Modern Machining Methods
More than conventional machining
By Robert B. Aronson
To meet the demand for production and precision, researchers and equipment builders are looking outside the bounds of conventional milling, drilling, turning, and grinding. New cutting and machining processes continue to emerge along with methods for modifying the conventional techniques. Here's a look at some that are here and others that are on the way.
Lasers have proliferated in type, size, and use since they were first demonstrated in the '60s. Now the emphasis is on improving user friendliness and ease of operation, as well as the following:
- Ease of maintenance
- Plug-and-play capability
- Smaller units to serve the medical and electronics industries
- Higher accuracy
- Ability to handle both niche and volume production
For more than two decades, lasers have been a substantial part of the sheetmetal fabrication business. Increasing laser power has driven a tremendous productivity gain. There are other features that distinguish one design from another. "We are stressing versatility and have always provided 2-D as well as 3-D cutting," explains Holger Schleuter, vice president, lasers, Trumpf Inc. (Farmington, CT).
The laser is very flexible and has done a good job of solving the dilemma that faces many managers: Should we go after niche markets or volume? The answer provided by Trumpf lasers is a 3-D system that can be easily customized to handle either type of job. In a volume automotive operation it is currently used to modify body segments. The laser makes clean cuts with no HAZ problem. "With this capability it's possible to simplify the manufacture of car bodies of the same model," says Schleuter. "For example, in the final production step the laser cuts out the area for the steering wheel, either right or left side. A sedan can become a convertible by cutting off the roof. Sedans can become station wagons by adding a few weldments.
"This reduces the number of stamping dies and handling and yields a large productivity gain. We offer flexible manufacturing without tools."
For job shops, Trumpf also offers cutting machines and press brakes, so it is possible to make box-like structures easily. The laser cuts the sheetmetal and the press brake forms the part. The newest development is the addition of a laser-based welding station for the sheetmetal shop.
An additional application of the 3-D cutting system for the auto industry is the precision cutting of high-strength steel. Automakers are using a tougher metal to gain this material's strength and weight advantages. The downside is that it is difficult to cut. Tool wear would be so high that conventional methods are not economically practical.
The Trumpf laser can easily and rapidly make the complex cut with no secondary processes needed. According to Michael Fritz, program manager, a big problem in the system's development was tolerance stackup, which included errors contributed by the part, the laser, and the part-holding system.
The fixturing issue was further complicated by the fact that the parts had no natural holes that could be used to secure them. Special fixtures were therefore required to secure the parts without causing any deformation.
CO2 lasers with a five-axis arm are used and the power can range from 3000 to 6000 W, depending on the material type and thickness.
To achieve the production speeds needed by the automotive operation, the laser system is integrated with a turntable part-holding system so the laser is constantly working.
Laser-assisted machining is a process that has long been researched and is now ready to come out of the lab, according to Dr. Yung C. Shin, professor mechanical engineering, Purdue University (West Lafayette, IN).
In operation, a laser beam is projected onto the part through fiber optics or some other optical-beam delivery unit, just ahead of the tool. The laser-induced heat softens the workpiece and makes it easier to cut.
"We use this process for very difficult-to-machine materials such as Inconel, Waspaloy or ceramics" says Shin. "For example, with our machine you can cut through ceramics like butter. It also offers a good surface, usually 0.5 µm in Ra or better.
Conventional CBN or ceramic cutting tools are used and have much longer life because the material cuts so easily.
Because the heating laser beam is tightly focused, heating is localized around the actual cut. Heat is carried off in the chips and there are no changes in the physical properties of the material cut due to heat.
During a cut, laser power is varied to match the profile being cut. Diode and CO2 lasers are used with power levels from 200 to 500 W.
"There are two problems with the process," explains Shin. "There is a physical problem initially of establishing the cutting parameters [laser power and focus, cutting feeds and speeds, etc.] and the psychological problem of convincing people it works as well as it does and has so many advantages."
MC Machinery Systems Inc. (Wood Dale, IL) offers a unique combination of two nonconventional cutting processes, waterjet and EDM. The Mitsubishi waterjet is specifically designed to work with an EDM with the combined machines functioning as a single manufacturing unit. The idea is to use the waterjet for roughing and bulkmaterial removal, then move the part to an EDM for final finishing. Experiments showed that the combined system can cut the total process by 28%.
A series of waterjet systems is available with from two to five axes. All machines are powered by a 60-hp (45-kW) motor that delivers water at pressures up to 60,000 psi (420 MPa).
Most waterjet equipment makers see an expanding market as manufacturers realize the versatility of this technique. Jeff Day, waterjet product manager for Bystronic Inc. (Hauppauge, NY), sees more sophisticated control as an area of improvement, particularly in the diagnosis of machine behavior. This will include monitoring of pump performance and maintenance needs.
"As the machines have become more reliable and operating pressures higher, multiple cutting heads are becoming more common," he notes. "Water pressures of 60,000 psi are common throughout the industry, but higher pressures are on the way. But you have to look at operating costs to see what the higher pressure does for cutting speeds and machine performance."
Easier maintenance with a resulting decrease in downtime is also stressed in newer equipment.
Improved waterjet cut finish means surfaces that don't need finishing operations.
"We now have a control that is Windows-based that is easier to run and has diagnostic features," concludes Day.
As ultra-high-pressure water technology advances, pressures will increase beyond what we have today. Additional changes will include improving the design of existing components and parts as we learn more about the impact of ultra-high pressure on the performance and life of these components and parts. For example, Jet Edge (St. Michael, MN) recently introduced a redesigned ultra-high-pressure intensifier on the iP60-50 intensifier pump. "The unit is physically smaller in size and provides easier servicing as well as extended parts and component life such as seals and check valves," explains Thomas MacGibbon, Jet Edge company vice president. "This results in lower operating costs for the users without compromising the performance of the waterjet system.
Although waterjet technology is relatively new among cutting methods, it is now considered a "mainstream" process that is rapidly expanding into the traditional metalcutting markets. More importantly, waterjet and abrasive waterjet also are expanding beyond the traditional cutting applications and are used in many other markets, with applications that include the cutting of stone and tile, soft materials such as gaskets and textiles, and food. As the technology advances with higher pressure levels, productivity also will have the opportunity to increase as more cutting heads can be added to each table.
"In the near future we may see the integration of additional motion control to the cutting heads allowing customers even more flexibility to make cuts that today require multiple machines or multiple setups," says MacGibbon. "We are also seeing an increasing demand for multitasking systems that incorporate waterjet and abrasive-jet cutting heads as well as plasma, laser, and other cutting technologies. We also expect to see a shift toward the STEP technology programming language."
Some of the advances in machining come from modifications of existing technology. Makino (Mason, OH) engineers had to make such changes when faced with problems in machining compacted graphite iron (CGI).
This metal is one of those advances that looks good on paper, but has a number of challenges when it comes to manufacturing something useful. The pressure is on to simplify machining of CGI, particularly for diesel engine blocks. The wear characteristics of this metal give it a major advantage over other types of cast iron.
In an effort to simplify the machining of CGI, Makino engineers have been working with Sandvik Coromant Co. (Fair Lawn, NJ) tooling engineers to modify conventional diesel milling and adding a boring technique based on their Flush Fine finishing process.
It's a new method for finishing cylinder bores using a precise boring process. It enables the manufacturer to operate in CGI at speeds approaching the machining of gray cast iron.
The tight tolerances necessary are achieved by a combination of programming and a high-accuracy machine, helical path operation, and tight control of the honing operation's bite. The result is a fine scalloped finish that needs no other finishing operation prior to honing.
According to Dave Woodruff, CMfgE, Makino process engineer, this technique has great possibilities when it comes to eliminating semifinishing operations. All tooling and machines for that process are eliminated. Cutting is done with specially made carbide tools utilizing Sandvik serration technology.
Another unique use of a fluid, in this case liquid nitrogen, is offering a modification of traditional turning to improve manufacturing operations. Using -320°F (-195°C) liquid nitrogen as a coolant (as opposed to water, oil or synthetic flood coolant) as a means to lower the heat in the cutting zone offers a number of advantages, particularly when working with hard materials and as a means of extending tool life. Now the process has become commercially available.
Hardinge Inc. (Elmira, NY) has introduced the process as Project Icefly, a cryogenic hard-turning technique using a Quest CNC turning center.
Icefly makes the ceramic insert stronger through the introduction of liquid nitrogen at the cutting zone. The process reduces cutting time and increases tool life, and works with hard-to-machine materials like hardened steels, wear-resistant alloys, and tungsten carbide parts.
Cubic boron nitride (CBN) and polycrystalline cubic boron nitride (PCBN) inserts have traditionally been the tools of choice for hard-turning applications. But, they are considered too costly for many operations. The Icefly cryogenic coolant system was originally developed by Air Products (Lehigh Valley, PA) and is now being refined for real-world machining by Hardinge. The system delivers a jet of -320°F liquid nitrogen directly to the insert during turning operations. This raises insert hardness, which significantly reduces the thermal softening effect that an insert may experience as a result of hard turning's inherent high cutting temperatures. The steep temperature gradient between the chip/tool interface and insert body also helps remove heat from the cutting zone. In addition, the significant cooling maintains insert edge integrity to prevent "smearing" a part's hot, compressed surface layer, thus providing a quality surface finish.
The process also allows greater use of low-cost ceramic inserts for hard-turning operations.
CBN and PCBN ceramic inserts tend to wear unevenly and are prone to fracturing when hard turning dry or with water or oil-based coolants. Increased fracture toughness resulting from low-temperature liquid nitrogen cooling provides more predictable, gradual flank wear for ceramic inserts, as well as increasing cutting speeds up to 200%. This predictable flank wear also allows alumina ceramic inserts to be used in critical finishing operations.
The liquid nitrogen may be stored in a small, dedicated cylinder near a machine, or in a supply tank that would serve multiple machines. Programming is similar to a traditional coolant delivery system. A flexible liquid nitrogen line attaches to a lathe's turret via a rotational coupling. This line feeds a delivery nozzle clamped to the tool, which directs the liquid nitrogen to the insert tip.
The system works with hardened steels, hard composites, and powder metal parts. Because the inert nitrogen vaporizes after contact with the insert, it doesn't leave residue behind. This is particularly helpful for porous powder metal parts, which often require subsequent partcleaning operations to remove coolant residue.
Nitrogen is a safe, noncombustible, and noncorrosive gas. It quickly evaporates and returns back into the atmosphere, leaving no residue to contaminate the workpiece, chips, machine tool, or operator and eliminates disposal costs.
According to Tom Sheehy, Hardinge team leader and application engineer for the project, "A supplier was having trouble machining a carbide part that was 11% cobalt, roughly 8.5" [216-mm] long and 2.75" [70-mm] diam. It was being rough-ground then finish-ground, in a four-hour rough-grinding operation. We decided to see what we could do with the part using the Icefly system. We were able to take six complete passes down the length of the part for about 48" [1219 mm], using only one edge of the insert. We ended up with around 80 fpm [24 m/min] and each pass was taking about four minutes. To take 0.00060" [0.015 mm] off, would be three passes, or about 12–15 minutes hard-turning time. This is versus four hours of rough grinding and then the additional finish-grinding operation.
The National Institute of Science and Technology (Gaithersburg, MD) is looking at a number of advanced machining and cutting processes. One that has significant influence in the nano and micro area is the focused ion beam (FIB) milling process. In this operation, a focused beam of ions "chips away" the workpiece surface to create forms and make holes. An ion beam down to 6 nm in diam is focused on the workpiece and can be controlled with submicron resolution. It can be used to remove material with a process called sputtering. There are few material restrictions. In industry, the process is used by the semiconductor industry for diagnostics and repair of integrated circuits, and researchers currently use the process to make 3-D nano structures, for example.
According to Bradley Damazo, a mechanical engineer in the Manufacturing Metrology Division. "We are looking into using this process as a means of adding very fine features to a part. FIB can make holes as small as 10 nm and removes material at a speed ranging from minutes to hours. The slow removal rate is currently one of the disadvantages, and at these levels, redeposition of the sputtered material is a problem."
"A part might be created by conventional processes to near-net shape. Then the FIB would add the micron and submicron-sized fine details. The FIB machining is certainly not in the mainstream, but it's beyond academia for creating very fine features," Damazo concludes.
This article was first published in the October 2006 edition of Manufacturing Engineering magazine.