Chapter 7: Computer Numerical Control

Three Type of Software Programs used for CNC Systems

- Operating System Software -Machine Interface Software -Application Software

Types of Memory

- Read Only Memory (ROM) -Random Access Memory (RAM)

Advantages of CNC Machine

-Can be operating during two shifts -Can combine several operations into one setup -Setup time set to a minimum

Disadvantages of a CNC Machine

-Expensive -Higher Maintenance -Part Programming

Basic Components of a CNC System

-Part Program -Machine Control Unit -Process Equipment

Four Common Machining Operations

-Tooling -Drilling -Turning -Grinding

Three Possible Mechanical Errors

1- Play between the screw and the worktable 2-Backlash between gears 3-Deflection of machine components

Machine Control Unit

A microcomputer and related control hardware that stores the program and executes it by converting each command into a mechanical action

Open Loop

A positioning system w no feedback

CAD

Computer Aided Design

CAM

Computer Aided Manufacturing

Microcomputer

Computer Numerical Control is a Numerical Control systems whose MCU is a dedicated

Facing and Shoulder Facing

Cutting down an object in size

Parabolic and Cubic Interpolation

Free form curves using higher order equations

CAD/CAM

Integrating the design engineering and manufacturing engineering functions

Operating System Software

Interpolate the NC Part programmers and generates the corresponding control signals to drive the machine

ROM

Main memory, operating system software and machine interface programs are starting here

Two Types of Positioning Systems

Open Loop Close Loop

Process Equipment

Performs the actual productive work

Cutter Offset

Positioning your cutter with respect to its radius

RAM

Secondary Memory

Application Software

The NC Part programs that are written for machining operations

Origin

The part programmer must decide where the _____ of the coordinates axis system should be located

Part Program

The set of detailed step by step commands that direct the actions of the processing equipment

Feed Rate

The travel rate of the machine tool cutter or machine tool table

Concerning interpolation, the maximum error between the nominal (desired) surface and the actual (machined) surface can be controlled by the lengths of the individual line segments (T OR F)

True

Contouring is used for simultaneous control of two or more axis in machining operations (T OR F)

True

Close Loop

a positioning system with feedback

Threading

adding a thread to a hole

Circular Interpolation

circular arc path, tool path consists of a serious of straight line paths

Helical Interpolation

combines the circular interpolation schemes for two axises with w/ linear movement of the third axis

Optical Encoder

is a device for measuring rotational speed, position, and direction that consist of a higher source and a photodetector on either side of a disk

Computer Numerical Control (CNC)

is a form of programmable automation in which the mechanical actions of machine tool or other equipment are controlled by a program containing coded alphanumeric data

Mastercam

is a leading CAD/CAM software package for CNC part programming

CNC Turning Center

is a machine tool capable of performing multiple machining operations on a single workpiece in one setup

Machining

is a manufacturing process by which the geometry of the work is produced by removing the excess material

Repeatability

is the ability of the positioning system to return to a given addressable point that has been previously programmed

Interpolation

is the capability of moving only in discrete, small steps, rather than continuous motion

Operator Control Panel

is the interface by which the machine operator communicates to the CNC system

Accuracy

is the maximum possible error that can occur between the desired target point and the actual position taken by the system

Distributed Numerical Control (DNC)

is the networking of CNC Machine tools

Pocket Milling

is the type of milling when machines a shallow into a work part

Cutting Speed

is the velocity of the milling cutter relative to the work surface

Post Processor

is when a program is converting into a low level code that can be interpreted by the CNC controller

Adaptive Control

is when the MCU measures and analyzes machining variables such as spindle torque, power, and tool tip temperature, and adjust cutting speed and or feed rate to maximize machining performance

Computer Assisted Part Programming

is when the machine instructions are written in English like statements that are subsequently translated by the computer into the low level machine code that can be interpreted and executed by the machine tool conroller

Postion Set

is when the machine tool axis are referenced to the location of the fixture using a target point or set of target points on the part or fixture

Incremental Positioning

is when the position of the tool is defined relative to the previous location of that tool

Absolute Positioning

is when the position of the tool is defined relative tot he origin of the coordinate system

Contour Turning

making a three dimensional reproduction of the shape of a template

Linear Interpolation

straight line path

Depth of Cut

the distance the tool penetrates below the original surface of the work

Control Resolution

used in optical coders, is the distance separating two adjacent addressable points in the axis movement

Machine Interface Software

used to operate the communication link between the CPU and the machine tool to accomplish the CNC Auxiliary functions