Machining Small Parts
Specialized microtools machine tiny parts for medical, die/mold, aerospace, and electronics
By Patrick Waurzyniak
Machining small parts requires specialized micromachining cutting tools and processes. Micromachining applications are mostly found in the medical and die/mold industries, as well as aerospace for avionics parts, and for manufacturing the small electronics components found in increasingly miniaturized high-tech gadgets and cell phones.
Microtools predominantly are made of carbide, often for inserts used in turning applications and on Swisstype machining. But many tooling suppliers also offer microsized drills, end mills, and boring bars using coatings optimized for HSM and precision machining microcutting processes.
Definitions of micro cutting tools vary but most suppliers consider 2-mm diam tools or smaller to be micromachining tooling. "How you define a micro range is all relative," says Rob Keenan, product manager for Seco Tools' (Troy, MI) Jabro mini line. "When we talk to some customers, you could talk about a 0.25" [6.4-mm] cutter and they think that's small, and then some other companies that do a lot of micromachining think that's huge, so it's all relative to what you're doing. But for all intents and purposes, we describe it as anything 2 mm in diam and below."
Medical and die/mold currently make up the vast majority of parts machined with micro cutting tools, Keenan adds. "What we see is the need to do more direct machining on much smaller parts, and with much more intricate detail," he states. "This would be parts that, in the past, would have been accommodated through the EDM process, or parts that we would not even have been able to make in the past."
Solid-carbide end mills make up Seco's Jabro tooling line that initially was used more in die/mold, but now are increasingly used in medical to make surgical tools and equipment, and implants such as hip, knee, and elbow-replacement parts. "We're seeing more medical direct parts being machined, and we've added a lot of new geometries and styles based on the need to machine difficult materials required in the medical market," he adds. "With surgeries becoming less invasive with endoscopy and arthroscopic surgeries, there's a need for very small, intricate instruments. We've developed application-specific tooling for this market, as the parts are not only smaller but they're taller, deeper, and more intricate. We've had to do a lot of development to make tools of small diameters that will not snap and break, with these extensive reaches."
Controlling tool runout with extremely small tools is a challenge, notes Keenan. "Everything is amplified. Again, if you have half a thousandths runout on a 1" [25-mm] diameter cutter, you can probably live with that. If you have that with a 0.010" [0.3-mm] tool, you've got 10% runout and it's catastrophic; everything's exponential in that case. What we've seen in the last four years is not only parts getting smaller and deeper, and we need more reach on the tooling, but the materials are getting much more difficult, especially in medical, with cobalt chrome, titanium, the different stainless steels, and alloys."
Until recently, most companies had one geometry that covered all applications, Keenan adds. "We had one carbide in one geometry and one coating that was to be used with graphite, hardened steel, titanium—whatever the application, you had only one choice," he says. "We saw that that wasn't effective, and in order to help our customers be more competitive, reduce cycle times, and then increase their ability to machine these components, we developed multiple geometries based on the material group, so we'll have a specific carbide geometry and coating for most materials encountered."
Precision micromachining applications like those involving medical implants require minimizing tool runout as much as possible. "When you get down into the micro range, less than 0.030" [0.76-mm] for which you would think fairly good runout is maybe only 0.0002" [0.005 mm], that's still an unacceptable amount," notes Jack Burley, vice president, engineering, BIG Kaiser Precision Tooling Inc. (Elk Grove Village, IL). "You have to approach less than 0.0001" [0.003-mm] runout for the cutting tool, because as there's a ratio of its chip load—it's huge, if it's two tenths runout or more. Because your chip load might only be 0.0001" or less, and now we're getting into even less than that, when you get into the real small tools, it almost goes without saying that the smaller the tool, the runout has to be controlled to that much finer precision."
BIG Kaiser's latest addition to its Sphinx micro drill line includes a new drill made of fine-grained solid carbide that delivers long tool life and consistent process reliability. The micro drills are available in diameters from 0.30 to 1.50 mm, in steps of 0.01 mm, with a 3-mm reinforced shank. Offered in standard lengths of 6xD, the micro drills can be run with or without coolant, and are recommended for steel (including stainless), titanium and nickel alloys, aluminum and copper materials, with less than Rc50. The specially designed two-flute geometry delivers a point angle of 130° and helix angle of 35° for extreme precision and close tolerance.
Exotic materials for medical applications also pose technical hurdles for tooling companies. "We're seeing more materials with names I've never heard before, on the engineering side, for medical companies," Burley adds, "materials that are proprietary from their suppliers and adapted for their applications. It's very tasking to understand how these materials may machine, and how do we approach the job, as far as finding the right geometry and/or coating. These materials are not so much hard, but abrasive. You're getting into some of the carbon fibers, some of the polypropylenes that maybe you haven't used before, and ceramics—it's not always so much just the ferrous material."
In such cases, tooling companies work with coating suppliers to determine which coatings deliver the longest tool life, adds Burley. "We'll approach our coating suppliers and they'll ask us to tell them what this material is; in a lot of cases, we don't know what it is, and the customer really doesn't know what's in it. From the manufacturing side, they're just giving a spec, like PEEK, or tape gloss, or ceramic. We're seeing more of the orthopedic device requirements using newer materials in composites that are not so well-understood from a machining side."
Small part machining tooling from Sandvik Coromant (Fair Lawn, NJ) covers a wide range of tools including inserts for turning on Swiss-type machines, and for drilling, milling, parting, grooving, and boring, according to Jimmy Fleming, Sandvik Coromant productivity engineer. Orthopedic applications include bone screws, plates, and implant parts for replacement hips, knees, finger joints, elbows, ankles, and shoulders.
Tool life, chip control, and surface finish are some of the obstacles with small part machining of medical-grade materials, Fleming says. Most small-part machining is performed on Swisstype machines, he adds, and medical-grade materials machined include cobalt chrome, stainless steels, titanium, and plastics. "We provide solutions for the special circumstances that arise when machining these materials," he says. "We offer solutions, not just tools, to give customers a competitive edge in small-part machining.
Fine surface finishes are often dictated by depth of cut and feed rate, adds Fleming, who's based in Warsaw, IN, an area noted for medical parts manufacturing. "With small parts, you've got to control center line height, taper runout and, of course, finish," Fleming notes. "One of the biggest issues is chip control. It's difficult to manage the chip on these small parts, on Swiss-type machines."
High spindle speeds are also crucial for many small tooling applications, notes Bernie Carroll of Iscar Metals Inc. (Dallas). "It's about small tooling and high rpm. We're coming out with smaller and smaller tools to run on the highspeed spindle machines, which are rated at 30,000 rpm and above," Carroll says. "Normal machines that you see on the shop floor today are up in the 10,000–12,000-rpm range. Now, the medical industry is starting to focus on the higher-rpm machines, the 30,000-rpm machines.
"If you go to smaller tooling, the limiting factor is usually not horsepower, it's rpm, because to generate surface footage on a smaller tool, it all relates to the rpm of the spindle available," Carroll adds. "In most cases, tools cannot be run at the proper speeds and feeds as you go smaller. It sounds crazy, but as you go smaller, to generate that surface speed, you have to rotate faster. Most machining people understand this. The opposite is also true—as you go larger, in most cases, you can't use the spindle speed and you don't make the most of the tool's capabilities.
"In many medical applications, they're really looking at much smaller components, and trying to produce much tougher materials, not tougher than aerospace, but tougher than one sees in a general machine shop. What they're trying to machine—titanium and stainless and even plastics—even though they are not what we would consider exotics or tough, they can tend to be very abrasive and tools are worn very quickly."
New micromachining end mills from Emuge Corp. (West Boylston, MA) were co-developed with Emuge's parent company in Germany and a German university using FEA tools to optimize shapes. "With our finite element modeling, we engineered a range of micro tools that are as near as possible to the ideal shape," notes Stephen Jean, Emuge milling products manager. "This modeling technique identified the tool shape that provides the most rigidity and can handle the relatively high torque at the neck of the tool. This was the key to maximizing the overall length and the effective reach or cutting depth of these tools."
Emuge's Solid Carbide Micro End Mills are offered in cutting diameters from 0.2 to 2 mm (0.008 to 0.080"), with effective cutting length-to-diameter ratios of 2.2:1, 5:1, and 10:1. The Micro End Mill line uses a single geometry in ballnose, torus, and flat-end styles in 24 designs, either uncoated, TiAlN, or diamond-coated.
Micro threading applications can employ tools including the new line of orbital thread mills from Walter USA Inc. (Waukesha, WI) optimized to offer chip clearance from very small threads in medical parts. The solid-carbide thread mills use helical interpolation to make threads suitable for small medical components.
"Customers typically use taps for producing threads down to this size, and usually thread mills haven't really quite come into the equation for them," notes Mark Hemmerling, Walter product manager. "One of the disadvantages of a tap, in small threads as well as in larger threads, is that in small threading-type applications, there's not a whole lot of room for chip clearance, and chips can build up and break the tool. If you break the tool itself, you've typically broken a tap off in the component, and you've scrapped the component.
"With medical components, high dollar-volume components, it's expensive workpiece material. By the time you get to the threading operation, you're close to finishing the component, because threading's one of the final operations, so you already have a lot of investment in that component—the last thing you want to do is break off a tool inside it."
Milling, drilling, and turning tools for micromachining applications are available from Sumitomo Electric Carbide Inc. (Mount Prospect, IL), which has introduced a new micro CBN ballnose end mill for medical and die/mold work.
"When the workpiece gets a little harder [>50 HRc], especially in the die-and-mold industry, you have to use CBN," according to Rich Maton, assistant manager, engineering. "Carbide just won't cut it, or you'd have to index the carbide end mill often, whereas with the CBN, you're getting 10–30 times the tool life."
Mitsubishi Materials offers micromachining tools including turning inserts, micro drills, and solid-carbide end mills, says Chris Wills, product specialist, CBN and small tools, Mitsubishi Materials USA Corp. (Schaumburg, IL). Drills include its Super Long Micro Drills ranging from 1.2.95-mm diam with 20, 25, and 30x depth-to-diameter ratio. "This allows customers to use smalldiameter micro drills to do deep-hole drilling in one operation," Wills says. "Milling has become very important in micromachining, with milling using solid-carbide end mills. In micromachining, it's expected to complete a finished part, eliminating secondary operations like removing burrs caused by standard end mills. Eliminating all secondary handling is vital to being cost-effective."
This article was first published in the May 2009 edition of Manufacturing Engineering magazine.