Making a hole is more than just drilling
One of mankind’s oldest manufacturing methods—making a hole—is now challenged by rapid advances, including new and harder materials, increased part complexity, ever smaller component parts, and the need for shorter cycle times. What had been a fairly routine procedure now demands tooling with stronger substrates, better—and in some cases, customized—coatings, along with improved geometries and specialized designs.
The need for stronger and lighter components in critical applications has prompted the use of non-traditional materials. These include new specialty steels, newer alloys, ceramics, composites, and glass. Difficult to machine and to penetrate, they are frequently used in parts requiring exceptionally tight tolerances and a fine surface finish.
Edwin Tonne, training and technical specialist for Horn USA Inc., Franklin, Tenn., defines some of the problems in making holes with drills in materials such as titanium and austenitic stainless steel grades. “The problems start at the very beginning in that it requires more effort to get into the material, resulting in a tendency to run higher in thrust,” he said. “Further, these materials frequently clamp back on the drill and don’t cut cleanly, wearing the drill faster than normal. Problems derive from the materials’ toughness rather than simply hardness. Some of the other materials, such as plastics, are harder and that brings about its own set of problems.”
Drilling More Than Metal
Craig Ewing, national product specialist–drilling at Iscar Metals Inc., Arlington, Texas, discussed the difficulties inherent in composites and ceramics. “The main challenge with non-traditional materials is the need for balance of both higher performance and longer tool life. Often, this is a function of carbide grades and edge preparation. The challenges with ceramics and composites are two-fold. They are extremely abrasive, which adversely impacts tool life. Composites also have a tendency to delaminate or fray without the correct geometry.”
For example, Iscar has ten standard geometries, but is often called on to create specialized designs. “Additional solutions use diamond-tip inserts or diamond-like coating, which improve tool life and can be a cost-effective alternative to coated carbide,” said Ewing.
When cutting hard materials, heat is the enemy. It not only affects accuracy but substantially shortens tool life, according to Manfred Lenz, product manager–holemaking for Seco Tools LLC, Troy, Mich. “With metals such as high-temperature alloys, built-up edge is a problem,” he said. “So, sharp edges and coatings that can withstand the higher temperatures and reduce or eliminate built-up cutting edges are essential. The main problem with composites is the abrasion that can wear a cutting edge and therefore create tear-out. A solution here is to keep the edge sharp with specially formulated coatings or PCD-tipped drills.”
David Maunu, applications specialist–medical at Mitsubishi Materials USA Corp., Schaumburg, Illinois, noted the economic considerations brought about by harder, non-traditional materials. “Given the competition that exists in the world market, one of the great challenges for toolmakers is the need to develop a standard tool capable of running in multiple materials with repeatability and reliability,” he said. “Although manufacturers need specialized tooling in any number of cases, economies dictate the need for a product that can run well, deliver satisfactory results, and achieve a reasonable life in a wide range of materials.”
Performance criteria has to include the correct geometry for proper chip control, he noted. “The specialized drills demanded for composites require specially formulated substrates and multiple coatings and, as such, are more expensive. By combining standardized drills where possible and specialized units where necessary, manufacturers and customers can average costs to achieve better economic performance.”
The requirement for smaller-diameter holes has proliferated throughout manufacturing, ranging from the smallest parts made on Swiss-style machines to some of the largest components in aircraft engines and elsewhere. In addition to simply avoiding drill breakage, tolerances and finish have become major points. The solution to the problem extends well beyond the tooling and requires an optimized combination of factors.
Lenz noted the issues. “Below 2 mm, you need quite a bit more rpm—generally in the range of 10,000 to 20,000, depending on the material,” he said. “More than just the spindle speed, the machine has to be stable, and both the tool and the workpiece require rigid workholding so that there is no run-out or instability. Obviously, the geometry must be correct, and the proper coating is important. But, I think the main takeaway is that, if you’re going to do this kind of work, you need the correct options on the machine to increase your chance for success.”
Tonne cited several means of combining speed and accuracy in smaller diameters. He noted that Horn offers precision collet systems from Fahrion, with concentric accuracy down to 2 µm. When combined with through-coolant and jet collets, higher cutting speeds can be achieved even in difficult-to-machine materials.
“We also have our DD series of high-performance drills, which drill in difficult materials,” said Tonne. “We achieve this through higher accuracy runouts and finely polished flutes. Our new Supermini HP tool can drill, bore, face, and turn. This approach allows the end user to rough out small holes and then precision bore. This saves tool change time and reduces tool stations, while providing the best accuracy possible.”
In short, the performance in drilling does not come from any one aspect of the system but from each contributing part—from the spindle to the tool tip.
Indexables and Inserts Advance
The economy, flexibility, and convenience of indexable tooling has also become relevant in small-hole drilling. Ewing cited Iscar’s SUMOCHAM, a line of replaceable-head drills, which are now available down to 4.0 mm in diameter. “We’ve developed three different standard geometries, including one for cutting exotic materials. The pocket design converts the cutting forces into gripping forces without potential plastic deformation. As drills get longer, self-centering geometries, with lower cutting forces on hole entry, improve drill performance regarding hole position and quality.”
Maunu from Mitsubishi cited the importance of the basics when it comes to speed and accuracy in small diameters. “We are always engineering new substrates, coatings, and geometries capable of dealing with today’s non-traditional materials,” he said. “Depending on the diameter, the complexity inherent in tool selection changes by what machines the customer is using and what features we can make available. At present, 0.019" (0.48 mm) is the smallest diameter capable of delivering coolant, which is as important for chip evacuation as it is for thermal control, and chip control is one of the biggest problems in small-hole drilling.”
Manufacturers have found that the elimination of postprocessing procedures (including deburring) offers multiple advantages, including savings in time and tooling, elimination of hand work, and improved product quality. Holemaking has been responsible for many of these innovations, ranging from such value-based engineering techniques as combination drills that produce multiple diameters to specialized coatings capable of generating an improved finish.
The most basic and frequent demand is for a “clean cut” that eliminates deburring and improves accuracy. Horn USA’s Tonne cited the importance of programming. “The most common technique for avoiding tear-out as the drill exits the hole is to reduce the feed. Likewise, in more difficult materials, reducing feed on entry can reduce the burr at the top of the hole. We also offer special drills in our DD series that can be engineered with additional chamfering, guidance and deburring built into the tool.”
Lenz of Seco Tools noted the relevance of chamfer size. “By increasing chamfer size, we have been able to reduce the creation of burrs,” he said. “This holds true especially with some of the newer, more abrasive materials. The larger the chamfer, the better—and cleaner—the cutting operation. In certain applications, we’ve actually increased the radius to an almost ball-nose design. Of course, a lot depends on the tool itself and its composition and coating. For instance, a diamond coating will work best on composites.”
The chamfer is also emphasized by Iscar’s Ewing. “We employ a number of different techniques regarding chamfering, including mounting drill bodies inside a chamfering collar, to drill and chamfer in a single operation,” he said. “We have also devised special tooling that drills and chamfers on one side, and also chamfers on the back side, by circular interpolation. This is particularly favored by the automotive industry, as it saves both time and tooling.”
When the goal is improved surface finish, a number of process elements must be incorporated. According to Maunu, “As the demand for improved finishes grows, both toolmakers and manufacturers are having to consider a variety of variables, including the tool substrate, the coating, the geometry, and the speed and feed. While the coating is generally considered to be a primary contributor to the process, all of the inputs have to be properly dealt with to obtain the optimum setting. When it comes to secondary operations, some manufacturers are finding ways to sidestep them through multi-axis equipment and tooling with select tool geometries.”
Advanced Materials, Advanced Tools
Advances in manufacturing—including materials, machines, design capabilities, and part configurations—have prompted the development of corresponding strategies on the part of toolmakers. In some cases, these are the work of a single company, but more frequently they are cooperative efforts with outsourcing of coatings, components, and accessories.
Maunu cited both basic tool composition and design. “As cutting tools have caught up with machines in terms of performance technologies, several new strategies have emerged,” he said. “The use of rare earth carbon substrates is increasingly necessary, as is the ability to handle different forms of coolant, depending on the operation. For instance, one of our new drills features triangulated holes for better chip control. Throughout the development phase, we were aware of the need for proper coolant flow. The reality is that successful tool manufacturers now have to update and extend their standard product lines to compete economically.”
Ewing likewise emphasized a continual need for research on new materials for both drill bodies and coatings. “The term ‘carbide substrate’ is no longer enough,” he said. “Thanks to nanotechnology and other developments, the library of substrate choices has increased substantially. As for coatings, we’re increasingly seeing multiple coats that have to be sequenced in terms of type and amount for maximum performance.”
Lenz noted the effort—and effectiveness—required for the development of “targeted” coatings. “Seco Tools is doing more R&D every year, especially in the area of addressing particular problems,” he said. “For instance, we have a new coating that almost eliminates built-up edge, and we’ll be using it on both our MS (milling) and DS (drilling) grades. The performance has been extraordinary, and thanks to the coating and a geometry that includes a very sharp edge and a positive rake, we were able to go from 500 to 4,000 holes. This took quite a bit of research, not just in terms of the formula but also the means of application.”
In addition to tooling components (including substrates and coatings), electromechanical devices and on-board machine capabilities are increasingly essential in improving holemaking. Tonne commented, “In certain applications, we recommend the use of force sensors. While we do not manufacture them, they are available from Kistler and perform well. Also, more machines have on-board monitoring to sense drill resistance. What was originally called ‘adaptive control’ has become much more sensitive, and Makino has developed features that actually monitor performance. In helping our customers select the proper drill or other holemaking tool, we have to be aware of the capabilities of their equipment in terms of the application that they are pursuing.”
Advancements in holemaking in the past decade have largely been due to methods, materials, and technologies that originated in other areas of manufacturing. Moving forward, toolmakers are incorporating their experience in getting “ahead of the curve” with new products and parameters.
“The past ten years have evolved tremendously,” said Maunu. “The development of small drills that can deliver coolant to the cutting edge and new coatings and geometries have enabled customers to successfully complete multiple holes in hard-to-machine materials. As we move into the future, drills will doubtless become more accurate, and improved coatings will play an increasingly larger part.”
The depth/diameter ratio will continue to increase, and there will be even greater emphasis on eliminating processes, according to Maunu. “For instance, one of our customers formerly had to send out parts for gundrilling and lost up to eight weeks in the process,” he said. “We developed a tool for him that enables him to complete the process in-house, thereby saving all that time and enabling him to take on additional work. Is the newer tool more expensive? Yes, but the price is more than justified in terms of increased production and time savings.”
Lenz foresees incremental developments in virtually all areas of holemaking. “Toolmakers will be working toward the development of more effective standard drills that can perform well in a range of materials and applications,” he said. “This will necessitate a heavy emphasis on chip evacuation, coatings, and larger flutes. At the same time, it will be impossible to avoid those specialty applications that require individual attention, including optimized geometries for special material configurations such as customized alloys derived through nanotechnology.”
The rapid growth of additive manufacturing in both metals and composites, and the continuing refinement of the process, will exert a major effect on holemaking operations. In the opinion of Tonne, this may well result in a subset differing from traditional drilling operations. “Holemaking will be more of a finishing operation, as the holes will already exist in the printed piece,” he said. “Techniques similar to boring and honing may well be used to refine hole diameters in close-tolerance operations and to provide finishes in the micron range. Achieving the desired result will require the combination of spindle technology, workholding, and tooling.”
Ewing summarized the future of holemaking in two uncompromising directives: “Get smaller, and get faster.”
What is historically one of the oldest and most common machining processes has evolved, of necessity, into an ever-growing array of complex designs, materials and parameters. In an effort to meet the ever-expanding demands of an increasingly complex marketplace, it can be tempting for toolmakers and manufacturers alike to become overly enmeshed in choices and details.
It is well to remember an old, but still true, adage: “The customer does not want drills. He wants holes.”
Meeting Threadmaking Challenges With New Designs, Substrates, Coatings
Just as holemaking has undergone multiple advancements, threading has likewise followed suit. For more than 30 years, Carmex Precision Tools LLC, Richfield, Wis., has specialized in the development of tooling for thread milling. The wide variety of thread types and sizes has meant developing ancillary equipment, including specialized toolholders and compound tools for both drilling and thread milling.
“Certainly, new high-temperature materials pose challenges for thread making—whether milling or tapping,” said Jim White, national sales manager at Carmex USA. As important as new carbide substrates and coatings are, design also plays a critical part, he added. “The helical contours of many of our tools not only result in a more accurate thread profile but deliver a better finish as well. We’ve also developed refinements such as our DMT center cutting end mill that starts on uneven surfaces which, with conventional tooling, could result in an elliptical hole.”
As with holemaking, smaller diameter holes pose their own problems for threading. Historically, tapping was the preferred solution. Unfortunately, when it comes to high-value-added parts, a broken tap could cause immediate scrappage. “Even today, high-temperature materials break taps, and thread milling is now the preferred procedure,” said White. “Besides breakage, taps wear out fast and require frequent changeover time. Thanks to our new substrates and the use of nano coatings, our tools are much stronger and more heat resistant. Further, the use of coolant through the thread mill extends tool life. On some of our smaller tooling, we’ve incorporated three flutes for improved chip evacuation. In any small-hole threading, deflection is the biggest problem, but we have satisfactorily cut threads as small as 00-96 UNS.”
When it comes to post-processing, it is often more effective to incorporate preventative measures rather than after-the-fact treatments. For example, the use of combination tools can eliminate costly finishing operations, and cutting off a partial thread in the initial pass is effective in preventing cross-threading.
Looking toward the future, White cited the importance of R&D in the development of new and better threading tools. “We work in an area where the thread configurations—as wide and different as they are—have been well established. Our responsibility includes developing tooling that will work for the customer in specific applications. For instance, we developed tooling that’s been successfully used in fracking operations, where pumps have to be quickly and efficiently restored to working order after handling highly abrasive slurries,” he said.
White also noted that the development of new substrates and the definition of new geometries will enable faster production without sacrificing precision and finish. He believes that, in the near term, finishing will become even more important, as his company deals with a larger population of printed and composite parts. “We’re also increasing the number and variety of products we create for Swiss-style machining and for specialty applications in medical and aerospace,” he said. The company’s newly introduced CIM toolholder permits the changing of Swiss-style tools without removing the holder, thus saving changeover time.
“When it comes to the new generation of applications, it is our responsibility not just to be able to perform the operation successfully, but to provide our customers with tooling that will generate higher performance and achieve economies over a longer tool life,” he stated.
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