Metal Machining - Theory and Applications Episode 1 Part 2 pps

If Figure 1.16b is compared with Figure 1.8b it is seen that for given workpiece size cross-traverse or work diameter a milling machine is likely to have from one fifth to one half the p

In Figures 1.16(a) and (b) the capacity of a milling machine is measured by its cross-traverse capacity This defines maximum workpiece size in a similar manner to defining the capacity of a turning centre by maximum work diameter (Figure 1.8) Figures 1.16(a) and (b) show that torque and power increase as cross-traverse cubed and squared respec-tively An assumption that machines are designed to accommodate larger diameter cutters

in proportion to workpiece size yields the D3and D2 relations derived in the previous paragraph

Machine tool technology 13

Fig 1.13 A traditional – column and knee – design and (right and below) partly-built and complete views of a modern

(bed) design of milling machine

If Figure 1.16(b) is compared with Figure 1.8(b) it is seen that for given workpiece size (cross-traverse or work diameter) a milling machine is likely to have from one fifth to one half the power capacity of a turning machine, depending on size This means that milling machines are designed for lower material removal rates than are turning machines, for a given size of work Figure 1.16(c), when compared with Figure 1.10(a), shows that milling machines are up to twice as massive per unit power as turning machines, reflecting the greater need for rigidity of the (more prone to vibration) milling process Figure 1.16(d), admittedly based on a rather small amount of data, shows little difference in price between milling and turning machines when compared on a mass basis Combining all these rela-tionships, the price of a milling machine is about 2/3 that of a turning machine for a 200

mm size workpiece but rises to 1.5 times the price for 1000 mm size workpieces The consequences for economic machining of these different capital costs, as well as the differ-ent removal rate capacities that stem from the differdiffer-ent machine powers, are returned to in Section 1.4

The D3and D2torque and power relationships found for milling machines are also observed, approximately, for drilling machines In this case, size capacity can be directly related to the maximum drill diameter for which the machine is designed Motor torques and powers, from catalogues, typically vary from 1 N m to 35 N m and from 0.2 kW to 4

kW as the maximum drill diameter that a drilling machine can accept rises from 15 mm to

50 mm The ranges of torques and powers just quoted are respectively 20% and 10% of the ranges typically provided for milling machines (Figure 1.16) In drilling deep holes, there

is a real danger of breaking the tools by applying too much torque, so machine capacity is purposely reduced Drilling machines also have much less mass per unit power than

14 Introduction

Fig 1.14 A 5-axis milling machine with interchangeable work tables

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milling machines: there is less tendency for vibration and the axial thrust causes less distortion than the side thrusts that occur on a milling cutter The prices of drilling machines are negligible compared with milling or turning On the other hand, the low power availability implies a much lower material removal rate capacity It is perhaps a saving grace of the drilling process that not much material is removed by it This too is taken up in Section 1.4

1.2 Manufacturing systems

The attack on non-productive cycle times described in the previous section has resulted in machine tools capable of higher productivity, but they are also more expensive If they had been available in the late 1960s, they would have been totally uneconomic as the manu-facturing organization was not in place to keep them occupied The flow of work in progress was not effectively controlled, so that batches of components could remain in a factory totally idle for up to 95% of the time, and even the poorly productive machines that were then common were idle for up to 50% of the time (Figure 1.3) Manufacturing tech-nology has, in fact, evolved hand in hand with manufacturing system organization, some-times one pushing and the other pulling, somesome-times vice versa

Manufacturing systems 15

Fig 1.15 A milling machine tooling magazine

In the late 1960s there were two standard forms of organizing the machine tools in a machine shop At one extreme, suitable for the dedicated production of one item in long runs – for example as might occur in converting sheet metal, steel bar, casting metal, paint and plastics parts into a car (Figure 1.17) – machine tools were laid out in flow lines or transfer lines One machine tool followed another in the order in which operations were performed on the product Such dedication allowed productivity to be gained at the price

of flexibility It was very costly to create the line and to change it to accommodate any change in manufacturing requirements

At the other extreme, and by far the more common, no attempt was made to anticipate the order in which operations might be performed Machine tools were laid out by type of process: all lathes in one area, all milling machines in another, all drills in another, and so

on In this so-called jobbing shop, or process oriented layout, different components were

16 Introduction

Fig 1.16 (a) Torque and (b) power as a function of cross-traverse capacity and (c) mass/power and (d) price/mass

rela-tions, from manufacturers’ catalogues, for mechanical (•) and basic CNC (o) milling machines and centres (+) Childs Part 1 28:3:2000 2:34 pm Page 16

manufactured by carrying them from area to area as dictated by the ordering of their oper-ations It resulted in tortuous paths and huge amounts of materials handling – a part could travel several kilometres during its manufacture (Figure 1.18) It is to these circumstances that the survey results in Figure 1.3 apply

It is now understood that there are intermediate layouts for manufacturing systems,

Manufacturing systems 17

Fig 1.17 Transfer line layout of an automotive manufacturing plant (after Hitomi, 1979), with a detail of a

transmis-sion case machining line

appropriate for different mixes of part variety and quantity (Figure 1.19) If a manufac-turer’s spectrum of parts is of the order of thousands made in small batches, less than 10

to 20 or even one at a time, then planning improved materials handling strategies is prob-ably not worthwhile The large amounts of materials handling associated with job shop or process oriented manufacture cannot be avoided Investment in highly productive machine tools is hard to justify Such a manufacturer, for example a general engineering workshop tendering for sub-contract prototype work from larger companies, may still have some mechanically controlled machines, although the higher quality and accuracy attainable from CNC control will have forced investment in basic CNC machines (As a matter of fact, the large jobbing shop is becoming obsolete Its low productivity cannot support a large overhead, and smaller, perhaps family based, companies are emerging, offering specialist skills over a narrow manufacturing front.)

18 Introduction

Fig 1.18 Materials transfers in a jobbing shop environment (after Boothroyd and Knight, 1989)

Fig 1.19 The spectrum of manufacturing systems (after Groover and Zimmers, 1984)

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If part variety reduces, perhaps to the order of hundreds, and batch size increases, again

to the order of hundreds, it begins to pay to organize groups or cells of machine tools to reduce materials handling (Figure 1.20) The classification of parts to reduce, in effect, their variety from the manufacturing point of view is one aspect of the discipline of Group Technology Almost certainly the machine tools in a cell will be CNC, and perhaps the programming of the machines will be from a central cell processor (direct numerical control or DNC) A low level of investment in turning or machining centre type tools may

be justified, but it is unlikely that automatic materials handling outside the machine tools (robotics or automated guided vehicles – AGVs) will be justifiable Cell-oriented manu-facture is typically found in companies that own products that are components of larger assemblies, for example gear box, brakes or coupling manufacturers

As part variety reduces further and batch size increases, say to tens and thousands respectively, the organization known as a flexible manufacturing system becomes justifi-able Heavy use can be justified of turning and/or machining centres and automatic handling between machine tools Flexible manufacturing systems are typically found in companies manufacturing high value-added products, who are further up the supply chain than the component manufacturers for whom cell-oriented manufacture is the answer Examples are manufacturers of ranges of robots, or the manufacturers of ranges of machine tools themselves (Figure 1.21) (Figure 1.19 also identifies a flexible transfer line layout – this could describe, for example, an automotive transfer line modified to cope with several variants of cars.)

The work in progress idle time (Figure 1.3) that has been the driver for the development

of manufacturing systems practice has been reduced typically by half in circumstances suitable for cell-oriented manufacture and by a further half again in flexible manufactur-ing systems (Figure 1.5(b)), which is in balance with the increased capacity to remove metal of the machine tools themselves (Figure 1.5(a))

1.3 Materials technology

The third element to be considered in parallel with machine technology and manufactur-ing organization, for its contribution to the evolution of machinmanufactur-ing practice, is the proper-ties of the cutting edges themselves There are three issues to be introduced: the material

Materials technology 19

Fig 1.20 Reduced materials flow through cell-oriented organizations and group technology (after Boothroyd and

Knight, 1989)

properties of these cutting edges that limit the material removal rates that can be achieved

by them; how they are held in the machine tool, which determines how quickly they may

be changed when they are worn out; and their price

1.3.1 Cutting tool material properties

The main treatment of materials for cutting tools is presented in Chapter 3 As a summary, typical high temperature hardnesses of the main classes of cutting tool materials (high speed steels, cemented carbides and cermets, and alumina and silicon nitride ceramics; diamond and cubic boron nitride materials are introduced in Chapter 3) are shown in Figure 1.22 The temperatures that have been measured on tool rake faces during turning various work materials at a feed of 0.25 mm are shown in Figure 1.23 If the work mater-ial removal rate that can be achieved by a cutting tool is limited by the requirement that its hardness must be maintained above some critical level (to prevent it collapsing under the stresses caused by contact with the work), it is clear that carbide tools will be more produc-tive than high speed steel tools; and ceramic tools may, in some circumstances, be more productive than carbides (for ceramics, toughness, not hardness, can limit their use) Also, copper alloys will be able to be machined more rapidly than ferrous alloys and than tita-nium alloys

Tools do not last forever at cutting speeds less than those speeds that cause them to collapse This is because they wear out, either by steady growth of wear flats or by the accumulation of cracks leading to fracture Failure caused by fracture disrupts the machin-ing process so suddenly that conditions are chosen to avoid this Steady growth of wear eventually results in cutting edges having to be replaced in what could be described as preventative maintenance It is an experimental observation that the relation between the

lifetime T of a tool (the time that it can be used actively to machine metal) and the cutting speed V can be expressed as a power law: VT n = C It is common to plot experimental

life/speed observations on a log-log basis, to create the so-called Taylor life curve Figure 1.24 is a representative example of turning an engineering low alloy steel at a feed of

20 Introduction

Fig 1.21 Flexible manufacturing system layout

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0.25 mm with high speed steel, a cemented carbide and an alumina ceramic tool (the data for the ceramic tool show a fracture (chipping) range) Over the straight line regions (on a

log-log basis), and with T in minutes and V in m/min

for cemented carbide VT0.25 = 150 (1.3b)

These representative values will be used in the economic considerations of machining in Section 1.4 A more detailed consideration of life laws is presented in Chapter 4 The

constants n and C in the life laws typically vary with feed as well as cutting speed; they also

depend on the end of life criterion, reducing as the amount of wear that is regarded as allow-able reduces At the level of this introductory chapter treatment, it is not straightforward to discuss how the constants in equations (1.3) may differ between turning, milling and drilling practice It will be assumed that they are not influenced by the machining process Any important consequences of this assumption will be pointed out where relevant

Materials technology 21

Fig 1.22 The hardness of cutting tool materials as a function of temperature

Fig 1.23 Maximum tool face temperatures generated during turning some titanium, ferrous and copper alloys at a

feed of 0.25 mm (after Trent, 1991)

1.3.2 Cutting tool costs

Apart from tool lifetime, the replacement cost of a worn tool (consumable cost) and the time to replace a worn-out tool are important in machining economics Machining economics will be considered in Section 1.4 Some different forms of cutting tool have already been illustrated in Figure 1.12 High speed steel (HSS) tools were traditionally ground from solid blocks Some cemented carbide tools are also ground from solid, but the cost of cemented carbide often makes inserts brazed to tool steel a cheaper alternative Most recently, disposable, indexable, insert tooling has been introduced, replacing the cost and time of brazing by the cheaper and quicker mechanical fixing of a cutting edge in a holder Disposable inserts are the only form in which ceramic tools are used, are the domi-nant form for cemented carbides and are also becoming more common for high speed steel tools Typical costs associated with different sizes of these tools, in forms used for turning, milling and drilling, are listed in Table 1.1

There are three sorts of information in Table 1.1 The second column gives purchase prices It is the third column, of more importance to the economics of machining, that gives the tool consumable costs A tool may be reconditioned several times before it is thrown

away The consumable cost Ct is the initial price of the tool, plus all the reconditioning costs, divided by the number of times it is reconditioned It is less than the purchase price (if it were more, reconditioning would be pointless) For example, if a solid or brazed tool can be reground ten times during its life, the consumable cost is one tenth the purchase price plus the cost of regrinding If an indexable turning insert has four cutting edges (for example, if it is a square insert), the consumable cost is one quarter the purchase price plus the cost of resetting the insert in its holder (assumed to be done with the holder removed from the machine tool) If a milling tool is of the insert type, say with ten inserts in a holder, its consumable cost will be ten times that of a single insert

In Table 1.1, a range of assumptions have been made in estimating the consumable costs: that the turning inserts have four usable edges and take 2 min at £12.00/hour to place in a holder; that the HSS milling cutters can be reground five times and cost £5 to

£10 per regrind; that the solid carbide milling cutters can also be reground five times but the brazed carbides only three times, and that grinding cost varies from £10 to £20 with

22 Introduction

Fig 1.24 Representative Taylor tool life curves for turning a low alloy steel

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