The [micro] World of Medicine
Fifty-thousand revolutions per minute – so fast the eye can’t tell it’s turning. Yet the machine’s tiny 10-thou’ tool severs 60RC stainless with a fierce precision unimaginable just months before. “It’s pretty unbelievable,” admits veteran micro-machinist Matt McCormick. “There was a time,” he remembers, “when we would have called it a miracle.”
“Pretty unbelievable” is a fitting depiction of McCormick’s working world: the research and development shop within the Keck School of Medicine at the University of Southern California. More specifically, it’s the Eye Concepts Laboratory of the school’s Doheny Eye Institute. From a quiet tree-lined campus near downtown Los Angeles, the institute’s ophthalmologists, bioengineers and researchers are in the daily business of performing miracles; and McCormick’s micro-machine shop is an integral part of it.
The world-renowned Doheny organization is intensely committed to helping people with progressive eye diseases by quickly bringing new techniques and tools to the medical marketplace. The laboratory is a unique blend of extraordinarily talented people. And, in what must be an equally extraordinary arrangement for any medical facility, the machine shop is located directly across the hall from the surgical operating rooms.
The shop, with its right-in-the-middle layout, has been politely called unusual – even praised with the term inventive. But either way, McCormick quickly corrects those understatements. “This,” he says, spreading his arms to include all around him, “is simply the coolest place in the world! It’s a place of excitement,” he smiles. “It’s a place of ideas. Here, machinists work directly with top-tier doctors developing amazing new instruments for eye surgery. We’ve got all these brilliant surgeons with new ideas, and all these sharp biomedical grad-students with fresh ways of seeing things.
“Some of the MDs around here have master’s degrees or doctorates in engineering – they’re all bona fide geniuses – but they don’t have the machining skills to make what they’ve thought up. Eye Concepts was founded to do just that,” continues McCormick, “make what the surgeons imagined.” With necessity being the mother of invention, the need for machinists to work next to surgeons dictated the across-the-hall location of the machine shop.
“Every day when the doctors come out of surgery we sit down at a big conference table and discuss new or different instruments that could have made their procedure better,” says McCormick. “A surgeon might comment, ‘If this pair of tweezers could actually lift when they grip, then we’d have something!’ We then discuss exactly how to tackle it; what kind of dangers are involved; what size constraints we must consider.” Finally, the machinists turn that “what if” vision into something the surgeons can actually touch – and always, the sooner the better.
The lab initially searched for laser-sintering or photopolymer-based rapid-prototyping equipment to fill their need for quick turnaround. But after considering the instrument quality needed to prove their concepts, the lab soon decided nothing short of actual finished parts would do. Small, manual machines got things started, but highlighted the lab’s need for programmable machine tools with greater precision and repeatability. The directors initially thought they were facing a half-million-dollar expenditure to buy micro-capable CNC machines, along with the expensive prospect of tearing up their hospital to build a shop and install them. Both scenarios seemed impossible. Then, at exactly the right time, they found an ideal solution – new, reasonably priced, precision CNC machines compact enough to roll through the doors.
“Those new Office Machines really saved our life,” says McCormick, referring to the lab’s OL-1 Office Lathe and matching OM-2 Office Mill from Haas Automation. “We became one of the first shops to acquire these extraordinary units. The small-tool capability and the high-speed spindle, combined with good rigidity, was exactly what we needed. These machines allowed us to turn the ‘nice idea’ of a quick response lab into a feasible operation.
“I’ve been a machinist for 33 years now,” muses McCormick, “and throughout my career I’ve always specialized in really small stuff. I’ve found that when working in micro sizes, many of the tried-and-true traditional machining techniques are no longer valid. Normally rare moves, like plunging or ramping down, or trochoidal cutting where the tool disengages from the workpiece in a circular path, are often done – usually with a ball endmill, keeping the depth to less than 20% of the tool’s diameter. Additionally, cutting speeds and feeds must be very high, or you can’t remove material at an acceptable rate, or achieve the proper chip size to avoid overheating. And, while rigidity is necessary to hold tight tolerances in all types of work, it’s especially important in micromachining. Small vibrations are amplified relative to tool diameter as the size of the tool is reduced, and a vibration as small as 0.0001" can cause major problems. If tool runout isn’t held exceptionally tight – typically to 4 µm or less – an ordinarily small vibration will cause a lot more than just reduced precision. It may even fracture an endmill.
That’s why I’m excited about how this new Haas equipment allows me to work. Machines like this have just never been available before. The performance differential between these and other small machines is like the difference between day and night.
“I can now do things like this,” says McCormick, holding up a machined sheet of miniature surgical probes. The eight thin probe beams with their 3-D contoured tips are barely an inch-and-a-half long, and each measures only 0.007" wide. These pieces would ordinarily be cut using EDM and abrasive-polished for commercial production. But for rapid design-evaluation, the Eye Concepts Lab machined them from 316-stainless shim stock – only 0.015" thick!
McCormick used a 0.020" ball endmill in the Office Mill’s NSK electric spindle to take progressive 2-thou’ cuts in the rapidly work-hardening steel. Air-gun cooling was used because the sample needed to remain uncontaminated for FDA (Food and Drug Administration) purposes. However, similar stainless setups are often cooled by misting or low volume flood coolant.
Workholding was elegantly simple: The sheet was attached to a vise-mounted aluminum block with doublestick tape. However, a soft wax (Mitee-Grip CompoundTM) was used to prevent the thin legs from flexing during the final cut. First, one side of each leg was machined; there was still plenty of material on the other side to prevent movement. Then, the hot wax compound was dripped into the pocketed cavities to provide support while the second side was machined. This uncomplicated technique allowed a cut tolerance of ± 0.0005". After machining, the wax was easily removed with mineral spirits and warm water, and the sheet was ultrasonically cleaned.
“For every prototype piece we make,” notes McCormick, “we have to keep in mind that someone else is going to have to mold it, or stamp it or coin it. These probes will be used to evaluate and perfect the concept, then they’ll become the basis for commercial production. The major medical companies we work with can take our designs and commercially produce them with a minimum of the usual development delay.
“A primary goal of this institution is to cure or prevent our patients’ progressive blindness. Time is important – of the essence, you could say. While there are many other exceptional medical facilities doing long-range research in this field,” observes McCormick, “here at Doheny, where machinists sit next to surgeons, an idea can be turned into something to show by the end of the day.”
To gain proficiency with the new Office Mill, Matt McCormick devised a demonstration to test its capabilities: He machined a flock of miniature butterflies. Cut from small blocks of aluminum and acrylic, the finished pieces (with wing thicknesses of only 0.004") are in some ways even more delicate than the real thing.
“People do seem fascinated with them,” says McCormick, “and everyone wants to know my secrets. Actually, the butterflies were simple, requiring only a bit of micro-machining street-smarts. And, if there is a secret,” he offers, “it’s Mitee-Grip CompoundTM; but you could just as easily use candle wax.”
How It Was Done
Since contours were cut on both sides of the thin wings, accurate re-positioning was important, and wax support was required to stabilize the piece against work forces and vibration during the second op. Each pin-registered block was first vise-mounted in the Office Mill, and the butterfly top-surface was rough profiled using a 0.062" ball endmill. All cutting paths were generated directly with Mastercam-XTM software. Finish passes were taken with a 0.010" ball endmill.
The resulting cavity was then filled with Mitee-Grip Compound and the workpiece was flipped over and perfectly re-registered to the vise pins. The second-op roughing and finishing passes, which shaped the butterfly bottom, were taken with parameters and tools identical to the first. However, during the final contour pass, the mill was taken around the entire perimeter of the part, cutting it free from the workpiece. The butterfly was still held solidly in position inside the block by the wax compound, until it was freed with hot water and mineral spirits.
Performing eye surgery is a lot like building a ship in a bottle. Not to diminish the seriousness of the doctor’s task, but in both exercises everything required to do the job must fit through a very small opening. For delicate work inside the eyeball, the surgeon often inserts a small grommet-like TROCAR through the white of the eye (conjunctiva), and passes the instruments and canulas through it. Probes, forceps and cutters are constructed of extremely thin tubes and beams. Nearly all motion within the tools must be co-axial, with operating parts sliding within the minute outer tube. The surgeon sees what he’s doing by looking through the patient’s cornea and dilated iris with the aid of a special external optical viewer.