CNC Machine Software and Machine Control Unit

• CNC machines operate using software.
• There are three software program types used in CNC systems.

Operating System Software

• It interprets NC part programs.
• It generates control signals to drive machine tool axes.
• Contains an editor, a control program, and an executive program.
• It includes diagnostic routines.

Machine Interface Software

• It facilitates communication between the CPU and the machine tool.
• It accomplishes CNC auxiliary functions.
• I/O signals are associated with auxiliary functions.
• This is sometimes implemented via a programmable logic controller interfaced to the MCU.
• This type of software is often written as ladder logic diagrams.

Application Software

• It consists of NC part programs.
• It programs for machining or other applications in a user's plant.

Machine Control Unit

• CNC machines operate as soft-wired controllers.
• The MCU is the heart of a CNC system.
• Once a CNC program is loaded into the computer's memory, no hardware is needed to transfer the NC codes to the controller.
• Functions include reading and decoding instructions, implementing interpolations to generate axis motion commands, and feeding axis motion commands to amplifier circuits.
• It drives axis mechanisms and receives feedback signals of position and speed for each drive axis.
• It implements auxiliary control functions like coolant, spindle on/off and tool change.
• The controller uses a permanent resident program, the EXECUTIVE PROGRAM, to process codes into electrical pulses for machine control.
• The EXECUTIVE Program is also called "executive software" or "executive firmware".
• The EXECUTIVE program resides in ROM (Read Only Memory).
• NC code resides in RAM (Random Access Memory).
• Firmware refers to routines of software including low-level instructions stored in ROM only for reading.

Configuration of CNC Machine Control Unit

• The MCU is the hardware that distinguishes CNC from conventional NC
• The MCU consists of the following components and subsystems:
• Central Processing Unit
• Memory
• I/O Interface
• Controls for machine tool axes and spindle speed
• Sequence controls for other machine tool functions

Central Processing Unit (CPU)

• The brain of the MCU.
• It manages other MCU components based on memory software.
• It can be divided into control section, arithmetic-logic unit, and immediate access memory.
• Control Section: Retrieves commands and data from memory and generates signals to activate other MCU components.
• It sequences, coordinates, and regulates the activities of the MCU computer.
• Arithmetic-Logic Unit (ALU): performs calculations and logic functions.
• Immediate Access Memory: Provides temporary storage for data being processed.
• It is connected to main memory by the system data bus.

Memory

• CNC memory is divided into main memory and secondary memory.
• Main Memory: ROM (read-only memory)
• Secondary Memory: RAM (random access memory) devices.
• Operating system & machine interface programs are generally stored in ROM (installed by the manufacturer).
• NC part programs are stored in RAM.
• Programs in RAM can be erased and replaced as jobs are changed.

ROM (Read Only Memory)

• Is an electronic chip that can be accessed by a computer but is not altered.
• Its contents cannot be erased or written without special equipment.
• The EXECUTIVE program cannot be erased.
• The EXECUTIVE program is always active when machine is on.

RAM (Random Access Memory)

• RAM can be altered by the computer.
• NC code is written into RAM by keyboard or external source.
• RAM content is lost when the controller is turned off.
• Battery backup systems are used to save the program in the event of power loss.
• CMOS (Complementary Metal-Oxide-Semiconductor) memory retains content in case of power loss.

Input/Output Interface

• Provides communication between the CNC system components, other computer systems, and the machine operator.
• It transmits and receives data and signals to and from external devices
• The operator control panel is the basic interface for machine operator communication.
• It is used to enter commands for part program editing, MCU operating mode, speeds and feeds, cutting fluid pump on/off, etc.
• An alphanumeric keypad or keyboard is usually included in the operator control panel.
• A display is used to communicate data and information from the MCU to the machine operator.
• The display indicates the program's current status and warns of malfunctions.

Input Media

• Electronically or mechanically stores the NC programs.
• An NC program is read from the input medium when it is loaded into the machine.
• Old NC machinery read programs from punched tape or DNC.
• CNC machines process multiple means of program input.
• Manual Part Programming: Manual programming of the machines
• Computer Aided Programming (CAP): Programming done by a computer
• Manual Data Input (MDI): A manual program is entered into the machine's controller via its own keyboard
• Traditional and modern CNC input and storage methods include: Punched Tape, Diskette, ZIP disk, CD-ROM, Flash Drive, and Portable External Drive

Controls for Machine Tool Axes and Spindle Speed

• These hardware components control the position and velocity (feed rate) of each machine axis and the spindle speed.
• Positioning systems can be classified as open loop or closed loop, requiring different hardware components in each case.

Sequence Controls for Other Machine Tool Functions

• In addition to controlling table position, feed rate, and spindle speed, several additional functions are accomplished under part program control.
• These auxiliary functions generally involve on/off (binary) actuations, interlocks, and discrete numerical data.
• The functions include cutting fluid control, fixture clamping, emergency warnings, and interlock communications for robot loading and unloading of the machine tool.

Examples of CNC Machines

• CNC controls are used to control various types of machine tools
• Regardless of which type of machine tool is controlled, it always has a slide table and a spindle to control of position and speed.
• The machine table is controlled in the X and Y axes.
• The spindle runs along the Z axis.
• Examples machines listed:
• LBO-L1060
• MICRON UCP 1150,5-axis vertical machining center
• OKUMA MCR-A5C 3 axis machining center
• STUDER
• HAAS

Direct Numerical Control (DNC)

• In DNC, multiple machine tools are controlled by a single (mainframe) computer through direct connection and in real time.
• The program was transmitted to the MCU directly from the computer, one block of instructions at a time; This form of communication is two-way.
• It was a 1960s technology.
• Distributed numerical control (DNC) refers to a network of a central computer connected to CNC machines (MCUs).

Computer Aided Programming Languages

• Programming languages allow the development of an NC program using a set of universal "pidgin English" commands.
• The computer translates commands into machine codes which are then punched into tapes.
• A computer is used as a partial or complete controller of one or more NC machines.
• Expensive mainframe or minicomputers were required in the past, due to cost the use of DNC was limited to large companies.
• Powerful PCs given rise to affordable PC-based DNC systems, most of which runs on MS Windows OS. Connection to MCU is behind the tap reader (BTR)
• In distributed NC, entire programs are downloaded to each CNC MCU.

Distributed Numerical Control Configurations

• Consists of a network of computers to coordinate the operation of a number of CNC machines.
• Can coordinate an entire factory.
• Alternative System 1: The NC program is transferred in its entirety from a host computer directly to the machine's controller.
• Alternative System 2: The NC program is transferred from a mainframe or host computer to a PC on the shop floor, stored, and used when needed and then transferred to machine controller.
• Complete part programs are sent to the machine tools, not one block at a time.
• The distributed NC approach permits easier and less costly installation overall.
• The individual CNC machines can be put into service and distributed NC can be added later.
• Switching Network Configurations: The DNC computer is connected to multiple Machine Tools via a central Data Switching Box and individual Machine Control Units (MCU).
• Local Area Network (LAN) Configurations: The DNC computer is connected to multiple Machine Tools via Satellite Computers and individual (MCU).

Table of Data Flow in DNC

• Data and information is downloaded from the central computer to the machine tools consisting of NC part programs, list of tools needed for job, machine tool setup instructions, machining cycle time for part program, data about when program was last used, and production schedule information.
• Data and information is uploaded from the machine tools to the central computer consisting of piece counts, actual machining cycle times, tool life statistics, machine uptime and downtime statistics, product quality data, and machine utilization.
• Two way communication of data and information occurs.

Applications of NC

• Milling, drilling, turning, boring, grinding
• Machining centers, turning centers, mill-turn centers
• Punch presses, presses for sheet metal bending, welding machines, tube bending and wire bending machines, wire EDM, thermal cutting machines, etc.
• Each of the machining operations is carried out at a certain combination of speed, feed, and depth of cut, collectively called the cutting conditions
• Rapid prototyping and additive manufacturing like Fused deposition modeling, 3-D printing, Selective laser slintering, stereo lithography, etc

NC Application Characteristics (machining) where NC is most appropriate:

• Batch production
• Repeat orders
• Complex part geometries
• Much metal needs to be removed from the starting workpart
• Many separate machining operations on the part
• The part is expensive

Advantages of NC

• Nonproductive time is reduced through fewer setups, less setup time, reduced workpiece handling time, and automatic tool changes.
• Parts are made by creating greater accuracy and repeatability.
• There is less dimensional variation among parts in the batch.
• Lower scrap rates as greater accuracy and repeatability is achieved, and human errors, so more parts are produced within greater tolerance.
• Inspection requirements are reduced.
• More complex part geometries are possible due to accurate positioning of the tool.
• Engineering changes are easier to make, and fixtures are simpler since more accurate positioning of the tool is accomplished. Since less setups are required shorter turnaround times tend to result for production.
• This leads to a reduction of parts inventory and less floor space.
• Operator skill level requirements are reduced.

Disadvantages of NC

• Higher investment cost.
• CNC machines are more expensive.
• Higher maintenance effort is required since they are more technologically sophisticated.
• Part programming issues, since there is a need for skilled programmers, and time investment for each new part
• Repeat orders require a part program that is already available.
• Higher utilization is required.

Drive System

• It consists of amplifier circuits, drive motors, and ball lead-screws.
• The MCU feeds the control signals (position and speed) of each axis to the amplifier circuits.
• Control signals are augmented to actuate drive motors.
• Drive motors rotate the ball lead-screws to position the machine table.

NC machining operations:

• The cutting speed is the velocity of the milling cutter relative to the work surface, m/min (ft/min); this is usually programmed into the machine as a spindle rotation speed, rev/min.
• In milling, the feed usually means the size of the chip formed by each tooth in the milling cutter, often referred to as the chip load per tooth.
• Since the chip load per tooth must normally be programmed into the NC machine, this sets the feed rate (the travel rate of the machine tool table).
• The depth of cut is the distance the tool penetrates below the original surface of the workpiece (mm or in)
• CNC horizontal milling machine is displayed
• There is an image for common NC machining operations displayed that includes the workpart, chip, new surface, depth, cutting tool, and feed; which are a components of turning.