Grinding Wheels for OD Processes

Grinding is a machining process that is best defined as "machining with a large unknown number undefined cutting edges." Hence, it differs from other machining processes such as milling and turning that can be defined as "machining with a known number of defined cutting edges"; see Illustration 1. First and foremost, grinding delivers finer surface finishes and higher accuracies than turning or milling ever could. Furthermore, grinding is more suitable for machining very hard materials, such as hardened tool steel or tungsten carbide, both of which cannot be turned or milled.

Illustration 1: Grinding vs. hard turning

Furthermore, it is safe to say that most materials can be ground, from the soft to the very hard. The range of materials ground spans all ferrous and non-ferrous metals, glass, tungsten carbide, to name the most common. As the name implies, cylindrical grinding refers to grinding cylindrical parts such as shafts, rings, or disk-type workpieces. Cylindrical grinding is also referred to as OD (outside diameter) grinding and ID (inside diameter grinding). Cylindrical grinding, therefore, imparts "roundness" to workpieces. However, as modern cylindrical grinding machines have many CNC-controlled axes that can be synchronized; they can also grind "out-of-round" shapes such as cams, orbital geometries such as crankshaft pins, and even threads, for example.

 The modern grinding wheel

The vitrified grinding wheel consists of three elements, all of which fulfill a specific purpose. (see Illustration 2). It is important to note that today's wheels use synthetic abrasive grits such as aluminum oxides (Al2O3), silicon carbide (SiC), CBN (cubic boron nitride), and diamond. Of all the four abrasive types, only diamond exists as a natural product. Nevertheless, for grinding processes, only synthetic diamonds are used. Natural diamonds are used in fixed or rotary dressing tools for the dressing grinding wheels.

Illustration 2: Grinding wheel structure

Apart from the abrasive grits, the other two elements that make up the grinding wheel are the bond and the pores. (See Illustration 3) In short, the abrasive grits' task is to perform the cutting of a specific material at an acceptable material removal rate, to give a surface finish within a defined range, and to make sure that there are no grinding burns during the process. The bond holds the abrasive grits in place so that the grits do not break out under heavy cutting loads in the roughing passes. Even though the bond does not aid the cutting process, it is of the utmost importance as it determines the cutting action indirectly by holding the grit firmly in place. Furthermore, and very importantly, the bond must ensure the grinding process's safety at high speeds. The pores give the grinding wheel an open structure to ensure that the coolant gets into the contact area during grinding and that the chips generated during the process can be transported away from the grinding zone.

Illustration 3: Elements of the grinding wheel structure

In an ideal world, one would aim for a grinding wheel with as low a percentage of bond as possible, with as many pores as possible and with an adequate portion of abrasive grits within the wheel's matrix. Realistically, looking at the available options, vitrified grinding wheels have a specific "window of feasibility," as shown in Illustration 4, and which is represented by the so-called phase diagram. The white section within the chart's triangle represents the possible combinations of percentages of Abrasive Grits – Pores – Bond. Compared to a surface grinding wheel, a cylindrical grinding wheel has a higher portion of grits and bond and tends to have low porosity.

Illustration 4: Phase diagram of grinding wheels

 Illustration 4 shows a combination of 55% abrasive grits, 25% bond, and 20% pores, which would be typical for a cylindrical grinding wheel. A surface grinding wheel used in aerospace applications, for example, may have the following composition: 40% Grit / 10% Bond / 50% Pores.

Grinding Wheel Specification

Grinding wheel specifications are only standardized to some degree and are often interpreted loosely by the wheel manufactures. The next diagram, Illustration 5, shows the most general rules on how to read specifications, which applies to most wheel makers.

Illustration 5: Typical grinding wheel specification

Starting from the left, the above specification can be interpreted as follows

32A: a combination of numbers and letters describes the type of abrasive grit used. The number cannot be freely interpreted, but the "A" stands for aluminum oxide.

80: this figure designates the grit size in mesh (number of linear openings in a sieve)

J: a letter defines the wheel's hardness, ranging from very soft (A) to very hard (Z). While a specific letter may not correspond from one wheel maker to another, the tendency from soft to hard is common to all.

8: this number defines the structure from closed (1) to very open (20). Again, these figures do not entirely correspond from one wheel maker to another.

V: this letter sets out the type of bond. Here, "V" stands for vitrified. This nomenclature is common to all wheel makers.

"XYZ": These letters, or a combination of letters and numbers, refer to a manufacturer's specific bond formulation.

Types of Grits

On the very left side of Illustration 5, a grinding wheel specification starts with the abrasive grits under the header "A," which stands for aluminum oxide (Al2O2). As a rule, the most commonly used abrasive for cylindrical grinding is some form of aluminum oxide. The many wheels specifications encountered in the marketplace are all sub-variants or blends of different aluminum oxide types. They range from:

WA = white aluminum oxide (pure white aluminum oxide, highly friable)

PA = pink aluminum oxide (the pink color comes from adding chromium oxide to toughen the grain)

RA = red colored aluminum oxide (the red color comes from adding chromium oxide to toughen the grain

Ceramics = this a micro-crystalline form of aluminum oxide, also known by their commercial names such as

- St. Gobain’s SGB, 3SG, 5SG, 1TGP, Quantum

- 3M's Cubitron

- Tyrolit's and Radiac's CSS

- Noritake's CX

- Kure's SG, SGF, SGX, TG, and TCX

The abrasive nomenclature varies from manufacturer to manufacturer. Illustration 6 lists several abrasives' codes as given by various manufacturers.

Illustration 6: Abrasive designation by various wheel manufacturers

Of course, there are other abrasives used, each with its specific header, which is a single letter of the alphabet:

B = cubic boron nitride (CBN), also called a superabrasive. CBN is a fully synthetic abrasive.

C = silicon carbide (SiC), also a fully synthetic abrasive

D = diamond. For grinding wheels, only synthetic diamonds are used. Natural diamonds are reserved for dressing tools.

Illustration 7 shows three different abrasives in vitrified bond:

Illustration 7

Illustration 8: Grinding wheel composition

Grit Size

The grit size is given in "mesh." In other words, the grit size refers to the openings in a sieve per linear inch, as illustrated in Illustration 8. A grit size 80 relates to a mesh size of 80 grits per linear inch. The 80 grit translates into an average grit size of 0.15 to 0.21 mm, the most commonly used abrasive grit size for cylindrical grinding. Illustration 9 shows a grit size of 12; hence, on this particular sieve, there are 12 openings per linear inch. 

Illustration 9: Definition of grit size

The finer the chosen grit size, the better the surface finish, and the better the form holding. However, at the same time, too fine a grit increases the risk of grinding burn and reduces the material removal rate potential. The most commonly used grit sizes of cylindrical grinding wheels range from 60 to 120. For a 500 mm (20 inches) diameter wheel, an 80 grit would be a good starting point in the wheel selection. For a larger diameter wheel of 750 mm (30 inches) wheel, a 60 grit may be more appropriate. Grit sizes above 120 are for fine grinding purposes with high demands on the surface finish and often used in combination with fine profiles such as small corner radii. When changing grits size, it is essential to consider that the grit size does not change linearly. Changing from a 100 grit to a 60 grit.

Illustration 10: Change in grit size

Illustration 11: Change of grit size

Illustration 12: Number of grits on the wheel's periphery

The above table shows the phenomenal difference of grit sizes on the number of grits on the wheel's periphery. All wheels are 500 mm in diameter and 50 mm wide. An 80 grit wheel has some 14 million grits on the periphery, while a 120 grit wheel has about 40 million grits!

Illustration 13: Grit size and surface finish

Wheel Hardness and Structure

The letters of the alphabet denote the wheel's hardness. The closer the wheel designation is to the letter "A," the softer the wheel, the closer the wheel designation is to the letter "Z" the harder it is. It is essential to know that one manufacture's designation is not the exact equivalent to another's. However, the tendency from soft to hard in the letter range is common to all manufacturers. A given hardness is always chosen as the best balance between self-sharpening and form holding. The softer the structure of the wheel, the cooler the grinding wheel cuts. However, form holding decreases with increased softness. The inverse holds, too: The harder the wheel structure, the better the form holding is. However, this also increases the risk of burning. The wheel's hardness is controlled by the amount of bond added to the mix and the pressure applied to the mold's mixture during pressing.

All grinding wheels feature some natural porosity. As mentioned before, porosity aids in getting the coolant into the grinding zone and removing the chips from that zone. Moreover, porosity reduces the contact area between the wheel and the workpiece. Porosity is particularly crucial for ID grinding or grinding of shoulders in stepped shafts. A porous wheel ensures that there is more pressure on an individual grit. This increased pressure on the individual grit makes the wheel more friable, i.e., it tends to self-sharpen more efficiently and results in a cooler cutting process. Porosity is introduced artificially into a grinding wheel by adding pore inducing agents into the wheel mixture before pressing the wheels in a mold and subsequently firing them in a kiln. The pore inducing agents evaporates completely, leaving a void. Grinding wheels are categorized into closed (natural) and open (pore induced) structures as illustrated in Illustration 14:

Illustration 14: Closed and open structure

Grinding Wheel Bond Designation

Bonds are defined by letters. These are in order of importance and shown in Illustration 10:

V: Vitrified bond. Used for all abrasives, conventional and superabrasives

B: Resin bond (B = Bakelite, the first plastic ever used) Resin bonded wheels are mostly used for superabrasives, often in tool grinding. However, resin bonded superabrasive wheels are also used for cylindrical grinding, particularly for polishing operations.

G. Electro-plated bonded (G comes from galvanic). This bond is only used with superabrasives, with a single layer of abrasive electro-plated onto a steel wheel body.

M: Metal bond. This bond is used only with superabrasives. The abrasives are mixed with metal powder, mostly bronze, placed in a mold and fired in a kiln at the bronze combination melting temperature. These wheels are mostly used with diamond grits for grinding tungsten carbide workpieces with complex profiles. However, today metal bonded wheels with CBN are used in conjunction with EDM dressing for grinding high precision workpieces made of hardened steel. 

Today, the vitrified bond is the most common bond used in the industry. The reason is simple: Vitrified wheels are easy to dress with diamond tools, and they have a controlled porosity that makes them free- and cool-cutting. "Easy to dress" means that the wheels can also be easily re-sharpened and profiled, all with a few passes of a diamond-dressing tool. Resin-bonded wheels have to be first dressed and subsequently conditioned or opened up in separate operations that are hard to automate. Electro-plated wheels cannot be dressed or profiled and, therefore, are limited to specific uses. Metal-bonded wheels are difficult to dress or require EDM dressing systems, and for that reason, are only used in specific applications.

Ceramic Abrasives in Vitrified Bonds

Today, vitrified bonded micro-crystalline aluminum oxides, generally called "ceramics, " are universally applied in many grinding applications. The term "ceramic" is somewhat misleading. To clarify, ceramic grains also consist of aluminum oxide just as white, pink, or red aluminum oxides do. The main difference is that ceramic grits are made up of many sub-micron particles that continuously fracture in the grinding process. This constant fracturing is referred to as "self-sharpening." One distinguishes between the various forms of self-sharpening of the individual type of grit, as shown in Illustration 15:

Illustration 15: Self-sharpening mechanism of different abrasives

 As soon as the grinding pressure increases above a certain level, the grains fracture to some extent at its worn points to produce new and sharp cutting points. In the case of standard aluminum oxide grit, this fracturing is in macro particles greater than 50 microns (μm). Ceramic abrasives, or micro-crystalline Al2O3, have particle sizes < 1 micron (μm).

Ultimately, the choice of abrasive depends on the specific needs of each customer. The micro-crystalline grit is the most aggressive on diamond dressing tools, and this must be factored into the overall economics of the process. Furthermore, the higher the grit performance, the higher the wheels costs as the grit becomes more complex to manufacture. Fig. 16 illustrates the grinding performance and the resistance to wear of the highlighted grits used for cylindrical grinding.

Illustration 16: Relative grinding performance of different abrasives

Resistance to wear and friability is always a trade-off when looking at grinding performance. High performance is not always required, and wheel costs must be factored into the overall calculation.

?Part 2 and Part 3 will describe the properties of different abrasives and finally focus on using ceramic abrasives, their high performance, and their limitations.

Walter Graf, Copyright, October 2020, The Philosopher's Grindstone