Abrasive Machining and Finishing Operations

Abrasive Machining and Finishing Operations

• A machining operation systematically removing material from the surface in very small amounts.
• Utilizes abrasive particles, small, hard particles with sharp edges and irregular shapes.
• Examples include sand, sandpaper, and emery cloth.
• Used to smooth surfaces, remove sharp corners, improve surface finish, and create dimensional accuracy.

Types of Abrasive Machining Operations

• Grinding: The predominant abrasive machining operation.
• Honing: Improves surface finish.
• Lapping: Achieves precise dimensional accuracy and highly polished finishes.
• Buffing: Removes fine scratches and imperfections, improving surface finish.
• Polishing: Creates a high-quality, smooth, mirror-like finish.

Applications of Abrasive Operations

• Finishing heat-treated hard surface metals and alloys.
• Finishing ceramic surfaces and sunglasses.
• Cutting off lengths of parts, structural shapes, masonry, and concrete.
• Removing unwanted weld beads and splatters.
• Creating smooth and flat surfaces on silicon wafers used in the semiconductor industry.
• Polishing bearings and races.
• Cleaning surfaces with jets of air and water containing abrasive particles.

Abrasive Materials

• Conventional Abrasives: Aluminum oxide and silicon carbide.
• Super Abrasives: Cubic boron nitride and diamond, exceptionally hard, enhancing surface quality.

Grinding Wheels

• Formed by bonding abrasive grains.
• Each particle removes tiny chips from the workpiece.
• The wheel wears out due to particle removal.
• Wheels require reconditioning.

Grinding Ratio

• Relationship between workpiece and grinding wheel material removed.
• Ranges from 2 to 200, depending on materials and parameters.
• Grinding fluids significantly improve the ratio (by a factor of 10 or more).
• High ratio means efficient material removal and less wheel wear.

Reconditioning Grinding Wheels

• Dressing: Conditions the wheel to create fresh, sharp grain edges.
• Truing: Restoring a true circular shape to the wheel surface.

Grinding Wheel Problems

• Glazing: Shiny surface due to excessive abrasive particle wear.
• Loading: Workpiece material clogging the wheel pores.
• Both glazing and loading require wheel reconditioning.

Dressing Grinding Wheels

• Uses a dressing tool to create a fresh abrasive surface on the wheel.

Grinding Operations

• Surface Grinding: Grinding flat surfaces with a horizontal spindle.
• Cylindrical Grinding: Grinding external cylindrical surfaces with parallel axes.
• Centerless Grinding: High-production cylindrical surface grinding, workpiece supported by a blade and regulating wheel.
• Internal Grinding: Grinding internal diameters and profiles with a smaller wheel.
• Creep Feed Grinding: Large depth of cut (up to 6mm) and slow feed rate for large-scale metal removal.

Grinders

• Universal Tool and Cutter Grinders: Grinding single and multipoint tools, cutters precisely.
• Toolpost Grinders: Attaches to lathes to grind lathe-mounted tools.
• Swing Frame Grinders: Used in foundries for large casting grinding.
• Portable Grinders: Handheld for weld bead grinding and flash cutting.
• Bench or Pedestal Grinders: General-purpose for off-hand grinding of tools and small parts.

Grinding Fluids

• Reduce workpiece temperature, improve finish and accuracy, increase grinding ratio, reduce wheel wear, minimize loading, and lower power consumption.

Ultrasonic Machining

• Material removal: microchipping and erosion using vibrating abrasive grains in a water slurry.
• Tool (sonotrode) vibrates at high frequency (20 kHz) with small amplitude (10 micrometers).
• Vibration gives high velocity to abrasive grains, causing erosion.
• Can create intricate shapes and small-diameter holes in hard materials (e.g., glass, graphite, epoxy composites).

Rotary Ultrasonic Machining

• Variation using a metal-bonded diamond abrasive tool instead of slurry.
• Tool vibrates and rotates simultaneously.

Finishing Operations

• Refining surface finish and reducing roughness for improved part quality.

Coated Abrasives

• Sandpaper: Backing material with abrasive coating and sizing to hold grains.
• Manual surface finishing.

Belt Grinding

• Uses a rotating abrasive belt on rolls to remove material for high-speed, good-finish removal.

Power Brushing

• Circular wire brush creates longitudinal scratches, improving surface finish.

Honing

• Improves surface finish of inside diameters, holes, and internal shapes.
• Rectangular honing stones reciprocate to create smoother, more precise circular surfaces.

Superfinishing

• Similar to honing, but with lighter pressure and shorter strokes for an ultra-fine finish.

Honing

• Uses a honing stone oscillating or reciprocating on the workpiece with light pressure for surface finish enhancement on cylindrical and centerless workpieces.

Lapping

• Uses a soft, porous tool (lap) like cast iron, copper, leather, or cloth to finish flat, centered, and curved surfaces with embedded or slurry-carried abrasive particles, often used for glass lenses. The lap moves parallel to the workpiece surface enhancing surface finish.

Polishing

• Produces a smooth finish by softening and smearing surface layers with fine abrasives through friction. The shiny appearance results from this smearing action.

Chemical Mechanical Polishing (CMP)

• Polishing semiconductor substrate surfaces in integrated circuits.
• Rotating workpiece disc with abrasive slurry between. Involves both physical abrasion and chemical reactions with the slurry. Creates perfectly flat surfaces.

Electropolishing

• Produces a mirror-like finish, the reverse process of electroplating.

Buffing

• Similar to polishing but with finer abrasives on soft cloths or felt disks; abrasive is supplied externally from a stick.

Deburring

• Removes thin, triangular ridges (burrs) from workpiece edges.
• Methods include vibratory deburring, thermal energy deburring, and robotic deburring.

Cost of Finishing Operations

• Finishing cost increases with surface finish quality.
• Rough turning is least costly, with finishing turning increasing the cost.
• Grinding and honing are the most expensive finishing operations, with finishing operations only carried out if necessary for required tolerances.