Machining Fundamentals 396 Copyright Goodheart-Willcox Co., Inc. Tooling changes are done by the operator. The big- gest advantage of the basic CNC lathe is its cost. Since tool changes are not automated on a basic machine, the cost of the hardware required for such automation is avoided. CNC lathes work well in a toolroom environment. Traditional machinists can operate them manually or, if a special run of multiple pieces is required, operators can be trained to create programs using conversational language. This allows operators to program machines without an intri- cate knowledge of CNC programming. The disadvantage of basic CNC lathes is that they are not suitable for high-volume production. Their speed, pre- cision, and repeatability are better than in manual machin- ing, but they cannot match the capabilities of more advanced CNC machines. CNC lathes can be equipped with gang-tool setups or automated turrets, described later in this chap- ter, to increase speed. However, their lack of precision and repeatability can create quality problems in high-precision manufacturing. 21.4.2 Gang-Tool Lathes Gang-tool lathes are a step up from the basic CNC lathe. Gang-tool lathes have multiple tools mounted along the cross-slide, Figure 21-11. This setup allows different tools to be used in a single program cycle. The tool to be used is controlled by the position of the cross-slide relative to the workpiece mounted in the spindle and collet. Although this type of setup can be added to a basic CNC lathe, the cross-slide travel on a basic machine is usually lim- ited to a maximum of 12″ to 18″, which does not allow room for several tools to be set up. Therefore, gang-tool setups typi- cally require a larger, more expensive machine. The need for a larger cross-slide travel and for higher-volume production usually drive the need to upgrade from a basic CNC lathe to a gang-tool lathe. The advantages of a gang-tool lathe include the ability to produce more parts per hour than a basic CNC lathe. Its cost, while higher than the basic CNC lathe, is still within the range of most manufacturers. The gang-tool setup allows for rapid tool changes and, as long as the different parts being machined are of the same or similar diameter, low change- over times. The disadvantages of the gang-tool lathe are its limited flexibility and the space required between the tools in the setup. If a changeover is required between parts whose diame- ters are drastically different, the entire toolholder setup on the cross-slide must be adjusted, adding to the time required to execute the changeover. This is especially true when there are several tools set up along the cross-slide. Gang-tool setups work well for machining small-diameter parts. Because the part turns between the tools on the cross- slide, the smaller diameters allow for more tools to be set up in gang-type fashion. Larger-diameter parts do not work well with a gang-tool setup, because the space needed between tools for the larger-diameter workpiece restricts the number of tools that can be set up on the cross-slide. 21.4.3 Turret Lathes CNC lathes that have a rotating turret attached to the cross- slide are called turret lathes. The computer controls the rotation of the turret and the movement of the cross-slide based on the information written into the CNC program. The part is held in a collet or chuck on the spindle. As the spindle is rotated, the turret rotates the appropriate tool into place, and the cross-slide feeds the tool into the workpiece, Figure 21-12. The apron feeds the cross-slide along the axis of the workpiece as it rotates. When this machining step is complete, the cross-slide withdraws from the workpiece, the turret rotates the next tool into place, and the next machin- ing step begins. This process repeats until the machining process is complete. Bridgeport Machines, Inc. Figure 21-10. A basic CNC lathe with 2-axis CNC control. Gang-tool setup Goodheart-Willcox Publisher Figure 21-11. Multiple tool setup on a CNC gang-tool lathe. The workpiece is held in a collet chuck.