MFE1202 - Fundamentals of Manufacturing and Machining - Grinding and Other Abrasive Processes

Introduction: Process Planning

• MFE1202 – Fundamentals of Manufacturing and Machining

• Grinding and Other Abrasive Processes

• Dr Ing. Emmanuel Francalanza and Dr Ing. Pierre Vella from DIME, UoM are the instructors.

Fundamentals of Abrasive Processes

• Abrasive machining involves material removal using hard, abrasive particles, typically in a bonded wheel.

• Grinding is the most significant abrasive process.

• Other abrasive processes include honing, lapping, superfinishing, polishing, and buffing.

• Abrasive machining is generally used for finishing, but some processes can achieve high material removal rates.

Grinding

• Grinding is a material removal process achieved by abrasive particles in a bonded grinding wheel rotating at high surface speeds.

• The grinding wheel is usually disk-shaped and precisely balanced for high rotational speeds.

• The wheel contains many cutting teeth (abrasive particles), and the workpiece is fed relative to the wheel for material removal.

• Grinding can be applied to various materials, from soft metals to hardened steels and hard non-metallic materials like ceramics and silicon.

• Some grinding processes can produce extremely fine surface finishes, down to $0.025 \,\mu m$.

• Grinding can achieve very close dimensional tolerances.

Grinding vs. Milling

• Significant differences exist between grinding and milling:

  • Abrasive grains in grinding wheels are much smaller and more numerous than milling cutter teeth.

  • Grinding cutting speeds are much higher than in milling.

  • Abrasive grits in grinding wheels are randomly oriented and have a very high negative rake angle on average.

  • Grinding wheels are self-sharpening; as the wheel wears, abrasive particles fracture to create fresh cutting edges or are pulled out to expose new grains.

• Tolerance Comparison:

  • Grinding a 10mm block:

    • Minimum Tolerance: $\pm 0.0005$

    • Maximum Tolerance: $\pm 0.002$

  • Milling a 10mm block:

    • Minimum Tolerance: $\pm 0.005$

    • Maximum Tolerance: $\pm 0.02$

Grinding Applications

• Grinding is traditionally used to finish parts whose geometries have already been created by other operations.

• Grinding machines have been developed to grind:

  • Plain flat surfaces

  • External and internal cylinders

  • Contour shapes such as threads

• Contour shapes are often created by special formed wheels that have the opposite of the desired contour.

Grinding Wheel Composition

• A grinding wheel consists of:

  • Abrasive particles

  • Bonding material

• The bonding material holds the particles in place and establishes the shape and structure of the wheel.

• Five basic parameters of a grinding wheel:

  • Abrasive material

  • Grain size

  • Bonding material

  • Wheel grade

  • Wheel structure

Abrasive Materials

• Different abrasive materials are suitable for grinding different work materials.

• General properties of an abrasive material used in grinding wheels include high hardness, wear resistance, toughness, and friability.

• Friability is the capacity of the abrasive material to fracture when the cutting edge of the grain becomes dull, thereby exposing a new sharp edge.

• The abrasive materials of greatest commercial importance are aluminum oxide, silicon carbide, cubic boron nitride, and diamond.

• Abrasive Materials and Their Uses:

  • Aluminum Oxide:

    • Use: Steel and other ferrous, high-strength alloys.

    • Knoop Hardness: 2100

  • Silicon Carbide:

    • Use: Ductile metals such as aluminum, brass, and stainless steel, as well as brittle materials.

    • Knoop Hardness: 2500

  • Cubic Boron Nitride:

    • Use: Hardened tool steel and aerospace alloys.

    • Knoop Hardness: 5000

  • Diamond:

    • Use: Hard and abrasive materials such as ceramics, cemented carbides, and glass.

    • Knoop Hardness: 7000

Grain Size

• The grain size of the abrasive particle is important in determining surface finish and material removal rate.

  • Small grit sizes produce better finishes.

  • Larger grain sizes permit larger material removal rates.

• The selection of grit size also depends to some extent on the hardness of the work material.

  • Harder work materials require smaller grain sizes to cut effectively.

  • Softer materials require larger grit sizes.

• Thus, a choice must be made between these objectives when selecting abrasive grain size.

Bonding Material

• The bonding material holds the abrasive grains and establishes the shape and structural integrity of the grinding wheel.

• Desirable properties of the bond material include strength, toughness, hardness, and temperature resistance.

• The bonding material must be able to:

  • Withstand the centrifugal forces and high temperatures experienced by the grinding wheel

  • Resist shattering in shock loading of the wheel

  • Hold the abrasive grains rigidly in place to accomplish the cutting action

  • Allow those grains that are worn to be dislodged so that new grains can be exposed.

Types of Bonding Materials

• Vitrified bond (V):

  • Description: Consists of baked clay and ceramic materials. Most commonly used. Strong and rigid, resistant to high temperatures.

• Silicate bond (S):

  • Description: Limited to applications where heat must be minimized, e.g., grinding cutting tools.

• Resinoid bond (B):

  • Description: Very high strength and used for rough grinding and cutoff operations.

• Metallic bond (M):

  • Description: Used for diamond and cBN grinding wheels.

Wheel Structure

• Wheel structure refers to the relative spacing of the abrasive grains in the wheel.

• In addition to the abrasive grains and bond material, grinding wheels contain air gaps or pores.

• Generally, open structures are recommended in situations in which clearance for chips must be provided.

• Dense structures are used to obtain better surface finish and dimensional control.

• The volumetric proportions of grains, bond material, and pores can be expressed as:

  • $P<em>g + P</em>b + P_p = 1.0$

    • $P_g$ = proportion of abrasive grains in the total wheel volume

    • $P_b$ = proportion of bond material

    • $P_p$ = proportion of pores (air gaps)

• An open structure is one in which $P<em>p$ is relatively large, and $P</em>g$ is relatively small.

• A dense structure is one in which $P<em>p$ is relatively small, and $P</em>g$ is larger.

Wheel Grade

• Wheel grade indicates the grinding wheel’s bond strength in retaining the abrasive grits during cutting.

• This is largely dependent on the amount of bonding material present in the wheel structure, $P_b$.

• Grade is measured on a scale that ranges between soft and hard.

• ‘‘Soft’’ wheels lose grains readily and are generally used for applications requiring low material removal rates and grinding of hard work materials.

• “Hard’’ wheels retain their abrasive grains and are typically used to achieve high stock removal rates and for grinding of relative soft work materials.

Grinding Wheel Types

• Peripheral Grinding Wheels

• Face Grinding Wheels

• Edge Grinding Wheels

Grinding Wheel Marking System Example

• Example: 2C 120 J5 VB8

  • Abrasive type: C (Silicon Carbide)

  • Grain Size: 120 (Fine)

  • Grade: J (Medium)

  • Structure: 5 (Dense)

  • Bond Type: V (Vitrified)

Analysis of the Grinding Process

• $v = \pi DN$

  • N = Spindle Speed

  • D = Wheel Diameter

• d = Infeed

• w = Crossfeed

• $RMR = vwd$

• In a grinding operation, key factors are:

  • Surface finish

  • Forces and energy

  • Temperature of the work surface

  • Wheel wear

Number of Active Grits

• The number of active grits (cutting teeth) on the outside periphery of the grinding wheel is denoted by C. Smaller grain sizes generally give larger C values.

• Based on the value of C, the number of chips formed per time \n_c is given by:

  • $n_c = vwC$

    • v = wheel speed, mm/min

    • w = crossfeed, mm

    • C = grits per area on the grinding wheel surface, grits/mm$</p></li></ul></li></ul></li><li><p>Surface finish will be improved by increasing the number of chips formed per unit time on the work surface for a given width w.</p></li><li><p>Therefore, increasing v and/or C will improve finish.</p></li></ul><h4 id="a5d469ad-9eda-4079-9824-5fba51059ca8" data-toc-id="a5d469ad-9eda-4079-9824-5fba51059ca8" collapsed="false" seolevelmigrated="true">Specific Energy in Grinding</h4><ul><li><p>In grinding, the specific energy is much greater than in conventional machining, due to:</p><ul><li><p>Size Effect</p></li><li><p>Negative Rake Angles</p></li><li><p>Not all grits are engaged in actual cutting</p></li></ul></li></ul><h4 id="f6cae052-200d-4ae6-8843-e7b54a0abee7" data-toc-id="f6cae052-200d-4ae6-8843-e7b54a0abee7" collapsed="false" seolevelmigrated="true">Rake Angles in Grinding</h4><ul><li><p>The individual grains in a grinding wheel can possess extremely negative rake angles.</p></li><li><p>The average rake angle is about -30, with values on some individual grains believed to be as low as -60.</p></li><li><p>These very low rake angles result in low values of shear plane angle and high shear strains, both of which mean higher energy levels in grinding.</p></li></ul><h4 id="05d6f256-a0aa-47a4-a7a4-4067caedb150" data-toc-id="05d6f256-a0aa-47a4-a7a4-4067caedb150" collapsed="false" seolevelmigrated="true">Grain Actions</h4><ul><li><p>Three types of grain actions can be recognized:</p><ul><li><p>Cutting:</p><ul><li><p>The grit projects far enough into the work surface to form a chip and remove material.</p></li></ul></li><li><p>Plowing:</p><ul><li><p>The grit projects into the work, but not far enough to cause cutting; instead, the work surface is deformed, and energy is consumed without any material removal.</p></li></ul></li><li><p>Rubbing:</p><ul><li><p>The grit contacts the surface during its sweep, but only rubbing friction occurs, thus consuming energy without removing any material.</p></li></ul></li></ul></li></ul><h4 id="cb978103-c06b-4ec3-926c-0d23b8a86669" data-toc-id="cb978103-c06b-4ec3-926c-0d23b8a86669" collapsed="false" seolevelmigrated="true">Surface Temperature</h4><ul><li><p>Grinding is characterized by high temperatures because of the size effect, high negative rake angles, and plowing and rubbing of the abrasive grits against the work surface.</p></li><li><p>Unlike conventional machining operations, much of the energy in grinding remains in the ground surface, resulting in high work surface temperatures.</p></li><li><p>The high surface temperatures have several possible damaging effects, primarily surface burns and cracks.</p></li><li><p>Surface temperature$\T_s$is related to grinding parameters as follows:</p><ul><li><p>$Ts = Ks d^{0.75} rg C \frac{v}{vw}^{0.5} D^{0.25}$</p><ul><li><p>$K_2$= a constant of proportionality</p></li></ul></li></ul></li><li><p>Surface damage owing to high work temperatures can be mitigated by:</p><ul><li><p>Decreasing depth of cut d, wheel speed v, and number of active grits C.</p></li><li><p>Increasing work speed$v_w$$

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  Cutting Fluids

  • The proper application of cutting fluids has been found to be effective in reducing the thermal effects and high work surface temperatures.

  • Reducing friction and removing heat from the process are the two common functions.

  • Washing away chips and reducing the temperature of the work surface are very important in grinding.

  Grinding Wheel Wear

  • Three mechanisms are recognized as the principal causes of wear in grinding wheels:

    1. Grain fracture: when a portion of the grain breaks off.

    2. Attritious wear: when dulling of the individual grain occurs.

    3. Bond fracture: when the individual grain is pulled out of the bonding material.

  • Wheel Loading

    • Accumulation of worn grains

    • Wear accelerated by grain fracture

    • Attritious wear with some grain and bond fracture

    • Grains becoming dull

  Grinding Operations and Machines

  • Surface grinding is normally used to grind plain flat surfaces.

  • It is performed using either the periphery of the grinding wheel or the flat face of the wheel.

  • Peripheral grinding is performed by rotating the wheel about a horizontal axis, and face grinding is performed by rotating the wheel about a vertical axis.

  • Types of Surface Grinding Machines:

    • Horizontal Spindle Reciprocating Worktable

    • Horizontal Spindle Rotating Worktable

    • Vertical Spindle Reciprocating Worktable

    • Vertical Spindle Rotating Worktable

  Cylindrical Grinding

  • External cylindrical grinding (also called center-type grinding) is performed much like a turning operation.

  • The grinding machines used for these operations closely resemble a lathe in which the tool post has been replaced by a high-speed motor to rotate the grinding wheel.

  • Centerless grinding is an alternative process for grinding external and internal cylindrical surfaces.

  • As its name suggests, the workpiece is not held between centers, resulting in a reduction in work handling time.

  • Centerless grinding is often used for high-production work.

  • The setup for external centerless grinding consists of two wheels: the grinding wheel and a regulating wheel.

  Disk Grinders

  • Disk grinders are grinding machines with large abrasive disks mounted on either end of a horizontal spindle.

  • The work is held (usually manually) against the flat surface of the wheel to accomplish the grinding operation.

  Other Abrasive Processes

  Honing

  • Honing is an abrasive process performed by a set of bonded abrasive sticks.

  • A common application is to finish bores of engines, hydraulic cylinders, and gun barrels.

  • The motion of the honing tool is a combination of rotation and linear reciprocation, regulated so that a given point on the abrasive stick does not trace the same path repeatedly.

  Lapping

  • Lapping is an abrasive process used to produce surface finishes of extreme accuracy and smoothness.

  • It is used in the production of optical lenses, metallic bearing surfaces, gauges, and other parts requiring very good finishes.

  • Metal parts that are subject to fatigue loading or surfaces that must be used to establish a seal with a mating part are often lapped.

  • Instead of a bonded abrasive tool, lapping uses a fluid suspension of very small abrasive particles between the workpiece and the lapping tool.

  Polishing

  • Polishing is used to remove scratches and burrs and to smooth rough surfaces using abrasive grains attached to a polishing wheel rotating at high speed (around 2300 m/min).

  • The wheels are made of canvas, leather, felt, and even paper.

  Buffing

  • Buffing is similar to polishing in appearance, but its function is different.

  • Buffing is used to provide attractive surfaces with high luster.

  • The abrasives are very fine and are contained in a buffing compound that is pressed into the outside surface of the wheel while it rotates.

  Summary

  • The lecture introduced the process planning activity.

  • Reviewed the fundamentals of the grinding process.

  • Described other abrasive processes used for finishing operations.