System Concepts

The key results of successful abrasive finishing or machining are improvements in:

• Surface quality

• Retained strength

• Tolerances/finish

• Production rate

• Product performance

These results are affected by four categories of factors. Following is a list of these factors and some of the variables that influence them:

Machine tool factors

• Design: rigidity, precision, dynamic stability

• Features: controls, power/speed, slide movements/axes, truing and dressing equipment

• Coolant: type, pressure, flow, filtration system

Work material factors

• Properties: mechanical, thermal, chemical, abrasion resistance, microstructure

• Geometry: wheel-part conformity, access to coolant, shape/form required

• Part quality: geometry, tolerances, consistency

Wheel selection factors

• Abrasive: type, properties, particle size, distribution, content/concentration

• Bond: type, hardness/grade, stiffness, porosity, thermal conduction

• Wheel design: shape/size, core material, form or profile

Operational factors

• Wheel balancing

• Truing, dressing, and conditioning: techniques, devices, parameters

• Grinding cycle design

• Coolant application

• Inspection methods

Regardless of the choice of variables in the four input categories, for every abrasive machining process it is possible to visualize four interactions between the abrasive product and the work material (Fig. 20):

• Abrasive/work interaction

• Chip/bond interaction

• Chip/work interaction

• Bond/work interaction

Of these, the abrasive/work interaction is the most critical, analogous in many respects to machining processes with cutting tools (Fig. 21). The interactions between the abrasive product and the work material may be categorized as cutting (material removal process), plowing (material displacement process), and sliding (surface modification process) (Fig. 20). Every abrasive machining process is an effort to balance cutting (surface generation) and plowing/sliding (which controls the characteristics of the generated surface).

Fig. 20 Interactions in the grinding zone. (a) Abrasive/work cutting (material removal process). (b) Abrasive/work plowing (material displacement process). (c) Abrasive/work sliding (surface modification process). (d) Chip/bond sliding. (e) Chip/work sliding. (f) Bond/work sliding. See text for details.

Fig. 20 Interactions in the grinding zone. (a) Abrasive/work cutting (material removal process). (b) Abrasive/work plowing (material displacement process). (c) Abrasive/work sliding (surface modification process). (d) Chip/bond sliding. (e) Chip/work sliding. (f) Bond/work sliding. See text for details.

Fig. 21 Schematics of cutting and abrasive "machining" processes. (a) Ideal process. (b) Practical process. (c) Modified processes (diamond turning). (d) Abrasive process

Thus, every abrasive machining process can be thought of as an input/output process with defined microscopic interactions of cutting and tribological aspects of plowing and sliding. These interactions can be measured or monitored using macroscopic process variables such as force, power, and temperature. These result in certain technical outputs, and based on the rules of manufacturing economics, these in turn result in economic or system output. Figure 22 is a representation of the systems approach and illustrates the use of the principles of machining and tribology to manage and/or improve abrasive machining processes. Details of the systems approach can be obtained from the references to this article.

Fig. 22 A systems approach for abrasive finishing processes

The four interfaces in the grinding zone,listed above, may be elaborated as shown in Fig. 20. The presence of coolant or other liquids influences the nature of these sliding interactions. From this point of view, every abrasive finishing process becomes a situation of maximizing the cutting action of the abrasive (Fig. 20a) and minimizing all the sliding or tribological interactions (Fig. 20b to f):

• Minimizing abrasive/work plowing (Fig. 20b) implies proper choice of abrasive/work combination, appropriate size or shape of abrasives, and suitable chip thickness.

• Abrasive/work sliding (Fig. 20c) is minimized when the abrasive used is self-sharpening and wear flats are not generated in the abrasive during setup or truing.

• Chip/bond sliding (Fig. 20d) is minimized by suitable selection of bond/work combinations. Changing coolant, porosity in the bond, or lubricants in the bond matrix are other means of reducing this interaction.

• Minimizing chip/bond sliding has the complementary effect of simultaneously reducing chip/work sliding (Fig. 20e). From the principles of tribology, the sliding interaction between like materials has the highest coefficient of friction; hence, a poor combination must be avoided at all costs. In practice, this simple principle is often missed or ignored, resulting in high grinding forces, extensive abrasive product wear, or poor work surface quality.

• Bond/work sliding (Fig. 20f) is generally minimal when porous abrasive products are used or when vitrified or resin bond is used. However, they are very pronounced when metal bond abrasive products are used. This interaction becomes critical when fine abrasive grits are used or during finish grinding processes, when the abrasive exposure is deliberately small and the bond matrix is very close to the work surface.

The cutting action can be maximized using the principles of machining described in the article "Finishing Methods Using Defined Cutting Edges" in this Volume.

Managing sliding or tribological components while maximizing the cutting component determines whether finishing is rough, precision, or high-precision. For instance, rough finishing processes minimize the tribological components as much as possible. Precision finishing processes minimize the tribology during the rough grinding portion of the cycle and selectively use them during the semifinish and finish grinding portions of the cycle to achieve the desired surface finish or similar surface features. High-precision processes, such as lapping and polishing, depend entirely on the tribological components to achieve the desired results of surface finish, surface texture, luster, and so on.

The results of both cutting and tribological interactions are surface generation, surface deformation, or mechanical-to-thermal energy conversion. Maximizing the cutting component and minimizing tribological interactions are often associated with minimum use of forces or energy. Coolants dissipate the thermal energy to minimize the heat dissipation through the work material, if it affects the integrity of the parent material.

All of the above observations are true for a wide range of abrasive machining processes, independent of the work material type. Hence, the science of grinding may be expressed as simultaneous manipulation of four categories of inputs (i.e., machine tool factors, work material factors, wheel selection factors, and operational factors) to maximize the tribological components with the minimum force and at the minimum expenditure of energy. The balance between cutting and tribological components may depend on both technical and economic considerations. When all four categories of inputs are varied simultaneously, the benefits achieved are quantum improvements. If only one category is manipulated, the results generally are small or incremental in nature. When the microscopic interactions are not understood or considered, abrasive finishing processes are reduced to empirical and statistically managed operations, with extreme cost and quality penalties.

Nonabrasive Finishing Methods

K.P. Rajurkar, Jerzy Kozak, and Arindam Chatterjee, Nontraditional Manufacturing Research Center, University of Nebraska-Lincoln

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