One orthopaedic company is already producing a third of its femoral nails by the hollow bar method thus avoiding any need for drilling or machining the bore.”

As a result of the new methods on femoral and tibial nails, Mollart's application engineering team has now developed the process for producing much smaller bone screws which, like other trauma products, are beginning to move away from 316 stainless steel in favour of titanium.

Traditionally, stainless steel bone screws are mainly produced on sliding head lathes where the complex thread form, screw head and through hole are produced in a single cycle. However, like femoral nails when produced in titanium the drilling of the central hole – often just 2mm diameter and between 40mm and 120mm in depth – tends to govern the output of the machine.

Also problems of tool life, part geometry and surface finish also exist. “The development of gundrilling will offer the same capability to lift productivity, ensure quality through geometric, size and surface finish,” concludes Mr Mollart.

Mollart Engineering
www.mollart.com