HIGH SPEED MACHINING

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UNIT 6 : HIGH-SPEED MACHINING

6.1 INTRODUCTION

Machining with high speeds (HSM) is one of the modern technologies, whichin comparison with conventional cutting enables to increase efficiency,accuracy and quality of workpieces and ay the same time to decrease costsand machining time.

The first definition of HSM was proposed by Carl Salomon in 1931. He hasassumed that at a certain cutting speed which is 5 – 10 times higher then inconventional machining, the chip tool interface temperature will startdecrease.

Practically, it can be noted that HSM is not simply high cutting speed. Itshould be regarded as a process where the operations are performed withvery specific methods and production equipment. HSM is not only machiningwith high speed spindle because many applications are performed withconventional spindle speeds. HSM is often used in finishing in hardenedsteels with both high speeds and feeds. HSM can be called rather the HighProductive Machining when machining components in roughing to finishingand also in finishing to super-finishing in components of all the sizes.

6.2 LEARNING OUTCOMES

After completing the unit, students should be able to:1. Define High Speed Machining (HSM)2. Explain the potential of High Speed Machining (HSM)3. Describe the requirement of High Speed Machining (HSM)4. Discuss some common applications of High Speed Machining (HSM)5. Explain the High Speed Machining (HSM) processes6 Discuss some advantages of High Speed Machining (HSM)

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6.3 WHAT IS HIGH SPEED MACHINING

Definition of high-speed machining (HSM) means using cutting speeds that are significantly higher than

those used in conventional machining operations.

Some examples of cutting speed values for conventional and HSM arepresented in Table 6.1, according to data compiled by Kennametal Inc.1

TABLE 6.1 Comparison of cutting speeds used in conventional versus high-speedmachining for selected work materials.

With increasing demands for higher productivity and lower production costs,investigations have been carried out since the late 1950s to increase thematerial removal rate in machining, particularly for applications in theaerospace and automotive industries. One obvious possibility is to increasethe cutting speed.

The term high speed is relative. As a general guide, an approximate range ofcutting speeds may be defined as follows:

1. High speed: 600-1,800 m/min,2. Very high speed: 1,800-18,000 m/min,3. Ultrahigh speed: > 18,000 m/min.

Spindle rotational speeds today may range up to 40,000 rpm, although theautomotive industry, for example, generally limits them to 15,000 rpm forbetter reliability and less downtime should a failure occur. The spindle powerrequired in high-speed machining is generally on the order of 0.004 W/rpm(0.005 hp/rpm), whereas in traditional machining, it is in the range of 0.2 to0.4 W/rpm (0.25 to 0.5 hp/rpm).

Spindle designs for high speeds generally involve an integral electric motor.The armature is built onto the shaft and the stator is placed in the wall of thespindle housing. The bearings may be rolling element or hydrostatic; the latterrequires less space than the former.

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Other definitions of HSM have been developed to deal with the wide variety ofwork materials and tool materials used in machining.

One popular HSM definition is by the DN ratio — the bearingbore diameter (mm) multiplied by the maximum spindle speed(rev/min).

For high-speed machining, the typical DN ratio is between500,000 and 1,000,000.

This definition allows larger diameter bearings to fall within theHSM range, even though they operate at lower rotationalspeeds than smaller bearings.

Typical HSM spindle velocities range between 8,000 and 35,000 rpm,although some spindles today are designed to rotate at 100,000 rpm.

Another HSM definition is based on the ratio of horsepower tomaximum spindle speed, or hp/rpm ratio. Conventionalmachine tools usually have a higher hp/rpm ratio thanmachines equipped for high-speed machining. By this metric,the dividing line between conventional machining and HSM isaround 0.005 hp/rpm. Thus, high-speed machining includes 50hp spindles capable of 10,000 rpm (0.005 hp/rpm) and 15 hpspindles that can relate at 30,000 rpm (0.0005 hp/rpm).

Other definitions emphasize higher production rates andshorter lead times. In this case, important noncutting factorscome into play, such as high rapid traverse speeds and quickautomatic tool changes ("chip-to-chip" times of 7 s and less).

Requirements for high-speed machining include the following: high-speed spindles using special bearings designed for high

rpm operation, high feed rate capability, typically around 50 m/min, CNC motion controls with "look-ahead" features that allow the

controller to see upcoming directional changes and to makeadjustments to avoid "undershooting" or "overshooting" thedesired tool path,

balanced cutting tools, toolholders, and spindles to minimizevibration effects,

coolant delivery systems that provide pressures an order ofmagnitude greater than in conventional machining, and

chip control and removal systems to cope with the much largermetal removal rates in HSM.

Also important are the cutting tool materials. As listed in Table 6.1, varioustool materials are used for high-speed machining, and these materials arediscussed in the following chapter.

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6.4 APPLICATION OF HIGH SPEED MACHINING

Much research and development work has been carried out in the area ofhigh-speed machining (turning, milling, boring, and drilling) of aluminumalloys, titanium alloys, steels, and superalloys. Much data have beencollected regarding the effect of high cutting speeds on

(a) the type of chips produced,(b) cutting forces and power,(c) temperatures generated,(d) tool wear,(e) surface finish, and(f) the economics of the process.

These studies have indicated that high-speed machining can be economicalfor certain applications. Consequently, it is now implemented for themachining of aircraft-turbine components and automotive engines with five toten times the productivity of traditional machining. High-speed machining ofcomplex 3- and 5-axis contours has been made possible only recently byadvances in CNC control technology.

A major factor in the adoption of high-speed machining has been the desire toimprove tolerances in cutting operations. With high-speed machining, most ofthe heat generated in cutting is removed by the chip, so the tool and (moreimportantly) the work-piece remain close to ambient temperature. This isbeneficial because there is no thermal expansion or warping of the workpieceduring machining.

Applications of HSM seem to divide into three categories. One is in the aircraft industry, by companies such as Boeing, in

which long airframe structural components are machined from largealuminum blocks. Much metal removal is required, mostly by milling.The resulting pieces are characterized by thin walls and large surface-to-volume ratios, but they can be produced more quickly and are morereliable than assemblies involving multiple components and rivetedjoints.

A second category involves the machining of aluminum by multipleoperations to produce a variety of components for industries such asautomotive, computer, and medical. Multiple cutting operations meanmany tool changes as well as many accelerations and decelerationsof the tooling. Thus, quick cool changes and tool path control areimportant in these applications.

The third application category for HSM is in the die and moldindustry, which fabricates complex geometries from hard materials.In this case, high-speed machining involves much metal removal tocreate the mold or die cavity and finishing operations to achieve finesurface finishes.

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Much research and development work has been carried out on high-speedmachining (turning, as well as milling, boring, and drilling) of aluminum alloys,titanium alloys, steels, and superalloys. Considerable data have beencollected regarding the effect of high speeds on the type of chips produced,cutting forces, temperatures generated, tool wear, surface finish, and theeconomics of the process. These studies have indicated that high-speedmachining can be economical for certain applications, and consequently, it isnow implemented for machining aircraft turbine components and automotiveengines with five to ten times the productivity of traditional machining.Important factors in these operations are the selection of an appropriatecutting tool, the power of the machine tools and their stiffness, the stiffness oftoolholders and the workholding devices, spindle design for high power andhigh rotational speeds, the inertia of the machine-tool components, fast feeddrives, and the level of automation.

It is important to note, however, that high-speed machining should beconsidered basically for situations in which cutting time is a significant portionof the floor-to-floor time of the operation. Other factors such as noncuttingtime and labor costs are important considerations in the overall assessmentof the benefits of high-speed machining for a particular application.

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6.5 THE HIGH SPEED MACHINING PROCESSES

The following are important machine factors in high-speed operations:1. Power and stiffness of the machine tools,2. Stiffness of tool holders and workholding devices,3. Spindle design for high power and high rotational speeds,4. Inertia of the machine-tool components,5. Fast feed drives,6. Level of automation, and7. Selection of an appropriate cutting tool.

It is important to note, however, that high-speed machining should beconsidered almost exclusively for situations in which cutting time is asignificant portion of the floor-to-floor time of the operation. Other factors—such as noncutting time and labor costs—are also important considerationsin the overall assessment of the benefits of high-speed machining for aparticular application.

6.5.1 MACHINING PROCESS CAPABILITIES

Relative production rates in turning, as well as other cutting operationsthat are discussed in the rest of this chapter, are shown in Table 6.1.These rates have an important bearing on productivity in machiningoperations. Note that there are major differences in the production rateamong these processes. These differences are not only due to theinherent characteristics of the processes and machine tools, but arealso due to various other factors such as setup times and the typesand sizes of the workpieces involved. The proper selection of aprocess and the machine tool for a particular product is essential forminimizing production costs.

As stated above, the ratings in Table 6.1 are relative and there can besignificant variations in specialized applications. For example, heat-treated high-carbon cast steel rolls (for rolling mills) can be machinedon special lathes at material removal rates as high as 6000 cm3/min(370 in3/min) using multiple cermet tools. The important factor in thisoperation (also called high removal rate machining) is the very highrigidity of the machine tool (to avoid tool breakage due to chatter) andits high power, which can be up to 450kW.

The surface finish and dimensional accuracy obtained in turning andrelated operations depend on factors such as the characteristics andcondition of the machine tool, stiffness, vibration and chatter, processparameters, tool geometry and wear, cutting fluids, machinability ofthe workpiece material, and operator skill. As a result, a wide range ofsurface finishes can be obtained.

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Following are special features for HSM

FIGURE 6.1 Cusps produced by ball nose end mill

The smoothness of the machined surface is determined in large partby the height of the cusps between adjacent passes with a ballnosetool. Take a small step over and cusp height goes down. In this way,lighter depth-of-cut can contribute to drastically reduced polishingtime.

FIGURE 6.2 Headlamp reflector mold

The smooth surface of a headlamp reflector was milled in thehardened state at depth on the order of 0.004 inch axial and 0.002inch radial. Machining at 22,000 rpm and 600 ipm made this finecutting productive. Smoothness was also affected by the machiningtechnique. For the best finish, milling followed parallel passes that allwent in the same direction.

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FIGURE 6.3 Copper electrode for EDM

Here is an extreme example of repeatability in electrode machining.Each of the 192 identical, 0.0449 inch diameter dome in this copperelectrode were machined to a tolerance of ± 0.0002 inch. The domewere finished using a 0.079 inch diameter tool at 25,000 rpm. Totalmachining time including roughing was 1 hour, 44 minutes. PhotoCourtesy: Mikron Bostomatic.

FIGURE 6.4 Dies for forging operation

HSM can let milling serve as an alternative to EDM for making dies ormolds from the hardest material (50 + rc). HSM allows a forgingsupplier to produce die like this one in a single setup on a machiningcenter, where once a combination of milling and EDM was required.HSM produces the dies faster. HSM is also more accurate, becausefewer steps results in reduced error stacking. Tolerance band of 0.005inch have been reduced to 0.001 inch.

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FIGURE 6.5 Typical workpieces for HSM forging die for anautomotive component, moulds for a plastic bottle and aheadphone.

FIGURE 6.6 Thin wall EDM electrode

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Electrode with thin walls, ribs or other delicate features can bemachined in one piece in HSM, because the light cuts so littlepressure between the cutter and the workpiece.

FIGURE 6.7 Landing gear bulk head for a C-17 cargo plane.Photo courtesy Boeing.

This landing gear bulk head for a C-17 cargo plane used to beassembled from extrusions and sheet metal component. HSM allowedit to be produced complete at the machine tool. Total machining timewas about 12 hours. Doing away with casting can cut production leadtime in half. It can also make the process more flexible for designchanges. Changing the design of a casting requires hard tooling to bechanged. With a machined part, many changes require only that theprogram be changed.

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FIGURE 6.8 Aluminum fuel control housing

HSM enable to eliminate the process of casting and machining foraluminium internal part such as the above fuel control housing in asingle step starting from a solid block.

FIGURE 6.9 Spindle tradeoff

A high speed present a tradeoff between cutting force and cuttingspeed. First, the size of the motor is limited. High-speed spindlesgenerally have direct-drive motor, meaning the motor must fit insidethe spindles housing. Another limiting factor is the bearing. Highspeed spindle bearings trade stiffness for speed. This is one morereason why high speed machining generally employed light depth ofcut.

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6.5.2 HYBRID BALL BEARINGS

• Hybrid ball bearings take the place of all-steel ball bearings inmost high speed spindles.

• In a hybrid ball bearing, the race is still steel but the balls areceramic.

• Ceramic balls deliver more stability at high speeds.• The balls are lighter and stiffer, so they deflect less from

centrifugal force.• This improves efficiency and quiets vibrations.• Ceramic balls also deliver longer life.• The reduced deflection reduces stresses.• Plus the harder ceramic balls interact less with the surface of

the steel raceway.• The longer life is particularly important.• In any high speed spindle using ball bearings, the bearing is

generally the component that fails first.• Some machine tool and spindle makers offer spindles with

non-contact bearings that overcome some of the limitations oftraditional bearings.

• Many of these non-contact spindle designs are still beingproven.

• There are three non-contact bearing types:• Hydrostatic Bearings

• Definition: A fluid such as water supports the spindleshaft.

• Strength: Stiffness and low runout— Fluid pressureholds the shaft on centerline.

• Limitation: Inefficiency from viscosity— A largefraction of the motor’s power is lost just to overcomethis resistance.

• Air Bearings• Definition: Air pressure supports the spindle shaft.• Strength: Use delicate tools more effectively—• An air bearing spindle's superior runout characteristics

make it possible to machine very effectively withdelicate tools.

• Tiny holes can be drilled to an L/D ratio of well over 10without breaking the tool.

• Combining the low runout with high speeds can makemilling with small tools an effective alternative to EDMeven for very narrow or intricate features.

• Limitation: Low stiffness—• An air bearing is a low stiffness bearing best suited for

the lightest cuts.• Where high speed drilling generally implies light cuts

taken quickly, effective milling with an air bearing islimited to very light cuts.

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• Magnetic Bearing• Definition: The spindle shaft is supported by a

dynamic magnetic field.• Strengths:• Speed and low runout— The ability to use delicate

drilling and milling tools at high speed is comparable tothat of air bearings.

• Stiffness—The stiffness of a magnetic bearing can be digitallycontrolled. The magnetic field can be modeled to offerstiffness comparable to that of a ball bearing forexample.

• Simplicity—Electrical power creates the force supporting the shaft.Therefore, magnetic bearings have no need for aseparate system to deliver air or hydraulic fluid.

FIGURE 6.10 Hybrid bearing photo courtesy Fadal Machining Centers.

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6.6 ADVANTAGES OF HIGH SPEED MACHINING

One persistent trend throughout the history of metal machining has been theuse of higher and higher cutting speeds. In recent years, there has beenrenewed interest in this area due to its potential for faster production rates, shorter lead times, reduced costs, and improved part quality. Taking the weight out of some parts by machining them with thinner

walls is not a new idea. But the thinnest walls require light cuts thatmay be too time-consuming at lower speeds. HSM changes theequation by combining light cuts with high feed rates, HSM permitsthin-wall machining with a more attractive metal removal rate.

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6.7 SUMMARY

In this unit we have studied that1. Hard competition causes rapid development of the machining

technology and design of new solutions. High Speed Machining isproposed as an example.

2. HSM ensures high metal removal rates, boost productivity, improvesurface finish and eliminates the need of coolant.

3. In spite of high requirements of machining tools, HSM gives numerousbenefits. It allows shortening the production time and eliminates sometreatment (e.g. manual finishing) besides simultaneously retaining theaccuracy.

4. These advantages are decisive for the use of HSM for machining thepress dies.

5. Even though HSM has been known for a long time, the research is stillbeing developed for further improvement of quality and minimization ofcosts.

6.8 SELF TEST

1. List three common application of HSM.2. Important machine factors in high speed machining.3. List three types of non-contact bearing.4. What are the advantages of high speed machining?

6.9 REFERENCES

Serope Kalpakjian, Steven R. Schmidt (2001). Manufacturing Engineeringand Technology, (4th Edition), state: Prentice Hall.

Mikell P. Groover (2002). Fundamentals of Modern Manufacturing Materials,Processes, and Systems, (2nd Edition), state: John Wiley & Son, Inc.

John A. Schey, (year). Introduction to Manufacturing Processes, (3rd Edition),state: Mc Graw Hill.

E. Paul Degarmo, J T. Black, Ronald A. Kohser (2003). Materials andProcesses in Manufacturing, (9th Edition), state: John Wiley & Son, Inc.

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6.10 ANSWER

1. List three common application of HSM. Aircraft industries Mold and dies industries Machining of aluminium by multiple operations

2. Important machine factors in high speed machining. Power and stiffness of the machine tools, Stiffness of tool holders and workholding devices, Spindle design for high power and high rotational speeds, Inertia of the machine-tool components, Fast feed drives, Level of automation, and Selection of an appropriate cutting tool.

3. List three types of non-contact bearing. Hydrostatic bearing Air bearing Magnetic bearing

4. What are the advantages of high speed machining? faster production rates, shorter lead times, reduced costs, and improved part quality. HSM permits thin-wall machining with a more attractive

metal removal rate.