Machining Fundamentals, 10th Edition Page 397 (419 of 672)

Chapter 21 Introduction to CNC Machining 397 Copyright Goodheart-Willcox Co., Inc. The advantages of a turret lathe are its accuracy and speed. With the proper toolholding technology, the machine can be changed over from one part to another in a minimal amount of time. The disadvantages of turret lathes include their cost and the skill set needed by operators to operate the equipment effectively. Tooling changes and part setups and change- overs are part of the daily operation of this machine, so the operator must be familiar with the machine controls. Often, the operator has to make frequent adjustments to the pro- gram to modify tooling offsets. The level of operator train- ing required is therefore higher than for basic and gang-tool CNC lathes. 21.4.4 Swiss-Type Turning Centers Complex details require a high level of precision in their machining. When a part to be machined includes elaborate detail, Swiss-type turning centers are used, Figure 21-13. These machines use a guide bushing to hold the workpiece tightly at the point of machining. The tools are not fed along the cylindrical axis of the part being machined. Rather, the main spindle behind the guide bushing moves the part along the cylindrical axis. The tools are fed along a direction per- pendicular to the cylindrical axis of the part, as with cross- slide motions on other lathe types, Figure 21-14. The tools on a Swiss-type turning center can be set up gang-tool style, or a turret can be used. The advantage of a Swiss-type machine is its ability to accurately machine small, complex parts with intricate details, Figure 21-15. The biggest disadvantage is cost. The machines are extremely expensive, and the raw materials needed to take advantage of the capabilities of this type of equipment are also expensive. The guide bushing diameter is not adjustable. To ensure that the part does not deflect during the machining operation, the stock must be ground to a precise diameter before machining with the Swiss-type turning center can begin. This increases the cost of the raw materials significantly. Swiss-type turning centers should be considered when the parts being produced require a high level of accuracy and precision. Tool presetter 12-station turret CNC Software, Inc. ©Copyright 1983–2013. All rights reserved. Figure 21-12. A large turret lathe machining a camshaft. The workpiece is held in a 3-jaw universal chuck. The cutting tools are held in a 12-station turret and are rotated into place. The tool presetter measures the offset lengths of the tools held in the turret for locating purposes. Swiss-type turning centers Bar feeders Sub spindle Gang-tool style setup Mack Molding Co., Arlington, Vermont Figure 21-13. These 7-axis Swiss-type turning centers have 21 tools each and are coupled with bar feeders to automatically load precision-ground stock into the machine. Mack Molding Co., Arlington, Vermont Figure 21-14. This Swiss-type turning center comes equipped with an additional axis on the sub spindle, allowing the center to machine both ends of the workpiece simultaneously. This feature, along with a rapid feed rate, signifi cantly reduces cycle times.

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Chapter 21 Introduction to CNC Machining 397 Copyright Goodheart-Willcox Co., Inc. The advantages of a turret lathe are its accuracy and speed. With the proper toolholding technology, the machine can be changed over from one part to another in a minimal amount of time. The disadvantages of turret lathes include their cost and the skill set needed by operators to operate the equipment effectively. Tooling changes and part setups and change- overs are part of the daily operation of this machine, so the operator must be familiar with the machine controls. Often, the operator has to make frequent adjustments to the pro- gram to modify tooling offsets. The level of operator train- ing required is therefore higher than for basic and gang-tool CNC lathes. 21.4.4 Swiss-Type Turning Centers Complex details require a high level of precision in their machining. When a part to be machined includes elaborate detail, Swiss-type turning centers are used, Figure 21-13. These machines use a guide bushing to hold the workpiece tightly at the point of machining. The tools are not fed along the cylindrical axis of the part being machined. Rather, the main spindle behind the guide bushing moves the part along the cylindrical axis. The tools are fed along a direction per- pendicular to the cylindrical axis of the part, as with cross- slide motions on other lathe types, Figure 21-14. The tools on a Swiss-type turning center can be set up gang-tool style, or a turret can be used. The advantage of a Swiss-type machine is its ability to accurately machine small, complex parts with intricate details, Figure 21-15. The biggest disadvantage is cost. The machines are extremely expensive, and the raw materials needed to take advantage of the capabilities of this type of equipment are also expensive. The guide bushing diameter is not adjustable. To ensure that the part does not deflect during the machining operation, the stock must be ground to a precise diameter before machining with the Swiss-type turning center can begin. This increases the cost of the raw materials significantly. Swiss-type turning centers should be considered when the parts being produced require a high level of accuracy and precision. Tool presetter 12-station turret CNC Software, Inc. ©Copyright 1983–2013. All rights reserved. Figure 21-12. A large turret lathe machining a camshaft. The workpiece is held in a 3-jaw universal chuck. The cutting tools are held in a 12-station turret and are rotated into place. The tool presetter measures the offset lengths of the tools held in the turret for locating purposes. Swiss-type turning centers Bar feeders Sub spindle Gang-tool style setup Mack Molding Co., Arlington, Vermont Figure 21-13. These 7-axis Swiss-type turning centers have 21 tools each and are coupled with bar feeders to automatically load precision-ground stock into the machine. Mack Molding Co., Arlington, Vermont Figure 21-14. This Swiss-type turning center comes equipped with an additional axis on the sub spindle, allowing the center to machine both ends of the workpiece simultaneously. This feature, along with a rapid feed rate, signifi cantly reduces cycle times.

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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.

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Machining Fundamentals 398 Copyright Goodheart-Willcox Co., Inc. 21.5 CNC Safety The only difference between CNC machines and their manu- ally operated counterparts is that the CNC machines are automated. They are controlled by a computer system as it executes a computer program written by a human. There- fore, the safety warnings and cautions related to manual machines are applicable to CNC equipment as well. Review Chapter 3, Shop Safety, for more information. The following are additional safety concerns related solely to CNC equipment: • Before operating a machine for the first time, obtain training on the machine. You must have a working knowledge of the machine and safe operating prac- tices. Different machines may have different operat- ing mechanisms. Read the operation and maintenance manuals provided with each machine. • Do not remove or bypass guards, doors, and other safety devices on any CNC machine so that they do not work as they were designed. • Know the sequence of operations executed by any program run by the machine you are operating. This allows you to recognize problems with the machine when it does something out of the ordinary. • When changing tools manually, wait for the machine axes and the spindle to come to a complete stop. • Know where the emergency stop buttons are located. One will always be located on the control panel and others may be located in other areas around the CNC Software, Inc. ©Copyright 1983–2013. All rights reserved. Figure 21-15. A small, intricate component made of brass being machined on a Swiss-type turning center with a gang-tool setup. machine. The emergency stop brings the program to an immediate halt. • Know how to manually jog the machine to a point clear of the workpiece. • Know how to restart the program from its beginning and, if multiple tools are part of the program, how to manually change the tools. This will have to be done in the event of an emergency stop. • Be familiar with all error codes and cautions that may appear on the control screen. Know what steps to take to correct each error code. • Obtain training in the use of all auxiliary devices on the machine. Many machines have coolant systems, automated lubrication systems for spindles and other moving parts, and automated chip handlers. Know how to maintain these. • Never try to remove chips while the machine is run- ning, even if the chips appear to be clear of the work area. • Cycle all programs once without a part in place dur- ing setup (this is called a dry cycle). It is best to run the program in single-step mode, with the feed rate overridden to its lowest range. This can prevent an accidental machine crash. • CNC programs can also be verified by machining a sample part from plastic, wax, or a similar inexpen- sive material, Figure 21-16. • Be careful around automated tool changers. They can move without warning and trap an operator against a part of the machine. Automated tool changers are usually pneumatically powered, so take care around compressed air systems as well. CNC Software, Inc. ©Copyright 1983–2013. All rights reserved. Figure 21-16. CNC programs can be verifi ed by producing a part in an inexpensive material, such as plastic or wax.

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