Advances in electrical discharge machining (EDM) improve cost-performance and complex shape capabilities, making the process a viable alternative to broaching for bone screws and other medical fasteners.
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.
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.
The MV series features improvements in machine construction, auto- threading, internal machine communication, power supply technology, and operating costs. It uses the M700 series control with a 15" touchscreen and includes Mitsubishi’s non-contact cylindrical drive system, delivering advancements in wire EDM technology. https://goo.gl/wiQQvh
MV4800 is capable of submerged cutting up to 20" deep with auto- threading system support. With an annealing length of more than 27", the system is capable of threading the maximum workpiece height both at the start point and through the gap if needed for a broken wire recovery. The MV4800 features a non-contact cylindrical drive system and the M700 series control with a 15" touchscreen. https://goo.gl/ccQZSl
The FA40V Advance performs up to 36in2/hour in hardened D2 steel. A faster, large workpiece, high-accuracy wire EDM, the FA40V also features the M700 series control and a 15" touchscreen. https://goo.gl/1fVkbT
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.
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. www.njpt.com
About the author: Bob Tarantino, president of New Jersey Precision Technologies, can be reached at or bob.tarantino@njpt.com or 800.409.3000.
NEW JERSEY PRECISION TECHNOLOGIES INC. (NJPT) is one of the largest all-Mitsubishi EDM machining service shops in the northeast with more than 40 machines, including many MV2400-R, MV4800, and FA20V machines running and staffed 24-hours a day, 7 days a week.
Starting off in a high-tech business incubator on the campus of the New Jersey Institute of Technology in 1989, they began doing prototype work in electronics and the medical device field.
“When the company was created, as a team we looked at different brands of EDM machines to purchase,” says NJPT President Bob Tarantino. “We went with Mitsubishi machines because of their features and their reliability. Other brands just weren’t as easy to use or had the dependability we needed.”
Since 1995, when NJPT reorganized into an EDM company specializing in medical device wire EDM and high-precision tooling, they have become more diverse. Although most of its work is centered in orthopedic and cardiovascular instrumentation, roughly 70% is in the medical device field. The company also serves as a contract manufacturer for the aerospace, tool and die, plastic extrusion tooling, and defense industries. Success allowed the purchase a 10,000ft2 building in Mountainside, New Jersey, in 1999, later expanding to 21,000ft2.
Two of the advantages that keep NJPT at the forefront are expert software system they developed to program EDM machines and a focus on employee training. Software extracts geometries, notes about material types, heat-treating requirements, and labels from drawings, then interprets the data to create process decisions without operator intervention. This simplifies program development, getting parts to the machine faster.
NJPT is also an intense engineering training company, with 75 points of training engineers must go through to be certified in EDM – believing that the only way to grow the business and continue delivering the best service is by offering continuous learning.
“Mitsubishi machines are very reliable and easy to program and use,” Tarantino says. “From a practical point of view, we can move our operators from previous to newer-generation models, as machines continue to use the same terminology. This helps us increase our throughput and provide faster turnaround times, which improves our customers’ shrinking time-to-market and reduces their related cost-to-market.”
In the rapidly changing medical device industry, part geometries are becoming increasingly complex and, in many cases, can only be produced using the EDM process. NJPT has used Mitsubishi machines to adapt quickly to these changes in medical devices and special features. The 45-degree angle wire-cutting special features on medical devices doesn’t require special fixturing. and 45° angle wire-cutting of features does not require special fixtures.
“The Mitsubishi EDM machines are easy to operate and reliable, but also the service department is by far the best in the industry,” Tarantino says. “We have achieved impressive up-time due to their service department which provides scheduled preventative maintenance or in the uncommon event of any emergency repair.”
NJPT also uses MC Remote 360, which delivers real-time machine data to mobile devices and desktops. Operations can increase productivity and efficiency while preventing disruptions and escalating service issues.
MC Machinery Systems Inc. www.mcmachinery.com