Small Parts Machining
Advanced technology for
milling, turning, drilling
The future is now for small parts machining in the US, where advances in machine capability, toolholding, workholding, and programming software are converging to make milling the smallest features for medical, biomedical, dental, electronics, engraving, jewelry, and institutional-research applications possible. That acceptance hasn’t always been the case. In fact, the rest of the world has generally been a step or two ahead in recognizing the potential of small-part machining, whether milling, drilling, turning (Swiss and diamond turning), among other processes.
According to Ralph Picklo, vice president, Cimatron Technologies (Novi, MI), milling part features to tolerances of 0.0001 mm and surface finish of Ra <0.2 µm with a 10-µm tool are well within reach of machines with high spindle speeds, even some in excess of 250K rpm. “The keys to successful micromilling are found in the stability of the machine process, including toolholding and workholding, positioning repeatability, and CNC controls capable of processing vast amounts of data very quickly.
“Micromilling is simply much less forgiving than traditional milling. Machines must be very accurate, vibration-free, and thermally controlled with minimum spindle runout. Programming strategies to achieve the tightest tolerances require tool control, knowledge of stock, spindle tracking, and especially tool monitoring for wear and growth,” Picklo explains.
The secret of small parts machining in a production environment lies in creating a machine platform that is stable and repeatable, according to John Bradford, micromachining R&D team leader, Makino (Mason, OH). “For the iQ300 precision micromachining center, we start with mass, and control thermal growth with core-cooling, under-race lubrication, and jacket spindle cooling to eliminate thermal growth, deflection, or vibration, during long runs of high-speed machining. The key to achieving surface finish at a high level is the ability to place the cutting tool repeatably at the same depth of cut (Z level), to get as close as possible to zero rotational tool runout."
Bradford says that micromachining is only as good as the quality of the cutting tools. The shank to cutting-edge runout from the primary holding diam to the cutting edge of the tool is critical. The closer you get to zero runout the longer the tool life and the better the surface finish that can be achieved. Of equal importance is cutting-edge geometry and what you might call the slope geometry of the microdrill. “We’re interested in cutting material, not rubbing, or pushing material,” he asserts. “You have to have the proper cutting-edge geometry. When you get into these small sizes, the ability to provide correct tool geometry becomes more and more difficult without specialized equipment to grind the tool, and, of course, to inspect the final result.”
Primary concerns for micromachining are found in such considerations as vibration, sharpness of tools, and stack tolerances inherent to replaceable insert-type tools, according to Matthew Schmitz, national product manager, Iscar Metals Inc. (Arlington, TX). “Machines have to be dynamically balanced with spindles rated upwards of 20K rpm. Runout is critical. I may get away with 0.002" [0.05-mm] TIR on a ½" [12.7-mm] diam SumoCham drill, for example, but that is absolutely not acceptable if you are drilling with a 0.020" [0.51-mm] drill.”
Schmitz says that the rules for micromachining are really not much different than any other form of metalworking. “It’s just that errors are amplified tremendously. Much like my example with the drill, operators need to ensure the tools are nearly perfectly adjusted for center height in turning environments. In the case of parting, Iscar now performs a special grind to inserts with large lead angles.”
Solid carbide tooling, whether large or mini-sized, depends on a three-legged stool of geometry, substrate, and coating. Seco Tools Inc. (Troy, MI) transferred its success with new carbide grades, geometry, and coatings for its larger tools to its Jabro mini carbide tools. “We’ve divided our micromilling products into three application-specific product ranges,” explains Eric Gardner, product manager milling Jabro. Products include the JM 400 tools in 0.2–2-mm diam for aluminum and nonferrous applications; the JM 900 tools from 0.1 to 2-mm diam for softer tool steels, stainless, and titanium; and the JM 100 tools from 0.2 to 2 mm for heat-treated tool steels, mainly for mold and die applications.
“Micromilling is simply much less
forgiving than traditional milling.”
For cutoff applications, Seco Tools has launched its MDT 2 multidirectional turning tool, which can be as small as 2-mm diam, that can cut off, side turn, and face turn on Swiss machines. “The MDT 2 has the advantage of using our Jet Stream high-pressure coolant system that directs coolant to the cutting edge to break chips, extending tool life,” says Don Halas, product manager for grooving, cutoff, and turning.
The newest addition to the CoroMill Plura solid carbide end mills from Sandvik Coromant (Fair Lawn, NJ) are small ballnose end mills to 0.004" (0.1-mm) diam. “We see the potential of micromachining for small medical parts, electronic parts, and small motors with these tools. The tools range from 0.004 to 0.472" (0.1–12 mm). The tools feature a new carbide substrate over 2000 Vickers in hardness and a new PVD coating. The coating is multilayer PVD titanium silicon nitride, which is extremely hard. We can machine material to Rc 73 with the tools,” explains Bill Durow, industry and application specialist for mlling.”
For turning, Sandvik Coromant has added high-pressure coolant to 1000 psi (6.89 MPa) through the tool for its quick-change QS Holding System. “That capability is especially important when machining difficult-to-chip materials like stainless and heat-resistant alloys or other long-chipping materials.," explains John Dotday, industry and product specialist.
The Threads-All line of solid carbide thread mills from Emuge Corp. (West Boylston, MA) has been expanded to meet the demand for smaller-parts machining solutions. “The miniature thread mills are changing the rules on how screw threads down to #0 are being produced on CNC machines, especially for difficult-to-machine materials,” says Mark Hatch, thread milling manager. Precision-ground rake and relief angles allow Threads-All tools to be used in all material groups to Rc 58 simply by adjusting operating parameters based on the metals’ chemical properties and hardness range.
“One Threads-All tool is used for through and blind holes with full bottom threading to within one pitch, with precise thread depth control, and thread depths to 2xD,” says Hatch. “The thread mills produce excellent thread finish and gaging, eliminating the possibility of tap breakage and the need for hand tapping and thread-cutting oil.” Threads-All miniature solid carbide thread mills are available in sizes from #0 to STI 8.
For parts that are less than 0.125" (3.18-mm) diam, Tungaloy America Inc. (Arlington Heights, IL) introduced a series of high-positive, low-cutting-force, screw-down, dead-sharp inserts and toolholders at IMTS. There are four chipbreakers: JS features a molded chipbreaker and the JRP, JPP, and JSP inserts are fully ground with accuracy of ±0.0002" (0.005 mm) from the base of the tool to the end of the insert’s tip. “These inserts are designed for parts that require critical centerline accuracy and low cutting force,” explains Brian Sawicki, turning products manager.
The JS chipbreaker is a bidirectional insert with an aggressive, molded chipbreaker. The insert is designed with a tightly placed breaker near the cutting radius. It also features a high positive, inclined cutting edge. “If this insert can’t break the chip, it will direct it away from the workpiece. The JS type is most applicable for very small components in the medical, aerospace, and automotive industries. The other three inserts, JRP, JPP, and JSP, were intended for small components made for the disk drive industry. They’re designed to take a lighter cut, but have potential for machining pacemaker parts, fuel system components, and even watch components,” says Sawicki.
“Boring very small holes requires that the tool is on center and very sharp,” Duane Drape, vice president, Horn USA Inc. (Franklin, TN). “For that reason we grind the geometries into carbide or even CBN tools. For boring tools of the smalllest sizes, we use very fine micrograin carbide and don’t coat the tools, because that may make them duller, shortening tool life.”
For microgrooving, the smallest off-the-shelf tool is 0.008" (0.20-mm) diam, and the smallest through-bore is 0.078" (2 mm). “At this small size, the biggest problem with getting smaller with a grooving-type tool is that the tool becomes unstable due to the radial pressure put on the tool. The biggest challenge is coming from materials, though not necessarily in the microrange.”
About 30–40% of Horn’s business is made-to-order product. “We are application and engineering-based, and we love to grind specials rather than wanting to be a supplier to every project. Our product ranges from the smallest solid carbide 0.008" [0.20-mm] boring tool up to the larger Super Mini system, which features a screw-on front insert."
In microdrilling products, careful attention is paid to developing tools that are versatile, especially when it comes to holemaking for medical applications. “What we’ve done is take some of our circuit board-style drills and made specials down in the 0.005" [0.13-mm] range,” explain Messrs. Joe Kueter and Patrick Krogman of M.A. Ford (Davenport, IA). “We’ve adopted special carbide grades to add toughness, and developed flute forms and point designs that are more robust. For the die and mold industry, we’re launching a product down to 1/64" [0.39-mm] diam with a tip strong enough to withstand the pressures.”
“Quality of the coating and its thickness in microtools are important considerations. Just as important is the ultrafine-grain carbide that is used primarily in tools for machining hardened steel in mold and die applications,” say Kueter and Krogman. In addition to hardened steel, M.A. Ford works with materials like cobalt chrome and stainless for medical applications. Some of its microtools are used for avionics, where microtools down to 0.003" (0.08-mm) diam are used to make holes with 0.0001" (0.003-mm) tolerance required.
Precision Dormer (Crystal Lake, IL) has introduced its Elect APP range of microdrills for small-diameter drilling applications in a wide range of materials and components. The microdrills are designed for modern rigid and high-performance CNC machining centers, and available in two standard lengths for drilling up to 8xD, 12xD, and specials up to 15xD. The design features Dormer’s ACM (Advanced Chip Management) flute geometry, and a micron-thin layer of its proprietary Super-Flow coating to optimize performance through enhanced wear resistance and productivity.
“With microdrills, cutting conditions are very critical. We recommend to our customers to run the drills on machines with high spindle speeds and good vibration damping characteristics. It’s recommended that Elect APP microdrills are used in applications where the drill rotates, and that static drilling is avoided. This is due to the fragile nature of micro geometries on carbide drills. It’s also important the machines have good centering capability, preferably no more than 3–6 µm TIR, and it is highly advisable that runout on the tool be kept to a minimum,” explain Messrs. Prashanth Jaipal, David Goulbourne, and Alex Traianopoulos.
Coolant-through capability has been introduced to all Phoenix 6xD high-performance drills from 1 to 12.7-mm diam. The line, available from BIG Kaiser Precision Tooling Inc. (Hoffman Estates, IL), comes with one caveat. “Very small-diam drills have very small coolant holes, and as a result they require a higher coolant pressure for small diameters from 1 to 2.5-mm to achieve maximum performance,” explains Mike Bojanowski. “We recommend a minimum of 725 psi [5 MPa], and we’d really like to see 1000 psi [6.89 MPa] or higher.” For end users who don’t have high-pressure coolant capability, Sphinx has introduced a secondary 6xD Phoenix microdrill (#50941) that is a noncoolant-through drill of 0.5–2.4-mm diam. It features the same Phoenix geometry and coating, and marks the first time that the Phoenix drill line has been extended below 1-mm diam.
For external turning in Swiss-style machines, Mitsubishi Materials USA Corp. (Fountain Valley, CA) provides tools for radial and gang-style tool post and turret-style machines. For internal turning, Mitsubishi tools such as the SBAH and Micro-Dex provide indexable insert boring as small as 3.0 mm. For bores even smaller the Micro-Mini Twin boring bars are capable of boring holes as small as 2.2 mm. Micro-Mini Twin bars are available in boring, grooving, and threading configurations. Mitsubishi Micro-MWS, solid carbide, coolant through, twist drills from 0.020" (0.51 mm) through 0.120" (3-mm) diam and 0.5 to 0.99 mm are capable of drilling hole depths of up to 12xD. Larger sizes, from 1 mm through 2.95 mm, are capable of drilling hole depths of up to 30xD. Micro-MWS drills feature unique cutting edge geometry optimized to produce the smallest possible chips for easy disposal.
The Mold Finish Master CBN end mill from Sumitomo Electric Carbide Inc. (Mount Prospect, IL) is intended for high-speed precision machining of pre-hardened and hardened steel <Rc 70 for die/mold applications. Available in 0.2 to 2-mm diameters, the end mill is designed to extend tool life in a wide range of material hardness. Rich Maton explains: “The CBN end mill offers significant improvement in tool life for Rc 42–45 metals and up to Rc 55, in place of coated carbide. The end mills produce a shiny finish on die/molds, and enable users to machine at higher speeds 20K rpm and higher available on modern machining centers.”
Normally, CBN wouldn’t be recommended for material hardness below Rc 45, says Maton. “The Mold Finish Master CBN end mill, however, can deliver 30x tool life in Rc 20 metals, D2-type material, and 3x to 10x tool life all the way up to Rc 70. Typically when we shoot for a hard-turn application vs. ceramic, our target is 10x the tool life. If you can do that or reduce cycle time, the key calculating factors, the cost difference can be justified.” The end mill line has been extended to include radius tools with 0.05, 0.1, 0.2, 0.3, and 0.5 options. ME
This article was first published in the April 2011 edition of Manufacturing Engineering magazine. Click here for PDF.