Taking just small amounts of material from the workpiece is how grinding can maintain generally superior surface finishes and closer tolerances than other chip making processes. Single point cutters are less effective in creating very small chips.

This article looks into the grinder cutting zone to identify what's happening when an abrasive cutter contacts a workpiece. Understanding this microscopic interaction is key to understanding the grinding process.

To find out what happens in the grind and how it relates to the process, we talked to Dr. K. Subramanian. He is the director of the World Grinding Technology Center at Norton Company (Worcester, Massachusetts).

In The Zone

A grinding cutting tool is a combination of an abrasive grain and the bond that holds it. It is analogous to a single point cutter and toolholder.

There are three fundamental interactions that occur between an abrasive tool and the workpiece. These are cutting, plowing and sliding. Because there are so many cutting tools at work in a grinding operation, all of these interactions occur to some degree at all times during the process. Grinding process control is an effort to balance these interactions.

Cutting a clean chip is interaction number one. Ideally, the abrasive grain is sufficiently exposed to penetrate the workpiece material and curl a good chip. In this condition, there is sufficient clearance between the grain, bond and workpiece for the chip to escape by being flushed out with coolant or thrown away from the cutting zone by wheel action.

Process parameters that contribute to this condition include a proper choice of abrasive and work combination reflecting the right size and shape of abrasive grains coupled with correct settings of feed, speed and depth to enable the cut chip to evacuate the cutting zone.

"This is what you want happening in the grind," says Dr. Subramanian.

Due to the dynamic nature of the grinding process, other interactions also occur in the cut zone. Plowing of the workpiece material can occur in the cut zone. In this cutting interaction, the abrasive grain is unable to get sufficient penetration into the material to lift a chip. Instead, it pushes the material ahead of the abrasive edge, in effect, plowing it.

The third cutting zone interaction is sliding. This condition can occur for several reasons. A lack of cut depth can cause the abrasive grain to slide across the workpiece surface.

Insufficient clearance between the abrasive grit and the workpiece can trap the chip, causing it to slide on either the grinding wheel or the workpiece.

In cases where a grit stays bonded to the wheel too long, and it doesn't break off soon enough, the binder can come into contact with the workpiece and create slide marks on the surface.

Every grinding operation is an effort to balance these cutting zone interactions. "They occur in every grinding process including centerless, internal, cylindrical, and even snag grinding," says Dr. Subramanian. "Get a handle on these interactions. You'll maximize the good, and you'll control your grinding process."

Grinding Talk

Understanding the basic cutting interactions gives shops a vocabulary to discuss different grinding processes. The three cut-zone interactions are involved, to differing degrees, in all grinding processes. The degree to which each of the three interactions occurs determines the kind of grinding being done.

Commercial grinding processes break down into three general categories. These are rough grinding, precision grinding and high or ultra precision grinding.

The differentiating factor for each of these categories is the amount of metal removed. Metal removal is balanced against the desired tolerance or finish. In grinding, like turning and milling, high metal removal rates are generally in inverse proportion to close tolerances. Which is why shops use roughing and finishing passes.

In rough grinding, the desired workpiece/wheel interaction is focused on cutting. In these applications, maximum metal removal is the goal. Cutting off billets, snagging gates and risers from castings, grinding weld beads smooth, are all processes where the maximum amount of metal removal is the goal. Precise control of the size and surface finish is a secondary consideration.

To create size and surface finish control for high metal removal--the precision grinding application--roughing passes are generally followed by finish passes. During rough grinding operations, stress is often loaded into the machine tool structure.

To alleviate this load, a spark-out pass is often programmed. This final pass is designed to take out the built-up stress in the machine tool structure and impart a good surface finish and size tolerance to the workpiece. A spark-out pass often results in some plowing of the workpiece material, because the depth of cut is shallow.

"An ideal spark-out pass," says Dr. Subramanian, "occurs when the abrasive grain finishes the chip making on the workpiece coincidental with the machine tool's return to a relaxed state from being stress loaded."

Precision grinding applications combine high metal removal with good part size control. "Creep feed grinding, high efficiency deep grinding, and slot grinding are processes developed to increase the chip making interaction during the grind, while delivering very good tolerance and part geometry," says Dean Arvidson, technical marketing director at Norton.

In ultra precision grinding operations, little or no actual cutting is done. Instead, the workpiece surface is in effect rubbed clean primarily by sliding action from very fine abrasive grains. Ultra precision grinding is the surface finishing of a very precisely sized workpiece. Most surface finishing processes generally fall into this category. These include lapping and polishing.

Other Inputs

Interaction between the workpiece and the abrasive cutter defines the grinding process. Moving away from the cutting zone brings other influences into the process. How successfully a shop can control what happens in the grind is dependent on hundreds of other variables.

These hundreds of variables can be categorized into just four process inputs. They are the machine tool, workpiece material, wheel selection and operational factors.

• Machine tool factors: A machine's design deals specifically with a machine's rigidity, precision and dynamic stability. A machine that cannot mechanically hold the close relationship between a grinding wheel and the workpiece is less likely to deliver consistent, predictable results.

  Other machine tool factors that influence the grind are machine controls, power/speed capability, slide movements, truing and dressing mechanisms. These control how closely the grinding wheel and workpiece can be positioned on a repeatable basis. Leaving the workzone to true the grinding wheel and then returning precisely is an important machine tool factor.

  The capability of the machine tool's coolant delivery system is also a factor in the successful grind by maintaining interface temperature, providing lubricity and flushing of chips. Pump capacity, coolant type, pressure ranges, filtration system and flow all play large in the cutting zone.

• Work material factors: Defining those aspects of the workpiece that influence the grind are especially critical to successful cutting. Workpiece characteristics include the material's mechanical propertiesits machinability, thermal stability, abrasion resistance, its microstructure and chemical resistance. For instance, you can't cut steel with diamond where the abrasive/workpiece contact pressure is high because of the chemical reaction between the carbon in both. However, for lower pressure grinding operations such as honing, steel and diamond can be used together.

  Geometry is another work material factor for grinding, determining how well the grinding wheel and workpiece conform to each other. Workpiece shape can restrict coolant flow to the workzone. Sharp edges or very tight radii are special considerations that should be included in the analysis of the workpiece.

  Part quality requirements are critical factors to the successful grinding operation. These include feature tolerances and surface finish requirements. The consistency by which these tolerances and finishes are produced gage the success of the application process.

• Wheel selection factors: Obviously, selecting the abrasive is an important factor in grinding. Grain types, properties, size, distribution and concentration need to be identified.

  The toolholder for these grains (bond) is an equally important factor. Bond characteristics can be classified by their type, hardness, stiffness, porosity and thermal conductivity.

  How the grinding wheel is constructed influences the cut. Shape and size of the wheel should be matched to the workpiece geometry. Core material of the wheel and its form or profile need to co-exist with the other selection factors.

• Operational factors: These factors include fixturing, wheel balancing, the frequency of truing and dressing, the application of coolant, and whether the part is gaged in-process or off-line.

To achieve maximum influence on the grinding process, Dr. Subramanian suggests that shops deal with the grinding process as a whole. "It's more effective to look at all of the factors: machine tool, work material, wheel selection and operation in terms of their contribution to the grinding system. Understanding how each of these factors influences the cutting zone helps predict behavior within the grind. This approach permits a shop to focus on a few critical factors in each of the four categories instead of trying to deal with a large number of variables on a hit-or-miss basis."

How Do You Know?

Because the cutting zone interactions between grinding grains and work-pieces occur on a microscopic level, monitoring what's happening in the cut must be done indirectly. Each of the three cutter/workpiece interactions--cutting, plowing and sliding--generates a signature that can be measured and monitored.

Based on the desired workpiece finish requirements, the optimum combination of cutting, plowing and sliding that produces those results becomes a baseline measurement. Deviation from that base can be measured and monitored using force, power consumption on the machine tool, and on occasion, temperature measurements. Wheel wear, patterns, workpiece geometry, finish and surface quality also serve as evidence about the interactions taking place in the cutting zone.

In application, using the four categories of machine, material, wheel and operational factors, a shop can set up an initial part run that fits within the known parameters of all four categories. Changing one or more categories will produce results either positive to the process or negative. Making workpieces that meet or exceed their technical and economic standards is the ultimate result of the grinding process.

Results

Successful grinding is a continuous effort to produce improvements in several key areas. Some of these deal with the technology of grinding a good part and others cover the economics of producing a part profitably. "When the results are tallied," says Dr. Subramanian, "grinding technology and economics are combined. Each influences how a part is made."

Technology results include meeting or exceeding the specifications required for the finished workpiece. Surface finish is one. This is accomplished by using the process inputs in a combination that delivers the required surface finish specification.

Likewise, part tolerances are an output of the grinding process. Tolerance is generally described as deviation from the nominal value. It quantifies dimensions such as length, diameter and thickness. Tolerances also describe surface features such as finish, flatness and form.

The process must hit these technology results more than once. Most parts are manufactured in batches. If one imagines a tolerance as a point in space, consistency is how close a process can stay to that point. Measurement of the results of consistency are done through statistical process control (SPC).

On the economic side of the result equation are factors that determine efficiency of the process and profitability. These are summarized as production rate, cost per part and finished product performance.

"If a shop looks at these results while keeping in mind the different factors that influence the interaction between the abrasive tool and the workpiece, the shop will be able to make improvements in the grinding process," says Mr. Arvidson. "Trying to address individual factors without an understanding of the overall interaction of the process is why grinding seems so complex to many."

It's About Today

Today's grinding scene has changed dramatically. Many of these changes are fundamental, reflecting shifts in how manufacturing as a whole is done. Long profitable runs are shorter. This has necessitated changing over to make different parts more often to fit just-in-time inventory requirements.

Responsibility for making the machine tool grind successfully on new and different jobs, falls squarely on the shoulders of the shop. Therefore, the shop needs to have the technical expertise in-house. For example, most shops have a person versed in turning. Fewer have an equivalent expert in grinding. This lack of technical expertise has driven many shops to look at alternative processes to grinding.

"To help grinding remain a viable chip making process, we and companies like ours need to help fill the grinding skills void," says Dr. Subramanian. "That's much of the motivation behind Norton's World Grinding Technology Center. A big part of its charter is to study grinding and disseminate the results to the grinding marketplace. Our goal is to work with builders to help educate shops about the grinding process. We want to take the mystery out and replace it with process knowledge."