When to EDM Medical Parts

Over the last few years more and more large medical companies and their suppliers shifted part productions from conventional machining to EDM. This article explores growth of EDM as a manufacturing technique for medical implants and other medical device components.

When we refer to EDM Electrical Discharge Machining in general, it is important that we differentiate between the two different methods. The first EDM technique to emerge was die sinking or ram EDM; wire EDM was developed when CNC controls became available. Both technologies have become quite advanced over the last two decades in terms of cutting speeds, accuracy and surface finish quality.

Compared to other machining processes, EDM offers a variety of advantages when cutting complex medical components including surgical tools and implants. Let's take a closer look at the process characteristics before we delve into specific examples.

The following bullet points highlight the critera by which to measure overall performance areas in which EDM consistently outperforms conventional machining:

• Surface cleanliness
• Tight tolerances
• Small parts (<10 mm 2 surface area)
• Sharp inside corners
• No burrs
• Automation
• No deformation
• Small holes or openings

Surface Cleanliness

Lets start with the surface cleanness -- a most important criteria in the manufacture of implants and surgical tools. If this is a criterion by which you measure your parts, Charmilles' new CC - EDM generator technology ensures a maximum surface cleanness, avoiding “back-plating” (polluting) of the wire material onto the surface finish. This technology also ensures that the cleaning process is minimal when compared to the milling process, which uses oils and other coolants. The graph shows how the surface cleanliness is improved to older generator technologies.

Tight tolerances

The EDM process performs particularly well when the part requires tight tolerances. EDM operators can use Charmilles' interactive EXPERT systems already integrated in the wire EDM CNC by answering some simple questions, e.g. workpiece material, part accuracy, desired surface finish and smallest inside radius.

The EXPERT system then automatically generates a specific cutting technology, including offsets for each skim pass, so that the final part is exact. Since the wire EDM always uses a new tool (continuously unspooling EDM wire) wear does not influence the tolerance of the final part.

Small Parts

Non-invasive surgical tools demanding ‘micro' parts are impossible to produce with milling or grinding. Twin Wire EDM technology now allows manufacturers to cut these parts unattended, not too mention quickly, by automatically switching between large diameter wire and small diameter wire.

Sharp inside corners for Bone Saw Guides

These surgical tools require a clean surface and accurate taper angles with small radii in the corners. A Wire EDM with a B-Axis can produce these parts in one setup. The part can be fixtured in a simple vice connected to the B-Axis and positioned to the required angle so the wire is vertical to achieve maximum cutting speed. The only necessary preparation of the part would be to drill a start hole to automatically thread the wire in the

individual slot. After the first slot is cut the B-Axis positions to the next position and the process is repeated.

Material and other cost savings

Medical implants like spinal implants, trauma implants, micro implants and dental implants are often made out of expensive alloys. The wire EDM process cuts only the shape desired out of a block of material, saving the rest of the material for other parts.

Titanium alloys can be flammable if milled or ground. The wire EDM process, which occurs fully submerged under water, prevents this fire hazard altogether, and can result in significant cost savings.

Micro implant production

Micro implants are often made out of very thin (< 0.02") material, and are very complex in shape. By stacking several (e.g. 300) pieces of thin sheet metal, the wire EDM can cut all of the parts at once in one setup to the same size and surface finish. This technique is very cost effective compared to building an entire punch and die set for a small series of parts.

Injection Needles

We often think an injection needle is just a small tube with a small opening at the end. However, recent medical advancements require complex and delicate procedures, such as injecting medicine into internal organs e.g. the liver.

To maximize the amount of medicine distributed through the needle without injecting the needle several times into the organ, needles are produced with several openings around the circumference of the needle. This produces an even distribution of the liquid medicine. The die sinking EDM process allows users to burn (cut) small rectangular openings (e.g. 0.02” x 0.013”) through the needle wall without burrs, eliminating the need for additional deburring.

Summary

If we summarize the benefits of the EDM process as they pertain to the medical industry, we can say that the non-force or friction that an EDM process affords allows for easy, inexpensive fixturing of complex parts.

The programming is simple because the tool is always the same diameter, compared to milling or grinding. Every electrically conductive material can be cut including surgical stainless steel and titanium alloys.

The ease of use of the integrated cutting technology ensures consistent surface finishes and accuracy, ensuring product quality and process description for FDA approval.

The EDM production process can be easily automated, guaranteeing consistent results without relying on operator experience.

And finally, rapid turnaround time for prototypes without building expensive fixtures or purchasing specialized, expensive end mills will help users remain competitive and innovative in this fast changing industry. TMD