Tools Smaller Than a Hair Fit the Job
Careful attention to process is required when using micro tools
By Jim Lorincz
Parts continue to shrink in size in the medical and dental, telecommunications, mold and die, and aerospace industries, creating a demand for micro cutting tools, some smaller than a human hair (average size 0.0035" [0.09-mm] in diam). Available for drilling, milling, and turning, these micro tools depend on developments in submicron-grain carbide and coatings, and careful attention to toolholders and spindle and machine rigidity, to be effective metal-removing instruments.
Classifying tools as mini or micro is a relative call, but generally speaking tools range from as small as 0.002" (0.05 mm) to no more than 2 mm in diam. There is no shortage of machine tools that can meet the requirements for rotating tools fast with minimal runout, and toolholding that can hold them securely and accurately. High-speed machines from Makino, Roku Roku (MC Machinery), Roeders, Kern, Chiron, Mikron, Hermle, and Yasda are readily cited for their micromachining capability. As a class, Swiss turning centers are all capable of machining the smallest parts with extreme precision using micro tools.
Special attention must be paid to the machining processes involved in machining with these micro tools. Some critical points:
- Controlling runout is challenging.
- Toolholding and tool handling require attention to rigidity and cleanliness.
- Machines and spindles require careful maintenance.
As parts get smaller and machining requirements go deeper, more difficult-to-machine materials like cobalt chromes, titanium, stainless, and alloys require submicron-grade carbides and coatings tailored to the application, and tools must retain the sharpness required. Medical, mold and die, and circuit board (drilling) continue to dominate in the use of micro cutting tools. Reduction in cycle times and elimination of the need for secondary EDM machining of finer details continue to head the list of reasons for innovations in micro cutting tools, including drills, end mills, and inserts for Swiss machining.
Seco Tools (Warren, MI) has expanded its Jabro mini end mill product lines for machining metals like hardened steel and titanium and soft metals like aluminum and copper. William Bogue, manager of Jabro products, explains: "New tools are available from 0.1 to 2 mm diam, expanding our ability to machine medical devices from metals like titanium, cobalt-chromes, and superalloys. Machining materials in the hardened state for mold and die applications has been achieved with predictable and extended tool life, and with better surface quality."
Two new Jabro Mini solid-carbide end mill lines include the JM100 series for machining tool steels hardened to RC 65 and the 400 series intended for machining aluminum and copper. "We are continuing our R&D and the effort to educate users about the potential of machining complex shapes with small-diam tooling," says Bogue. "Machining with small tools just takes a totally different mindset and attention to process capabilities, including machine rigidity, toolholding, and noncontact laser presetting."
US industry may be coming a little later to the widespread adoption of micro cutting tool technology than Japan or Europe, but applications like medical/dental, electronics, and optics are becoming targets for tools that have been successfully used for mold and die machining. MicroTool micro end mills from Niagara Cutter (Amherst, NY) are available in two, three, or four-flute configurations in squareend, ball-end, stub, regular, and long-reach ranging in size from 0.005 to 0.120" (0.13–3 mm). From 0.005 to 0.055" (0.13–1.4-mm) tools are available in increments of 0.001" (0.03 mm), and in increments of 0.005–0.120" (0.13–3-mm) diam. Specials in diam smaller than 0.005" (0.13 mm) are available.
With micro tools, the quality of the tool and machine/toolholding are paramount. Niagara Cutter's William Sebring explains: "The main driver is runout, TIR from the shank to the cutter edge. Half a thousandth may be acceptable for a half-inch diam mill, but if you have five tenths runout on a 0.010" (0.25-mm) diam tool, that isn't acceptable. Cleanliness of the tool when assembled into the toolholder, and using the right toolholder with low runout and high repeatability in a rigid machine, are extremely important when using these small tools at high rpms. The ability of the control to process accel/decel information in terms of how many blocks ahead the control can read is also critical."
New tool introductions by Emuge Corp. (West Boylston, MA) are aimed at expanding micro machining to specific applications like milling injection molds and extrusion dies, or graphite and copper electrodes. "The Micro end mills feature a geometrically optimized cutting edge combined with a slim, concave tapered neck to provide high-performance milling in materials from RC 65 hardened steel to graphite," explains Stephen Jean, milling products manager. "The neck design allows for a 10:1 reach ratio. This aspect of the tool was developed in conjunction with a university study in Germany that used the FEM (finite element method) parameter for modeling the tool to develop and incorporate the ideal neck shape to maximize length and rigidity while minimizing vibrations," Jean explains.
"Micro tools are also well suited for high-precision surface structuring of macroscopic components and micromechanical parts production," says Jean. "They have proven themselves highly effective in machining coinage dies and medical parts [both molds and devices], especially where EDM would require moving parts to another machine to finish details." Jean credits micro tools with overcoming the difficulties that manufacturers sometimes face in machining complex parts.
Emuge's end mills are available from 0.2 to 2 mm with effective cutting length-to-diam ratios of 2.2:1, 5:1, and 10:1 in ballnose, torus, and flat-end styles in 24 different designs, uncoated or with TiAlN coating or diamond coating. Emuge's solid-carbide micro end mills are designed for machining in a wide range of materials from graphite and aluminum to steel hardened to RC 66.
The need for drilling deeper and smaller holes is being met by the Walter Titex X-treme DM20 and DM 25 solid carbide high-performance micro drills with internal coolant from Walter USA (Waukesha, WI). "Applications for the DM 20 and 25 can be found in the fiber-optics, mold and die, and hydraulics industries," says Patrick Nehls, product manager. "In hydraulic valves, for example, multiple ports may be needed, and oil holes in crankshafts require longer drilling depths or longer drill reach."
The X-treme DM 20 drill is available in 2–2.9-mm diam able to drill up to 20 x D; the DM 25 is available in 2.5–2.9-mm diam with ability to drill 25 x D. Toolholding is important in terms of reducing TIR, with shrink fit providing the highest accuracy. Hydraulic chucks and high quality collets can be applied with acceptable results. The advantage of the DM series of drills is their ability to reduce cycle time. Nehls explains: "In small-diam drilling the normal procedure is to use high-speed steel twist drills that run a bit slower and require peck drill cycles, especially if coolant-through is not available. Or you can use gundrilling that requires special machines and fixturing. Typically, we can drill 5 to 10 times faster with the DM tools than with HSS drills, reducing cycle time." Nehls does point out, however, that HSS is more forgiving, especially when it comes to rigidity of the setup and fixturing.
For threading applications, especially in high-value workpieces, Nehls recommends orbital thread milling. "For a high-value part, you want to minimize the risk of scrapping the part out with the threading, which is typically one of the last operations performed. Using Walter Prototyp solid-carbide orbital thread mills, the user has the advantage of controlling the process through the CNC program. He can control pitch diam, produce threads of different diam with the same pitch, produce left or right-hand threads with the same thread mill and thread closer to the bottom of the hole than would be possible with a forming tap or cut tap."
Walter Prototyp offers two versions of the thread mills, one for most materials, the other for hard materials up to RC 65. Walter USA offers its CCS software to facilitate calculations for tapping, milling, and thread milling, and for drilling its TEC software, both free of charge.
Manufacturers have come to expect that micro drills will perform very much like any other drill, according to Mike Bojanowski, assistant product manager for Sphinx drill line at BIG Kaiser Precision Tooling Inc. (Elk Grove Village, IL). "For a long time, the common thought was that below a certain size, it wasn't possible to drill accurately with any acceptable tool life. If micro tools are used properly, they get the same performance of larger drills, including tool life, full repeatability, and dimensional accuracy," says Bojanowski.
BIG Kaiser's Sphinx micro drills are made from fine-grained, high-strength carbide and are available in sizes from 0.05 mm diam, which is slightly smaller than a human hair, to 2 mm in diam. Drills are offered in standard lengths of 6 x D, as well as a 2 x D drill from 0.3 to 1 mm diam that can be used as a center drill or spot drill for micro drills.
"Good results machining with micro tools begins with using high-precision collet chucks," says Bojanowski. "In addition, regular machine maintenance, including checking and greasing bearings, can go a long way to ensuring precision drilling results." BIG Kaiser offers the New Baby collet chuck for reducing runout.
M.A. Ford Manufacturing Co. Inc. (Davenport, IA) offers micro end mills down to 0.005" (0.13-mm) diam for medical applications, including two-flute, and four-flute square end and ball nose end mills. The company is currently developing micro drilling technology intended to move its customers away from the old circuit-board drilling platform to a micro drill that is specifically suited for metalcutting. Joe Kueter, manufacturing and engineering director, explains: "We are looking at a distinctly different geometry, carbide, and coating technology for customers who are focused more in the medical and metalworking industries. In some cases, we have produced tools below 0.003" [0.08-mm] diameter and produced drills to 0.0001" [0.003-mm] diam tolerance and 0.0001" maximum point runout."
Circuit board drills are notable for their favorable price. Kueter feels, however, that what is becoming more important to users is consistency from drill to drill and hole to hole, and the fact that the new drills could be used on metalcutting, ceramics, and medical applications. Says Kueter: "The carbide used for certain diameters is a grade that provides a higher transverse rupture strength, while still providing the hardness necessary for keeping a sharp cutting edge in small-diameter drilling. When adding coatings to small-diameter drills, the coating must be thin to retain the sharp cutting edge. We're in the process of testing a TiAlN coating specifically developed for micro machining that would combine the thin coating necessary to keep a sharp cutting edge, while still providing the benefits of TiAlN for heat resistance."
For Swiss-turning applications, Tungaloy America (Wood Dale, IL) has introduced its ISO-positive screw-down inserts, including the new AH725 grade with 0.0005" (0.013-mm) corner hone with TiAlN-coating, and the SH73 dead sharp insert. The bidirectional inserts allow facing, turning, and profiling of parts as small as 0.010" (0.25-mm) diam. According to Brian Sawicki, director of engineering-turning, the inserts feature low cutting force, bidirectional cutting capability (facing, turning, and profiling), heat-resistant coatings for nickel-based alloys, ability to break chips or direct them, and design suitable for low feed rates.
Small-diam drills include DSM Mini drills that are available in 0.1–3 mm diam in 0.01 mm increments. "They're known as circuit board drills or micron drills," says Sawicki. "They also use a micrograin carbide, are TiAlNcoated, and are noted for the ability to increase tool life in machining, especially of stainless [300 series], carbon alloy steel, and nickel alloys." Applications are small-diam drilling on Swiss lathes. For medical parts, you don't have to pre-drill a hole. The 140° point allows punching a hole, according to the accuracy of the machine, with a high degree of reliability in flat surfaces. The exception might be spherical surfaces or highly irregular surfaces.
"Typical parts include jet engine filters, fuel filters, separators or other sieve-like parts with a lot of small holes, and computer circuit boards. In the medical industry there are a lot of Z-axis holes that must be made when the design requires putting small holes into dental parts and implants that require longitudinal or cross holes. The drills work well in these applications," says Sawicki.
Mitsubishi Materials USA Corp. (Fountain Valley, CA) includes both small diameter Micro drills and short length solid-carbide end mills in its small tool manufacturing category for precision machining often performed on Swiss-style machines. "We've just launched our 11° P-style VPET and VPGT series inserts and our external SVLP and SVPP tool holders," says Chris Wills, product specialist CBN and small tools. "They are metric or inch standard turning tools with 11° positive, 35° rhombic inserts for machining high precision parts or small details in larger parts for medical and aerospace applications, among others."
The coated solid-carbide inserts feature the SRF and SMG breakers. "With the 11° positive rake, 35° inserts, profiling achieves surface finishes and chip control, which is expected of these high-end E classes and G class inserts. The E class SFR insert can hold higher tolerance than molded or ground inserts and allow higher accuracy when indexing from one corner to the other," says Wills.
"When you get into small tool manufacturing, the two things I find to be commonplace for manufacturing parts is that the tools have to work in higher volume production without operator intervention, and they have to work reliably, even if the cost is a little higher," says Wills. "The result is that one person can run a whole line of automated machines." Wills points to another important consideration: availability. Mitsubishi Materials has just opened a new distribution center in Fountain Valley, CA, for just that purpose.
This article was first published in the April 2010 edition of Manufacturing Engineering magazine.