The number of parts requiring tiny holes, channels and other features is growing, and the tools capable of creating those features are getting more capable and more sophisticated.
Solid-carbide micro cutting tools about the diameter of human hair or smaller—some producing parts visible only under a microscope—are making a huge impact on manufacturing highly advanced electronics, automotive and aerospace fuel injection systems, and medical instruments and implants.
Maintaining tighter and tighter tolerances at spindle speeds up to 150,000 rpm, micro drills, end mills, routers and other tools are breaking machining barriers thanks to improved clamping techniques and proprietary coatings applied via physical vapor deposition (PVD). More coolant-through drills and application-specific carbide grades and geometries are becoming the norm for toolmakers.
“As tighter tolerance demands have increased and miniaturization of parts has increased, our customers require many more options,” said Mike MacArthur, vice president of engineering for RobbJack Corp. (Lincoln, CA). “We have to offer tools in every thousandths of an inch increment starting at 0.005″ [0.13 mm] and ending at 0.062″ [1.57 mm]. Micro tool makers also must offer specific problem-solving geometries and the ability to follow high-efficiency toolpaths “using the same angles of engagement and Z-depths of cut as their full-sized counterparts.”
New Era of Micro
Symbolizing the increasing reliance on micro machining tools, the 2018 catalog for Richards Micro-Tool (Plymouth, MA) features 2200 new products, according to Engineering Manager David Paquette. The emphasis is on longer flute lengths and reach lengths and special coatings like aluminum titanium nitride (AlTiN). Among those new tools is a new line of tapered rib cutters that create draft on molds for hardened machine materials.
The company’s tools process everything from platinum to PEEK (polyether ether ketone) plastic, he said. While Richards has been a pioneer of micro milling—having always offered end mills at 0.005″ diameter and always employed microscopes in their facilities to evaluate their tools—Paquette noted that today’s tool repertoire includes diameters down to 0.0005″ (0.013 mm).
Companies are experimenting with ways to reduce tool vibration and chatter, Paquette explained, including odd numbers of flutes and various rake and relief angles—and sometimes not incorporating relief. “Traditionally you don’t relieve all the teeth, so it almost gives you somewhat of a balance to how the cutter works.”
And while “we don’t have our heads in the sand” regarding competition to the rotary cutting business from lasers and electrical discharge machining, micro tool makers have found symbiosis among other players in the broader stock removal business, Paquette noted. “We have a series of tool lines made for the EDM industry.”
Kyocera Precision Tools Inc. (Hendersonville, NC), which has “historical strength in drilling,” has been having success with its Titan AX three-flute and upcoming five-flute micro mills on 6-mm shanks, said Round Tool Product Manager Joe Negron. “With three flutes, you always have a cutting tooth engaged,” he said. Five-flute mills, already proven to allow higher-speed milling with Kyocera’s indexable tools, will bring that functionality to the company’s solid round tools.
With cutting diameters from 0.06″ (1.5 mm) down to 0.02″ (0.5 mm), they are made to process “super-tough material like Invar or any of the stainless steels, he said. “It was kind of a surprise when we put it out. We knew it would do well in precision tool and die applications, but it turns out it’s being used across multiple markets, such as medical and aerospace.”
Meanwhile, Kyocera’s coolant-through 10×D Hydros drills will soon be joined by 5×D and 7×D versions. “We initially came out with drills down to a 3 mm cutting diameter,” Negron explained.
“Now we’re down to 1 mm for two-flute drills of 8×D or 15×D.” These drills are particularly geared to aerospace materials like 17-4 PH, titanium and Inconel, said Brian Wilshire, technical applications manager for Kyocera.
Meanwhile, Kyocera’s super-micro drills are less than 0.003″ (0.08 mm) in diameter. Kyocera produces drills down to 0.0015″ (0.038 mm) in diameter and end mills at 0.002″ (0.05 mm), Wilshire added. “We’re finding more call in the advanced electronics industry and chip test-bed manufacturing with metals like Invar,” particularly in Asia, Negron noted.
In fuel-injection systems for automotive and, to a lesser extent, aerospace, “we’re seeing quite a bit of call for diameters anywhere between 0.008 and 0.010″ (0.20 and 0.25 mm) up to maybe 0.060″ (1.52 mm). While those diameters are not particularly noteworthy in the micro sense, he said, “the big deal is they are asking for super-precise diameter tolerances of plus or minus a micron.”
Such tools are not easy to make, Negron continued. “You can start one hundred and not end up with very many, so getting very good at blank prep and making sure you have the most accurate blank ready to go” is vital. “Fluting has been the trick with it.”
Steve Boss, product manager for Horn USA Inc. (Franklin, TN), noted several key improvements:
- Flute designs featuring material-specific cutting geometries and very short flutes with neck clearances for reach, as well as high-polish flutes for S and N group materials.
Materials: New carbide grades that can be material specific to optimize wear.
- Grinding process: Modern multiaxis tool grinders and refined grinding processes—high-tolerance blank and holding, grinding wheels and grinding oil filtration—have made tool consistency the norm.
- Because of the precision of today’s tool grinding machines, micro tools are as detailed as their larger-diameter counterparts, said RobbJack’s MacArthur, from the fluting to the gash to the relief and primary angles. The company’s Mirror Edge chatter-reduction geometry, for deep pockets or long-reach miniatures in aluminum, matches the vibration of the tool to that of the part, eliminating chatter and vibration.
Custom is King
In today’s complex micro machining world, it is not uncommon for 30 to 60% of a company’s tools to be custom, said MacArthur. About 60% of RobbJack’s business is custom. “We have the luxury of seeing some pretty amazing projects,” he said. RobbJack will re-create customers’ applications in its lab, run test cuts, then report back indicating the proper tool and process parameters.
Meanwhile, Paquette said maybe 15–20% of Richards Micro-Tools’ portfolio is engineered solutions. Such projects “don’t happen overnight,” instead developing over years as a relationship with a customer is built.
At Kyocera, a range of special end mills addresses medical industry demands, said Wilshire: “We do step drills to try to save cycle time,” aiming to process a part in one pass.
“Our focus outside the general catalog line … becomes a threefold process to improve performance,” explained Paquette, honing the proper mix of materials, geometries and coatings “that do not diminish what we’ve done with our cutting tools and our geometries.”
Extraordinary throughput gains can be achieved by dialing in the right process and materials, as Paquette illustrated in a recent case. A Richards client machining a feature into spools of laminated plastic material used in the printing industry would get about 200″ (5.1 m) of material before the cutter had to be changed. By refining a corner-radiused end mill with a diamond-like coating and switching from an upshearing cut to a downshearing cut, Richards’ tooling produced 800% more linear footage. The 2018 Richards catalog includes two new levels of diamond coating.
Coatings tailored to match the maximum cutting temperatures of many unique ferrous and nonferrous metals and carbon fibers allow higher working temperatures and can increase cutting speeds by up to 20%, noted Kyocera’s Negron. Experiments with PVD coating equipment abound to find the best process mix of gas, temperature and deposition rates.
Proprietary materials like aerospace stainless steels typically require proprietary coatings to increase tool longevity and foster chip evacuation by maintaining lubricity in drilled holes, according to Kevin Jackson, round tool product specialist for Kyocera. Coatings like Kyocera’s Megacoat series are applied at thicknesses based on tool diameter.
Where coatings won’t really help, Negron said, is in the super-micro world of tool diameters under 0.008″ (0.20 mm).
We get a lot of calls” for coating these tools, but “there’s no way something [that size] is going to generate sufficient heat” to require a protective shield 3–4 μm thick.
At RobbJack, every micro tool offered in increments of 0.0002″ (0.005 mm) is available in titanium nitride, titanium carbon nitride, AlTiN and the newer diamond-like carbon, MacArthur said. When cutting heat-generating metals like stainless steel or nickel alloys, AlTiN is the ideal choice, he noted, since it forms an aluminum oxide heat shield that keeps heat from going into the tool.
Ultimately, coatings produce cost savings, said Richards’ Paquette. In a medical production scenario, for instance, an enhanced-performance tool can process 200″ of material with parts having to be checked only 10% of the time.
Of course, micro tools are only as accurate as the equipment and software that produce and guide them.
The evolution of more accurate milling machines is a cyclical process, RobbJack’s MacArthur explained: “We’ll come out with a tool that’s better than the machine tools, then the machine tool makers will come out with a better machine that maxes out the tools. Then you have all the new CAD/CAM toolpaths that control engagement angles and how much contact the tool has with the cut.”
One key to machining success “is the equipment and how it is clamped,” Kyocera’s Negron said. “The best clamping is heat-shrink. End users are concentrating on buying very good machine equipment that has excellent clamping.”
Low runout is one of the key goals of acquiring high-end equipment, fostered by exceptionally stable spindle/tool combinations. “You can’t have a lot of slop and high runout because these tools just won’t put up with it,” he advised. “With a bigger tool, you’ll hear growling” under less-than-ideal conditions, “whereas a micro tool is just going to break.”
Spindle speed is another critical performance factor, Jackson added. “The smaller the tool, the more rpms you need.” To optimize its tools, Kyocera uses a spindle speed multiplier in its lab, Wilshire noted. While metalcutting generally requires 10,000 to 15,000 rpm, tools to produce printed circuit boards must run at 120,000–150,000 rpm. MacArthur puts the vast majority of application spindle speeds at 30,000–60,000 rpm.
Some newer machines have high-pressure coolant available, Wilshire noted, which is why Kyocera extended its coolant-through drilling line to smaller diameters. The coolant “is a great help in fishing the chip out of the hole and making sure we maintain hole quality and tool life.”
Meanwhile, current CAM software maintains equal chip load whether the tool is in a turn or corner, Negron said. Many customers preview their software by programming, for example, a high-speed milling routine, and making sure the process is optimized “no matter where you are in the part.”
For milling, “most CAM software has some version of high-efficiency machining built in,” Wilshire said. This facilitates lighter radial width of cuts, maintains strong feed rates and mitigates tool chatter or breakage as cut loads increase in corners. For drilling, Jackson added, newer and easier-to-program CAM software facilitates variable drill depths, shorter cycle time and chip evacuation.
Said Horn USA’s Boss, “New toolpath choices control radial engagement, ensuring the tool doesn’t come into an area with more material than it was programmed to take.” Even more important, he said, are better-educated programmers “who are gaining the understanding on toolpath selection and how and what parameters to select.”
Richards’ Paquette said that most tool companies have proprietary software specific to their CNC grinding machines that models optimum tool designs and performance. Two years ago, his company bought a miniature milling machine for testing geometries, particularly for PEEK applications.
Tips for Users
To protect these valuable tools, Boss said pre- and post-use inspection are as important for preventing breakage as adhering to proper usage parameters. Before use, he advises using a micron accuracy tool presetter with live-view camera and automation to inspect for runout and form. This not only affirms tool geometry and proper runout, but achieves tool-length offset with no touch-off required.
For proper machining, Boss stressed using a high-accuracy toolholding system like Horn USA’s Fahrion and a spindle speeder, if necessary. Maintain correct cutting speed, radial and axial depths and feed per tooth, and clear chips during operation with proper placement of compressed air or coolant to avoid double cutting chips, he advised.
Afterward, shops should use a loop, presetter or digital microscope to check for uniform wear, chipping or breakage. “Inspect finished parts with a 3D scanner or interferometer to assess surface finish and final form,” said Boss.