Grinding Flashcards
What is an abrasive?
A small, non-metallic hard particle with sharp edges, an irregular shape, and a high hot hardness value.
What are some examples of common abrasives?
Aluminum Oxide (HH 2100,) Silicon Carbide (HH 2480,) Diamond (HH 700,) Cubic Boron Nitride (HH 4700.)
Why do we use abrasives as opposed to other types of materials for grinding?
Grinding produces very large amounts of heat, therefore materials with high hot hardness values are required to withstand the temperatures.
What is abrasive machining?
When hard, abrasive particles are used in the form of a bonded wheel to perform material removal.
What are some other examples of abrasive processes?
Honing, lapping, superfinishing, polishing, and buffing.
What are abrasive processes/grinding typically used for?
Finishing operations (after part geometry has already been established,) and reducing part thickness.
Why do we use abrasive processes? (Importance)
Can be used on a wide variety of materials, can produce extremely fine surface finishes, and can hold dimensions to extremely close tolerances.
What is grinding?
An abrasive material removal process where abrasive particles are contained in a bonded grinding wheel, balanced for high rotational speeds in order to operate at very high surface speeds.
What are the parts of a grinding wheel and what are their functions?
Abrasive particles - accomplish cutting and material removal.
Bonding materials - hold the abrasive particles in place, establishes structure of wheel.
What are the necessary parameters for a grinding wheel?
The abrasive and bonding materials, grain size (size of particle, smaller grain size = better surface finish,) wheel grade, wheel structure.
What are the properties of abrasive materials?
High hardness, good wear resistance, toughness, and friability.
What happens if a material has poor friability?
Become loaded with the stock material, cut poorly (poor surface finish,) requires more cutting force, and generates heat.
How does grain size depend on the hardness/softness of a given material?
Harder materials require smaller grain sizes (because of how difficult it is to remove material,) and softer materials require larger grain sizes.
What are the key properties of bonding materials?
- Must withstand hight temperatues and centrifugal forces.
- Must resist shattering.
- Must hold abrasive grains rigidly in placee, yet allow worn grains to chip.
How is the structure of a grinding wheel determined?
The volumetric proportions of the grains, bonding materials, and pores; (overall relative spacing of grains.)
The sum of volume of grains, bonding material, and the pores of the grinding wheel is equal to…
1.0
(Pg + Pb + Pp = 1.0)
Why are pores/air gaps important in a wheel?
Allows for successful chip removal/workplace material removal.
What does an open structure mean in relation to wheel structure?
Pp is relatively large, and Pg is relatively small.
When is it recommended to use an open structure for a grinding wheel?
Recomended for when chip clearance must be provided.
What does a dense structure mean in relation to grinding wheel structure?
Pp is relatively small, and Pg is larger.
When is it recommended to use a dense structure for a grinding wheel?
To obtain a better surface finish and dimensional control.
What is grinding wheel grade, and what does it depend on?
It indicates the bond strength of retaining grains, depends on Pb.
What does a soft wheel mean in relation to grinding wheel grade?
A weaker bond, they lose grains readily, allowing for new grains to be exposed.
What are soft wheels used for?
Grinding hard materials, and applications requiring low material removal rates.
What does a hard wheel mean in relation to grinding wheel grade?
A stronger bond, they retain grains in place longer.
What are hard wheels used for?
Applications requiring high material removal rates, and grinding soft materials.
What are some of the standard grinding wheel shapes?
Straight, recessed two-sides, metal frame w/abrasive bonded to outer circumference, abrasive cut-off.
How is the best surface finish achieved through grinding?
Small grain sizes, high wheel speeds, and dense wheel structure.
How much higher is the specific energy for grinding compared to conventional machining?
Roughly 10x higher.
Why is the specific energy in grinding so high?
- The small chip size.
- Since grains have extremely neg. rake angles, shear plane angles are low -> more cutting force req.
- Not all grits are actually engaged in cutting, therefore more force req.
- Energy lost to plowing and rubbing
What are the three types of grain action in grinding?
- Cutting (material removal)
- Plowing (scratching the surface)
- Rubbing (just friction)
What are some of the damaging effects of high work surface temperatures caused by grinding?
- Surface burns/cracks
- Metallurgical damage immediately beneath surface
- Softening of work surface
- Residual stresses
What is friability?
Tendency to fracture - if the material wears out, it fractures, exposing a new sharp cutting edge.
How can we reduce grinding temperatures?
- Decrease depth of cut
- Reduce wheel speed, number of active grits/sq. inch
- Increase work speed
- Use a grinding fluid (usually oils)
What are the 3 main causes of wheel wear?
- Grain fracture
- Attritious wear
- Bond fracture
What is grain fracture?
When a portion of the grain breaks off, and the rest remains bonded to the wheel (fractured edges also become cutting edges.)
What is attritious wear?
The gradual dulling of individual grains, resulting in flat spots and rounded edges.
What causes attritious wear?
friction, diffusion, and chemical reactions
What is bond fracture?
When the individual grains are pulled out of the bonding material.
What causes bond fracture?
Grain has become dull, and resulting cutting force becomes excessive.
How is a wear curve plotted?
Function of volume of material removed.
What is the slope of a wheel wear curve?
Grinding ratio
GR = Vw/Vg
What are the 3 regions of a wear curve?
- Break-in period
- Steady-state wear
- Failure region
What is dressing the wheel?
holding a rotating disk, abrasive stick, or grinding wheel against the wheel being dressed as it rotates
What is the function of dressing a wheel?
- breaking off dulled grains to expose new, sharp ones
- removes clogged chips
During what stage/region is it required to dress a wheel?
3rd stage/region of wear curve
Why do we dress wheels?
to un-clog work material that gets stuck due to poor friability of wheel, and to expose new sharp cutting edges
Does dressing guarantee your desired shape of grinding wheel?
Nope, it sharpens - but does not guarantee shape.
What is truing?
Using a diamond-pointed tool and feeding it slowly + precisely across wheel as it rotates.
What is the depth that is taken against the wheel when truing?
0.025mm or less
Does truing guarantee your desired shape of grinding wheel?
Yes! Not only sharpens, but restores shape and ensures straightness on perimeter
How can you maximize MRR while grinding?
- Have a large grit size
- Have a more open wheel structure
- Vitrified (glass, ceramic) bond
For steel and most cast iron, use what kind of abrasive?
Aluminum oxide
For most nonferrous (not iron or steel) metal, use what kind of abrasive?
silicon carbide
For hardened tool steels and certain aerospace alloys, use what kind of abrasive?
cubic boron nitride
For hard abrasive materials (ceramics, cemented carbides, and glass) use what kind of abrasive?
diamond
Four types of surface grinding: ____ (1) spindle and ____ (2) worktable
(1) Horizontal, vertical
(2) Reciprocating, rotating
What are the 2 main types of grinding?
Surface and cylindrical
What are the 2 main types of cylindrical grinding?
Internal + external
In cylindrical grinding, what can the 2 feed directions be?
Traverse (wheel feed moving horizontally,) or plunge (wheel feed moving downward)
Which feed direction of cylindrical grinding is less time consuming?
plunge
What is centreless grinding?
removing material from the outside diameter of a workpiece without needing to centre it between two points (common in mass manufacturing)
What is creep feed grinding?
using a slow feed rate and a very deep depth of cut (1000-10,000x greater than conventional surface grinding) to perform material removal
What are the benefits of creep feed grinding?
MRR and productivity are increased, because the wheel is continuously cutting.
What are grinding belts?
Abrasive materials are bonded to belts instead of wheels to perform grinding.
What is honing?
abrasive process performed by a set of abrasive sticks, using rotational and oscillatory motion
What is the grain size range of honing processes?
Between 30 and 600
What kind of surface finish/quality does the honing process produce?
Surface finish of 0.12 micrometers or better, and a cross-hatch surface that retains lubrication and improves sealing
What is lapping?
a process that uses fluid suspension of very small abrasive particles between the workpiece and the lap (tool)
What fluid is used for lapping?
Lapping compound - a fluid with abrasives, generally a chalky paste
What is the typical grain size range of the lapping process?
Between 300 and 600.
What are some applications of lapping?
Glass - optical lenses, metallic bearing surfaces, gages
What is superfinishing?
Uses bonded abrasive stick pressed against surface and reciprocating motion.
What are the differences between superfinishing and honing?
Superfinishing has:
* Shorter strokes
* Higher frequencies
* Lower tool-surface pressure
* Smaller grit sizes
Than honing.
What are polishing + buffing?
Processes used to improve the physical appearance of workpieces
How does polishing/buffing work?
An abrasive is suspended in a compound/wax, and applied with cloth/leather/felt
What is the purpose of cutting fluid?
- Washing away chips
- Keeping the wheel from clogging
- Dust control
- Reducing temperature
What are the safety concerns surrounding grinding?
The small particle size, posing a risk to:
- workers health (breathing in particles)
- nearby machinery (can clog bearings)
- disposal - metals/bonding/materials/abrasive/coolant
How to combat the safety concerns surrounding grinding?
Grinding is usually done in an environment “sealed off” from the rest of a manufacturing plant.
