designing_for_machining-_general_guidelines.pdf

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28. DESIGNING FOR MACHINING: GENERAL

GUIDELINES

28.1. THE MACHINING PROCESS

All machining, whether heavy single-point planing or turning, form-tool

turning or milling, grinding, honing, or lapping, involves essentially the same

process at the point where the cutting tool meets the work. Figure 4.1.1

illustrates this process.

Material lying in front of the cutting tool is compressed as the tool advances

and fail in shear in a narrow zone extending at an angle from the cutting

edge to the surface of the workpiece ahead of the tool. For practical

purposes in single-point cutting this shear zone can be considered a plane.

As the cutting tool advances into the work, the shear plane also constantly

moves forward. The material that passes through the shear plane is

deformed. This material comprises the chip. In the case of ductile materials,

it is apt to consist of a continuous ribbon of deformed and heated metal

moving away from the workpiece along the face of the cutting tool. In the

case of nonductile or brittle materials, the shear action periodically causes

fracture, and the chips consist of discrete pieces rather than a continuous

ribbon of material.

Since the energy expended in cutting is manifested as heat, the chip, the

cutting tool, and even the workpiece experience a considerable rise in

temperature. This temperature rise can be reduced when a fluid coolant is

applied to the cutting tool. In addition to reducing temperature, the coolant

DESIGNING FOR MACHINING: GENERAL

GUIDELINES
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lubricates the tool in its movement against the workpiece and, more

important, the movement of chips against the face of the tool.

Figure 4.1.1. Action of a metal-cutting tool.

Grinding operations including honing and lapping exhibit the same basic

interaction between workpiece and cutter. However, the cutter in suchabrasive-machining operations is an abrasive particle, which may be very

small. The shape of the abrasive particle also may vary considerably from

that of the metal-cutting tool shown in Fig. 4.1.1.

28.2. TYPICAL MACHINED PARTS

Machined parts are universally used in industrial and consumer products of

every description. They are found in applications for which precision is

required. If high dimensional accuracy is not necessary, stampings, castings,

or stock shapes or molded parts used as is will be more economical. However,

if surface finish, flatness, roundness, circularity, parallelism, or close fit is

involved, some machining of the workpiece will practically always be

involved.

Almost invariably, if the part is in motion, is in contact with a part that is inmotion, or fits precisely with another part, machining operations will be

employed in its manufacture. For most interchangeable parts, machining is a

probable step in the manufacturing sequence.

Of course, wonders are worked with stampings and molded components and

with new precision techniques such as powder metallurgy, fine-blanking, and

investment casting, but these processes usually only reduce rather than

eliminate the need for machining if the part has a truly precision application.

Machined parts can be as small as the miniature screws, shafts, gears, and

other parts found in wristwatches and small precision instruments. They can
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be as large as the huge turbines, turbine housings, and valves found in

hydroelectric power stations.

Metals in a broad variety, both ferrous and nonferrous, are the normal

materials used for machined components. However, plastics (with or without

reinforcement), hard rubber, carbon, graphite, wood, and ceramics are also

employed.

28.3. RECOMMENDED MATERIALS FOR MACHINABILITY

Table 1.4.14 in Chap. 1.4 contains a summary of the machinability of common

metals, including those generally suitable for a broad cross section of

machining operations. The other chapters in this section cover parts

produced by specific machining operations and provide additional and more

specific materials recommendations. Chapters 2.2 to 2.4 in Sec. 2 include

additional recommendations of materials that can be machined

advantageously. Table 4.1.1 summarizes how changes in certain materials

properties affect machinability.

Table 4.1.1. Effects of Material Properties

Probable effect of decrease in material

factor on

Material factor Machinability Finishability Tool life

Strength/hardness Improves None Improves

Ductility Improves Improves Improves

Strain hardenability Improves Improves Improves

Coefficient of friction Improves Improves Improves

Heat conductivity None None Reduces

Heat capacity None None Reduces

Chemical reactivity None Improves Improves

Grain size Improves Improves Reduces

Abrasive insolubles Improves Improves Improves

a

b

c

b

d

e

e

f
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28.4. DESIGN RECOMMENDATIONS: MACHINED PARTS

1. If possible, avoid machining operations. If the surface or feature desired

can be produced by casting or forming, the cost is almost always lower. (SeeFig. 4.1.2.)

2. Specify the most liberal surface finish and dimensional tolerances possible,

consistent with the function of the surface, to simplify the prime machining

operation and to avoid costly secondary operations like grinding, reaming,

lapping, etc. (See Fig. 4.1.2.)

3. Design the part for easy fixturing and secure holding during machining

operations (see Fig. 4.7.4). A large, solid mounting surface with parallelclamping surfaces should be provided to assure a secure setup.

4. Avoid designs that require sharp corners and sharp points in cutting tools

because these make the tools more subject to breakage.

5. Use stock dimensions whenever possible if so doing will eliminate a

machining operation or the need for machining an additional surface. (See

Fig. 4.1.3.)

6. It is preferable in all single-point machining operations to avoidinterrupted cuts, if possible, because they tend to shorten tool life or

prevent the use of faster-cutting carbide or ceramic tools.

Machinabilityrefers to ease of chip removal.

Tensile strength and hardness are the simplest, but not always reliable, guides to

machinability. High-temperature alloys, for example, are difficult to machine in spite of

their low room-temperature hardness and strengths. High strain hardenability and

reactiveness to tool materials are the reasons.

While lower ductility seems to help machining, inadequate ductility (like that of

molybdenum and tungsten) can cause spalling at exit cuts or on clamped edges.

Low frictional resistance is desirable; hence the use of cutting fluids is recommended.

Low heat conductivity (especially if combined with low heat capacity, as in titanium)

contributes to high tool temperature and local high workpiece temperature.

Chemical reactivity of certain metals (such as titanium) can cause galling, smearing, and

welding of machined metal to the tool.

Source:Machine Design.

Free-machining additions Decreases Decreases Decreases

a

b

c

d

e

f
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Figure 4.1.2. Avoid tolerances that necessitate machining operations

if the as-cast, as-forged, or as-formed dimensions and surface

finishes would be satisfactory for the parts function.

Figure 4.1.3. Use stock dimensions whenever possible, and minimizethe amount of machining.

7. Design the part to be rigid enough to withstand the forces of clamping

and machining without distortion. The forces exerted by a cutter against a

workpiece can be severe, as can the clamping forces necessary to hold the

workpiece securely. Parts that may be troublesome in this respect are those

with thin walls, thin webs, or deep pockets and deep holes that require

machining. Also design the part


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Figure 4.1.4. Design the part to be rigid enough so that it will

withstand cutting and clamping forces without significant deflection

and so that cutting tools and toolholders also do not deflect.

so that a rigid cutter can be employed while still permitting access to the

surface. (Figure 4.1.4 illustrates this point. See also Fig. 4.4.8.)

8. Avoid tapers and contours as much as possible in favor of rectangular

shapes, which permit simple tooling and setups.

9. Reduce the number and the size of shoulders because they usually require

extra operational steps and additional material.

10. Avoid undercuts, if possible, because they usually involve separate

operations of specially ground tools. (See Fig. 4.1.5.)

11. Consider the possibility of substituting a stamping for the machined

component. If tooling is available, or if quantities are sufficient to amortize

the tooling cost, a stamped-sheet-metal part invariably will be lower in cost

than one made by machining, provided of course that the dimensional

accuracy and surface finish are adequate for the components function.

(Figure 4.1.6 illustrates one such example.)

12. Avoid the use of hardened or difficult-to-machine materials unless their

special functional properties are essential for the part being machined.

13. For thin, flat pieces that require surface machining, allow sufficient stock

for both rough and finish machining. In some cases, stress relieving between

rough and finish cuts also may be advisable. Rough and finish machining on

both sides is sometimes necessary. Allow about 0.4 mm (0.015 in) stock for
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finish machining.

14. It is preferable to put machined surfaces in the same plane or, if they are

cylindrical, with the same diameter to reduce the number of operations

required. When surfaces cannot be in the same plane, they should be

located, if possible, so that they all can be machined from one side or from

the same setup.

Figure 4.1.5. Avoid undercuts as much as possible because they

require extra machining operations, which may be costly.

Figure 4.1.6. Stampings are often less costly than machined castings.

15. Provide access room for cutters, bushings, and fixture elements.

16. Design workpieces so that standard cutters can be used instead of

cutters that must be ground to a special form. (See Fig. 4.1.7.)
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