What is Abrasive Machining?
This process can supplant conventional large-chip machining operations like milling, planing, broaching, and turning
Stuart C. Salmon, PhD, FSME
Advanced Manufacturing Science and Technology
Web site: www.ModernGrindingTechnology.com
There's precision grinding and then there's abrasive machining. So, what is the difference?
Insofar as grinding processes go, there couldn't be two processes that look so similar yet are so juxtaposed. The mere mention of the word "grinding," to some manufacturing professionals, conjures up nightmare scenarios of processes that take forever to remove hardly any material, at a stage where the part is of high value, and any mishap will be costly. Some have been known to break out in hives.
Abrasive machining is not precision grinding. The objective is neither super precision nor high-luster surface finishes. Abrasive machining first and foremost generates high stock removal. Abrasive machining is not considered to be a precision grinding process, but that's not to say it isn't precise. Abrasive machining can take the place of "large-chip" machining processes like milling, planing, broaching, and turning. Compare the surface finish and the precision achieved with the large-chip processes to the surface finish and precision achieved by abrasive machining, and there is no comparison—abrasive machining is far superior. Not only is abrasive machining more precise than large-chip processes (size tolerances within 0.001" or 0.025 mm and form tolerances to within 0.0005" or 0.0127 mm), it also produces a significantly better surface finish. An added bonus is that there is little to no burr generated. Abrasive machining has one other major feature—it's the means by which difficult-to-machine materials become machinable, be they metals or nonmetals.
Abrasive machining was rooted in the aerospace industry in the late 1950's when milling and broaching the dovetail and fir-tree root forms on the ends of compressor and turbine blades was considered difficult, if not impossible. It was during the late '50s that Edmund Lang (founder of ELB in Babenhausen, Germany) and his son Gerhard were experimenting with electrochemical grinding. One of their experiments appeared to go wrong when the grinding wheel was fed slowly through the workpiece with a large depth of cut, but without any electrical current. To their surprise and astonishment, the wheel walked right through the workpiece as though it was a milling cutter—Creep-Feed Grinding (CFG) was born.
Creep-feed grinding has shown how it can remove very difficult-to-machine materials quite easily and economically, with minimal burrs and with accurate form-holding capability. CFG was the first of the abrasive machining processes, though, as we might see later, abrasive cutoff may be considered abrasive machining too. Then, away from the aerospace industry, CFG began to spill over into other applications. A workpiece might have previously been rough-milled in its soft state, heat-treated, and hardened prior to a finish-grinding operation. CFG allows such parts to be through-hardened and creep-feed ground from solid. In those early days of CFG, the machining cycle was felt to be fast for machining the impossible to machine. The overall cost was reasonable, and the surface integrity was far superior to that of milling or broaching. Oftentimes, today, the overall cost of milling versus creep-feed grinding can be a wash, but it's the surface finish and the virtually burr-free nature of the process that nets a major saving in post-machining operations.
While CFG is much like milling, it uses a grinding wheel in place of a milling cutter. Unlike conventional surface grinding, CFG demands a machine tool of high stiffness and high power. The early creep-feed grinders used conventional vitrified bonded abrasives with aluminum oxide or silicon carbide grain, in a very open structure and having quite fragile bonds. Back then, producing such a tool was a challenge for the grinding-wheel manufacturers. The process also used crush dressers or diamond rolls to intermittently dress full-wheel-width forms onto the grinding wheels in very short dressing times. The grinding wheel would make one roughing pass through the material. It would then be dressed to sharpen the wheel surface, as well as refurbish the form, and a final, lighter cut was made to finish size. The cycle would then repeat.
The throughput and productivity of CFG needed to improve, as well as the ability to avoid surface cracks and workpiece burn. In the late 1970's, Continuous-Dress Creep-Feed grinding (CDCF) came onto the scene. Instead of dressing between parts or passes, the grinding wheel is constantly dressed while it's machining. Not only is the grinding wheel held continuously in a constant state of maximum sharpness, but the form is accurately maintained. The level of sharpness of the grinding wheel is such that stock removal rate could increase by a factor of 20 or more over conventional grinding, even in the most difficult-to-machine nickel and cobalt-based superalloys. What took minutes to achieve by the old CFG process takes seconds with CDCF. This new process revolutionized turbine blade manufacture, and spurred the development of automated grinding cells that took a rough cast turbine blade to a fully inspected finished part—without the workpiece being touched by a human hand.
Abrasives research was on the march too. Superabrasives (diamond and CBN) were making their mark, mostly in resin bonds and more for conventional precision grinding applications. Then vitrified superabrasive wheels appeared. Obviously, they were not candidates for any continuous dressing, due to the high cost of the abrasive, but the life of a CBN wheel was significantly greater than that of an aluminum oxide or silicon carbide wheel. High-production systems began to use intermittently dressed vitrified superabrasive wheels in a creep-feed mode. It was, however, necessary for the application to be high production, or at least have a common form, because the cost of frequently redressing a different form on a vitrified CBN wheel, versus an aluminum oxide wheel, is prohibitive.
An "intermediate" abrasive that appeared in the late 1970's is ceramic aluminum oxide. The 3M Co. called its product Cubitron, and Norton chose the name SG (for Sol-Gel or Seeded Gel). An aggressive shape abrasive, ceramic aluminum oxide has a longer life than fused aluminum oxide. The ceramic abrasive does, however, require a high force on the individual grains to initiate grain fracture and self-sharpening. CFG, on the other hand, creates very low forces on the individual grains. Initially, the ceramic abrasive was not well suited to CFG, so hybrid wheels that combined fused and ceramic aluminum oxide became popular. Later, ceramic technology allowed the production of high-aspect-ratio grains that were better suited to CFG, especially when machining the softer, more gummy materials, such as stainless steels and superalloys. The high aspect ratio can go anywhere from 4:1 to 8:1, giving the grain a directional friability. Depending on the complexity of the form, a ceramic aluminum oxide wheel can compete with CDCF.
Higher wheel speeds will typically produce faster cut times and longer wheel life. It has long been understood that aluminum oxide does not perform well at very high speed. In fact, speeds over 6000 fpm (30 m/sec) tend to cause accelerated attritious wear of the abrasive grain. In a plastic bond (not a resin bond, which employs a thermosetting plastic, but a thermoplastic plastic), however, aluminum oxide has performed well at higher wheel speeds.
At high speed, CFG is typically performed using plated superabrasive grinding wheels (12,000–24,000 fpm or 60–120 m/sec). This is called HEDG—High Efficiency Deep Grinding. Today, wheels may also be made with vitrified superabrasive segments bonded to the periphery of a metal core. To move grinding wheel speeds into the ultra-high speed grinding (UHSG) regime (above 40,000 fpm or 200 m/sec), the wheel core must be made of metal, and the abrasive is likely to be plated. Such wheels can run at nearsonic speeds (66,000 fpm or 335 m/sec) without the fear of bursting. The safety issue here is more of a "wheel off" situation. The likelihood of a metal-core wheel bursting is remote. But mounting a wheel to a spindle, on a taper, gives rise to weak design areas close to the bore of the wheel, where the stress is highest. Ultra-high-speed machines need to be designed with a "wheel-off" condition in mind. To date, UHSG has only been done in the lab. Few production systems today are running in excess of 30,000 fpm (150 m/sec).
As far as safety is concerned, human life is unlikely to be in danger during a UHSG operation, because the stock removal rate is so fast that loading and unloading of parts, as well as wheel changing, will be done automatically. Unlike the manual machines of yesteryear, there will be no personnel nearby to be injured.
Wheel balancing will be important at higher peripheral wheel speeds. At such speeds, balancing must be carried out properly, consistently, and quickly. Machine spindle designs will be quite different from those used on a conventional grinding machine. Abrasive machining grinding machines will incorporate hydrostatics, air, and combination squeeze-film technology. Such spindles will be transmitting far higher power (40 to over 100 hp, or 30–75 kW) than that employed in a conventional surface grinding system of similar table size. This is a major difference in precision grinding compared with abrasive machining.
Precision grinding takes very small depths of cut at a fast feed rate. The action is more of a polishing/rubbing action than stock removal. The specific energy (the energy required to remove unit volume of material) is high, and more of that energy transfers into the workpiece surface. It takes over 200 sec to remove 1 in.3 (16.4 cm3 of material. The spindle power required is only 12 hp (9 kW).
CFG has a higher specific energy due to the length of the arc of cut, the long thin swarf, and a wheel that is continuously degrading—and so limiting the part length (3" or 76.2 mm) that can be ground prior to the onset of thermal damage. Most of the energy in the grind is transferred to the swarf. It takes only 117 sec to remove 1 in.3 of material. The spindle power required is substantial—51 hp (38 kW) for conventional spindle speeds.
CDCF has the lowest specific energy due to the maximum state of sharpness of the grain and the lack of rubbing energy. The wheel "never gets dull," so there is no limit on part length. Most of the energy in the grind is transferred to the swarf. It takes only 17 sec to remove 1 in.3 of material. The spindle power required is high at 38 hp (28 kW), and low-to-conventional spindle speeds are required.
HEDG has a low specific energy due to the aggressive nature of the superabrasive grain, and can run longer part lengths than CFG. Most of the energy in the grind is transferred to the swarf and the grinding wheel. It takes 83 sec to remove 1 in.3 of material. The spindle power required is high—44 hp (33 kW) at high spindle speeds.
As for abrasive cutoff, cutting off a piece of 1" (25.4 mm) square bar stock has a moderately high specific energy due to the grinding energy used to "self-dress" the wheel while it's in action. The energy in the grind is transferred to the swarf and the grinding wheel, but moreover into the bulk of the workpiece material, which acts like a heat sink. It takes only 16 sec to remove 1 in.3 of material (eight cuts). The spindle power required is high, 41 hp (31 kW) at conventional wheel speeds.
UHSG has relatively low specific energy due to the brittle fracture mechanism of stock removal. Most of the energy in the grind is frictional energy, and is split between the workpiece and the wheel. It takes 41 sec to remove 1 in.3 of material. The spindle power required is high, at 52 hp (39 kW), at significantly high spindle speeds.
This contrasting of the processes is generalized for the sake of some basic understanding; results can be significantly different depending on the materials being machined, the grinding wheels used, and the dressing methods employed on the dressable wheels.
A major advantage of using plated superabrasive wheels in either HEGD or UHSG is that the wheels do not need to be dressed. No time has to be allotted for a dressing cycle, and no capital equipment or control system is required for wheel dressing. There can, however, be quite large variability in plated wheel life due to plating and processing irregularities. Wheel performance monitoring, in the form of force transducers, needs to be incorporated into the machine tool either at the spindle housing or in the part fixture, to enable personnel who monitor the wheel to decide when wheel changes need to be made.
Plated grinding wheels not only wear attritiously, but also clog and load with material. Hard-lubricant coatings have been used to provide a "slippery" surface between the grains, and so provide longer wheel life due to the additional grain protrusion achieved by the elimination of any wheel loading.
As grinding-wheel speed increases, grinding forces decrease, providing longer wheel life, yet generating more frictional energy as the wheel wears. Consequently, at high peripheral speeds cooling becomes more important than lubrication. Cryogenics have been used with success in very special applications where not only was part cooling important, but also part rigidity.
CFG, CDCF, HEDG and UHSG are considered to be abrasive machining processes, and so too is "peel grinding." Peel grinding was invented and patented by Erwin Junker Maschinenfabrik GmbH (Nordrach, Germany) in 1985 under the name Quickpoint. It's an abrasive machining process whereby a thin superabrasive grinding wheel is run at high speed and used as the nose of a "turning tool" to machine cylindrical components; even parts with high length:diameter ratios, like automotive valve stems, can be machined by peel grinding. It's a versatile process due to the common shape of the form on the wheel, though that can be modified or changed where necessary.
The abrasive machining process that removes material at a rate faster than any other is VIPER (Very Impressive Performance Extreme Removal). The first machine was installed, in production, in 1999 for the aerospace industry, again manufacturing turbine blades. VIPER combines open structure, aluminum oxide grinding wheels with high wheel speed, in a continuous-dress mode, and with CNCcontrolled, high-pressure, high-flow-rate nozzles that direct refrigerated grinding fluid to the exact position where it's required, just prior to the arc of cut and throughout the cut and all wheel diameters. This process is best performed on a machining center concept, where both wheel and dresser changes can take place automatically and under full CNC control. Patented by Rolls-Royce, the process uses special wheels manufactured by Tyrolit.
Faster wheel speeds seem worthwhile but, "Why ultra-high speed? Is it just a gimmick?" There is an additional benefit to moving into the ultra-high speed region (above 35,000 fpm or 178 m/sec). As wheel speed increases, chip morphology changes. Most metals machine in a ductile regime, whereas ceramics machine in a brittle regime. The swarf for a metal is long and thin, and can clog the pores of a grinding wheel, reducing the surface clearance between the abrasive grains. Ceramic (brittle) swarf is more like dust particles. It has been shown that when the peripheral speed of the wheel, and therefore the speed of the grain, exceeds the speed of stress propagation in the material, chip formation changes from a ductile to a brittle formation. An analogy might be to first visualize machining at normal speed, where the swarf is formed in a ductile mode, and then, as the cutting speed increases, there comes a point in time when the material compresses in front of the grain and cannot move out of the way and acts like a brittle material. At ultra-high wheel speeds, hard and soft materials will machine alike.
These very fast cutting operations will require automation for part loading and wheel changing. Looking back to when CDCF revolutionized turbine blade manufacture and grinding cells were born, it was the automation that took productivity to another level, but it was clustered around machine tool designs of a bygone era. For high-speed grinding systems, the machine tool has to be redesigned in combination with the necessary automation and safety. This will not be a new era, it will be a different world altogether.
Abrasive machining is a faster and more economical way to machine difficult-to-machine materials, and competes with milling, broaching, planning, and turning. A trend has developed over the past five or so years where machine tool builders have combined abrasive machining with large-chip machining on one unit that must handle large volumes of "Brillo" or "SOS" pad grinding swarf, and perhaps some loose abrasive grain, as well as the bulk chips from drilling and milling operations. It is, however, critical to have a fluid filtration system that can adequately accommodate the en tire range of swarf and chips.
Calamine lotion can sooth the pain from hives but to truly see high stock removal in virtually any material, with almost no burrs, you need abrasive machining.
This article was first published in the February 2010 edition of Manufacturing Engineering magazine.