Basics of cutting Flashcards
Cutting force
Depends on: - specific cutting force of the workpiece material
- cross section area of chip
cutting speed * cutting force = required power
Cutting energy and tool
- energy input converts to heat
- Heat and abrasion cause tool wear
- Tool economic lifetime is typically 15 - 30 minutes for basic cutting methods
Differences Turning and Milling
- Chip breaking
- Tool load (continuous and discontinous)
Tool geometry in turning (angles)
α Clearance angle β Wedge angle ε Included angle γ Rake angle κ Major cutting edge angle κn Minor cutting edge angle λ Inclination angle r Tool nose radius
(Freiwinkel, Keilwinkel, Eingeschlossener Winkel, Spanwinkel, Hauptschneidkantenwinkel, Kleiner Schnittkantenwinkel, Neigungswinkel, Radius der Werkzeugschneide)
Tool geometry in turning (points and surfaces)
a tool holder b cutting edge c clearance face d minor clearance face e minor cutting edge f rake face
(a Werkzeughalter b Schneide c Freifläche d Nebenfreifläche e Nebenschneidkante f Spanfläche)
Relation Rake angle and Angle of inclination
when inclination angle rises, the rage angle rises
Factors influencing the cutting force
- Workpiece material
- Tool geometry
- cutting parameters
Components of the resulting cutting force (turning)
- Axial/feed component
- Radial/passive component
- Tangential/main component
Relation cutting force and cutting speed
Very little influence
Formula for main cutting force
Fy ≈ f apkc
feed f, specific cutting force kc, depth of cut ap
Relation specific cutting force and chip thickness
kc increases when h decreases (exponential)
–> Small cutting depth requires sharp cutting edge
Formulas surface roughness
Ry=f^2/(8*r) (maximum peak to valley
Ra=f^2/(20*r) (arithmetic average)
Temperature and heat
- Most heat generated in shear zone and due to friction on the rake surface
- Workpiece material and cutting speed are the most important affecting factors
- thermal energy goes to: 80% chips, 10% workpiece, 10% tool
FEM modeling of cutting
- modelling of bodies as meshes
- balance equations mainly for force and heat –> minimization
- regeneration of mesh after every movement
Purposes of FEM
- Chip form
- Cutting forces
- Tool and workpiece temperature
- Tool wear
- Workpiece deflection
Where does tool wear affects?
- Workpiece surface and dimensions
- Chip form
- Cutting forces
Tool wear types and main cause
- abrasive wear (Abtrag) (low tool hardness)
- crater wear (high temperature)
- Notch wear (Kerben) (adhesion)
- Build-up edge (adhesion of workpiece material, low cutting speed and temperature)
- cracking (mechanical overload)
- plastic deformation (high temperature)
Characteristics rough cutting
• Objective is efficient material removal
• high cutting depth and feed considering rigidity limitations due to tool-
fixture-workpiece-machine tool system
• Cutting speed is optimized with the objective of minimizing total cost or sometimes
output (pieces/hour)
Characteristics finishing
• Objective is on surface quality and precision
• The allowance left from rough cutting determines cutting depth, which must be
small in order to keep cutting forces, deformation, and vibration low
• Feed is selected according to the defined surface roughness
• Cutting speed is optimized with the objective of minimizing total cost
Cost factors while machining
- Labor
- Depreciation
- Interest (Zins)
- Rent
- Energy
- Maintenance
- Tools