Material removal by action of hard, of hard, abrasive particles usually in the form of a bonded wheel
Generally used as finishing operations part geometry has been established by conventional machining is most important abrasive process
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honing, lapping, superfinishing, polishing, and buffing
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A small, non-metallic hard particle having sharp edges and an irregular shape.
1. 2. 3. 4.
Evidence of grinding dates back to 2000 BC when ancient Egyptian used to grind large stones for pyramid construction. Large water or wind driven grinding wheels used to make flour (in a gristmill) since the middle ages in Europe. The first modern bonded grinding wheel were made in the 1800’s in India to grind gem stones.
Can be used on all types of materials Some can produce extremely fine surface finishes, to 0.025 µm (1 µ-in) Some can hold dimensions to extremely close tolerances
Grinding operations generally involve abrasive discs. Fine grinding, honing and polishing will sometimes involve (loose). ◦
Examples: optical grinding (optical lenses, mirrors) and cylinder bores of combustion engines.
Abrasive material Grain size Bonding material Wheel grade Wheel structure
- capacity to fracture when cutting edge dulls, so a new sharp edge is exposed.
Materials with poor friability become loaded with the stock material, cut poorly and generate heat.
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Most common abrasive Used to grind steel and other ferrous high-strength alloys
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Harder than Al 2O3 but not as tough Used on aluminum, brass, stainless steel, some cast irons and certain ceramics
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Very hard, very expensive Used for hard materials such as hardened tool steels and aerospace alloys
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Even harder, very expensive Natural and synthetic Not suitable for grinding steels Used on hard, abrasive materials such as ceramics, cemented carbides and glass
- grit projects far enough into surface to form a chip - material is removed - grit projects into work, but not far enough to cut - instead, surface is deformed and energy is consumed, but no material is removed - grit contacts surface but only rubbing friction occurs, thus consuming energy, but no material is removed
(a) cutting
(b) plowing
(c) rubbing
Small grit sizes produce better finishes Larger grit sizes permit larger material removal rates Harder work materials require smaller grain sizes to cut effectively Softer materials require larger grit sizes
Grit size is measured using a screen mesh procedure ◦
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Smaller grit sizes indicated by larger numbers in the screen mesh procedure and vice versa Grain sizes in grinding wheels typically range between 8 (very coarse) and 250 (very fine)
Must withstand temperatures Must of wheel
and high
during shock loading
Must abrasive grains rigidly in place for cutting to expose new sharp grains
wheels.
– baked clay and ceramic. General purpose – sodium silicate. Minimizes heat generation. – most flexible. Good for cutoff wheels.
wheels.
– thermosets. Good strength for cutoff – not rigid. Good for surface finish. – bronze bond for diamond or cBN matrix.
Refers to the relative spacing of abrasive grains in wheel In addition to abrasive grains and bond material, grinding wheels contain air gaps or pores Volumetric proportions of grains, bond material and pores can be expressed as:
P g + P b + P p = 1.0
Measured on a scale that ranges between "open" and "dense." ◦
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means P p is relatively large and P g is relatively small - recommended when clearance for chips must be provided means P p is relatively small and P g is larger - recommended to obtain better surface finish and dimensional control
Indicates bond strength in retaining abrasive grits during cutting Depends on amount of bonding material in wheel structure (P b ) Measured on a scale ranging between soft and hard ◦
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lose grains readily - used for low material removal rates and hard work materials retain grains - used for high stock removal rates and soft work materials
Standard grinding wheel shapes: (a) straight, (b) recessed two sides, (c) metal wheel frame with abrasive bonded to outside circumference, (d) abrasive cut- off wheel.
Standard grinding wheel marking system used to designate abrasive type, grit size, grade, structure, and bond material
Material removal process in which rotate at very high surface speeds
Grinding wheel usually disk-shaped and precisely balanced for high rotational speeds
holds particles in place and establishes shape and structure of wheel
Usually a bonded wheel of abrasive material is used to grind material off of the workpiece. Slow but exact process to create a very high quality surface finish. with reciprocating
worktable, with rotating worktable, with reciprocating worktable, with rotating worktable.
Similar to using a lathe, a rotating tool (grinding wheel) is held to a rotating workpiece.
(a) External
(b) Internal
The
Traverse Feed
in grinding can be:
Plunge Feed
Tool post grinders are often mounted on lathes to perform additional work:
In centerless grinding, the stock material’s own centre is found. Allows for more “precision” grinding. (ie. The work is not held to the machines “center”.)
External centerless grinding
Depths of cut 1000 to 10,000 times greater than in conventional surface grinding ◦
Feed rates reduced by about the same proportion
Material removal rate and productivity are increased in creep feed grinding because the wheel is continuously cutting ◦
In conventional surface grinding, wheel is engaged in cutting for only a portion of the stroke length
Creep Feed Grinding
(a) conventional surface grinding
(b) creep feed grinding.
In grinding belts, the abrasive materials are bonded to belts instead of large wheels.
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Bench & Tool grinders are used sharpen (or re-sharpen) tools by hand.
Honing Superfinishing Lapping Polishing Buffing
By using a finer and finer grain, a smoother surface finish can be achieved Best
Abrasive process performed by a set of
Common application is to finish the bores of internal combustion engines or gun barrels Grit sizes range between 30 and 600 Surface finishes of 0.12 µm (5 µ-in) or better Creates a characteristic cross-hatched surface that retains lubrication
(a) the honing tool used for internal bore surface, and (b) cross-hatched surface pattern created by the action of the honing tool.
In honing a set of abrasive sticks is used to remove machining marks. VIDEO – Harley Davidson (HD) honing
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Uses bonded abrasive stick pressed against surface to remove ~ 1um layer Reciprocating motion with abrasive stick
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Shorter strokes Higher frequencies Lower pressures between tool and surface Smaller grit sizes
Superfinishing on an external cylindrical surface. Similar to honing, but a high frequency, short stroke is used.
of very small abrasive particles between workpiece and lap (tool)
– aluminum oxide, emery, SiC, diamond Typical grit sizes between 300 to 600 Applications: optical lenses, metallic bearing surfaces, gages
is a famous lapped instrument. Error in flatness of the primary mirror was 2.2 microns. Shape off spec by 10 nm!
Used to improve the visual appearance of the workpiece. An abrasive is suspended in a compound or wax and applied with a cloth/leather/felt.
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CLASS NOTES
- smal smalll chip chip size size energy to remove each unit volume of material is very high ◦
Roughly 10 times higher for grinding compared to conventional machining
Individual grains have , resulting in low shear plane angles and high shear strains Not all grits are engaged in actual cutting
Wear is plotted as a function of volume of material removed, rather than as a function func tion of time. (1) Initial wear; (2) Steady state; (3) Failure
Indicates slope of the wheel wear curve
GR =
V W V g
where GR = GR = grinding ratio V w = volume of work material removed V g = corresponding volume of grinding wheel worn
Grinding is characterized by Most of energy remains in the ground surface, resulting in high work surface temperatures : 1. Surface burns and cracks 2. Metallurgical damage immediately beneath the surface 3. Softening of the work surface if heat treated 4. Residual stresses in the work surface
Decrease infeed (depth of cut) d Reduce wheel speed v Reduce number of active grits per square inch on the grinding wheel C Increase work speed v w Use a grinding fluid
- when a portion of the grain breaks off, but the rest remains bonded in the wheel ◦
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Edges of the fractured area become new cutting edges Tendency to fracture is called friability
- dulling of individual grains, resulting in flat spots and rounded edges ◦
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Analogous to tool wear in conventional cutting tool Caused by similar mechanisms including friction, diffusion and chemical reactions
- the individual grains are pulled out of the bonding material ◦
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Depends on wheel grade Usually occurs because grain has become dull due to attritious wear and resulting cutting force becomes excessive
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Washing away chips,
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Keeping the wheel from becoming clogged,
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Control of grinding dust
4.
Reduces operating temperatures
INDEPENDENT STUDY
- accomplished by rotating disk, abrasive stick, or another grinding wheel against the wheel being dressed as it rotates to: ◦
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Break off dulled grits to expose new sharp grains Remove chips clogged in wheel
Required when wheel is in third region of wear curve Although dressing sharpens,
Due to poor friability, the grinding wheels often get loaded with workpiece material.
- use of a diamond-pointed tool fed slowly and precisely across wheel as it rotates
Very light depth is taken (0.025 mm or less) against the wheel Not only sharpens wheel, but
To optimize surface finish, select ◦
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Small grit size and dense wheel structure Use higher wheel speeds (v ) and lower work speeds (v w ) Smaller depths of cut (d ) and larger wheel diameters (D ) will also help
To maximize material removal rate, select ◦
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Large grit size More open wheel structure Vitrified bond
For steel and most cast irons, use ◦
For most nonferrous metals, use ◦
Silicon carbide as the abrasive
For hardened tool steels and certain aerospace alloys, use ◦
Aluminum oxide as the abrasive
Cubic boron nitride as the abrasive
For hard abrasive materials (e.g., ceramics, cemented carbides, and glass) use ◦
Diamond as the abrasive
