CHAPTER 11

COMPUTER NUMERICAL CONTROL MACHINES

CHAPTER LEARNING OBJECTIVES

Upon completing this chapter, you should be familiar with the following:

Set up CNC lathes and milling machines.

Develop CNC lathe and milling machine programs using CAD/CAM.

As the hardware of an advanced technologybecomes more complex, new approaches to theprocessing of materials into useful products come intocommon use. This has been the trend in machiningprocesses in recent years. Parallel development in othertechnologies such as electronics and computers havemade available to the machine tool designer methodsand processes that can permit a machine tool to farexceed the capabilities of the most experiencedmachinist. To state it very simply, the MR rate has gonefrom the horse and buggy days to the space shuttle daysalmost overnight.

This chapter will give you some information on theimpact numerical control (NC) has had in the field ofmachining. It is only an overview and cannot beexpected to take the place of a programming manual fora specific machine, on-the-job training, or MR “C”school.

With the development of NC, a variety of inputmedia has been used to convey the information from theprint to the machine. The most common types of inputmedia used in the past were magnetic tape, punchedcards, and punched tape. However, most of the newmachines are controlled by computers. All of themachines on board Navy intermediate maintenanceactivities (IMAs) are computer numerical control(CNC).

There are many advantages to using NC machines.The greatest advantage in NC machining comes fromthe unerring and rapid positioning movements possible.An NC machine does not stop at the end of a cut to planits next move. It does not get fatigued and is capable ofuninterrupted machining, error free, hour after hour. Inthe past, NC machines were used for mass production.With the introduction of CNC, a qualified machinist canprogram and produce even a single part economically.

NC MACHINE SAFETY

All safety rules that apply to conventional machinetools also apply to NC machines. Always wearappropriate eye protection. Remove all rings, bracelets,watches, necklaces, and loose clothing that may becaught by moving or rotating equipment. Always wearsafety shoes.

Always keep the work area around the machineclean and clear of obstructions. Do not use compressedair to clean the machine or the area around it; chips canbecome dangerous missiles. Never use machinesurfaces as worktables. Use proper lifting methods tohandle heavy workpieces. Take measurements onlyafter the spindle has come to a complete stop. Neverhandle chips with your bare hands.

Before you start the machine, make sure thework-holding device and the workpiece are securelyfastened. When changing cutting tools, protect theworkpiece from damage, and protect your hands fromsharp cutting edges. Make sure cutting tools areinstalled correctly.

Do not operate any machine controls unless youunderstand their function. All NC machines haveelectrical compartments; keep the doors closed to keepout dirt and chips. Only authorized technicians shouldperform maintenance or repairs in these compartments.

To avoid damage to the machine or workpiece,check the tool path by making a dry run through theprogram; that is, let the machine make all thepositioning and cutting moves without a cutting toolinstalled.

Above all, follow all safety precautions posted onthe machine and any listed in Navy Occupational Safetyand Health (NAVOSH) Program Manual for ForcesAfloat, OPNAV Instruction 5100.19B.

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Figure 11-1.—CNC vertical spindle milling machine.

MACHINES

With the rapid development of machiningtechnology in the last few years, many new types ofCNC machines have been introduced. Many of these

machines are not simply CNC machines, but are CNCturning centers. Turning centers may have one or moreturret-type toolholders or have an automatic tool changesystem. For illustration purposes we will discuss a CNCvertical milling machine and a CNC lathe.

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Figure 11-2.—CNC lathe.

CNC VERTICAL SPINDLEMILLING MACHINE

Many of the CNC vertical spindle milling machinesbeing introduced to the Navy today are similar to theone shown in figure 11-1. It is basically a verticalmilling machine that has an onboard computer tocontrol its motion. Most of these machines are manu-factured with what is known as an R-8 spindle taper thatemploys a quick-change tool system.

The quick-change tool system consists of a quickrelease chuck (which is held in the spindle) and a set oftoolholders that hold the individual tools needed for aparticular part program. The chuck is a separate tool-holding mechanism that stays in the spindle. During a

tool change, the toolholder is removed from the chuckand a toolholder containing the next required tool isinstalled in its place. Many varieties of quick-changetool systems are available on the market.

Since this is only a brief overview, we will notdiscuss each part of the machine. Refer to yourmachine’s technical manual for more information.

CNC LATHE

Many CNC lathes (fig. 11-2) look like traditionalengine lathes. The lathe carriage rests on the ways. Theways are in the same plane and are parallel to the floor.On other CNC lathes, the turret tool post is mounted on

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Figure 11-3.—Slant bed for CNC lathe.

Figure 11-4.—CNC programming station.

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Figure 11-5.—Direct numerical control.

the opposite side of the saddle (fig. 11-3) and themachine has a slant bed. The slant bed allows the chipsto fall into the chip pan, rather than on tools or bedways.Despite this odd appearance, the slant bed CNC lathefunctions just like a conventional lathe.

As with CNC milling machines, you must have thecontroller manual for your machine’s controller toprogram it and operate the machine.

CNC TOOLING

There are many different tool-holding devices usedfor CNC machines. They can be as simple as a quick-change tool post or as complicated as an automatic toolchange system, but they all serve the same purpose. Thetool-holding devices for each shop will vary since eachmachine comes with different tooling and because shoppersonnel will purchase the tooling they prefer.

Cutting tools are available in three basic materialtypes: high-speed steel, tungsten carbide, and ceramic.Since we covered cutting tools in chapter 5, we will onlybriefly discuss them now. High-speed steel is generallyused on aluminum and other nonferrous alloys, whiletungsten carbide is used on high-silicon aluminums,steels, stainless steels, and exotic metals. Ceramicinserts are used on hard steels and exotic metals.Inserted carbide tooling is becoming the preferredtooling for many CNC applications.

COMPUTER-AIDEDDESIGN/COMPUTER-AIDED

MANUFACTURING

Computer technology has reached the stage wherea machinist can use computer graphics (fig. 11-4) to

design a part. The computer is used to place the designelements (lines, curves, circles, and so on) anddimensions on the computer display screen.

The machinist defines the geometry of the part tothe computer using one or more input devices. Thesedevices may be the keyboard, a mouse, a digitizer, or apen light. When the design has been edited and proofed,the computer can be instructed to analyze the geometryof the part and calculate the tool paths that will berequired to machine the part. Each tool path is translatedinto a detailed sequence of machine axes movementcommands that will enable a CNC machine tool toproduce the part. No engineering drawing is required.

The computer-generated instructions can be storedin a central computer for direct transfer to a CNCmachine tool for parts manufacture as shown infigure 11-5. This is known as direct numerical control(DNC). The data also can be stored on disk for futureuse.

The system that makes all this possible is computer-aided design/computer-aided manufacturing (CAD/CAM). There are several CAD/CAM softwareprograms. They are constantly being upgraded andmade more user friendly.

To state it simply, CAD is used to draw the part andto define the tool path. CAM is used to convert the toolpath into codes that the computer on the machine canunderstand.

NUMERICAL CONTROL SYSTEMS

A CNC machine consists of two major components:the machine tool and the controller, which is an onboardcomputer. These components may or may not be

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Figure 11-6.—A typical CNC controller.

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Figure 11-7.—Point-to-point angles and arcs.

manufactured by the same company. Figure 11-6 showsa typical controller. Each controller is manufacturedwith a standard set of built-in codes. Other codes areadded by the machine tool builders. For this reason,program codes differ somewhat from machine tomachine. Every CNC machine, regardless of manu-facture, is a collection of systems coordinated by thecontroller.

TYPES OF CONTROL SYSTEMS

Point-to-point machines move only in straightlines. They are limited in a practical sense to holeoperations (drilling, reaming, boring, and so on) andstraight milling cuts parallel to a machine axis. Whenmaking an axis move, all affected drive motors run atthe same speed. When one axis motor has moved theinstructed amount, it stops while the other motorcontinues until its axis has reached its programmedlocation. This makes the cutting of 45-degree anglespossible, but not arcs or angles other than 45 degrees.Arcs and angles must be programmed as a series ofstraight line cuts, as shown in figure 11-7.

There are two types of control systems used on NC A continuous-path machine can move its drivemachines: point-to-point systems and continuous-path motors at varying rates of speed while positioning thesystems. machine. Therefore, it can more easily cut arcs and

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Figure 11-8.—Continuous-path angles and arcs.

angles, as shown in figure 11-8. Most newer NCmachines are the continuous-path type.

SERVOMECHANISMS

It will be helpful to understand the drive systemsused on NC machinery. The drive motors on a particularmachine will be one of four types: stepper motors, dcservos, ac servos, or hydraulic servos. Stepper motorsmove a set amount of rotation (a step) every time themotor receives an electrical pulse. DC and ac servos arewidely used variable-speed motors found on small andmedium continuous-path machines. Unlike a steppermotor, a servo does not move a set distance. Whencurrent is applied, the motor starts to turn: when thecurrent is removed, the motor stops turning. The acservo is a fairly recent development. It can developmore power than a dc servo and is commonly found innewer CNC machines. Hydraulic servos, like ac or dcservos, are variable-speed motors. Because they arehydraulic motors, they can produce much more powerthan an electric motor. They are used on large NCmachinery, usually with an electric or pneumaticcontrol system.

Figure 11-9.—Cartesian coordinate system.

Figure 11-10.—Three-axis vertical mill.

CARTESIAN COORDINATE SYSTEM

The basis for all machine movement is the Cartesiancoordinate system (fig. 11-9). Programs in either inchor metric units specify the destination of a particularmovement. With it, the axis of movement (X, Y, or Z)and the direction of movement (+ or –) can be identified.Some machining centers may have as many as five orsix axes, but for our purposes we will only discuss threeaxes. To determine whether the movement is positive(+) or negative (–), the program is written as though thetool, rather than the work, is doing the moving.

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Figure 11-11.—Incremental positioning system.

Spindle motion is assigned the Z axis. This meansthat for a drill press or vertical milling machine the Zaxis is vertical, as shown in figure 11-10. For machinessuch as lathes or horizontal milling machines, the Z axisis horizontal.

POSITIONING SYSTEMS

There are two ways that machines positionthemselves with respect to their coordinate systems.These systems are called incremental positioning andabsolute positioning. With incremental positioning (fig.11-11) each tool movement is made with reference tothe prior or last tool position. Absolute positioning (fig.11-12) measures all tool movement from a fixed point,origin, or zero point. Use absolute dimensioning wherepossible because a mistake on the dimensions at onepoint will not be carried over to the dimensions at otherpoints. It is also easier to check for errors.

SETTING THE MACHINE ORIGIN

Most CNC machinery has a default coordinatesystem the machine assumes upon power-up, known asthe machine coordinate system. The origin of thissystem is called the machine origin or home zero

Figure 11-12.—Absolute positioning system.

location. Home zero is usually, but not always, locatedat the tool change position of a machining center. A partis programmed independent of the machine coordinatesystem. The programmer will pick a location on the partor fixture. This location becomes the origin of thecoordinate system for that part. The programmer’scoordinate system is called the local or part coordinatesystem. The machine coordinate system and the partcoordinate system will almost never coincide. Beforerunning the part program, the coordinate system mustbe transferred from the machine system to the partsystem. This is known as setting a zero point. In otherwords, the machine has a zero point that is alreadyprogrammed into it, but you can place the part to bemachined either in the chuck or on the table andestablish your zero point, and then the machine will usethat point as its zero point. This eliminates having to usefixtures and other complicated setups. You just put yourwork in the machine and by using the proper codes youtell the machine where the zero point is located.

As stated earlier in the chapter, this has only been abrief overview of CNC. It is a complicated subject thatmany books have been written about. To cover itcompletely you will need to have formal training andextensive on-the-job training. As the Navy expands intothe world of CNC this training will become morereadily available.

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CHAPTER 12

METAL BUILDUP

CHAPTER LEARNING OBJECTIVES

Upon completing this chapter, you should be able to do the following:

Explain the thermal spray system.

Explain contact electroplating.

Metal buildup is a rapid and effective method thatcan be used to apply almost any metal to a base material.It is used to restore worn mechanical equipment, tosalvage mismachined or otherwise defective parts, andto protect metals against corrosion. Compared tooriginal component replacement costs, metal buildup isa low-cost, high-quality method of restoration.

As you advance in the MR rating you must knowhow to prepare a surface for metal buildup and to set upand operate the equipment used in the thermal spraysystems and the contact electroplating process. In thischapter, we will briefly discuss the thermal spraysystems and contact electroplating. We will not coverthe actual spraying or plating processes; you will learnthem in classes you need to attend before you can be aqualified sprayer or plater.

Additional information on metalizing is inMIL-STD-1687A(SH), Thermal Spray Processes forNaval Ship Machinery Applications.

Additional information on electroplating is inMIL-STD-2197(SH), Brush Electroplating on MarineMachinery.

As with any shop equipment, you must observe allposted safety precautions. Review your equipmentoperators manual for safety precautions and anychapters of Navy Occupational Safety and Health(NAVOSH) Program Manual for Forces Afloat,OPNAV Instruction 5100.19B, that apply to the equip-ment. Also, read the sections in MIL-STD-1687A(SH)and MIL-STD-2197(SH) that cover safety.

THERMAL SPRAY SYSTEMS

There are four different thermal spray processes:wire-oxygen-fuel spray, wire-consumable electrodespray, plasma-arc spray, and powder-oxygen-fuel

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spray. All four generally perform the same basicfunction: they heat the wire or powder to its meltingpoint, atomize the molten material with eitherhigh-velocity gas or air, and propel it onto a previouslyprepared surface. In this chapter we will discuss thewire-oxygen-fuel and powder-oxygen-fuel sprayprocesses, with emphasis on the latter.

The rapid rate at which metal coatings can besprayed and the portability of the equipment haveincreased the use of thermal spray processes. Metalcoatings are especially useful to (1) rebuild worn shaftsand other machine parts not subject to tensile stress,(2) apply hard surfacing that must resist wear anderosion, and (3) protect metal surfaces against heat andcorrosion. Navy repair facilities use thermal sprayprocesses to coat metallic and nonmetallic surfaces withpractically any metal, metal alloy, ceramic, or cermetthat can be made in wire or powder form. (Cermet is astrong alloy of a heat-resistant compound, a metal usedespecially on turbine blades.)

APPLICATIONS

Thermal spray coatings have been approved byNAVSEA for several applications. Case-by-caseapproval is not needed for the folllowing applications,but the procedures used for these applications arelimited to those approved by NAVSEA:

Repair of static fit areas to restore originaldimensions, finish, and alignment

Repair of seal (including packing areas) torestore original dimensions and finish

Repair of fit areas on shafts to restore originaldimensions and finish (except for motorgenerator sets)