Abrasive Machining Processes

• Usually brought into play after casting, forging, sheet metal forming, machining, etc.

• Required for:– tight tolerances

– smooth surface finishes

– hard materials

– brittle materials

• Not possible by conventional machining processes

Abrasive Machining Processes

Analogy• Think of “whetting” a knife with a stone or "sanding" wood 

–Abrasive machining or grinding is the automated version of whetting with an abrasive stone or sanding wood

Set-up• Abrasive machining typically uses specially made abrasive disks in which the abrasive powder protrudes a small amount from a matrix

• These disks rotate at high speed and are brought to bear on the material to be ground

Abrasive vs. conventional Machining • Many more cutting edges• Distribution of rake angles: +45o to ‐60o 

• The grains undergo much larger deformations than cutting tools

• Chips are tiny– 2 to 5% of grain surface area is operative at any time

– DOC for an individual grain w.r.t grain diameter is very small

Abrasive-Work-piece Interaction

• Abrasives (grits) withlarge negative rakeangles or roundedcutting edges do notform chips but will plowor rub

• Proportion of cutting,plowing and rubbing willchange continuously asthe abrasives wear

Abrasive Properties: Requirement

• Hardness• Friability• Grain shape

Abrasive Hardness

Knoop Hardness

Aluminum Oxide 2000‐3000

Silicon Carbide 2100‐3000

Cubic Boron Nitride 4000‐5000

Diamond 7000‐8000

Abrasive Friability

• The ability of the individual grains to fracture or split into smaller pieces

• Gives a self sharpening  capability– By having the grain split or fracture, new sharp edges are created to replace rounded or blunt edges

• Advantages– Abrasive wheel does not need sharpening or dressing”

• Disadvantages– Abrasive wears more rapidly than non‐friable materials

Abrasive Shape• Grain shape determines tool geometry

Sharp particles remove more materialRounded particles give smoother finishes

Abrasive Materials

• Hard to superhard ceramics                                       

• Common abrasives are:– Garnet  

– Aluminum Oxide

– Silicon Carbide (SiC) or Silocon Nitride (Si3N4)

– Cubic Boron Nitride (superabrasive)

– Diamond (superabrasive)

Classification: Abrasive Tools

Bonded Loose

Bonded Abrasives• A composite of the abrasive powder and a matrix

• Bonding material can be glass, resin, rubber• Can be solid discs (grinding wheel) or bonded to paper/cloth which is then stuck to a backing disc.

Grinding wheel

• Cavity or voids should be large enough to hold the chips during the cut

Grinding Processes

Grinding Process

Centreless grinding

• Workpiece is not supported by centers or chucks, but by ablade.

• Grinding is done by larger wheel• Smaller wheel – mounted at an angle wrt grinding wheel –

controls the rotational and longitudinal motion of theworkpiece

• Reduced cycle time and hence higher productivity

Machines for Bonded Abrasive‐ Surface Grinder 

Finishing Operations

Coated Abrasive• Sandpaper and emery cloth• Abrasives (mostly Al2O3, ZrO2, SiC) deposited on flexible 

backing material (paper, cotton, rayon polyester). Matrix made of resins

• Coated abrasives are also used as belts for high‐rate material removal.  Belt speeds ~ 700 – 1800 m/min

Lapping

Lap usually made of cast iron, copper, leather, or cloth

Abrasive particles are embedded in the lap

Dimensional tolerance ~ 0.0004 mm

Surface finish ~ 0.025 – 0.1 μm

Lapping of flat and cylindrical surfaces

Lapping

Silicon chips

Heat sink

Honing

• Honing primarily gives holes a fine surface finish

• Honing tool consists of a set of aluminium‐oxide or silicon‐carbide bonded abrasives called “stones”

• Stones mounted on a mandrel that rotates in a hole

Honing

• Surface grinding

• Honing

• Lapping

• Polishing

• Buffing

COMMERCIAL ABRASIVE MACHINING PROCESSES

IncreasingSurface Roughness

Decreasing material removal rate

Kalpakjain p 1060

PROS AND CONS OF ALL ABRASIVE PROCESSES

• Advantages

–Very precise material remove possible

–High tolerances

–Very smooth surfaces

• Disadvantages

– Low material removal rates

–Complex shapes not possible