Machining Fundamentals 394 Copyright Goodheart-Willcox Co., Inc. method, the control is equipped with software that allows the operator to select operations from menus. The software converts the operator’s input into standard programming code for the control, which then drives the equipment to machine the parts. Conversational language is described in Chapter 22, CNC Programming Basics. Basic CNC milling machines are usually equipped with quick-change collets to allow the operator to change tools during the programmed cycle, Figure 21-5. A CNC program can be written with pauses to allow an operator to change tools manually. Quick-change collets are less rigid than nor- mal collets and can only be used for light- to medium-duty operations. The biggest advantage of the basic CNC milling machine is its cost. It is well suited to the toolroom environment and allows traditional machinists to operate it manually or in full CNC mode to create small batches of parts. However, the basic CNC milling machine is not suitable for high-volume production because it lacks the necessary speed, precision, and repeatability. 21.3.2 Machining Centers The spindle of a machining center can be either horizon- tal or vertical. Horizontal machining centers (HMCs) have the same features as manual horizontal milling machines. HMCs offer rigid, horizontally mounted spindles and stable workholding methods, Figure 21-6. Vertical machining centers (VMCs) have vertically mounted spindles and feature the same capabilities as manual vertical milling machines with the added benefit of computer control and automatic tool changing, Figure 21-7. VMCs can be equipped with additional axes of rotation for machining parts with complex features that cannot be Royal Products, Division of Curran Manufacturing Corporation Figure 21-5. Inserting an end mill mounted in a quick-change collet. Quick-change tool systems provide a fast method of changing tools on basic CNC milling machines. Spindle Machine table MHI Machine Tool U.S.A., Inc. Figure 21-6. A fi xed-bed horizontal machining center. This CNC milling machine has a 40-ton automatic tool changer and can handle work weighing up to 17,600 lb (8000 kg). Mazak Figure 21-7. A vertical machining center (VMC) with a 24-tool capacity.