Karteikarten
Machining
Karten in diesem Stapel (31)
- 1Metal CuttingUsed as secondary process to:
Improve surface finish
Improve dimensional tolerance
Produce geometrical features that are difficult to produce in primary process - 2Cutting as a Primary ProcessLittle dedicated tooling hence good for low volume production as good flexibility
Required low material cost due to waste
Automation means cutting can compete with forming and casting - 3Feed ThicknessDistance below surface of metal at which tool is introduced
- 4Chip ThicknessThickness of chip sheared from surface
- 5Cutting Ratiotf/tc
Defines the efficiency of cutting process - 6Clearance AnglePrevents excess rubbing of surface to maintain good surface finish
- 7Bigger rake angle means...More pointy
More efficient
Less durable and more prone to wear - 8Shear Plane AngleThere will be an angle at which the shear stress is just equal to the shear strength of the work material
- 9What shear plane angle do we want?A high shear plane angle results in a smaller shear plane area and thus lower shear force
Gives less heavily strain hardened chip
Achieved by increasing the rake angle and decreasing the friction angle through lubrication - 10Oblique CuttingEasier to work diagonally up a hill
Effective rake angle is larger than normal
Lower cutting force - 11Orthogonal Cutting Primary Shear ZoneShear plane broadens into primary shear zone
Thickness of shear zone greater for materials with larger strain hardening and strain rate sensitivity
Strain hardening extends ahead and below shear zone
Newly formed surface is strain hardened - 12Orthogonal Cutting Secondary Shear ZoneIf pressure on the rake face is high and the friction stress on the rake face then exceeds shear yield stress sticking friction occurs
Flow occurs internally in workpiece
Hard to lubricate hence sticking friction is the norm in cutting - 13Orthogonal Cutting Tertiary Shear ZoneArea which is upset and ploughed by the tool edge rubbing against the freshly formed surface
- 14Effect of Cutting Speed on Cutting ForceHigh cutting speed means higher temperature, lower yield stress and hence lower cutting force
- 15Chip Formation At low speedsSevere strain hardening occurs until sufficient strain is accummulated to initiate shear
Elastic members in system e.g. tool mount, allow sudden acceleration and full separation of the chip followed by a new upsetting cycle - 16Chip Formation At moderate speedsWith presence of cutting fluid a continuous chip can be achieved
Newly formed surface is smooth
Top side of chip is jagged showing presence of shear - 17Chip Formation At higher speedsFriction increases until sliding at the rake face is arrested
A nose of stationary material accumulates on the tool forming a built up edge
- Increases rake angle
- Protects Tool
- Lower surface finish - 18Chip Formation At high speedsMaterial becomes hot enough to undergo recovery
No work hardening
No built up edge
- Degrades into secondary shear zone, a small distance above the rake face
- Depends on the temperature in the shear zone, adhesion between the tool and workpiece - 19Heat Balance at High SpeedsMajority of heat comes from primary shear zone
- Exposed to atmosphere/coolant
Secondary shear zone under the tool is hard to cool
- Adjacent, primary shear zone is hot hence limits conduction to chip
- Chip is removed however tool is constantly in contact with the workpiece hence high T develops in tool - 20Why is dry machining becoming increasingly attractive?Cutting fluid acts as coolant and lubricant
- Still have maximum temperatures in the tool of 900'C hence the effect of coolant on the tool wear is minimal
- Difficult to ensure coolant reaches necessary area
- Main benefit is it minimises thermal stresses in workpiece
Coolant is expensive and environmentally dangerous
Chip is easier to re-use / sell if it is dry rather than covered in coolant - 21Flank WearIntense rubbing of the clearance face
Surface finish deteriorates
Dimensional control lost
Heat generation increases
Small increase in cutting speed causes substantial decrease in tool life shown by the Taylor tool life eq.
- 22Crater WearCrater forms in rake face
Most influential factors
- Temperature
- Degree of chemical affinity
(Tool dissolves into workpiece)
- Abrasion, adhesion, thermal softening and plastic deformation - 23MachinabilityHardness - Softer materials shear easier
Surface texture - Materials with high work hardening rates can produce built up edges
Max rate of material removal
Tool life
Chip formation - 24Machinability Rating
- 25Lead as a workpiece additiveSoft
Fractures easily
Becomes smeared on tool/chip interface
- Smaller chips
- Lower cutting force
Lower temperature - 26Resulphurised SteelsMnS inclusions act as stress raisers in primary shear zone resulting in small chips
- 27Tool material propertiesHardness at elevated temperatures
Toughness
- Especially for interrupted cutting
Thermal shock resistance from coolant
Wear resistance
Chemical stability / inertness - 28High Speed Steel ToolsMolybdenum (M Series)
Tungsten (T Series)
Secondary hardening steels strengthened by complex carbides of Fe, W & C. Mo can be substituted for W.
Can be hot rolled or forged in the annealed condition
- Followed by heat treatment to HRC > 63
Can be repeatedly reground
M series has higher abrasion resistance, undergoes less distortion during heat treatment and is less expensive
A greater carbide content, finer carbides and more uniform carbide distribution can be obtained from powder met processing - 29Cemented Carbide ToolsTo improve performance of HSS we incorporate a large volume fraction of hard stable carbides
Why don't we bind carbides together with a metal using powder met processing
Produces carbide, cemented or sintered carbides - 30Cemented Carbide Tools WC in Co Matrix3 - 6% CO for greater hardness
6 - 15% for greater toughness
WC dissolves readily in steel at about 1200K - crater wear - 31Cemented Carbide Tools TiC in Ni-Mo MatrixHigher wear resistance than WC but not as tough
Used for hard materials - Steels, cast irons
Higher cutting speeds can be used
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- Beschreibung
- Karteikarten am Machining, erstellt von tom8390 am 09/01/2015.
- Erstellt von
- tom8390
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