Chapter 21 Introduction to CNC Machining 393 Copyright Goodheart-Willcox Co., Inc. The biggest disadvantage of using CNC is the initial cost of the machine. As costs continue to decrease, CNC technology is becoming more affordable. However, some small companies still lack the resources to purchase the necessary equipment. 21.3 CNC Milling Machines A basic CNC milling machine is a traditional milling machine with built-in CNC capabilities. Machining centers are CNC milling machines equipped with an automatic tool changer (ATC), a device that automatically changes and stores the tools, Figure 21-3. The machining cycle pauses for the automated tool change process, but the automated tooling change takes much less time than a manual tooling change. Most machining centers are enclosed. The advantages of CNC milling machines are that they increase part output through faster operation compared to manual machining, and their reduced variation yields a higher quality product. Options such as additional rotating axes can be added to increase the versatility of the machine, but these options also significantly increase the cost. The greatest disadvantage of CNC milling machines is their high cost. Various types of CNC milling machines are available, with prices ranging from moderately expensive to hundreds of thousands of dollars or more. 21.3.1 Basic CNC Milling Machines The basic CNC milling machine, like its manual counter- part, has three axes of movement and requires tooling to be changed manually. The machine shown in Figure 21-4 is an example of a basic CNC milling machine. It can be operated both manually and in full CNC mode. Traditional machinists can operate a basic CNC milling machine manually, or they can be trained to create programs using a method known as conversational language. In this Rotary table Trunnion table CNC Software, Inc. ©Copyright 1983–2013. All rights reserved. Figure 21-2. A helical impeller machined on a vertical machining center. The complex contours of this part can be machined thanks to the additional axes of rotation provided by the tilting trunnion table and rotary table under CNC control. Tool changer Tool magazine Goodheart-Willcox Publisher Figure 21-3. This automatic tool changer (ATC) holds the tools in a tool magazine and uses a pivot insertion system to quickly and automatically change tools on a horizontal machining center. Bridgeport Machines, Inc. Figure 21-4. A Bridgeport 3-axis CNC vertical milling machine. This basic CNC milling machine can be controlled both manually and in full CNC mode.