Ch18- GRINDING AND OTHER ABRASIVE PROCESSES
Silicon carbide (SiC)
-harder than Al2O3 but not as tough -Used on aluminum, brass, stainless steel, some cast irons and certain ceramics
Aluminum oxide (Al2O3)
-most common abrasive -Used to grind steel and other ferrous high‑strength alloys
dressing of grinding wheels function
1. Break off dulled grits to expose new sharp grains 2. Remove chips clogged in wheel
To reduce grinding temperature
1. Decrease infeed (depth of cut) 2. Reduce wheel speed 3. Reduce number of active grits per square inch on the grinding wheel 4. Increase work speed 5. Use a grinding fluid
bond fracture depends on what
1. Depends on wheel grade, among other factors 2. Usually occurs because grain has become dull due to attritious wear i. And resulting cutting force becomes excessive
mechanisms of wheel wear
1. Grain fracture 2. Attritious wear 3. Bond fracture
Superfinishing differences with honing
1. Shorter strokes 2. Higher frequencies 3. Lower pressures between tool and surface 4. Smaller grit sizes
Water Jet abrasive main problems
1. Very Noisy-supersonic valve 2. Expensive equipment
different types of abrasives
1. grinding 2. Honing 3. Lapping 4. Superfinishing 5. Water Jet Abrasive Machining
grain sizes 5 things
1. small grit produce better finish 2. larger grit sizes permit larger material removal rates 3. harder work material requires smaller grain sizes to cut effectively 4. softer material requires larger grit sizes 5. larger number is smaller grit size
Traditional Abrasive Materials
1.Aluminum oxide (Al2O3) 2. Silicon carbide 3. Cubic boron nitride (cBN) 4. Diamond
high temperature grinding damaging effects
1.Surface burns and cracks 2. Metallurgical damage immediately beneath the surface 3.Softening of work surface if heat treated 4. Residual stresses in work surface
bonding types
1.Vitrified bond 2. Silicate bond 3. Rubber bond 4. Resinoid bond 5. Shellac bond 6. Metallic bond
effects on chip formation
1.cutting with abrasive causes chips and material is removed 2. open structures for clearance for chips 3. dense structures for better surface finish and dimensional control
honing grit size
30 and 600
lapping grit size
300 to 600
Grinding wheel structure, typical vitrified wheel
50% particles 10% bond or adhesive 40% Cavities or pores
Vitrified bond
A type of bonding material, which is made of clay and water. most common because strong and rigid creep resistand
attrition and tool wear
Analogous to tool wear in conventional cutting tool Caused by similar mechanisms as in conventional tool wear, including friction, diffusion, and chemical reactions
lapping application and whats needed to it
Clean glasses, gauges parting powder needed for work piece and material
Resinoid bond
Consists of various thermosetting resin materials, such as phenol-formaldehyde High strength and used for rough grinding and cutoff operations
newer abrasive materials
Cubic boron nitride (cBN) Diamond
effect of temperature induced on workpiece
Damaging effects include 1. Surface burns and cracks 2. Metallurgical damage immediately beneath the surface 3. Softening of work surface if heat treated 4. Residual stresses in work surface
grain fracture edges and tendency of grain to fracture
Edges of the fractured area become new cutting edges Tendency for the grain to fracture is called friability
Most important abrasive process
Grinding
mechanics of how abrasive machining takes place
Material removal by action of hard, abrasive particles usually in the form of a bonded wheel
Grinding
Material removal process in which abrasive particles are contained in a bonded grinding wheel that operates at very high surface speeds Grinding wheels balanced precisely for high rotational speed and disks
Rubber bond
Most flexible of the bonding materials and used as a bonding material in cutoff wheels.
surface finish
Most grinding operations performed to achieve good surface finishing Best surface finish achieved by: 1. small grain sizes 2. higher wheel speeds 3. Denser wheel structure i. means more grit per wheel area
Superfinishing/microhoning
Similar to honing - uses bonded abrasive stick pressed against surface and reciprocating motion
Turning vs grinding
Turning: more material, changes D, need chip breaker Grinding: finishing, small amount of material, no long chips
Lapping Definition
Uses fluid suspension of very small abrasive particles between workpiece and lap (tool)
truing of grinding wheels depth
Very light depth is taken (0.025 mm or less) against the wheel
Surface Grinding
a. Horizontal spindle with reciprocating worktable b. horizontal spindle with rotating worktable c. Vertical spindle with reciprocating worktable d. vertical spindle with rotating worktable
Attrotious wear
affects individual wear
abrasive machining used
as finishing operations after part geometry has been established by conventional machining
bond fracture occurs because
because grain has become dull due to attritious wear And resulting cutting force becomes excessive
Honing common applications
bores of internal combustion engines for final finish
Honing characteristics and its use
characteristic cross‑hatched surface that retains lubrication
dressing of grinding wheels
def: Resharpening the wheel - accomplished by rotating disk, abrasive stick, or another grinding wheel held against the wheel being dressed as it rotates §Required when wheel is in third region of wear curve
mechanisms of wheel wear bond fracture
def: the individual grains are pulled out of the bonding material
mechanisms of wheel wear: attrition def
dulling of individual grains, resulting in flat spots and rounded edges
effect of residual stresses induced on workpiece
grinding causes high surface temperatures (work peice)
centerless grinding function
hard to hold work piece because no room to hold work piece
Abrasive Material Properties
high hardness, wear resistance, toughness, friability
what bonding types do in a grinding wheel
holds particles in place and establishes shape and structure of wheel
what to do about vitrified wheel if want to catch more chips
increase percent of bond/adhesive
Lapping applications
optical lenses, metallic bearing surfaces, gages
Honing
performed by a set of bonded abrasive sticks using a combination of rotational and oscillatory motions
residual stress induced on workpiece and effect from what
possibly decreasing the fatigue strength of the part. is a thermal effect
Shellac bond
relatively strong but not rigid often used for good finish
Silicate bond
sodium silicate (Na2SO3). and used when heat generation must be minimized such as grinding cutting tools
Grinding wheel structure
typical vitrified wheel spacing for abrasive particle posts of binders
truing of grinding wheels def
use of a diamond‑pointed tool fed slowly and precisely across wheel as it rotates
Water Jet abrasive
used mainly in cutting and deburring usually just water sometimes abrasive
Metallic bond
usually bronze common bond for diamond for cBN used to bond the metal matrix and abrasive grains to the outside periphery of the wheel.
Cubic boron nitride (cBN)
very hard, very expensive §Suitable for steels §Used for hard materials such as hardened tool steels and aerospace alloys
mechanisms of wheel wear grain fracture def
when a portion of the grain breaks off, but the rest of the grain remains bonded in the wheel
When is dressing required
when wheel is in third region of wear curve
Diamond
§Even harder, very expensive §Occur naturally and also made synthetically §Not suitable for grinding steels §Used on hard, abrasive materials such as ceramics, cemented carbides, and glass
What is grinding ratio
§Indicates slope of the wheel wear curve where GR = grinding ratio; Vw = volume of work material removed; and Vg = corresponding volume of grinding wheel worn
cylindrical grinding
§Two types of cylindrical grinding: (a) external, and (b) internal
truing of grinding wheels does what besides sharpening wheel
§restores true disk shape and insures straightness across outside perimeter §Although dressing sharpens, it does not guarantee the shape of the wheel