CNC Machine Tools

CNC Machine Tools

Xun Xu (University of Auckland, NZ)
DOI: 10.4018/978-1-59904-714-0.ch008
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Abstract

The introduction of CNC machines has radically changed the manufacturing industry. Curves are as easy to cut as straight lines, complex 3-D structures are relatively easy to produce, and the number of machining steps that required human action has dramatically reduced. With the increased automation of manufacturing processes with CNC machining, considerable improvements in consistency and quality can be achieved. CNC automation reduced the frequency of errors and provided CNC operators with time to perform additional tasks. CNC automation also allows for more flexibility in the way parts are held in the manufacturing process and the time required to change the machine to produce different components. In a production environment, a series of CNC machines may be combined into one station, commonly called a “cell”, to progressively machine a part requiring several operations. CNC controller is the “brain” of a CNC machine, whereas the physical configuration of the machine tool is the “skeleton”. A thorough understanding of the physical configuration of a machine tool is always a priority for a CNC programmer as well as the CNC machine tool manufacturers. This chapter starts with a historical perspective of CNC machine tools. Two typical types of CNC machine tools (i.e. vertical and horizontal machining centres) are first discussed. Tooling systems for a CNC machine tool are integral part of a CNC system and are therefore elaborated. Also discussed are the four principal elements of a CNC machine tool. They are machine base, machine spindle, spindle drive, and slide drive. What letter should be assigned to a linear or rotary axis and what if a machine tool has two sets of linear axes? These questions are answered later in the chapter. In order for readers to better comprehend the axis and motion designations, a number of machine tool schematics are given.
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A Historical Perspective

Computer numerical control refers specifically to a computer “controller” that reads some sort of machine control (e.g. G-code) instructions and drives the machine tool. The controller does numerically directed interpolation of a cutting tool in the work envelope of a machine. Numerical controllers (NC) were developed in the late 1940s and early 1950s by John T. Parsons in collaboration with the MIT Servomechanisms Laboratory.

The first NC machines, which are frequently referred to as being of the first generation, had been previously designed for manual or fixed cycle operations. These machines had numerical control systems added, but only for numerical control on positioning the work relative to the tool. Considerable time was saved, yet the operator had to select the tools, speeds and feeds.

Second-generation machines are those on which material removal occurs at the same time as control of the work/tool relationship. These NC machines were also termed tape-controlled machines, because the information was stored on either punched tape or magnetic tape. Figure 1 illustrates the characteristics of a punched type. It is very cumbersome to edit the information at the machine; the machines had only very limited memory capacity.

Figure 1.

NC punched tape. ©1969 (or 2003) Industrial Press. Used with permission.

The development of computers has created the third-generation machines which are capable of an extended range of machining operations. These machines are commonly referred to as Computer Numerical Control and sometimes Direct Numerical Control (DNC) machines.

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Principles Of Numerical Control

A CNC system usually contains a machine-control unit (MCU) and the machine tool itself. The MCU is further divided into two elements: the data-processing unit (DPU) and the control-loops unit (CLU). The DPU processes the coded data and passes information on the position of each axis, its direction of motion, feed, and auxiliary function controls signals to the CLU. The CLU operates the drive mechanisms of the machine, receives feedback signals about the actual position and velocity of each of the axes, and announces when an operation has been completed. The DPU sequentially reads the data when each line has completed execution as noted by the CLU.

A DPU consists of some or all of the following parts,

  • data-input device such as a paper-tape reader (as in the old says), RS-232-C port, and so on;

  • data-reading circuits and parity-checking logic;

  • decoding circuits for describing data among the controller axes; and

  • an editor.

A CLU, on the other hand, consists of the following,

  • an interpolator that supplies machine-motion commands between data points for tool motion;

  • position-control-loops hardware for all the axes of motion, where each axis has a separate control loop;

  • velocity-control loops, where feed control is required;

  • deceleration and backlash take-up circuits; and

  • auxiliary function control, such as coolant on/off, gear changes, and spindle on/off control.

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Typical Cnc Machine Tools

CNC machine tool can be categorized as CNC mills, CNC lathes and CNC mill-turning centres. In fact, it has become customary to referring to the machines that are capable of multiple operations such as turning, milling, drilling, boring and tapping operations, and can work on more than one face of a component, as machining centres. They are among the most popular types of CNC machine tools these days. As the name itself implies, machining centres have a great deal of machining versatility.

Complete Chapter List

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Dedication
Table of Contents
Foreword
A.Y.C. Nee
Acknowledgment
Xun Xu
Chapter 1
Xun Xu
One of the key activities in any product design process is to develop a geometric model of the product from the conceptual ideas, which can then be... Sample PDF
Geometric Modelling and Computer-Aided Design
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Chapter 2
Xun Xu
Today, more companies than ever before are involved in manufacturing various parts of their end products using different subcontractors, many of... Sample PDF
CAD Data Exhange and CAD Standards
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Chapter 3
Xun Xu
Products and their components are designed to perform certain functions. Design specifi- cations ensure the functionality aspects. The task in... Sample PDF
Computer-Aided Process Planning and Manufacturing
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Chapter 4
Feature Technology  (pages 75-89)
Xun Xu
Throughout the course of the development of CAD, CAPP, and CAM systems, unambiguous representation of a design’s geometry and topology remain an... Sample PDF
Feature Technology
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Chapter 5
Feature Recognition  (pages 90-108)
Xun Xu
Conventional CAD models only provide pure geometry and topology for mechanical designs such as vertices, edges, faces, simple primitives, and the... Sample PDF
Feature Recognition
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Chapter 6
Feature Interactions  (pages 109-125)
Xun Xu
Feature interaction tends to have a wide range of consequences and effects on a feature model and its applications. While these may often be... Sample PDF
Feature Interactions
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Chapter 7
Xun Xu
Integrated feature technology promotes a closer connection between design and manufacturing through features. When machining features are... Sample PDF
Integrated Feature Technolog
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Chapter 8
CNC Machine Tools  (pages 165-187)
Xun Xu
The introduction of CNC machines has radically changed the manufacturing industry. Curves are as easy to cut as straight lines, complex 3-D... Sample PDF
CNC Machine Tools
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Chapter 9
Program CNCs  (pages 188-229)
Xun Xu
A CNC machine can be programmed in different ways to machine a workpiece. In addition to creating the cutting program, many other factors also need... Sample PDF
Program CNCs
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Chapter 10
Xun Xu
Technologies concerning computer-aided design, process planning, manufacturing and numerical control, have matured to a point that commercialized... Sample PDF
Integration of CAD/CAPP/CAM/CNC
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Chapter 11
Xun Xu
The integration model (Model B) as discussed in the previous chapter makes use of exchangeable neutral data formats such as IGES (1980). Neutral... Sample PDF
Integration Based on STEP Standards
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Chapter 12
Xun Xu
Function blocks are an IEC (International Electro-technical Commission) standard for distributed industrial processes and control systems (IEC... Sample PDF
Function Block-Enabled Integration
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Chapter 13
Xun Xu
In order to prepare manufacturing companies to face increasingly frequent and unpredictable market changes with confidence, there is a recognized... Sample PDF
Development of an Integrated, Adaptable CNC System
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Chapter 14
Xun Xu
A logical step after CNC machining is inspection. With inspections, Closed-Loop Machining (CLM) can be realized to maximize the efficiency of a... Sample PDF
Integrating CAD/CAPP/CAM/CNC with Inspections
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Chapter 15
Xun Xu
Today, companies often have operations distributed around the world, and production facilities and designers are often in different locations.... Sample PDF
Internet-Based Integration
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Chapter 16
Xun Xu
Companies that have been practicing CAD, CAPP, CAM, and CNC integration have now realized that there is a need to operate in a much broader scope... Sample PDF
From CAD/CAPP/CAM/CNC to PDM, PLM and Beyond
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Chapter 17
Key Enabling Technologies  (pages 354-393)
Xun Xu
While computers have proven to be instrumental in the advancement of product design and manufacturing processes, the role that various technologies... Sample PDF
Key Enabling Technologies
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