Medical component machining contracts are highly sought, for parts that include interventional cardiology devices (catheters, surgical tools), orthopaedic devices (bone screws, implants, joint replacement parts), minimally invasive surgical devices and equipment (laparoscopic parts), diagnostic instruments, wound care parts (staples, suture anchors and clips), and dental equipment and implants, to name just a few.
So what special considerations does the medical market hold for those who wish to make medical parts or devices? At a recent open house held by sliding-head turning centre supplier Tornos (01530 513100), the company's application engineering team gave a presentation on the special machining processes used to make turned components for the medical market.
The presentation provided an insight into the machining of PEEK (polytheretherketone), for example, which is used widely for medical component applications. PEEK doesn't dissipate heat like metal does, so pure water coolant is essential. Alternatively, PEEK can be machined dry – Tornos says it has used special cold air lines and dust collection systems with great success at customers such as Metronic and Smith & Nephew.
Other topics covered by the Tornos presentation included micro-machining, thread whirling (used typically for bone screws), special workholding, such as step collets, and gundrilling on parts such as cannulated or hollow bone screws.
Image: Tornos technology is helping to manufacture parts used in dental implants
AHEAD OF THE GAME
One subcontract manufacturer convinced about the benefits of sliding-head technology is Staffordshire Precision Engineering (SPE) of Newcastle-under-Lyme, which has recently invested in an SR-20J, supplied by Star Micronics (01332 864455).
Image: Star technology helped Staffordshire Precision Engineering win medical instrument business
The contract that clinched the acquisition was for the manufacture of a medical instrument – a device used in shoulder repair operations. Tolerances on the surgical component were tight, but shortly after SPE started machining the component on the Star, a quality control engineer from the customer visited with a digital micrometer to measure a critical width dimension. He noted that on every part he tested, the measurement fell right in the middle of the 20-micron tolerance band, which was not the case before.
Maintaining the sliding-head theme, Birmingham-based Rowan Precision is using its four recently installed M32-V CNC machines (three fitted with the CoolBlaster III 2,000 high pressure coolant system) from Citizen (01923 691500) to manufacture a 316 stainless steel medical industry component in a single operational cycle time of 13 minutes.
The part is 7 mm diameter by 24 mm long, requiring a considerable amount of the outside diameter to be milled away. It is initially spot-drilled and drilled through at 4.8 mm diameter, with a 2 mm wide longitudinal keyway milled in the OD that breaks into the bore. There is a further spot and cross-drilled hole of 2 mm diameter and another long keyway form, 1.65 mm wide, that has to be milled into the OD.
Transferred to the sub-spindle, a very complex milled profile using a 2.5 mm diameter end mill removes a quadrant of the OD over two-thirds of its length, with the spindle then indexed 180° and the operation repeated on the adjacent quadrant to match and line up with the first profile. When completed, almost three-quarters of the material has been removed from the wall of the component at one end and half the material in the centre.
PUT THROUGH THE MILL
It is a known fact that a large number of turned medical parts feature relatively heavy milling content. For this reason, a leading medical sector OEM, Smith & Nephew Orthopaedics Ltd of Leamington Spa, has recently invested in its tenth Doosan multi-axis mill-turn centre from Mills CNC (01926 736736).
The machine, an MX 2000ST, is being used to manufacture precision orthopaedic ball (femoral) and socket (acetabular) components used in BHR (Birmingham Hip Resurfacing) surgical procedures, which uses low wear, metal-on-metal bearing resurfacing technology. BHR components are made from cobalt chrome castings: roundness tolerances are generally less than 2 micron, while surface finish requirements are in the region of Ra 0.015 micron.
For medical components that are predominantly prismatic in shape, machining centre technology is still the preferred option, as demonstrated recently by Newcastle-under-Lyme subcontractor Greenway Pepper Precision Engineering, following its installation of a Brother TC-32BN QT 4-axis vertical machining centre from Whitehouse Machine Tools (01926 852725).
As an example of the savings being enjoyed at Greenway, a side plate (made from 6082 T6 aluminium) for a new instrument that allows on-the-spot DNA profiling by medical staff and police forces was previously produced in one hour and 10 minutes on a standard, one-year-old VMC – it now takes just 26 minutes to machine on the Brother. Overall, cycle times were more than halved for producing this and five other components transferred in the first month.
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 new series of options for component supply.
The change to the highly specialised grade of titanium Ti-6-4 ELI, from the traditional 316 stainless steel, has already led to 80 per cent 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.
However, the downside in meeting the 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 in excess of 400 mm in depth to be produced. While the tensile strength of titanium is ideal for its intended application, it forces down the speed at which the material can be machined using conventional tooling.
However, 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 18 mm diameter, with a penetration rate of 12 mm per minute. Such is the technology that a gundrill will maintain concentricity and straightness within 0.015 mm TIR over a depth of 400 mm.
Moulding a healthy future
Technology from Matrix MicroScience, innovative product design from DesignEdge and manufacturing expertise from Midas Pattern (01234 358394) have created a laboratory testing unit that reduces the time needed to detect pathogens. One of the first uses of 'Pathatrix Auto' has been for the pre-admission screening of elective hospital patients for the MRSA bacteria, thus preventing the introduction of the disease to the wards.
The design features five sample 'cassettes', each of which holds a plug-in cartridge of test consumables. Over the course of several meetings between the designers and Midas Pattern, the details of the housing moulding were refined to ensure trouble-free manufacturing, as well as optimising the position and angle of the touch-screen, which, due to its location on the upper curved face, is not in the line of draw of the moulding.
It was decided to manufacture the mouldings using the Midas composite resin MRIM tooling process, which uses a blend of laminated and cast resins, incorporating areas of CNC machined aluminium and steel to produce high quality polyurethane mouldings on short production lead-times, and which can accommodate both thick and thin sections, square faces, undercuts and cast features.
First published in Machinery, July 2010