Broaching has traditionally been considered the most economical means of producing medical screws, such as cortical, cancellous, lag bone, and transfixation that are used to secure implants in the body and accurately position devices used during surgical procedures. Yet today, sinker electrical discharge machining (EDM) – also known as traditional EDM, or ram EDM – has proven to offer performance advantages compared to broaching, becoming a cost-competitive alternative.
Typically, the screws that remain in the body following a surgical procedure, bone screws, are made from either titanium or cobalt chrome, while screws only used during the surgical process are made from stainless steel. While machining on a computer numeric control (CNC) Swiss-type machine is the most commonly used process for generating medical screw threads, there are alternative methods for creating the screw’s driving feature, the internal shape of the screw head including hex, hexalobular (Torx), squares, and many customer-specified shapes.
Broaching screw heads
Manufacturers often use broaching techniques to create screw head internal shapes. Broaching uses a tool to remove material in a linear or rotary fashion. In linear broaching, the tool runs linearly against the surface of the workpiece to produce a cut. In rotary broaching, the broach is rotated and pressed into the workpiece to cut a shape.
To create that screw-head internal shape with sinker EDM, an electrode and the workpiece are submerged in a dielectric liquid and subjected to an electric voltage. Although the electrode and workpiece do not make contact, current flow between the two causes reversely charged materials to be removed, creating a wide range of shapes in many types of conductive materials.
Medical screw-head internal shapes must be completely burr-free, accurate, consistent, structurally sound, and economically producible in short turnaround times. The screws also need to be produced in a wide variety of sizes, shapes, and materials. While many manufacturers use broaching, the sinker EDM process meets all of these requirements and exceeds the capabilities of broaching.
Burr-free – Any type of burr left behind after a manufacturing process has the potential to eventually detach and cause patient discomfort (or worse). Broaching, a high-impact method of metal removal, has the potential for generating burrs as tools wear. Sinker EDM is a material-erosion process, so no chips are produced. Also, with no contact between the sinker EDM tool (the electrode) and the workpiece, there is no tool wear that can cause burrs in the bone screw’s driving feature.
Accuracy – Broaching has proven to be an accurate means of production as long as the broaching tool remains in good condition, but sinker EDM is more accurate. The sinker EDM process can hold 0.0002" total tolerance on size and 0.0005" total tolerance on depth. This capability is becoming more important as customers tighten their product tolerances to assure the driving tool inserts the screw as effortlessly and effectively as possible during surgery.
Consistency – Broaching yields good part-to-part consistency but is highly dependent on the material being cut and the broaching tool’s condition. Sinker EDM yields more consistent results due to the absence of tool wear. Electrodes are redressed after every run to assure optimum performance. Mitsubishi sinker EDM machines provide consistent spark gaps during the production process, while System 3R tooling provides positioning repeatability due to pallets that align and secure electrode holders with screw holders.
Structural integrity – A fast, high-impact manufacturing process, broaching can introduce stress to the work piece, potentially leading to material degradation that often cannot be seen immediately but can lead to premature part failure. With no tool contact or heat-affected zones created on a part, sinker EDM does not introduce such stresses, improving structural integrity of medical devices.
Economical, short turnaround time – Linear and rotary broaching are fast operations that can meet the needs of low-cost, short-turnaround production. However, advances in sinker EDM technology and tooling have reduced processing costs for this alternative. For example, custom-designed, multiple-electrode part holders accommodate multi-up applications where as many as 100 parts can be completed in one production cycle. Companies such as New Jersey Precision Technologies Inc. (NJPT) have numerous machines that can produce multi-up jobs, assuring short production turnaround time. Likewise, tooling costs – always a source of concern on sinker EDM operations – have been greatly reduced due to the large inventory of electrode sizes and shapes. For example, with more than 1,000 screw and shape configurations already part of NJPT’s tooling library, it is likely most customers would not incur tooling costs. If a customer requires an electrode size larger than one in inventory, the Mitsubishi CNC Sinker EDM machines can orbit in any pattern to achieve the specified size.
Variety of part sizes, shapes, and materials – Metal cutting processes, including broaching, have traditionally faced numerous challenges when producing titanium and cobalt-chrome parts for the medical industry. The difficulty of handling these materials diminishes with sinker EDM handling any conductive material equally well. Broaching can only machine internal shapes that are produced on the broach (and in the case of rotary broaching, can be rotated) and is restricted to producing flat or uniform bases in a driving feature.
Sinker EDM can offer any shaped driving feature such as a three-dimensional side-wall shape with tapers and non-perpendicular side walls. It can also make the base of the screw-driving feature any three-dimensional contour or shape and can even produce a multi-step feature if required.
Moving to EDM
There is no doubt that sinker EDM has found a place in the manufacture of multi-feature screws used in the medical industry. Many medical device manufacturers are already calling for sinker EDM as the preferred means of manufacturing the internal features of medical screw heads.
The constraints of sinker EDM as a viable alternative to broaching have all but disappeared because of numerous advances in technology and tooling. State-of-the-art power supplies in the latest sinker EDM machines have greatly reduced heat at the point of metal removal. As a result, designers can specify unique, thinner-walled features in the screw head to reduce cost without adversely affecting a product’s performance throughout its life cycle (or the fear of cracking or snapping off a screw head when broached).
New Jersey Precision Technologies Inc.