Abrasive Machining Overview

Abrasive Machining

• Abrasive machining processes remove material using abrasive particles.
• Nearly all surfaces have peaks and valleys.
• The difference in height between the peaks and valleys is usually about one micrometer.
• A good surface finish has smaller peaks and valleys.
• Surface finish is characterized by parameters like roughness average, root mean square roughness, maximum peak-to-valley height, etc.
• Different machining processes result in different roughness parameters.
• Grinding, honing, and lapping are surface finishing processes.

Abrasive Machining Processes

• Grinding
• Honing
• Lapping
• Buffing/Polishing
• Abrasive jet machining
• De-burring (shot blasting, vibratory, and barrel finishing)

Grinding

• Grinding removes metal by abrasive particles (irregular shape).
• The abrasive grains use a zero to negative rake angle, creating short, small, curved, or wavy chips.

Types of Grinding

• Purpose-based:
  • Non-precision grinding: Primarily removes material, accuracy and finish are secondary concerns.
  • Precision grinding: Focuses on achieving good surface finishes and precise dimensions.
• Process/Procedure-based:
  • Surface grinding (flat surfaces)
  • Cylindrical grinding (cylindrical surfaces, may be centered or centerless)
  • Form grinding (gears, threads etc.)
  • Abrasive belt grinding (flats, curves, or cylinders)
  • Manual grinding (workpiece or grinder held by hand)
  • Special grinding processes (cam/crank shafts, gear grinders)

Grinding Machines

• Surface grinders
• Cylindrical grinders
• Centerless grinders
• Internal grinders
• Special types of grinders

Classification

• Based on purpose (non-precision, precision)
• Based on process/procedure (surface, cylindrical, form, belt, manual, special)

Centerless Grinding

• Eliminates the need for center holes in the workpiece.
• The workpiece rests on a workrest blade and is backed up by a regulating wheel.
• The regulating wheel rotates in the same direction as the grinding wheel and controls the workpiece feed.

Internal Grinding

• Used to finish straight, tapered, or formed holes in workpieces.
• The workpiece is held by a chuck or collet and revolved by a motorized headstock.
• The grinding wheel is fed in and out, adjusted for depth of cut.

Special Grinding Processes

• Tool and cutter grinders: Sharpen milling cutters, reamers, taps, etc.
• Jig grinders: Precisely grind tapered or straight holes.
• Thread grinding machines: Produce precise threads.
• Crankshaft grinders
• Piston grinders
• Cam grinders
• Tool-post grinders (lathe grinders)

Machining Parameters

• Cutting speed (V): Calculated using wheel diameter and rotational speed.
• Metal removal rate (Z): Calculated using depth of cut, cross feed, and table speed.
• Machining time (tm): Calculated using thickness, width of work piece, cross feed, depth of cut etc.

Abrasives

• Natural abrasives (e.g., sandstone, quartz, emery, corundum, diamond)
• Manufactured abrasives (e.g., aluminum oxide, silicon carbide, cubic boron nitride, diamond)
• Grain sizes range from 10 (very coarse) to 600 (very fine)
• Grit number relates to sieve mesh size.

Bonding Materials

• Vitrified
• Silicate
• Shellac
• Resinoid
• Rubber
• Metal bonds
• Electroplated bond
• Brazed bond

Grades of Hardness

• Grinding wheel hardness, measured using a letter system (softest 'A', hardest 'Z').
• Hardness considerations should match the grain size.

Glazing and Loading

• Glazing: Wheel face becomes smooth, cutting ability decreases.
• Loading: Metal particles adhere to the wheel face, reducing cutting ability.

Dressing and Truing

• Dressing: Cleaning and restoring the grinding wheel's cutting surface.
• Truing: Ensuring the cutting surface is smooth and true, crucial for precision grinding.

Grinding Ratio (G)

• G = Volume of material removed / Volume of wheel wear
• Varies depending on the type of wheel, grinding fluid, and process parameters.
• Higher forces typically reduce the grinding ratio.

Abrasives - Workpiece Material Compatibility

• The reactivity between abrasives and workpieces influences wear.
• Certain combinations are commonly used based on material compatibility.

Mechanics of Grinding

• Chips form due to the negative rake angle of the abrasive grains.

Application Guidelines

• Consider factors like grit size, wheel structure, wheel speeds, work speed, depth of cut, wheel diameter, bond type, to select right wheel for application.

Lapping

• Final finishing operation for extreme dimensional accuracy, correcting imperfections, refining surface finish, and achieving close fits between surfaces.
• Uses loose abrasives like emery, corundum, iron oxide.
• Materials lapped range from optical lenses to precision plug gages.

Honing

• Abrasive process with low velocity.
• Primarily used for correcting internal cylindrical surfaces, tool marks, taper, and axial distortion
• Produces a cross-hatched surface.

Superfinishing

• Extremely high-quality surface finish process.
• Minimizes defects and removes a very thin layer of metal.
• Used for external and internal surfaces of parts.
• Uses fine grit abrasive sticks and spring pressure.