Feature Article - Abrasive Machining - It's Not the Same Old Grind

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Abrasive Machining - It's Not the Same Old Grind

By Dr. Stuart C. Salmon

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For more information contact Dr. Stuart C. Salmon, president of Advanced Manufacturing Science and Technology (Ross-ford, OH), at (419) 662-9551.

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Grinding processes traditionally have been used for the fine finishing of hardened surfaces and for the inherent dimensional precision the process yields. The super precision and surface quality required by the moldmaking industry demands the use of grinding processes to create flat, rounded and complex surfaces. The expertise necessary to conduct efficient and productive grinding processes often lies in the "hands-on" knowledge and experience of the journeyman. With the lack of apprenticeship programs and the age of the fully-skilled moldmaker, moving closer and closer to retirement, grinding is proving to be a practical as well as a technical challenge. Grinding and abrasive processes are regarded as necessary evils by the industry. If anything can be done to take the place of a grinding operation it is felt that it's for the better. This is a grave mistake.

Traditional Grinding Technology
Grinding technology has advanced significantly during the past 25 years. Unfortunately, the great majority of the machine tool industry has not kept up with those advances and has fed the industry with poor, lackluster designs that will not allow the new technologies to be exploited to their fullest potential. There are, however grinding processes operating reliably in the industry today, successfully competing with "large chip" processes like broaching, milling, planing and even some turning operations. The principles of creep-feed grinding, high-speed grinding and the proper use of superabrasives are not only better alternatives to the conventional "large chip" machining processes, they also allow the next generation of materials like ceramics, whisker-reinforced metals and fiber-reinforced polymers to be machined easily where there is no alternative. That is an important point to ponder. As materials become more difficult to machine, it is of paramount importance that the principles of grinding technology are understood. Abrasive machining practices cannot be brushed aside.

Much of the manufacturing industry today is entrenched in the traditional methods of machining. Actually, their rationale is that they are being conservative by maintaining the "status quo." This is not being conservative at all, it is settling for mediocrity and that breeds the loss of profit and the ability to compete in new markets. Typically, if a company's operation is milling, then it looks for better ways to mill; if it is broaching, then it looks for better ways to broach, etc. etc. Grinding today is often the overall better alternative method. This is a most difficult message to get across.

Historically, grinding has been most problematic and is traditionally visualized as the process for fine finishing and providing a close tolerance part. Modern grinding processes can offer substantial stock removal rates coupled with high dimensional tolerances and superior, virtually burr-free, surface integrity. Just because abrasive machining is neither milling, nor broaching, nor turning, the conservative traditionalists sweep it aside, mostly because it is something unknown - something new to have to learn and master. There is inertia and great apathy to want to adopt what appear to be unusual methods. This is generally what the industry calls "being conservative." Let someone else try it first. Many years ago, EDM and wire EDM were designated "non-conventional" or "non-traditional" processes; today they are well-established techniques and part of the everyday manufacturing world. It is strange that the basic principles of grinding - a process used by Stone-Age man - are viewed by many with such disbelief and contempt.

Modern Grinding Process Technology
Modern grinding process technology lies in the areas of creep-feed grinding, high-speed grinding and grinding with super-abrasives.

Abrasive Types
The basic abrasive types are aluminum oxide, silicon carbide, cubic boron nitride (cBN) and diamond. Aluminum oxide is the all-purpose abrasive with the widest variety of applications. Due to its inherent sharp shape, silicon carbide will be used to grind aluminum, magnesium and titanium alloys, as well as polymeric materials and rubber. Silicon carbide works well on hard materials; however, diamond is a better abrasive for grinding hard and ultra-hard materials like carbides, glass and ceramics. Diamond is carbon and has a chemical affinity for iron - resulting in heavy wheel wear and poor performance when machining ferrous materials. Diamond is always used on non-ferrous materials.

Cubic Boron Nitride (cBN) has been available since 1969. It is an extremely hard and abrasion-resistant material. It can be used to great advantage in the machining of ferrous materials. It is significantly harder than aluminum oxide, but moreover it is a conductor of heat. Aluminum oxide and silicon carbide - the conventional abrasives - are refractory materials and act as insulators. cBN and diamond - the two superabrasives - are conductors of heat. Diamond conducts heat by a factor of six times over that of copper. These superabrasives will always inherently grind cooler than the conventional abrasives.

Bond Types
There is an array of bond types. The most common bond for a grinding wheel is vitrified - a glassy, brittle bond allowing the wheel to "self-sharpen" by mechanical breakdown. There are resin bonds (thermo-setting resins) that hold the abrasive grains in a virtually impervious bond with little chip clearance. These resin bonds "self-sharpen" by heat coking the resin, which becomes brittle and crumbly - releasing the worn grains. This is a major and fundamental difference in bond types. Vitrified wheels "self-sharpen" by the increase in mechanical force whereas resin bonds "self-sharpen" by the action of heat. A relatively new bond type is plastic bond (a thermo-plastic plastic). That means when the bond heats up it doesn't coke, it becomes pliable and allows the grains to move in the bond system and re-set to present new and sharper edges to the arc of cut. This is a particularly useful phenomenon for high-speed grinding applications when using conventional abrasives.

There are two types of metal bond - plated and sintered. In both cases the wheels are impervious to fluid. The wheels have very little chip clearance. They are used for ceramic and glass grinding; however, when grinding metals they would be best run at high-speeds (above 12,000 sfm). The higher wheel speed decreases the individual grain depth of cut, making it less important for a large chip clearance within the grinding wheel. The harder the material, the better the performance of high-speed grinding.

Creep-Feed Grinding
Creep-feed grinding is the process that competes with the "large chip" machining processes. Consider creep-feed grinding when milling, broaching, planing or turning. If the parts have complex forms and demand high surface integrity, creep-feed grinding is a good candidate. Something to consider is how much easier it is to modify the profile on a grinding wheel by dressing, than it is to re-manufacture a set of broach tooling.

Creep-feed grinding can be used with high-induced, porosity-vitrified aluminum oxide, silicon carbide and superabrasive grinding wheels in an intermittent dressing mode or with continuous dressing. The continuous dressing operation however, would be used with conventional abrasives only.

Continuous-dress, creep-feed grinding is a creep-feed process where the grinding wheel is dressed using a diamond roller at the same time the grinding wheel is machining the workpiece. It would not be at all cost-effective to continuously dress superabrasive wheels. There is heavy wheel usage per unit time, but the stock removal rate is phenomenal. Wheel usage in terms of G ratio is two to three times that for intermittent dressing, but the stock removal rate increases by a factor of 10 to 20 times. Surprisingly, the surface integrity is excellent and there are virtually no burrs. Parts that would have taken minutes to machine have their machining time reduced to seconds. In fact, where the process once caused major problems and received a bad name, it is now found that the parts cannot be handled fast enough and so automation for part loading and unloading is essential.

Creep-feed grinding also is performed with plated cBN wheels at high speeds - often termed HEDG (High Efficiency Deep Grinding). High-speed is generally run at around 30,000 sfm for cBN, and typically, 9,000 sfm for diamond. Diamond performance drops off above 9,000 sfm whereas cBN continues on as high as 60,000 sfm. At high (20,000 to 35,000 sfm) and very high speeds (45,000 to 66,000 sfm, close to the speed of sound) safety becomes a major issue. The machine design is critical. Normally insignificant areas become major factors. At high speeds the grinding debris leaving the grinding wheel can quickly erode the machine guarding and enclosure. A major advantage of high-speed grinding is the long life afforded by the plated cBN with no cost incurred for a dressing system or its associate consumable dressers. Creep-feed principles at high-speed work well on all materials and both efficiency and competitiveness improve, as the materials become more difficult to machine by the conventional means.

Remember high-speed grinding (10,000 to 12,000 sfm) can be achieved using aluminum oxide wheels in combination with the special plastic bonds. Plated wheels perform best when an accurate intricate form is required, whereas the plastic-bonded aluminum oxide wheels would be best suited to pure stock removal. Both plastic- and resin-bonded wheels add a little "cushion" to the process, allowing some vibrationally unstable machines to perform a little better than they would using a solid metal bond wheel.

High-speed grinding works well in hardened tool steels, the harder the material the better. Unless however, the wheel speed is very high - in the range of 35,000 to 40,000 sfm or higher. Only then does high-speed grinding work well in softer or gummy materials like stainless steel and aerospace alloys and only plated cBN would then be used.

Softer materials are better machined using creep-feed grinding with highly-induced porosity grinding wheels. The arc of cut is long. Spreading the load over a large arc of the wheel shares cutting force over many grains. The force on each individual grain is so low that even the most complex, delicate form is maintained accurately for a significant time over that of reciprocating and cylindrical grinding where the arc of contact is very short and the force on each individual grain is very high. A comparable increase in the length of the arc of cut for creep-feed over conventional grinding is in the order of 10 to 100.

Dressing
Dressing vitrified grinding wheels with diamond roller dressers is becoming increasingly popular due to the ease with which a form can quickly be dressed into a vitrified wheel. Rotary dressers are cost-effective when machining large batch quantities of the same part or form. Diamond rollers offer a more consistent wheel surface, better repeatability and better control of the wheel topography than any other dressing method.

Plated Wheels
The use of plated wheels negates the need for a dresser. Plated, metal-bonded wheels may be used to their full advantage, cutting naturally cooler and so transferring 80 percent of the energy away from the arc of cut and only 20 percent typically being transferred into the workpiece. Comparable ratios of energy for conventional grinding may be as much as 40 percent into the workpiece and 60 percent into the surroundings. Be careful here because the plated cBN running at higher wheel speeds demands a very rigid structure and one that is vibrationally and thermally stable. Few machines are up to the challenge.

Dry Machining
With all of the talk about dry machining, will we ever see dry grinding? It is a sure bet that grinding will remain substantially a wet process. Resin bond wheels tend to break down and "self-sharpen" by heating the bond system - so cooling is less of a factor, lubrication is more important. Some oils react with the resins and can assist in the "self-sharpening" effect, however watch for fluid reactions when a fine form is needed to be held. The better cutting fluid choice for use with superabrasives is oil. Fluids based on straight mineral oils are the cause of industrial workplace hazards in the area of fires, health problems in the areas of fumes and particulate in the atmosphere as well as the need to use a solvent for the cleaning or de-greasing of parts. New fluids, like ester oils and glycol-based fluids, dramatically decrease these risks and in the case of the glycol fluids actually show cBN grinding wheel performance to be significantly better when using sulfur-chlorinated straight mineral oils.

In order to facilitate dry (or what is more properly described "quasi-dry" or "micro-lubrication" machining, cutting tools are coated with a multi-layer of PVD coatings - wear-resistant, thermal barrier and hard lubricant coatings. The coatings applied to "large chip" cutting tools have not shown promise on grinding wheels. Plated wheels offer the best avenue for PVD coatings. TiN (Titanium Nitride) coatings have been tried and showed no advantage over having no coating at all. The hard lubricant coatings of MoS2 (Molybdenum Disulphide) might show an advantage. The application of hard lubricants to plated superabrasive wheels has yet to be tested. The fundamental mechanism of cutting with "large-chip" formation over that of "micro-chip" formation in grinding is vastly different. The effects of coatings on "large-chip" making tools do not necessarily follow for grinding wheels.

Key to Successful Grinding
The key to successful grinding is the proper selection and proper application of the cutting fluid. Generally, the longer the arc of cut, the greater the need for more of a wetting/cooling, synthetic fluid, as opposed to the smaller arc lengths where highly lubricating fluids with EP (Extra Pressure) additives will be necessary. Companies are becoming more discerning with regard to the performance as well as the environmental impact of the cutting fluids they purchase. It is peculiar that such importance is given to the chemistry, yet the mechanics of actually applying the fluid correctly is almost reckless. Large volumes of fluid are squirted toward grinding wheels in the vain hope that with enough pressure the fluid might just get between the grinding wheel and the workpiece and even into the arc of cut. The matching of the speed of the fluid exiting the nozzle to the velocity of the grinding wheel ensures proper fluid application. So, few companies design their nozzles scientifically, settling for a bent piece of copper pipe or a flexi-joint piece of plastic hose to direct the fluid in some "good looking" direction.

Ideally, the grinding machines of the day need to have higher wheel speeds, be constructed with higher rigidity and exhibit good vibrational stability - all in a small footprint. They also need to be fully enclosed, yet easy to clean and service as well as not leak fluid or fumes. A precious few machine tool builders are embracing those ideals.

The potential for advances in grinding applications, especially over the conventional "large-chip" making processes is phenomenal, but they are not for everyone. Look to the types of materials and the complexity of the form on the surface of those parts. Look carefully at the condition of those machine tools. The next time you are looking for a new "large-chip" making machine, perhaps consider grinding as the alternative.