Understanding Numerical Control (NC) and CNC

• Numerical Control (NC) is a form of programmable automation where machine tool mechanical actions are controlled by a program.
• The program contains coded alphanumeric data and instructions.
• NC is the predecessor to CNC.

Alphanumeric Data

• Alphanumeric data represents the relative positions between a workhead and a work part.
• A new program can be entered after the current job is done.

Applications of NC

• Machine tool uses include drilling, milling, and turning.
• Other applications include assembly, rapid prototyping, and inspection.
• The common feature is the work head movement control relative to the work part.
• NC concept dates to the late 1940s.

Computer Numerical Control (CNC)

• CNC is a form of electromechanical motion control used on machine tools.
• It uses a computer and computer program to perform machining operations.

Historical Notes on NC

• 1947: J. Parsons experimented with using 3-axis curvature data to control machine tool motion for aircraft components.
• 1949: Parsons was awarded a US Air Force contract to build the first NC machine.
• 1951: MIT was involved in the project.
• 1952: NC was achieved when MIT demonstrated simultaneous 3-axis movements using a controller and a Cincinnati HYDROTEL vertical spindle.
• 1955: NC became available to the industry after refinements.
• 1959: MIT announced Automatic Programmed Tools (APT) programming language.
• 1960: Direct Numerical Control (DNC) eliminates paper tape punch programs, allowing programmers to send files directly to machine tools.
• 1968: Kearney & Trecker machine tool builders marketed the first machining center.
• 1970s: CNC machine tools were introduced.
• 1980s: Graphics-based CAM systems were introduced, with Unix and PC-based systems available.
• 1990s: A price drop occurred in CNC technology.
• 1997: PC Windows/NT based "Open Modular Architecture Control (OMAC)" systems were introduced to replace "firmware" controllers.
• 2001-2007: Machine simulation, virtual prototyping, and WEB services were introduced.

Basic Components of an NC System

• The program of instructions is a part program in machining.
• The machine control unit controls the process.
• The processing equipment performs the process.

Part Program

• Includes set of detailed step-by-step commands to direct the actions of the processing equipment.
• Individual commands refer to positions of a cutting tool relative to the worktable.
• Additional instructions cover spindle speed, feed rate, and cutting tool selection.
• Programs are coded on media like 1-inch wide punched tape, magnetic tape, diskettes, and via electronic transfer from a computer.

How a Controller Processes Information

• Computers and computer-controlled machinery do not use Arabic symbols or numbers.
• All internal processing is done by calculating or comparing binary numbers.
• Binary numbers contain only two digits: ZERO (0) and ONE (1).
• Within a CNC controller, a binary digit "1" may represent a "positive charge" or "ON".
• A binary digit "0" may represent a "negative charge" or "OFF".
• The CNC program code, in binary form, must be loaded into the computer.
• Programming formats allow NC code to be written using alphabetic characters or base-ten decimal numbers.
• NC programs used to be stored as holes punched in paper or plastic tape.
• When an NC program is punched or recorded on tape, the information is translated to binary form.

Tape Format

• Punched tape is not often used for NC program storage today.
• Formats used in NC programming are still referred to as Tape Formats.
• Computer files containing NC programs are referred to as "tape files," "punch files," or "tape image files."
• Old machinery is still used in machine shops.

RS-274 Format

• Follows Electronics Industries Association (EIA) standard.
• Program information is contained in program lines called "Blocks".
• "Blocks" are punched into the tape using one or two tape code standards.
• RS-274 is a "variable block coding" format.
• The information contained in a block can be arranged in any order.

RS-244 Binary Coded Decimal (BCD)

• It is one of the two tape codes used for NC tapes.
• This became a standard early in the development of NC but had limited punctuation.
• Each hole represents the digit "1," while each blank represents the digit "0."
• The tape code translates alphabetic characters and base-ten numbers into binary code the controller requires.

RS-358 Format

• Government, telephone, and computer industries required a tape code containing upper and lower case letters.
• Existing tape coding formats were adequate for machining only.
• The American Standard Code for Information Interchange (ASCII) was adopted.
• EIA RS-358 was adopted and is also ISO, and ASCII is a subset of the ASCII code used in other applications, and it is dominant over RS-244.

Machine Control Unit (MCU)

• MCU is a microcomputer and related control hardware that stores and executes the program of instructions.
• It converts each command into mechanical actions of the processing equipment, one command at a time.
• The hardware of the MCU includes interface components, feedback control elements, and reading devices.
• Software in the MCU includes control system software, calculation algorithms, and translation software.
• MCU converts the NC part program into a usable format.
• MCU is a computer, the term computer numerical control (CNC) distinguishes it from older, hardwired electronics-based NC.
• Virtually all new MCUs are based on computer technology.

Processing Equipment

• The third basic component of an NC system that performs the actual productive work, such as machining
• Accomplishes the processing steps to transform the starting workpiece into a completed part
• Its operation is directed by the MCU, which in turn is driven by instructions in the part program.

Computer Numerical Control (CNC)

• Since the introduction of NC in 1952, there have been dramatic advancements in digital computer technology.
• The size and cost of digital computers have been reduced, while computational capabilities have increased.
• Today, NC means computer numerical control (CNC), an NC system whose MCU uses a dedicated microcomputer rather than a hardwired controller.
• Modern CNC controllers feature high-speed processors, large memories, solid-state memory, improved servos, and bus architectures.
• Early NC machines ran off punched cards and tape.
• Computers were introduced for programming due to the time and effort required to edit and change tapes.
• Computer is used as the Control Unit (CU) of modern Numerical Control machinery.
• The computer replaced the Tape Reader found on earlier NC machines.
• The program is loaded into and executed from the machine's computer.
• Computer Numerical Control (CNC) machines are the NC machines of today.
• Emphasis is on Manual Data Input (MDI) programming of CNC machinery.
• A Computer Numerical Control (CNC) machine has an on-board computer.
• The on-board computer is often referred to as the Machine Control Unit (MCU).
• Control units for NC machines are usually hard-wired.
• All machine functions are controlled by the physical electronic elements built into the controller.
• The on-board computer, is "soft" wired.
• In CNC, machine functions are encoded into the computer at the time of manufacture.
• Information will not be erased when the CNC machine is turned off.
• Computer memory holding such information is known as ROM (Read-Only Memory).
• The MCU usually has an alphanumeric keyboard for manually inputting part programs.
• Programs are stored in RAM.
• Programs can be played back, edited, and processed by the control.
• All programs residing in RAM are lost when the CNC machine is turned off.
• Programs can be saved on auxiliary storage devices such as punched tape, magnetic tape, or magnetic disk.
• Newer MCU units have graphics screens that can display the CNC program, cutter paths, and errors.

Goals of Numerical Control (NC) Development

• To increase production
• To reduce labor costs
• To make production more economical
• To do jobs impossible or impractical without NC
• To increase the accuracy of duplicate parts

Computer Numerical Control (CNC) – Additional Features

• Storage of more than one part program
• Various forms of program input
• Program editing at the machine tool
• Fixed cycles and programming subroutines
• Adaptive control
• Interpolation
• Positioning features for setup
• Acceleration and deceleration computations
• Communications interface
• Diagnostics
• Several DNCs can be networked, forming a large distributive NC system.
• A CNC machine consists of two major components: the Machine-Tool and the Controller, which is the Machine Control Unit (MCU).
• The MCU is an on-board computer.

Machine Control Unit (MCU)

• Generates, stores, and processes CNC programs.
• Machine also contains the machine motion controller in the form of an executive software program.

CNC Controller Manufacturers

• Fanuc
• Bridgeport
• Haas
• Cincinnati Milacron
• Mitsubishi
• Siemens

Machine Control Unit

• Each MCU is manufactured with a standard set of build in codes.
• Other codes are added by the machine tool builders.
• Program codes vary somewhat from machine to machine.
• Every CNC machine is a collection of systems coordinated by the controller.

Monitor/Keyboard/Control Panel for the 8025 CNC

• Function keys (SOFT-KEYS)
• Alphanumeric keyboard for editing programs
• ENTER: Allows information to be entered in the CNC memory
• RECALL: To call a program to access blocks within a program
• OP MODE: Allows a list of operating modes to be displayed on the screen.
• DELETE: Allows deletion of a complete program or a block of the programme.
• RESET: To revert the CNC to the initial conditions.
• CL: To delete characters one by one during the editing process
• INS: Allows characters to be inserted during the edition of a program block
• Arrow keys for moving cursor.

Operating Modes of the CNC

• AUTOMATIC: Execution of programs in a continuous cycle.
• Single BLock: Execution of part programs block by block.
• PLAY-BACK: Creation of a program in memory while the machine is being operated manually
• TEACH-IN: Creation and execution of a block without entering it into memory.
• DRY RUN: To check programs before actual execution of the first part.

CNC Machine

• CNC machine has more programmable features than older NC tape machinery.
• A CNC machine may be used as a stand-alone unit or in a network of machines like Flexible Machining Centers or Machining Cells.
• CNC machines are easier to program
• On board computer keyboard
• Electronic connector to transfer a program to CNC machine