By Robert B. AronsonSenior Editor
For much of the history of drilling, the drill bit was the dominant tool. Now lasers, EDM, ECM, waterjets, and a few other techniques all have some of the action. Here's a recap of the processes manufacturers and users are working with.
The CO2 lasers offered by Coherent Laser (Santa Clara, CA) produce small holes (100µm) in a variety of materials. A laser is often the only device that can achieve the desired hole size in the particular material. In addition, laser processing can yield high-quality holes, in terms of dimensional tolerances, and has very little effect on the material surrounding the hole. Finally, lasers offer good uptime because of their combination of fast processing speed and lack of tool wear.
Many hole drilling applications are cost-sensitive, making the sealed, pulsed CO2 laser usually the preferred technology. This laser offers long lifetime and the lowest overall cost per watt of any laser. In particular, pulsed CO2 lasers with output power in the 50–500-W range are preferred for small-hole drilling in organics because their infrared laser beam is wellabsorbed by these materials. Pulsed operation in this power range minimizes charring or discoloring of organic materials as compared to CO2 lasers that produce continuous output. Pulsed operation also gives good control of the total power delivered to the workpiece, making it possible to precisely maintain hole depth. This capability is often important when drilling multilayer materials.
According to Frank Gaebler, senior product manager of Coherent, "The Coherent Diamond Series lasers are optimized for processing of organic materials because they produce peak power up to three times their average power with fast pulse rise times; this yields minimal edge charring.
"Labor-intensive jobs are the most likely to go overseas," says Dan Capp, director, Laserage (Waukegan, IL), a service company with 50 lasers. But he finds that because laser drilling is not such a process, he's doing rather well. "We can drill quickly and accurately under CNC control," he says. "For example, we can make 12 holes per sec in a ceramic substrate.
"With metals we usually work with plates 0.5-mm thick making 30–50µm holes. A big part of our work is with medical devices such as stents requiring 100–125µm holes. We use a YAG or CO2 laser, depending on the material, its thickness, and the product end use. Unlike working with conventional drills, changing hole size with a laser is just a matter of changing beam focus."
"Penetration rate improvement for greater productivity is a big request from our customers," explains James Porter, applications engineering supervisor, Allied Machine and Manufacturing Corp. (Dover, OH). "How we achieve this depends on the product. It may be increased by higher coolant flow, higher [or sometimes lower] spindle speeds, or drill design—or a combination of the three.
Allied also sees a demand for drills that can handle deeper holes. This again is a matter of increased production speed, as opposed to a change in product. "With our designs we are able to beat out gundrills or similar specialized deep-hole designs," says Porter. "We have a double cutting edge and can cut at twice the speed rate of conventional gundrills."
One of the latest Allied designs is the Gen3sys line. It is a multifunction tool that often limits tool changes and repeated setups for the customer. It is said to provide penetration rates up to 35% faster than competitive products. These tools have a proprietary multilayer AM200 coating for increased tool life. Tool geometry is designed for good chip control, especially in elastic materials.
The SMD replaceable carbide-tip drill from Sumitomo Electric Carbide Inc. (Mount Prospect, IL) has a hardened-steel drill body that accepts multiple drill-size heads, for reduced costs and inventory. The carbide replaceable drill tip can be reground. The drill tips are made of carbide with ZX coating, a 2000-layer process that provides very good wear resistance.
Ground serrations on the back of the drill tip allow for precise assembly and hole accuracy. The SMD is available in metric and inch diameters ranging from 0.4688 to 1.2008" (12–30.5 mm) with 3 and 5x diameter coolant-through drill bodies. About five tip sizes are available for each body size.
When the drill tip becomes dull, it can be discarded or resharpened. According to Paul Ratzki of Sumitomo, "This line will be expanded. A new drill tip for cast iron applications is currently in production. The main goal of the new drill tip design is to eliminate breakout occurrences."
Drilling through composite materials has its own unique set of challenges requiring specially designed tools. According to Jay Rosenbluth, president, Starlite Industries (Rosemont, PA), "A big issue is the creation of burr-like 'fuzz' around the hole's entry and exit."
The company makes specially designed tools to handle carbon and glass fiber-based materials. Drills are usually either solid carbide or PCD-tipped. Routers, countersinks, and sawblades can be either diamond or carbide.
"The solid-carbide fluted drills generally give the best quality holes," says Rosenbluth. "Carbide can give it a reaming aspect, so you get closer to a near-perfect hole. It's the tool's geometry that creates the fuzz-free hole entrance and exit. The downside to carbide tools is they dull quicker than diamond. But when it's sharp it will give you a better cut. PCD is harder than carbide."
In diameters of 3/8" (10 mm) and above, the diamond hole saw can be used. It's a tube with diamonds along the rim, and is a less expensive way of making a larger hole.
ATI Stellram, an Allegheny Technologies company (La Vergne, TN), offers a new dual carbide drill featuring Hardcore technology—a core of micrograin carbide engineered to resist breakage at the point, surrounded by a carbide grade engineered to withstand the higher speeds at the cutting edge.
Werner Mueller, global product manager of drilling says, "A Hardcore drill may last up to four times longer than other 'high-performance' drills, and operate up to twice as fast as other drills."
The point absorbs high pressure, but is basically stationary while the outer edges are running at higher surface speeds. In single-carbide tools, the carbide grade chosen had to compensate for this speed difference, so there was a trade off—performance for durability. In the past, this has meant that the drillpoint is prone to chipping.
"When you use two grades of carbide—one designed to be resilient enough to handle the pressure applied to the tool point and one tailored for wear resistance to match the high speed of the tool's outer diameter—you've eliminated the need for compromise, and you've dramatically improved performance."
Tool grades are available for cutting steels and cast irons. Drill models are available in 3:1 and 5:1 configurations. Currently, tool grade configurations for 5:1 and 8:1 are planned for later this year. The tool's 140° point geometry is designed to "zero" runout for reduced wear forces, and the flute profile promotes the best possible chip evacuation.
It can be difficult to select the best tap geometry for optimum performance when machining a broad range of metals. "But with the thread mills made by Emuge (West Boylston, MA), most of the material variables are minimized because there are no problems with matching geometries," explains Mark Hatch, product manager. This tool combines a steel body with an indexable carbide insert having a PVD coating. The tool is available in sizes from 1.0" (25 mm) on up. One of the largest is 180 mm in diam.
This product is said to bypass many of the problems encountered when working with conventional taps. According to Hatch, "Machine taps are built to the finished size of the thread, so the tool is fully engaged in the workpiece and efficient chip removal is critical. That's why you have to have the correct cutting geometry, clearance/relief angles, flute shape, surface coating, and lubricant to make an efficient tapping process.
According to Daniel Wettermap, of DMG America (Chicago), "The harder the better," is the rule of thumb when considering ultrasonic holemaking. In this process a needle-like head oscillates, acting like a miniature jack hammer to move through the workpiece.
The model 20-5 ultrasonic machine offered by DMG can machine conventional metals as well as advanced materials (glass, ceramics, etc.) through its ultrasonic tool.
In operation, the spindle oscillates a diamond tool at a frequency between 17,500 and 48,000 Hz. This removes microparticles from the workpiece surface, which can improve material removal rates by up to five times.
Where to use ECM depends on type of material, the geometry, and the radius of the form required. "We have produced holes down to 100µm [diameter depending on material thickness] with our micro ECM process," explains Don Risko, director ECM technology (The ExOne Company, Irwin, PA). Holes have been made with a conical taper, or sharp edge on one end, to be used with fluid dynamic bearings or medical devices.
In the ECM (or electrolytic) process, metal is removed by dissolution of surface atoms of the workpiece without direct contact between the tool and workpiece material. The amount of material removed is proportional to the time and intensity of an electrical current flowing between tool and workpiece.
ECM can produce geometries that normally cannot be produced by conventional machining methods, and has been used in many high-volume applications.
Abrasive waterjet holemaking has become a significant technique, particularly where a heat-affected zone is a consideration. The trend for this process, as well as most others, is for ever smaller and faster holes. Currently, Flow International (Kent, WA) is making 0.015–0.030" diam (0.4–0.76 mm) cooling holes in jet-engine components.
These components have thermal barrier coatings (TBC) to resist heat, but the blades also need to have aircooling holes drilled. It's not necessary to clean the holes as it is when a laser is used.
Major problems in making holes in fragile and composite materials are chipping and delamination. "To eliminate these problems, we use a vacuum-assist that entrains the abrasive into the cutting head before the high-pressure waterjet starts," says Hashish. "This eliminates the chipping or delamination that might be caused by the impact of the jet when water is used alone.
"Fine-hole technology utilizing EDM is one of our better processes," says Lee Richmond, micro market manager for Makino (Auburn Hills, MI). "While it serves many general applications, it has specific benefits to two major markets. Aerospace needs consistency in process. Areas of the medical industry demand accuracy—hole roundness, size, and finish."
As in any EDM process, flushing is critical to remove the "chips." To achieve a high L/D ratio, tubing to flush through is essential. Now there are "pipe electrodes" available, as small as 75µm in diam. "Flushing through the pipe allows for 50:1 L/D," says Richmond. "Or, we can machine from the backside and meet the first hole with about 10 µm accuracy, essentially giving you 100:1 capability."
As for hole roundness, obviously the deeper the hole the harder it is to maintain. "But in practical applications, we can hold 2 µm roundness with this process," he says.
"More and more applications involve some kind of automation," says Walter Schnecker, president, Datron (Milford, NH). "This is an effort to counter the overseas moves of jobs with a high volume and high labor content. In turn thatmove has caused us to emphasize work handling reduction. Our customers want all aspects of their process automated." To limit work handling with CNC machines, add a 4th or 5th axis.
The major Datron product is small tools. "But," notes Schnecker, "the smaller the drill bit, the more critical the runout. With our drills we need 3–5 µm runout. "To minimize runout and provide even clamping force, we have HSK 15 or 25 spindles that give a runout of 3 µm. They provide direct shank-clamping and the tool will not pull out of the collet. Size range is 0.004–0.5" (0.1–13 mm).
"Some potential customers believe they can make small holes by simply putting small tools in the spindles of conventional machines," says Schnecker. "This doesn't always work. For example, with a bulky 45-hp [33.75-W] motor driving a small drill, there is a lot of inertia in the system, and the motor won't stop when you want it to. So, in the case of glass, ceramic, or silicon, you may destroy the crystalline structure. It's like trying to putt with a driver. We have more nimble machines to minimize such problems. And when necessary, special tools," he concludes.
This article was first published in the June 2007 edition of Manufacturing Engineering magazine.