What are the positive consequences of CNC?
Positive consequences of CNC
-
Automatic tool changing on machining centres and lathes
- More productive time in machining cycle
- Fully enclosed machines
-
Automatic workpiece loading/unloading
- Allowing unmanned production
- Bar feeder, Gantry robot, Pallet changing
-
In cycle measurement of tools and workpieces
- Program offsets to compensate for tool wear
- Mechanical error calibration and compensation
- Ability to control multiple axes
What are the processing drivers in today's environment
Processing drivers in today’s environment
Operational Cost
-
Labour
- Automated workpiece handling
- Fabrications being replaced by “machine from solid”
- More accurate machining to aid downstream assembly
-
Asset utilisation
- Raw material stock and WIP – reducing batch sizes
-
Shorter product life cycles, quicker product info, shorter lead times
- Flexible machines
- CAD/CAM, machining simulation
-
Energy efficiency
- Weight reduction
- Fewer but more complex parts
- Weight reduction
What are the major independent variables in the cutting process?
Major independent variables in the cutting process:
- Tool material and coatings: if not ductile material makes machining hard
- Tool shape, surface finish and sharpness
- Workpiece material and condition
- Cutting speed, feed, and depth of cut
- Cutting fluids
- Characteristics of the machining tool
- Work holding and fixturing
What are the major dependent variables in the cutting process
Major dependent variables in the cutting process:
- Type of chip produced
- Force and energy dissipated during cutting
- Temperature rise in the workpiece, the tool and the chip
- Tool wear and failure
- Surface finish and surface integrity of the workpiece
What are large shear strains associated with?
Large shear strains are associated with low shear angles or low or negative rake angles
What are the basic types of chip produced in metal cutting?
Basic types of chips produced in metal cutting and their micrographs
a) Continuous chip with narrow, straight primary shear zone
b) Secondary shear zone at tool chip interface
c) Continuous chip with build-up edge
d) Segmented or non-homogeneous chip
e) Discontinuous chip
What are the main points about continuous chips?
Continuous chips
- Formed with ductile materials machined at high cutting speeds and high rake angles
- Deformation takes place along a narrow shear zone called the primary shear zone
- Continuous chips may develop a secondary shear zone due to friction at the tool-chip interface
What are the main points about BUE and how to reduce them?
Build up Edge chips
- Consists of layers of material from the workpiece that are deposited on the tool tip
- As it grows, BUE becomes unstable and breaks apart
- BUE can be reduced by:
- Increasing the cutting speeds
- Decrease the depth of cut
- Increase the rake angle
- Use a sharp tool
- Use an effective cutting fluid
- Use cutting tool with low chemical affinity for workpiece
What are the main points about serrated chips?
Serrated chips
- Also called segmented or non-homogeneous chips
- They are semi continuous chips with large zones of low shear strain and small zones of high shear strain
- Chips have a sawtooth-like appearance
What are the main points about discontinuous chips?
Discontinuous chips
- Consists of segments that are attached firmly or loosely to each other
- Form under following conditions:
- Brittle workpiece material
- Materials with hard inclusions and impurities
- Very low or very high cutting speeds
- Large depths of cut
- Low rake angles
- Lack of an effective cutting fluid
- Low stiffness of the machine tool
How do chip breakers work?
Chip breakers:
- Chip breakers decrease the radius of curvature of the chip
- Grooves on the rake face of cutting tools, act as chip breakers
- Most cutting tools now are inserts with built in chip breaker features
What are the different power equations?
What are the different ways of measuring cutting forces?
Measuring cutting forces and power
- Cutting forces can be measured using:
- A force transducer
- A dynamometer or
- A load cell mounted on the cutting tool holder
- Cutting force can be calculated from the power consumption during cutting
- The specific energy in cutting can be used to calculate cutting forces
What are the three main sources of heat when cutting?
Cutting temperature
- In cutting, nearly all of the energy dissipated in plastic deformation is converted into heat that in turn raises the temperature in the cutting zone.
- Three main sources of heat when cutting:
- Plastic deformation by shearing in the primary shear zone (Q1)
- Plastic deformation by shearing and friction on the cutting face (Q2)
- Friction between chip and tool on the tool flank (Q3)
What are the different ways heat is dissipated?
Heat is mostly dissipated by,
- The discarded chip carries away about 60-80% of the total heat
- The workpiece acts as a heat sink drawing away 10%
- Cutting tool draws away 10%
- Any reduction of cutting temp will require substantial reduction in either the cutting speed or feed. However, cutting time and production rate decreases, coolants required.
Why do we want to avoid high cutting temperatures?
Why do we want to avoid high cutting temperatures?
- Affects the wear of the cutting tool. Cutting temperature is the primary factor affecting tool wear
- Can induce thermal damage to machined surface
- Causes dimensional errors in the machined surface
What are the different types of feeds in milling?
There are three types of feeds in milling
- Feed per tooth (fz): basic parameter equivalent to the feed in turning
- Feed per revolution (fr): determines the amount of material cut per one full revolution of the milling cutter
- Feed per minute (fm): calculated taking into account the rotational speed N and number of cutter’s teeth z
What are the different machining processes?
What are the different machining processes?
- Turning
- Boring
- Drilling
- Milling
- Planing
- Shaping
- Broaching
- Sawing
Explain cemented carbides and the effect of cobalt content on properties
Cemented carbides
- Co matrix with WC/TiC grains
- Effect of cobalt content. Hardness is directly related to compressive strength and hence, inversely to wear.
What is the different wear patterns of high speed steel uncoated and Ti N coated cutting tool?
What are the different types of tool wear?
What are the different types of tool wear?
- General wear
- Flank wear
- Crater wear
- Chipped cutting edge
- Flank wear and built up edge
Draw the different types of tool wear
Draw taylors tool life equation
What are the factors that affect tool wear?
Factors that feed into tool wear
- Contact stresses
- Due to cutting tool shape
- Cutting conditions
- Cutting temperature
- Depends on fluids
- Cutting conditions
- Cutting tool shape
- Tool material
- Work material
What is the difference between surface finish and surface integrity. What factors influence them both?
Surface finish and integrity
- SF – geometric features of surfaces
- SI – refers to properties such as fatigue life and corrosion resistance
- Factors influencing SI:
- Temperature
- Residual stresses
- Metallurgical transformations
- Surface plastic deform, tearing and cracking
- BUE has greatest influence on SF
What are the difference roughness parameters?
Measurements
- Ra is now the most commonly specified roughness parameter
- Rq is known as RMS
- Rz is the distance between average of peaks and valleys
- Rt is the largest. It is max/min reading
Show a representation of surface roughness
Roughness
Nominal Shape
Waviness
Desired Shape
What are the different measuring devices
Different measuring devices
- Stylus
- Interferometric microscope
What is taylor's tool life equation?
The ability to produce quality products hinges on which four key competencies?
- Process precision – the ability to produce quality products hinges on four key competencies:
- Modelling of process form and precision levels
- Design tolerancing of parts and products
- Selecting production processes that match part specifications
- Applying quantitative measurement methods for inspection and process control
- Modelling of process form and precision levels
- Design tolerancing of parts and products
- Selecting production processes that match part specifications
- Applying quantitative measurement methods for inspection and process control
Why High Precision
Why High Precision
- To create a highly precise movement
- To reduce the dispersion of the product’s function
- To make the parts interchangeable
- To make automatic assembly possible
- To make functions independent of each other
- To maintain the same level of relative precision for miniaturisation
- To reduce the number of required parts to minimise accumulated error
- To improve the efficiency of the machine
- To reduce the initial cost
- Extend the life span
- Enable safety factor to be lowered
How is stability of the process measured?
Stability of the process
- Stable processes are those that are free from special cause variation.
Statistical process control, scatter plots or other forms of statistical analysis are used to measure process stability
- Stability determination requires enough data sampled to cover a wide range of possible variation contributors that apply to the process being measured. What are the possible contributors to variation?
- Part variation: piece to piece, raw material lot to lot etc.
- Tooling variation: cavity to cavity, tool to tool, tool wear over time etc.
- Human variation: operator to operator
- Time variation: sample time to time, shift to shift
- Location variation
What are contributing factors to machine functionality (error generating process and machining accuracy)
Contributing factors to machine functionality
Error generating process
- Static deformation
- Dynamic deformation
- Thermal deformation
- Rotation accuracy
- Guiding accuracy
- Dynamic movement error
- Tool wear
Machining accuracy
- Contour accuracy
- Surface roughness
- Dimensional accuracy
What are the static and dynamic errors associated with tool design?
Tool design
Static effects
- Poor tolerance
- Material flow problems
- Set up
- Shrinkage
Dynamic effects
- Wear
- Non-linear coupling (chatter)
- Degradation (wear/erosion)
What are the static and dynamic errors associated with machine design?
Machine design
Static effects
- Precision
- Materials
- Environment
Dynamic effects
- Dynamic vibration
- Thermal stability
- environment
What are the static and dynamic effects of installation and material
Installation
Static effects
- Environment
- Location
Dynamic effects
- Dynamics thermal
- Operators
Material
Static effects
- Performance
Dynamic effects
- Poor fixture design
What are the sources of vibration in machine tools?
Sources of vibration in machine tools
- Force excitation
- Seismic excitation
- Inherent behaviour of the structure
Give examples of sources of vibrations in machine tools
Examples of sources of vibration in machine tools
- Unbalanced mass
- Alignment
- Pressure variations
- Teeth engagement faults
- Bearing defects
- Tool engagement
What are thermal errors are due to
Thermal errors
- For machine tools, thermal errors are due to a combination of effects:
- Room environment
- Coolants
- People
- Machine itself
- Machine process
- Energy transferred via conduction, convection and radiation
- Estimated to account for 40% of errors
How to avoid errors?
What is error compensation?
Error avoidance: eliminates the source of the error or alters the process whereby the source becomes an error
Error compensation sums an additional component to cancel an error
How do you minimise thermal expansion?
- Common solutions to minimise thermal expansion include choosing materials with low coefficients of expansion (Zerodur)
What are the mathematical ways of establishing the effect of thermal transients?
Establishing the effect of thermal transients in a structure
• A measure of thermal diffusivity, D = k/cp
- k = conductivity, c = specific heat and p = density
- Indicates the relative rate of settling of transients, faster is better to insure stability.
• A further indication of a material’s resistance to thermal effects is to divide the thermal diffusivity by the coefficient of thermal expansion, alpha.
- D/alpha
- A larger value is better (faster settling and lower expansion)
What are the different classifications of errors?
Part specific mechanical error
Static
- Machine variation
- Design deficiencies
- Deformation due to clamping
Dynamic
- Material variation
Machine specific mechanical errors
Static
- Elastic deformation of cutter
- Roll, pitch, yaw
- Squareness
- Parallelness
- Mechanical, structural and tooling deformation
Dynamic
- Deformation due to clamping
- Elastic deformation of cutter vibrations
- Moulding tool wear
- Pressure variations in injection sequence
- Load variations in forging
What are the principles of measurement
Principles of measurement
- Error reduction: brute strength approach to isolating error sources
- Error correction: calibrate then correct
- Error compensation: measuring error in real time and compensation for the perturbation
- First principle – when measuring the dimension one must know its temperature
What are the different measurement devices?
Laser scan micrometre
Gauges
Coordinate measuring machine
What is SPC?
Statistical process control
- SPC is used to detect and eliminate the sources of variation in the process that could not be attributed to the routine operation of the process
What is the control is QC? Give examples and testing points
Quality control
- Control: the activity of ensuring conformance to requirements and taking corrective action when necessary to correct problems
- Examples of QC
- Hazard analysis
- Critical control points
- Preventative measures with critical limits for each control point
- Procedures to monitor the critical control points
- Corrective actions when critical limits are no met
- Verification procedures
- Effective record keeping and documentation
- Inspection/Testing points
- Receiving inspection
- In-process inspection
- Final inspection
- What to inspect? – key quality characteristics that are related to cost or quality
- Where to inspect – key processes, especially high cost and value added
- How much to inspect? – all, nothing, or a sample
What are the two sources of process variation?
There are two sources of process variation
- Chance variation that is inherent in process, and stable over time – known as COMMON CAUSE VARIATION
- Assignable, or uncontrollable variation, which is unstable over time – the result of specific events outside the system – known as SPECIAL CAUSE VARIATION
- A process that is operating with only chance causes of variation present is said to be in statistical control
- A process that is operating in the presence of assignable causes is said to be out of control
- The eventual goal of SPC is the elimination of variability in the process
What are the different types of charts for SPC?
- Types of charts
- Attributes – product characteristic that can be evaluated with a discrete response (Y/N)
- P-chart – uses portion defective in a sample
- C-chart – uses number of defects in an item
- Variables – for variables that have continuous dimensions – weights, speed, length
- Range (R-chart)
- Mean (X bar – chart)
- Attributes – product characteristic that can be evaluated with a discrete response (Y/N)
- P-chart – uses portion defective in a sample
- C-chart – uses number of defects in an item
- Variables – for variables that have continuous dimensions – weights, speed, length
- Range (R-chart)
- Mean (X bar – chart)
How do you use SPC?
How to use SPC
- Before implementing SPC or any new quality system, the manufacturing process should be evaluated to determine the main areas of waste
- Some examples of manufacturing process waste are rework, scrap and excessive inspection time
- During SPC, not all dimensions are monitored due to the expense, time and production delays that would incur
- Prior to SPC implementation, the key or critical characteristics of the design or process should be identified by a cross functional team
- Data would then be collected and monitored on these key or critical characteristics
What are the important points about collecting and recording data in SPC
Collecting and Recording Data
- SPC data is collected in the form of measurements of a product dimension/ feature of process
- The data is then recorded on various types of control charts, based on the data being collected.
- It is important that the correct type of chart is used to obtain useful info
- The data can be in the form of continuous variable or attribute data. Can also be collected as individual value or average of a group of readings
What are the steps in building an X-bar and R chart?
The following steps are required to build an X-bar and R chart
- Designate the sample size (4/5) and determine the frequency the sample measurements will be collected
- Collect samples. General rule: 100 measurements in groups of 4, 25 data points
- Calculate the average value for each of the 25 groups of 4 samples
- Calculate the range of each sample
- Calculate the x-bar
- Calculate the average of the R values
- Calculate the upper and lower control limits.
What is the difference between Cp and Cpk?
What are the benefits of control charts