21st Century Manufacturing Episode 2 Part 6 pptx

Empirically, it can be shown that tool life decreases with increases in cutting speed, as shown in Figure 7.13.. Tool life,T,is also sensitive to feed rate,fwith Vanddheld constant, and

Secondary zone of hea

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Wmk materiale-e- Prim,." jl'l roo with bhrnt tool

•.- heat _.-_ • ~ Tertiary zone of heat generaI

source fheatgeneration.

F1pre7.12 Regions o

Experimental (dashedlines]

Theory (full lines)

Diamond is not the stable form of carbon at atmospheric pressure Fortunately, it 1,5OO°C.In contact with iron, however, graphitization begins just over 730°C,and oxygen begins to etch a diamond surface at about 830°C

It is also disappointing that diamond tools are rapidly worn when cutting nickel and aerospace alloys Generally, they have not been recommended for machining high-melting-point metals and alloys where high temperatures are generated at the interface The family with the highest hot hardness is the alumina-based (AI203)group, and these are favored for high-speed facing of cast iron Cast iron machines with a well-controlled "shower" of short chips that facilitate high-speed cutting However, the

Al203-based materials are also very brittle, and they have limited use for cutting steels Empirically, it can be shown that tool life decreases with increases in cutting speed, as shown in Figure 7.13

It turns out that the prolific F W Taylor also took great interest in this topic The optimization of cutting speeds fell in naturally with his interests in the principles

of scientific management By the time the results of hisTaylor equation were applied

to the Midvale Steelworks, a productivity gain of 200% to 300% was achieved on the machine tools, which also created a 25% to l00'Yo increase in the wages of the

is obtained for most tool-work combinations

This observation led to a wide series of plots of the type shown in Figure 7.14 The famous "Taylor equation" relates the cutting speed,V,and tool life,T,to the con-stants nand C, particular to each tool work combination

Tool life,T,is also sensitive to feed rate,f(with Vanddheld constant), and to

depth-of-cut (with V and fheld constant), see Figure 7.15.

Speed (It/min)

logT General observation

: straight line

logY Flgure 7.14Log-log plot of tool life versus cutting speed

1

T=(71)~anddheldcon't.nt

1

T=(72)~.ndfh"ldoon't.nt

Figure 7.15 Tool life variations with feed rate and depth-of-cut

However, it is found that

This physically means that with(n2>"1> n),changes in cutting speed, rather than feed rate or depth-of-cut, will result in greater amounts of tool wear 7.3.4Significance to Work Holding and FIX1uring

The forcesF c and F Tgenerated during milling or turning are resistedbya family of

work-holding devices called-depending on the context and the specific machining

process-c-nxtures, jigs, clamps, vises, and chucks, The accuracy that can be obtained

in a particular machining process is directly related to the reliability of these

work-parts Fixtures are a subset of work-holding units designed to facilitate the setup and

holding of a particular part The fixture must conform to specific surfaces on the part

so that all 6 degrees of freedom are stabilized Forces and vibrations inherent in the manufacturing process must be resistedbythe fixture Ajigsupports the work like a

might contain a hardened bushing to guide the drill to a precise location on the part being processed

Both fixtures and jigs are usually custom configured to suit the part being man-ufactured Hence tooling engineers have endeavored to give these devices flexibility and modularity so that they can be applied to the greatest possible set of part styles toward production in small batch sizes (Miller, 1985) Batch production represents 50% to 75% of all manufacturing, with 85% of the batches consisting of fewer than ulanzing fixtures and jigs can help to minimize the setup costs per unit produced Developments in microprocessor-based controllers, sensors, and holding devices in the last decade have made this goal more feasible

Today's fixture designers depend on heuristics such as the "3·2-1rule," which states that a part will be immobilized when it is rigidly contacting six points (Hoffman, 1985) Three points define a plane called the primary datum, and two point contact These six locations fix the part position relative to the cutter motions (see Figure 7.16)

If friction is considered, fewer contacts can be used, so long as the applied cut-ting forces are not excessive The choice of these datum points is often left up to the their datums explicitly stated in the part drawing These datums are also used to ness, or concentricity Information on tolerancing can be found in Hoffman (1985) Once a suitable set of contact locations on the part has been determined, a rigid structure must be devised to hold these contacts in space Also, the contact type must

be selected Finally, a set of clamps is chosen that apply forces to the part so that it will remain secured For complex parts, the final fixture will be a custom designed device that only works for that part with minor variations

A fixture is composed ofactive elements that apply clamping forces andpassive elements that locate or support the part For simple parts a custom designed fixture

is not needed Instead, simpler setups are built that use at least one active element located Since the loaded position of each part of the same type must be measured,

Tertrary datum poinr / Primary datum plane Secondary datum line

the time cost of using a simpler setup balances against the cost of building a special fixture

Figure 7.17 shows some common passive fissuring elements The primary datum can be definedbya subplate that is fixed to the machine tool When angled

features are called for in the part drawing, a sine plate may be used It can reorient

the primary datum to any angle from 0 to 90 degrees They are usually set manually Angle blocks or plates perform the same function but are not adjustable Parallel and riser blocks can lift the part up a precise amount Fixed parallels can be used as a

"fence" to prevent motion in the horizontal plane

Vee blocks give two line contacts so that cylindrical parts can be fixtured.

Spherical and shoulder locators are used to establish a vertical or horizontal position when the surface being clamped is wavy or when datums are explicitly defined in the part drawing

The parallel-sided machining vise is a versatile tool capable of both active

clamping and locating prismatic workpieces (Figure 7.18) Special jaws can be that is fixed and one that moves toward the fixed portion of the vise When the vise

Sine plate Right angle plate

"tiIIiJ 8 rJ e

Parallels Vee blocks Spherical Flat Shoulder

locator locator locator

~

Subplate

Figure7.}7 Passive Iixturing elernents

Toeclllmp Vise

jaws have a shoulder and one additional stop, all degrees of freedom are eliminated Under light machining loads, these additional locators may not be necessary

Chucks provide an analogous function for rotationally symmetric parts They have multiple jaws that move radially and, in some cases, independently A chuck is used in Figure 7.3 to locate and clamp the part Although such three-jaw chucks have limited accuracy due to finite rigidity and clearances similar to the vise, their flexi-bility makes them the standard lathe fitting

Toe clamps and side clamps provide a smaller area of contact and do not locate the part Toe clamps exert vertical forces on the workpiece and are often used when clamps provide supplemental horizontal forces that support the part against stops For safety reasons, they are rarely used alone since the part may become dislodged The nature of the contact between the part and the fixture or chuck establishes the maximum clamping force that can be exerted on the part without crushing it and the number of degrees of freedom effectively removed A greater area of contact means that the clamping forces can be lower One area of research has been in devel-oping conformable fixtures that increase the area of contact for irregular workpiece shapes Line contact and point contact induce greater stresses in the material but pro-sibility to the component being machined This is a measure of how many faces of the The capacity of the fixture to handle different part shapes is a measure of its reconfig-urability Other important qualities for fixtures are reliability, precision, and rigidity

The development of new workpiece fixturing devices is an important area of research As a first example, modular tooling sets (Figure 7.19) are used extensively

floor They were first invented in Germany in the 19405

The basic concept of "modular" fixturing is well known: these systems typically include a square lattice of tapped and doweled holes with spacing toleranced to ments that can be rigidly attached to the lattice using dowel pins or expanding man-fitted out with a complement of active and passive fixturing elements and fasteners The elements are assembled in "Erector set" fashion, using standard parts Extraordinary part shapes might require special elements to be machined Use of these sets can speed the design and construction of fixtures for small batch sizes.The sets can also reduce the cost of storing old fixtures, since they can be disassembled and reused The setups can be rapidly replicated, once they have been recorded with photographs and notes In order to achieve sufficient precision in the assembled fix-ture, all component surfaces are hardened and ground

When using modular fixturing, there is a general need for systematic algo-rithms for automatically designing fixtures based on CAD part models Although the lattice and set of modules greatly reduce the number of alternatives, designing a suit-able fixture currently requires human intuition and trial and error Furthermore, if the set of alternatives is not systematically explored, the designer may settle upon a

Figure7.!9 Modular tooling kit.

Goldberg and colleagues (Wagner et al., 1997) have thus considered a class of modular fixtures that prevent a part from translating and rotating in the plane The implementation is based on three round locators each centered on a lattice point, and one translating clamp that must be attached to the lattice via a pair of unit-spaced holes, thus allowing contact at a variable distance along the principal axes of the lattice World Wide Web users may now use any browser to "design" a polygonal

along with images showing the part as the fixture will hold it in form closure for each solution

The current version of FixtureNet is described in Section 7.12 The links on the Website include an online manual and documentation This initial service provides

an algorithm that accepts part geometry as input and synthesizes the set of all fixture first fixture synthesis algorithms that is complete, in the sense that it guarantees finding an admissible fixture if one exists Planning agents can call upon FixtureNet directly and explore the existence of solutions, practical extensions to three dimen-sions, and issues of fixture loading

As a second example, quick change tooling is helpful in factories that use

exten-sive automated material handling It can also reduce the setup time at the machining workstation For instance, the automated pallet changer receives pallets of standard size and connections, carrying a diverse array of part shapes It can act as the tool

to a mill with no refixturing time, potentially on material handling equipment with this same receiver Standard connections to the equipment can be made in seconds

In flexible manufacturing systems (FMS), these pallets are built up and loaded off-line at manual workstations

As a third example, hydraulic clamping systems have been developed to replace manually actuated active elements The oil charged cylinders provide a much created that result in self-leveling supports, sequenced clamping order, and precise clamping forces When accumulators are used, the hydraulic power source can be dis-connected without a reduction in clamping force

As a fourth example, the automatically reconfiguring fixture system described

by Asada and colleagues (1985) is intended for sheet-metal drilling operations The tical supports The supports feature a lock mechanism that permits them to be assem-slid into position along the tee slot The height of the locators can also be set by the the system decomposes this into a series of manipulation tasks

As a final example, the reference free part encapsulation (RFPE) system is

designed to "free up" the design space and greatly expand the possible range of the parts that can be designed and then machined (Sarma and Wright, 1997) RFPE allows the machining of parts with thin spars and narrow cross sections RFPE uses

a biphase material (Rigidax) to totally encapsulate a workpiece and provide support during the machining process (Figure 7.20)

After the first side of a component has been machined, the Rigidax is poured around the features, returning the stock to the encapsulated, prismatic, bricklike appearance that can be easily reclamped Machining then continues on the other sides This iterative process at the manufacturing level of abstraction (encapsulate! machine side-ltrepour-to-reencapsulate/repositionlmachine side 2, etc.) has a dra-matic "decoustraintng'' effect on the designer The RFPE fixturing rules are

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Ftpre7.zo Reference free part encapsulation (RFPE) "deconstrains the design space" during fixturing for macbining

The use of RFPE does decrease the achievable tolerances to some degree Without

RFPE a machine tool offers a daily accuracy of +/-O.CK)l inch (0.025 mm).Also Mueller the machiningPJ'OCeS8lto obtain tolerances down to +/-0.(0)2 inch (0.005 mm).During fabrication with RFPE, typical tolerances average +/-0.003 inch (0.075 nun) Ongoing research will aim to improve the machining accuracy using RFPE techniques

7.4 THE ECONOMICS OF MACHINING

7.4.1 Introduction

A method is now introduced to optimize the costs of operating the machine tools in

a production shop Actually, the general method is applicable to many variable cost

relevant to shop-floor microeconomics The general goals are to achieve one of the following'

• Minimize the production cost per component

• Minimize the production time per component

• Maximize the profit rate

The symbols shown in Table 7.2 are needed for the analysis

7.4.2 Production Cost per Component

The cost to produce each component in a batch is given by

In this equation, the symbols include

W==the machine operator's wage plus the overhead cost of the machine

WT L="nonproductive" costs,which vary depending on loading and fixturing

WT M ==actual costs of cutting metal

WT R = the tool replacement cost shared by all the components machined This cost is divided among all the components because each one uses

up T M minutes of total tool life, T, and is allocated of T MIT of WT R'

(7.16)

Using the same logic, all components use their share T MIT of the tool cost, y.

TABLE 7.2 Symbols and Explanations for the Analysis on the Economics of Machining

inches/rev rum/rev inches millimeters minutes minutes

minutes minutes

$/minute $/minute

f Feed rate for the turning operation in Figure 7.3 It has

been found empirically that speed is much more damaging

to the tool than either feed rate or depth-of-cut.Thus V

appears III the analysis more thanfor d.

d Depth-of-cut in the turning operation

T Tool life

T/,[ Time cutting metal

T Ii Replacemenl lime of a worn 1001

T, Part loading lime, which includes (loading +fixturing +

advancing +overrun +Innl withdrawal +pari unlnading)

W Average cost per minute of operating the machine plus the

operator's wage

Cost of the cutting edge of the tool For a cemented carbide

indexableinsert the cost ofa single edge is the cost of the

insert divided by the number of edges (usually 3, 4 6, or 8)