Chapter 18 - Grinding and other abrasive processes

How does grain size factor into abrasive procedures?

A small grain size will produce better finishes (larger grit number means smaller grains) A larger grain size permits larger material removal rates Hard work materials require small grains while soft materials require larger grains for effective cutting

Why are abrasive processes important?

Abrasive processes can be used on all kinds of materials, produce very fine surface finishes and hold extremely close to dimensions

Which abrasive materials work best for the variety of workpart materials?

AlO3 => steel and most cast irons SiC => most nonferrous metals cBN => hardened steels and aerospace alloys Diamond => hard and abrasive materials

What are the different kinds of wheel wear?

Attritious wear - dulling of the individual grains, resulting in flat spots and rounded edges from friction and chemical reactions Bond fracture - occurs when the individual grains are pulled out of the bonding material usually occurs because the grain has become dull because of attritious wear, and the resulting cutting force is excessive Grain fracture - occurs when a portion of the grain breaks off, but the rest of the grain remains bonded in the wheel (tendency called friability)

What is vitrified bond/ vitrified wheel?

Consists chiefly of baked clay and ceramic materials. Most grinding wheels in common use are vitrified bonded wheels. They are strong and rigid, resistant to elevated temperatures, and relatively unaffected by water and oil that might be used in grinding fluids.

What is creep feed grinding?

Creep feed grinding is grinding at very high depths (1,000 to 10,000 times greater than conventional) of cut at very low feed rates. Material removal rate and productivity increased with creep feed because it is continuously cutting, where the workpiece reciprocates in traditional processes (engages only a portion of stroke length)

What are the types of grain action?

Cutting - grit goes into work forms chip Plowing - only surface deformed, no chips produced Rubbing - rubbing friction on surface, no material removed, hardly any deformation

How are grinding temperatures reduced?

Decrease depth of cut, reduce wheel speed, reduce active grits per square inch, increase work speed, use grinding fluid

What are the types of cylindrical grinding?

External on the surface of the workpart and internally

What is the grinding ratio?

GR = Vw/Vg Gr is grinding ratio Vw is volume of work material removed and Vg is volume of grinding wheel worn in process This indicates the slope of the wear curve

How is grain size measured?

Grit is measured using a screen mesh, larger numbers means smaller grains and vice versa Grinding wheels range from 8 (very coarse) to 250 (very fine)

What are important material properties for abrasiveness?

High hardness, wear resistance, toughness and friability (capacity to fracture when cutting edge dulls, exposing new cutting edge). Without friability the process becomes very expensive because you have to replace the wheel.

Why do we aim to reduce temperatures while grinding?

High temperatures can cause surface burns and cracks, metallurgical damage beneath surface, softening of work surface and stresses in surface

What are the most commonly used abrasive materials?

In order from least to most hard: Aluminum Oxide (Al2O3) Silicon Carbide (SiC) - harder that Al2O3 but less tough Cubic boron nitride (cBN) very hard and expensive Diamond - used on hard and abrasive materials like ceramics, cemented carbides, and glass Depends on work material - more hard/abrasive? then grinding material must be more hard/ abrasive

What is the typical wear curve in grinding?

It has the same shape as the typical cutting tool wear, with a break-in period, steady-state wear and failure region

How do you maximize material removal rate in grinding?

Large grain size, open wheel structure and vitrified bond

What is abrasive machining?

Material removal by hard, abrasive particles (usually wheel-form) Can be used as finishing operation after conventional machining. Grinding most important, but there are also alternatives.

What is grinding?

Material removal process in which abrasive particles bonded into wheel shape that operates at high rotational speeds

What material properties make a good bonder for grinding?

Must withstand high centrifugal forces and temps Resist shattering during shock Hold abrasive grains rigidly but allow broken grains to be dislodged

What is dressing the wheel?

Resharpening the wheel by rotating a disk, abrasive stick or another grinding wheel held against wheel as it rotates Helps break off dulled grits and remove chips clogged in wheel, required in third region of wear curve

What is superfinishing?

Similar to honing, uses abrasive stick against surface in reciprocating motion, but differences include: shorter strokes higher frequencies lower pressure between tool and surface smaller grain size Makes very fine, mirror-like finishes and can be used for flat and external cylindrical surfaces

How are grinding wheels structured?

The are proportioned using a formula: %grain + %bond material + %pores/airgaps = 100%

What is a surface grinder?

The machine tool that holds the spindle and workpiece. The most common surface grinder is a horizontal spindle with reciprocating worktable

What is an example of transverse and plunge grinding accomplishing the same result?

Threads can be ground by transverse grinding with a high feed or by plunging with a large thickness wheel that has a thread pattern

What are the different types of feed action in grinding?

Transverse feed - wheel fed across rotating surface at a depth like a lathe Plunge cut - wheel fed directly downwards into rotating workpiece

What is lapping?

Uses fluid suspension of very small abrasive particles between workpiece and lap. The particles are not bonded and is used for optical lenses typically.

What is water jet abrasive machining?

Uses force of high pressure water to remove material, used in cutting and deburring on any kind of material. Variations: Abrasive water jet cutting- put abrasive particles in jet stream Abrasive jet machining - use jet of gas with abrasive particles in it (for surface treatment like blasting).

What is wheel grade?

Wheel grade indicates bond strength when retaining grains while cutting "Soft" wheels lose grains more readily, and are used for low material removal rates on hard materials "Hard" wheels are used for high removal rates on soft materials

How is wheel structure measured?

Wheel structure is measured on a scale ranging between "open" and "dense", where an open structure is one where %pores is large and %grain is small, and dense structures are the opposite. Open structures remove more material (pores collect chips and provide clearance) and dense structures are recommended for better surface finish and dimensional control

What is honing?

a grinding operation for internal surfaces performed after drilling, boring and reaming. Creates a cross-hatched surface that retains lubrication.

What are the four types of surface grinding?

a) Horizontal spindle with reciprocating (linear) worktable b) Horizontal spindle with rotating worktable c) Vertical spindle with reciprocating worktable d) Vertical spindle with rotating worktable

What are some standard grinding wheel shapes?

a) Straight - same thickness throughout b) Recessed two sides - thicker at outside and thin towards middle c) metal wheel frame with abrasive bonded to outside circumference d) abrasive cut-off wheel - thin for material separation a-c typical peripheral grinding wheels grind on outside Also face grinding wheels, where cutting is performed by grinding face of many shapes Also Bur/cone shape for internal grinding of material

What is centerless grinding and why is it important?

high production process for continuously grinding cylindrical surfaces in which the workpiece is supported by a blade and regulating wheel while it is ground down instead of centers or chucks. It is important because sometimes the workpiece is too large to be held by traditional means and this is the solution.

How is better surface finish accomplished in grinding?

small grains, higher speeds, denser wheel structure (more grits per wheel area)

What is truing the wheel?

use of diamond-pointed tool fed slowly and precisely across wheel as it rotates at very small depth Resharpens wheel and restores true disk shape and ensures straightness of outside perimeter (dressing cannot guarantee true shape of wheel)

What kinds of deviations can be corrected by honing?

waviness, undersize, barrel shapes, rainbow, taper, boring marks and misalignment, etc.