Small Parts, Smaller Features
By Jim Lorincz
The emergence of micro-cutting tools for industrial applications is a more recent evolution of a class of cutting tools that trace their popularity back more than a half century to use for drilling printed circuit (PC) boards. End mills and, more recently, thread mills have expanded the traditional reach of micro-cutting tools in dental, medical, and aerospace applications. Microtools generally fall into the range of 1-mm diameter and smaller, though there doesn’t seem to be a hard and fast rule with some tools in the 2-mm size range. The reality is that the most common cutter sizes used for micro-milling metals tend to fall within the 0.003" (0.08 mm) and up diameter range, while extraordinary manufacturing capabilities have produced some smaller than 0.001" (0.03 mm), even reaching as small as 5-µm diameter.
What has changed dramatically is the diversity of materials that are being micromachined. "Many of today’s shops involved with micromachining are doing so on extremely hard metals, such as CPM [1-V, 3-V, 9-V] D2, A2 and F7 tool steels, along with hard powdered metals, all of which can be as hard, or harder than, Rc 62-64. They also micro-machine stainless steels like 450, 410, and 17-4 and titanium and Inconel," says Gary Meyers, product manager for milling and Jabro cutters at Seco Tools Inc. (Troy, MI). "For these shops, micromills designed specifically for such tough materials reduce overall part production cycle times, help increase microtool life, and make micro hard-milling cost effective for more parts that would have otherwise been machined using different processes."
Seco like other toolmakers has tailored microtools with special combinations of substrates, coatings, and geometries aimed at optimizing cutter life and performance for specific groups of materials. "For shops producing powdered metal tooling, micro-hardmilling operations can involve imparting face detailing on punches and semifinishing and finishing the surfaces of tooling used in the process. Tooling components are often made from various powdered metals and tool steels such as CPM (1-V, 3-V,9-V), D2, and F7. Apparent hardnesses can be Rc 55–64. However, particles within such CPM materials are often more along the lines of Rc 70 in hardness," Meyers explains.
Pushing the Limits of Micromanufacturing
Performance Micro Tool (PMT; Janesville, WI) has pushed the limits of micro-cutting tool manufacturing with its proprietary manufacturing processes. It manufactures end mills in standard and stub lengths smaller than 0.001" (0.03-mm) diameter for machining ferrous or nonferrous materials in applications, including electronics, graphite electrodes, and plastics for medical and optical. Tools include size ranges from 0.0009 to 0.0002" (0.023–0.005 mm), even an end mill 5 µm in diameter. According to Dave Burton, PMT president, the company is a primary supplier of micro-cutting tools 0.004" (0.10 mm) and smaller to the market.
"There are a lot of challenging applications for micro-cutting tools in medical, electronics, microfluidics, and research. We call anything below 0.005" [0.13 mm] our nano line although it’s not really nano. I’d say that half of what we build is bigger than 0.010" [0.25 mm] and half of that goes to the electronics industry for circuit board manufacturers, prototypers, microwave boards and that kind of thing. When the iPhone came out, we made an 0.008" [0.20-mm] ball nose end mill strictly for engraving the plus and minus on the volume control button, a cosmetic feature. About 20% of our market is for cataract lens manufacturing for implants; another 5–10% goes to medical device manufacturing."
"What has changed dramatically is the wide diversity of materials that are being micromachined."
Burton says that a lot of people are afraid of using small micro-cutting tools. "I can usually allay most of the fears about using micro-cutting tools with three sentences of instruction. For success in using tools smaller than 0.001" [0.03-mm] diameter, I like to see less than 0.0001" [0.003-mm] runout. Our experience is that if you use the tool properly when the tool engages the part, the movement of the feed of the material tends to self-center the tool. Low runout, low vibration, maintaining proper chiploads, and good machine technique can avoid unnecessary tool failure and lead to successful micro-cutting results."
Small Tools in Large Numbers
Union Tool Inc. (Anaheim, CA), which began making small dental tools more than 50 years ago, manufactures micro drills for drilling abrasive PC boards, and manufactures metalcutting tools, primarily for the mold and die sector. Today, the company produces more than 25 million off-the-shelf tools per month on equipment of its own design and manufacture. "We manufacture as many as 300 different models below 1-mm diameter, including two different types of drills—coated for metalcutting and uncoated for PC board cutting," says Jonathan Hay, vice president.
"About 60–70% of the tools are 0.3-m diameter or smaller. The smallest tool we make is 60-µm diam, which is about 60% of a diameter of a human hair. Microtools require an incredibly accurate shank, tool concentricity, and repeatability running on machines that feature spindles with good runout, a stable environment with thermally controlled structure, and lack of vibration," says Hay. "Any eccentricity in spindle, toolholder, and cutting tool, can cause a problem. In fact, we ink jet markings on our shanks rather than ablation laser mark them to avoid any possible distortion or cause the tools to get stuck in the toolholder in shrink-fit systems."
Micro-machining applications for Union Tool’s Unimax end mills and drills feature coatings specially designed for specific material applications. "All of our tools, including our coated tools, feature very sharp edges. We have pioneered the use of coatings which have a low-coefficient of drag for hard-milling applications. For micro-machining alloys we use our chrome-based UT coating, which promotes cutting rather than rubbing. For machining precious metals, which can be very abrasive, we have PCD-tipped microdrills, 0.3 mm and larger, for durability of the tool and quality of the hole."
Design Options with Deeper Drilling
Drilling deeper is achieved with the expansion of the Sphinx line of high-performance microdrills with the addition of a 12×D microdrill available from BIG Kaiser Precision Tooling Inc. (Hoffman Estates, IL). "There have been more and more requests for different types of microdrills, simply because there are more applications , particularly in the medical, aerospace areas, and, increasingly, the automotive industries, for machining small features and small parts," explains Mike Bojanowski, product manager. The Sphinx 12×D line ranges from 0.2 to 2-mm diameter, features the same geometry as the popular 6×D line, and is intended for material applications including steel, cast iron, stainless, and some nonferrous metals like bronze and brass.
"Manufacturing technology has caught up with the design aspect giving design engineers more leeway to design features that couldn’t be easily manufactured in the past," says Bojanowski. "Using these micro-cutting tools is made possible by high-speed spindles, and toolholding is simplified because the drills all have a 3-mm shank, which can be handled by standard collet chucks." For older machines that don’t have a high-speed spindle capability, BIG Kaiser offers an Air Power Spindle that can be mounted in a legacy machine’s low-rpm spindle and driven with standard shop air at variable speeds to 80,000 rpm for micromachining.
In the micro-drilling arena, materials are changing and so are the expectations of customers. According to Mitsubishi Materials USA Corp. (Fountain Valley, CA), customers want to get away from gundrilling and drill deep and with drills that offer quicker solutions, higher productivity rates, higher feeds that can be used on both machining centers and lathes. Materials include exotics and difficult-to-machine HRSA metals. The drills are very popular in the medical industry where they are used for machining stainless, cobalt-chrome, zirconium and other materials used for implants and surgical devices.
Mitsubishi Materials offers solid-carbide end mills with the Impact Miracle high-resistant coating for high-temperature alloys in sizes as small as 0.1 mm. Typical applications include medical, aerospace, and tool and die which is the highest consumer of solid-carbide end mills. The Impact Miracle coating is an AlTiSiNi coating with silicone content that helps with heat resistance and tool wear. Aerospace applications for the solid-carbide end mills include fuselage, body parts, wing parts; while in the medical area, applications include prosthetic knees, shoulders, and screws.
Hardmilling for Mold and Die
Walter USA LLC (Waukesha, WI) offers micro-milling cutters in its Prototyp Proto max Ultra ballnose end mill line from 0.1 to 2.5 mm for machining hard materials from Rc 48 to 70. "There is a clear trend toward hard-part machining in the mold and die industry, currently the heaviest users of the Proto max Ultra micro-milling cutters," says Tom Benjamin, product manager-milling. The Proto max Ultra cutters feature a TiAlSiN-based coating (TAS for short) that is highly heat-resistant—"critical when machining very hard materials, boosting quality, productivity, and tool life," says Benjamin. "Also, the tools’ superior radius accuracy means that when a tool is changed the next tool is going to produce a very similar radius, resulting in greater accuracy and less rework."
"I can usually allay most of the fears
about using micro-cutting tools with
three sentences of instruction."
For deep-hole drilling, the Walter Titex X treme solid carbide microdrills cover drilling depths from 5×D to 25×D in diameters ranging from 2 to 2.95 mm. Point geometry is designed to ensure precise positioning on entry. Internal cooling and polished flutes with special profiling facilitate optimal cooling at the cutting edge and reliable chip removal. Applications include drilling cooling and ventilation holes for mold and die and in automotive for parts like fuel injectors, and medical parts for difficult-to-machine materials require precision machining. Coatings include the AlTiN-based coating for maximizing cutting performance and tool life, and an AML coating thinly applied to harmonize with microdrilling along the entire useful lengh of the tool. Pilot drills are toleranced to fit with the 20×D and 25×D drills. The pilot drills have a plus tolerance while the DM20 and DM25 drills have a minus tolerance in the micron range, eliminating the chance that the pilot hole would be slightly smaller than the DMD drill.
Design Capabilities are Critical
Richards Micro-Tool Inc. (Plymouth, MA), serves three different markets with micro-cutting tools: industrial, dental, and medical. "In industrial cutting tools, our sweet spot is in sizes from 3/8 to 0.005" [9.5–0.13 mm], in two, three, and four-flute end mills with square end, ball end and corner radius end mills, microdrills, and combined drill and countersinks," says Terry Leach, vice president-operations. Typical applications include general-purpose end mills, along with specialty tools used to cut graphite, plastics, Inconel, titanium, and special tools for soft wax.
In addition to proprietary manufacturing processes that Richards has developed, the company, which is a private label manufacturer in the medical and dental sector, offers design capability to enable it to maximize tool designs for its customers. "We can help make their tools better. Right now, we’re redesigning a full line of tools for one of our customers," says Mike Gambale, manufacturing manager. "A lot of the designs we do belong to us. We’ve just completed a design for a company that has gotten all the FDA approvals for cutting a notch in the spine without pushing bones apart. We designed a solution that can be inserted between the vertebrae so that the bone actually gets pulled together during the cutting process."
"Advanced technology has created an increased demand for high value parts requiring small holes, tight tolerances, and superior finish," says Jeff Dei, president, Carmex Precision Tools LLC (Richfield, WI). "To meet the requirements of application with hard materials like titanium, Inconel, or special alloys, we carry an extensive line of tooling, ranging from our 0.80 Mini Thread Mills to solid-carbide tiny tools. We’re continually shrinking conventional tooling to meet new application needs and we’ve recently extended our line of Swiss-type screw machines."
The superiority of thread milling in small-hole applications was illustrated in a recent customer application. Dei explains: "The material was titanium and the specifications called for 683 0-80UN 0.150" [3.8-mm] deep blind holes. On-machine tapping was not an option, and the shop owner had to come in weekends to hand-tap the parts due to breakage. Taps were inconsistent and lasted from 2 to 40 threaded holes before breaking. On top of that, roughing and finishing taps were required to achieve thread depth to the blind hole. The solution was using a Carmex Mini-Mill Thread [MTS 0250 C16 80UN] eliminating blind tapping, reducing tooling costs, and improving thread quality. Not only is productivity increased by eliminating the hand tapping and secondary operations, but a single MTS tool accomplished threading in all 683 holes."
In a second application, a shop in the aerospace industry required a ¼-20UN 0.450" (11-mm) deep through hole in Rc 40–42 stainless. Parts were core-drilled before heat treating, resulting in threading operations in Rc 56. The Carmex Mini-Mill Thread (MTSH 0250 C55 20UN), a solid-carbide tool featuring three flutes in a helical configuration, delivered 300–400 threaded holes per tool, resulting in a cost of 34¢ per threaded hole, compared with a range of $4 to $16 per holes tapped. ME
This article was first published in the April 2012 edition of Manufacturing Engineering magazine. Click here for PDF.