Milling microparts presents some mighty big challenges. Surface speeds are typically inadequate when cutting tools and parts are less than one millimeter (0.039 in.) or so in diameter. Runout that might be acceptable on a larger drill or end mill becomes a tool life killer. Tolerances are commensurately tighter, making effective process control even more important than usual. And workholding? It’s probably time to retire that 6-inch Machinist’s Vise.
The same holds for turning tiny parts, which is why shops that excel in this arena invest in high-rpm Swiss-style CNC lathes, equally high-end barfeeders, and jeweler’s microscopes to inspect details that are often too small to see with the naked eye.
Still, such compact components are all around us. From the microelectronics we depend on each day to the stents and micro-catheters that keep an increasing number of us in good working order, pint-sized parts and part features are only growing more prevalent as the world attempts to lightweight everything and fit more capabilities into smaller packages.
Making it with medical
Erik Poulsen knows all about it. The manager for medical segment marketing at Georg Fischer Machining Solutions, he seconds the challenges just listed, but points out that many of these small parts—especially those found in the medical and automotive segments—are also high volume. It’s for this reason that he offers the following advice: ask for help.
“Whether it’s a medical OEM or the job shop working for them, manufacturers often face long-term contracts and need to develop the most predictable, cost-effective process possible,” says Poulsen. “What’s more, that process is typically locked down after first-article inspection [FAI], so getting everything right the first time is critical. That’s why I tell people to find a partner with strong applications engineering support and the ability to deliver a system that can produce parts efficiently.”
Poulsen notes that GF Machining Solutions supplies CNC machine tools for the three methods listed earlier (milling, EDM, and laser). For shops that want to stick with the first and most common of these—milling—it’s crucial to invest in an extremely accurate machining center, one with high spindle speeds, responsive servo systems, and an HSK or equivalent toolholder interface that provides dual face and taper contact for minimal runout.
They might also consider five-axis machining. As with larger components, it’s often possible to machine a micropart complete in a single operation, reducing or in some cases eliminating secondary operations and the associated workholding headaches that come with gripping parts smaller than a fingernail. “You’re typically dealing with micron-level precision in this realm, so machine tool accuracy and proper setup is paramount,” says Poulsen. “At some point, though, it becomes necessary to adopt a different approach.”
Precision pulses
Enter Mike Lerner, head of business development for Laser Micro Machining at GF Microlution. As you might have guessed from his job title, Lerner will likely steer prospective micromachining customers to a technology that was still science fiction when many of us were born, and didn’t become an established machining process until the last decade or so: femtosecond lasers.
Unlike the continuous-wave lasers found in sheet metal cutting equipment, femtosecond lasers emit extremely brief but intense pulses of light, on the order of 10-15 seconds in duration. Because the pulse duration is so short, there’s less time for heat to transfer to the surrounding material during applications like cutting or drilling. This eliminates any heat-affected zone (HAZ), and when coupled with a highly accurate and responsive motion control system and optics, makes femtosecond lasers ideal for delicate or precision tasks.
“Radiopaque marker bands are another common medical example,” says Lerner. “In the past, you could either produce them mechanically—on a Swiss-style lathe, for instance—or use a wire EDM to slice these donut shapes off the end of a tube measuring maybe 5 mm (0.196 in.) in diameter. But newer generations of marker bands have micro-sized features that are very hard to produce with conventional technology, making it more economical to do them with a laser.”
People who’ve used lasers for marking parts or cutting sheet metal will tell you that lasers are limited. They require pulse widths and light wavelengths that the manufacturer has tuned to specific groups of materials—a laser that’s good at machining plastic will fare poorly with metal, for instance, and vice-versa.
Not so with a femtosecond laser, says Lerner. “The system actually comes with an adjustable pulse width, and as far as the wavelength, we offer infrared and green, which can cover pretty much all of the most common materials. And then the other point with femtosecond lasers is, they’re less dependent on the material’s absorption band than those with longer pulse lengths. It’s a cold machining process, so is far more material agnostic.”
Not one size fits all (but close)
That said, Lerner is quick to note that GF Machining Solutions does offer other laser types (nanosecond, for example) as well as a variety of machine tool platforms on which to mount them. These include three-axis and five-axis configurations, both of which GF can equip with five-axis “scanning heads” that allow the laser to create part features that would be impossible for “normal” laser systems.
One of the earliest (and highest volume) applications for ultrafast lasers was machining of the shaped holes used to control fluid flow in automotive fuel injectors. Since then, lasers have become the preferred method for creating the micro-textures found in many plastic injection molds (automobile dashboards is one example). Similarly, lasers with extremely short pulse widths are being used to generate hydrophobic, hydrophilic, and osseointegration-friendly surfaces on orthopedic implants, and to machine the cooling holes in the turbine blades that flew you and your family to Disney World last year.
The caveat to all this is
speed—the femtosecond lasers discussed here “ablate” (remove) extremely minuscule amounts of material with each pulse, making them a highly accurate and
extremely repeatable—albeit slower—machining technology than standard lasers. This helps explain their popularity in the micromachining world, where material removal is minimal, and most OEMs adopt a “whatever it takes” mentality. “In the micro-machining world, the lower ablation rate is less relevant, and the reductions in scrap and post processing can often make femtosecond lasers more economical than traditional machining,” Lerner adds.
The other good news is that you won’t need to hire an optical engineer to run one. “A well-trained machinist is perfectly capable of programming and setting up one of our laser systems,” says Poulsen. “And once the job is going, there’s no tool wear to worry about, or concerns about how much wire is left on the reel, so you can have a less skilled operator (or even no operator) tending to the machine.” SMT
