05 Numerical Control 1 - Structure, Programming, CAM Flashcards
Components of Control Systems
- Supervisory Control and Data Acquisition Layer (SCADA)
- Human Machine Interface Layer
- I/O Layer
- Drive Layer
Components of Control Systems: SCADA Layer
a. Manufacturing Execution System (MES) distributes manufacturing orders and corresponding data, incl. NC programs and cutting tool lists/tables
b. Network communication by e.g. Industrial Ethernet or PROFINET
Components of Control Systems: Human Machine Interface Layer
a. PC-based console with standard operating system allowing for a simplified exchange of data via internal network
b. Local interface for the machine operator
Components of Control Systems: I/O Layer
a. For communication at the sensor and actor layer standards like PROFIBUS-DP are utilized
b. NC and Programmable Logic Control (PLC) are physically separated, but synchronized and share tasks
Components of Control Systems: Drive Layer
Use of internationally normed (open or proprietary) protocols for high dynamics and precision at servo level
Core Task of Numerical Control
Generation of relative motion between the cutting tool and the (raw) workpiece
- NC programs can be edited directly using the NC panel at the machine
Process of the execution of an NC program
NC interpreter decodes the information and splits it into geometry and technology data as well as switching commands
Geometry Data: Comprised of all information describing the tool path to be moved in order to create the geometry of the finished workpiece.
Technology Data: Used to set e.g. spindle speeds and feed rates
Structure of a Numerical Control
(NC Program, PLC, NC-Interpolator)
- NC programs can be edited directly using the NC panel at the machine. However, many NC programs are created externally and then transferred to the machine
- PLC is used to map machine-independent NC functions to trigger machine-specific hardware components
- NC interpolator computes the sequence of synchronized movements of the feed drive axes -> needed to create the workpiece contour
o It outputs the setpoint values for the position control loops for feed drives
o The slowest axis (e.g. due to weight) sets the standard/maximum for the velocity parameters
Task distribution between NC and PLC in Machine Tools: Tasks of NC
- Geometrical path information
- Technical instructions
- Identification of switching commands inside the NC program
Task distribution between NC and PLC in Machine Tools: Tasks of PLC
- Link between switching controls and feedback from the machine
- Conversion of control commands for switching units
NC-internal Information Flow: NC-Interpreter
o Functions as a syntax analyzer (parser) translates different NC program formats and input data into a consistent internal form (kept in random access memory)
o NC interpreter sends commanded boundary coordinates, the interpolation mode and the feed rate for each NC sentence to the Geometry Data Preparation module
o Switching commands are transferred to the PLC -> Still have to be synchronized
NC-internal Information Flow: Geometry Data Preparation
o NC computes several geometric transformation (e.g. zero point offset, tool corrections) -> Tool Path offset is continuously calculated based on the current tool geometry
o Velocities and accelerations for speed profile preparation are adapted to the boundary conditions of the machine
o Look-Ahead functionality: Analyzes multiple NC sentences ahead of the movement
NC-internal Information Flow: Interpolator
o Computes intermediate points along the path, defined by the NC sentence -> Aggregated path movement is divided into its partial contributions of all single axes
o Linear, circular and spline segments are discretized by a set of interpolated points -> Distribution is equidistand in time (IPO cycle time), depends on the pre-processed speed profiles
The more points are calculated, the closer the approximation is to the real shape (-> also depends on the clock time of the processor)
o Integration of compensation functionality and compensation tables
NC-internal Information Flow: Axis Control
Feed drives are provided with cyclically transferred setpoint values
-> All steps from interpolation until output of axis setpoint are performed within NC channels. -> Each channel may contain multiple axes and spindles (synchronously or asynchronously controlled)
Instructions for Numerical Controls -> What needs to be described in the program?
Description of tool path
Indirectly: Workpiece geometry
Directly: Tool center point (TCP) trajectory including (position) compensations
Technology Information:
E.g. number of revolutions or feed rate
Switching commands
Spindle on (clockwise/counter-clockwise)
Tool changes
Synchronization (Different tools on the same workpiece)
Wait commands for multi-slide machining
Control commands for switching tables, loading systems etc.
NC-Programming Approaches
- Textual -> Drawing
- Shopfloor-oriented -> Drawing/CAD File
- CAx-based -> CAD file
NC-Programming Approaches: Textual
o Worker separates the final workpiece into manufacturing features based on a technical drawing (usually paper-based) -> Largest amount of work is consumed by the description of the tool path
Sampling points not depicted in the drawing have to be calculated manually by the programmer
o Tables, Experiences, Computations
o Programming of geometry
o Programming of technology
NC-Programming Approaches: Shopfloor-oriented (SFP)
Programmer gets to set up a sequence of manufacturing operations which he can parameterize with the help of graphical templates -> Corresponding NC code is automatically generated
o Simulation features
o Graphically-interactive, problem-oriented programming of a machine tool:
Predefined programming interfaces
Query and suggestions for parameters
Could allow programming without explicit knowledge of NC code
Textual and SFP yield a machine-dependent NC program output -> Direct transfer of NC programs onto other machines is usually not feasible
Changes done by machine operator are normally not fed back into planning department
NC-Cycles
Bundle specific commands and manufacturing operations (e.g. tap machining)
Sub-program-like structure
Contain a set of multiple NC lines that can be parameterized via input parameters
NC-Programming Approaches: CAx-Based
o Main idea: Utilizing the CAD model received from product design and augmenting it with manufacturing information and commands
o Advantage: High level of automation that can be realized by using existing models and connecting the CAM software with different databases
o Translation of CAM program into NC program is done via a processor and a subsequent post-processor
NC program (G-Code)
- NC code is organized in modal groups -> A code or variable stays in effect until replaced or cancelled by another permitted code
- A NC program consists of a series of information blocks, which correspond to a particular step in the working process
- The NC program contains all movements and switching operations which have to be executed in order to move the tool and the workpiece while machining the part -> Part’s geometry is simplified to single linear and circular movements
- In addition to the standard commands there are special commands, defined and provided by control vendors or developed by machine tool users (for special requirements) -> Cannot be exchanged and executed on different machine tools
NC in the production process: Development&Design
Definition of product shape
–> Workpiece Geometry
–> Workpiece Surface
–> Tolerances
–> Material
NC in the production process: Process Planning
- Definition of the manufacturing process
- Creation of NC programs
- Simulation of the manufacturing process
- Machine Scheduling
NC in the production process: Manufacturing
o (Automated) creation of the final product
o Execution of the created NC programs
o Problem: Programming errors can only be detected by deficient products
Process Data Feedback
Feedback and subsequent analysis can potentially unlock unused economic potentials (e.g. process-parallel quality analytics) -> Elimination of rejection costs and decreasing lead time
Process Data Feedback: Steps
- Process Data streaming (real-time)
- Process-parallel analytics of part & process quality (model-based)
- Evaluation assistance software
- Process monitoring and optimization
One of the biggest challenges in manufacturing data analytics
Synchronization and mapping of different data and information sources
Data Contextualization within PLM Cycle
Contextualization is needed to enable efficient automated optimization processes (deep learning, machine learning etc.) and to derive feasible use cases e.g.
o Monitoring of processes
o Monitoring of machine tool condition
o Workpiece quality estimation
o Cutting tool analytics
Computer Aided Manufacturing (CAM)
Describes methods and softwares used for the computer-based creation of NC programs
Advantage of CAM
Complex and time-intensive steps, that cannot be handled economically by humans, are automatically handled by the CAM system
Factors to determine if the investment in CAM is reasonable
o Number of parts and variants to be machined
o Frequency of changing parts
o Complexity of parts to be machined
o License cost of the CAM system
Application fields of CAM (5)
o Tool path generation and optimization
o Simulation
o Creation of manufacturing documents (machining plans, clamping plans, tool lists)
o Management of tool and equipment data
o Distributed Numerical Control (DNC)
Post-Processor
The Post-Processor is a parser which converts one code (CAM-internal tool path description) into another code (NC code). It adapts the machine-independent data to the NC of a specific machine tool.
Goal of the post-processor
Post-processors are used for translating general CAM tool path data into machine specific NC programs
Different Milling Setups
- 3-Axis Milling
- 3+2-Axis Milling
- Simultaneous 5-Axis Milling
3-Axis Milling
- Constant orientation along the entire path
- Cutting conditions cannot be adjusted locally
- Simple process planning
- Low risk of collision
3+2-Axis Milling
- Adaptable relative orientation between tool and workpiece
- Optimization of cutting conditions possible
- Medium effort for process planning
- Low risk of collision after initial orientation setting
Simultaneuous 5-Axis Milling
- Simultaneous adjustment of relative orientation may ensure optimal cutting conditions
- Increased accessibility features
- Significantly increased planning complexity
- High risk of collision
Advantages of 3- and 3+2-axis milling operations
o Smaller amount of axes simultaneously engaged -> Calculations for the path are less complex and less time consuming
o Collisions can easier be predicted
o Singularities (unique mapping of Cartesian positions to complementary axis positions) don’t exist
o Higher feed rates/velocities can be reached, since the positioning is only based on the linear axes
Advantages of five-axis milling
o Allow an optimal orientation of the cutting tool in relation to the workpiece contour -> May yield a better surface quality and consistent material removal conditions
o Complex workpieces can be manufactured in one or two clamping setups -> saving the time for re-clamping and adjusting the workpiece
Simulation
Simulation either takes place as a CAM-internal simulation before the post-processor run or as an external simulation after post processor run
Necessity for simulation
- Increasing complexity of product portfolio
- Complex machining technology
- Simulation of the manufacturing process
Simulation types - CAM internal Simulation
- Toolpath-based verification
- Toolpath-centred machine simulation
Simulation: Toolpath-based verification
No consideration of machine kinematics and fixtures)
Consideration of blank, tool and holder geometry in simulation
Simulation based on the paths generated by the CAM system in the CAM internal format
Simulation: Toolpath-Centred machine simulation
Extension of the tool-path-based verification
Consideration of machine workspace and kinematic behavior (machine model)
Additional collision checks with further components in the workspace
Boundaries of CAM-internal simulation
Errors caused by Post Processor cannot be detected
Cycle times obtained are not exact, because dynamic limits of the machine are neglected
Simulated path may deviate from real path
Simulation Types: Machine Simulation
- Machine simulation based on NC code
- Machine simulation with virtual numerical control (VNC)
machine Simulation based on NC code
Simulation in CAM system based on machine-specific NC programs after the post-processor run
Use of an NC emulation to interpret the G-Code
Transfer of setpoint values to machine model
Machine Simulation with virtual numerical control
Extension to toolpath-centered simulation
Provided by controller vendors
VNCs use the same software kernel for motion generation as the real machine’s NC -> Increased precision of the simulation
Transfer of realistic lead values from the virtual control machine models
Simulation Types - Virtual Machine
o May consist of machine model, drive model, control model and optional process model
o Virtual representation of machine control and HMI
o Consideration of the entire machining situation by using a machine and drive model with parameter settings corresponding to the real machine
o Avoiding extensive test using the real machine
o Modifications of individual components are possible
o Effects on other parts of the machine can be analyzed
o Small deviations of predicted and real manufacturing time
Benefits of Virtual Machines
o Virtual set-up planning
o Virtual process run-in and collision avoidance
o Optimization of NC programs
Challenges of Virtual Machines
Unknown process physics
Ideal material removal
Ideally rigid mechanical systems
Lead tool paths only
Fixture consideration
Machining forces
Vibrations
Overloads
Application of Virtual Machines
- CAM/NC Programming
- Machine Design
- Electronics Design
Opportunities of today’s NC simulations
o Damage-free testing of NC programs
o Virtual ramp-up of NC processes without occupation of machines
o Optimization or preventive compensation possible
o Increased understanding of the process through expert systems at different levels
o Detection of interactions by coupling of multiple simulation systems
o Teaching new employees on virtual machines
Limitations of today’s NC simulations
o Reliability of the results varies depending on the application
o Often results are not suitable for automated evaluation and must be interpreted by experts
o Modeling effort may be uneconomically high (depends on detail and scope)
o Models are not complete and neglect effects that can have significant impact
o Not fully mature coupling mechanisms between simulation systems
-> Understanding of the process
-> Technical and organizational implementation
NC-Internal Information Flow (Steps)
- NC Interpreter
- Geometry Data Preparation
- Interpolation
- Position Control
- Drive Control
