What Is Abrasive Machining?

This process can supplant conventional large-chip machJasa Machining ining operations like milling, planing, broaching, and turning

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 Jasa Machining Medan 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.

Jasa Machining Medan 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 paling aman 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 tertekan 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.

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