Abrasive Machining and Finishing Operations Flashcards
When using a computer controlled grinding machine, which features can be automated?
Load/unload workpiece
Clamp workpiece
Repeat cycle path
Sharpen wheel between passes
What does the selection of a grinding process depend on?
Part shape
Ease of fixing
Part size
Production rate
Describe lapping
A process wherein fine abrasive particles are embedded into a relatively soft material called lap
The lap can be made out of leather, cloth, cast iron, copper
It is used on flat or cylindrical surfaces of both metal and non-metal parts
Describe sanding
Uses coated abrasives i.e. sand paper and emery cloth
The grains are electro-statically deposited on backing material
Coating is made of resin
The process is widely used in finishing flat or curved surfaces of metallic and non-metallic parts
What’s the main difference between electro-polishing and other finishing operations?
For other finishing operations, the peaks of rough bits are grinded down. Whereas for electo-polishing the troughs are also broken down
List the general finishing operations by increasing machining cost
As cast or cut Rough turn Semi-finish turn Finishing turn Grinding Honing
Describe the grinding process
A chip removal process
The grinding tool is arranged of irregular shaped grains spaced randomly along the periphery of the cutting wheel
Briefly describe Ultrasonic machining
Material is removed from a surface by micro-chipping or erosion with fine abrasive grains
What material is ultrasonic welding best suited to?
Hard and brittle materials as the impact stresses are high enough to cause microchipping and erosion of the workpiece surface
e.g. glass, ceramics, silicon, carbides, precious stones, composites and hardened steels
What will happen if the bond between abrasives is too strong or too weak?
Too strong - Dull grains cannot be dislodged to allow other sharp grains to perform grinding
Too weak - Grains become dislodged easily, hence maintaining dimensional accuracy becomes difficult
What’s an abrasive?
A small, non-metallic hard particle with sharp edges and irregular in shape
What is abrasion used for?
To remove small amounts of material from a surface by a cutting process that produces tiny chips
To hone, lap, and polish workpieces
What is attritious wear?
The wear of the grains, the flank wear of a cutting tool
Caused by friction
It is the contact of grain and workpiece material involving physical and chemical reactions
What is honing used for?
To size and finish bored holes e.g. engine cylinders
To remove errors left by boring
To remove tool marks left by grinding
List 5 examples of abrasive finishing operations
Sanding Honing Lapping Polishing Electropolishing
Which has the slowest MRR, polishing and abrasive machining or electro-polishing?
Electro-polishing, but you get a smoother surface finish
Describe the vibrating process of ultrasonic machining
The tip of the tool vibrates at a frequency of ~20kHz and an amplitude of ~0.05mm
The vibration drives the abrasive grains at high velocity into the workpiece
Why is electro-polishing suitable for polishing irregular shaped workpieces?
Polishing is accomplished without mechanical contact with the workpiece
Describe bonded abrasives
Each abrasive grain removes only a very small amount of material at a time
Abrasives are boned together to form a grinding wheel for higher material removal rate
What’s abrasive machining?
A method used to produce parts with high dimensional accuracy
Describe honing
A stock-removal process using abrasive stones to remove very small amounts of metal
What is the limitation of ultrasonic welding?
Each job requires special tooling
Describe electro-polishing
A DC electric current is constructed using the workpiece as the anode, usually submersed in electrolyte
As current is applied, material is oxidised and then dissolved by the electrolyte , stripping it from the surface
Name and describe the bond type used for bonding abrasives
Clay and feldspar (crystalline structure)
Vitrification (substance to liquid) is used to produce a glassy, amorphous structure (removing heat quickly or using additives)
Strong, stiff, resistant to oil, acid and water
Why is the stress produced by abrasive particles hitting the workpiece high?
The contact time between particle and workpiece is short
The area of contact is very small
Why is porosity essential in bonded wheels?
It provides clearance for the minute chips being produced
It provides cooling
What type of material iselectro-polishing good for?
Brittle as it makes no contact
Describe grain fracture
Abrasive grains are brittle
If grains become dull, grinding becomes inefficient
Therefore the grains should fracture moderately to expose sharp grains
Why is electro-polishing expensive?
Surface needs to be as pure as possible (washing, cleaning, de-greasing etc.) before it goes into the electrolyte.
Then it has to be cleaned again to remove the electrolyte.
More steps makes it more epensive
Describe polishing
A process that produces a smooth, lustrous surface finish
It is done with disks or belts of fabric or leather coated with fine powders of aluminium oxide or diamond
