机械加工工艺装备夹具外文文献翻译、中英文翻译、外文翻译

外语文献翻译

摘自

:

《制造工程与技术（机加工）

》

（英文版）

《

Manufacturing Engineering and Technology

—

M achining

》

机械工业出版社

< br>2004

年

3

月第

1

版

P

560

—

564

页

美

s.

卡尔帕基安

(Serope kalpakjian)

s.r

施密德

(Steven )

著

原文

：

20.9 MACHINABILITY

The machinability of a material usually defined in terms of four factors:

1

、

2

、

3

、

4

、

Surface finish and integrity of the machined part;

Tool life obtained;

Force and power requirements;

Chip control.

Thus, good machinability good surface finish and integrity, long tool life, and

low force And power requirements. As for chip control, long and thin (stringy) cured

chips, if not broken up, can severely interfere with the cutting operation by becoming

entangled in the cutting zone.

Because of the complex nature of cutting operations, it is difficult to establish

relationships that quantitatively define the machinability of a material. In

manufacturing plants, tool life and surface roughness are generally considered to be

the most important factors in machinability. Although not used much any more,

approximate machinability ratings are available in the example below.

20.9.1 Machinability Of Steels

Because steels are among the most important engineering materials (as noted in

Chapter 5), their machinability has been studied extensively. The machinability of

steels has been mainly improved by adding lead and sulfur to obtain so- called

free-machining steels.

Resulfurized and Rephosphorized steels.

Sulfur in steels forms manganese

sulfide inclusions (second- phase particles), which act as stress raisers in the primary

shear zone. As a result, the chips produced break up easily and are small; this

improves machinability. The size, shape, distribution, and concentration of these

inclusions significantly influence machinability. Elements such as tellurium and

selenium, which are both chemically similar to sulfur, act as inclusion modifiers in

resulfurized steels.

Phosphorus in steels has two major effects. It strengthens the ferrite, causing

increased hardness. Harder steels result in better chip formation and surface finish.

Note that soft steels can be difficult to machine, with built-up edge formation and

poor surface finish. The second effect is that increased hardness causes the formation

of short chips instead of continuous stringy ones, thereby improving machinability.

Leaded Steels. A high percentage of lead in steels solidifies at the tip of

manganese sulfide inclusions. In non-resulfurized grades of steel, lead takes the form

of dispersed fine particles. Lead is insoluble in iron, copper, and aluminum and their

alloys. Because of its low shear strength, therefore, lead acts as a solid lubricant

(Section 32.11) and is smeared over the tool-chip interface during cutting. This

behavior has been verified by the presence of high concentrations of lead on the

tool-side face of chips when machining leaded steels.

When the temperature is sufficiently high-for instance, at high cutting speeds and

feeds (Section 20.6)

—

the lead melts directly in front of the tool, acting as a liquid

lubricant. In addition to this effect, lead lowers the shear stress in the primary shear

zone, reducing cutting forces and power consumption. Lead can be used in every

grade of steel, such as 10xx, 11xx, 12xx, 41xx, etc. Leaded steels are identified by the

letter L between the second and third numerals (for example, 10L45). (Note that in

stainless steels, similar use of the letter L means “low carbon,” a condition that

improves their corrosion resistance.)

However, because lead is a well-known toxin and a pollutant, there are serious

environmental concerns about its use in steels (estimated at 4500 tons of lead

consumption every year in the production of steels). Consequently, there is a

continuing trend toward eliminating the use of lead in steels (lead-free steels).

Bismuth and tin are now being investigated as possible substitutes for lead in steels.

Calcium-Deoxidized Steels. An important development is calcium-deoxidized

steels, in which oxide flakes of calcium silicates (CaSo) are formed. These flakes, in

turn, reduce the strength of the secondary shear zone, decreasing tool-chip interface

and wear. Temperature is correspondingly reduced. Consequently, these steels

produce less crater wear, especially at high cutting speeds.

Stainless Steels. Austenitic (300 series) steels are generally difficult to machine.

Chatter can be s problem, necessitating machine tools with high stiffness. However,

ferritic stainless steels (also 300 series) have good machinability. Martensitic (400

series) steels are abrasive, tend to form a built-up edge, and require tool materials with

high hot hardness and crater-wear resistance. Precipitation-hardening stainless steels

are strong and abrasive, requiring hard and abrasion-resistant tool materials.

The Effects of Other Elements in Steels on Machinability. The presence of

aluminum and silicon in steels is always harmful because these elements combine

with oxygen to form aluminum oxide and silicates, which are hard and abrasive.

These compounds increase tool wear and reduce machinability. It is essential to

produce and use clean steels.

Carbon and manganese have various effects on the machinability of steels,

depending on their composition. Plain low-carbon steels (less than 0.15% C) can

produce poor surface finish by forming a built-up edge. Cast steels are more abrasive,

although their machinability is similar to that of wrought steels. Tool and die steels are

very difficult to machine and usually require annealing prior to machining.

Machinability of most steels is improved by cold working, which hardens the material

and reduces the tendency for built-up edge formation.

Other alloying elements, such as nickel, chromium, molybdenum, and vanadium,

which improve the properties of steels, generally reduce machinability. The effect of

boron is negligible. Gaseous elements such as hydrogen and nitrogen can have

particularly detrimental effects on the properties of steel. Oxygen has been shown to

have a strong effect on the aspect ratio of the manganese sulfide inclusions; the higher

the oxygen content, the lower the aspect ratio and the higher the machinability.

In selecting various elements to improve machinability, we should consider the

possible detrimental effects of these elements on the properties and strength of the

machined part in service. At elevated temperatures, for example, lead causes

embrittlement of steels (liquid-metal embrittlement, hot shortness; see Section 1.4.3),

although at room temperature it has no effect on mechanical properties.

Sulfur can severely reduce the hot workability of steels, because of the formation

of iron sulfide, unless sufficient manganese is present to prevent such formation. At

room temperature, the mechanical properties of resulfurized steels depend on the

orientation of the deformed manganese sulfide inclusions (anisotropy).

Rephosphorized steels are significantly less ductile, and are produced solely to

improve machinability.

20.9.2 Machinability of Various Other Metals

Aluminum is generally very easy to machine, although the softer grades tend to

form a built-up edge, resulting in poor surface finish. High cutting speeds, high rake

angles, and high relief angles are recommended. Wrought aluminum alloys with high

silicon content and cast aluminum alloys may be abrasive; they require harder tool

materials. Dimensional tolerance control may be a problem in machining aluminum,

since it has a high thermal coefficient of expansion and a relatively low elastic

modulus.

Beryllium is similar to cast irons. Because it is more abrasive and toxic, though,

it requires machining in a controlled environment.

Cast gray irons are generally machinable but are. Free carbides in castings

reduce their machinability and cause tool chipping or fracture, necessitating tools with

high toughness. Nodular and malleable irons are machinable with hard tool materials.

Cobalt-based alloys are abrasive and highly work-hardening. They require sharp,

abrasion-resistant tool materials and low feeds and speeds.

Wrought copper can be difficult to machine because of built-up edge formation,

although cast copper alloys are easy to machine. Brasses are easy to machine,

especially with the addition pf lead (leaded free-machining brass). Bronzes are more

difficult to machine than brass.

Magnesium is very easy to machine, with good surface finish and prolonged tool

life. However care should be exercised because of its high rate of oxidation and the

danger of fire (the element is pyrophoric).

Molybdenum is ductile and work-hardening, so it can produce poor surface

finish. Sharp tools are necessary.

Nickel-based alloys are work-hardening, abrasive, and strong at high

temperatures. Their machinability is similar to that of stainless steels.

Tantalum is very work-hardening, ductile, and soft. It produces a poor surface

finish; tool wear is high.

Titanium and its alloys have poor thermal conductivity (indeed, the lowest of all

metals), causing significant temperature rise and built-up edge; they can be difficult to

machine.

Tungsten is brittle, strong, and very abrasive, so its machinability is low,

although it greatly improves at elevated temperatures.

Zirconium has good machinability. It requires a coolant-type cutting fluid,

however, because of the explosion and fire.

20.9.3 Machinability of Various Materials

Graphite is abrasive; it requires hard, abrasion-resistant, sharp tools.

Thermoplastics generally have low thermal conductivity, low elastic modulus,

and low softening temperature. Consequently, machining them requires tools with

positive rake angles (to reduce cutting forces), large relief angles, small depths of cut

and feed, relatively high speeds, and

proper support of the workpiece. Tools should be sharp.

External cooling of the cutting zone may be necessary to keep the chips from

becoming “gummy” and sticking to the tools. Cooling can usually be achieved with a

jet of air, vapor mist, or water- soluble oils. Residual stresses may develop during

machining. To relieve these stresses, machined parts can be annealed for a period of

time at temperatures ranging from

80

?

C

to

160

?

C

(

175

?

F

to

315

?

F

), and then

cooled slowly and uniformly to room temperature.

Thermosetting plastics are brittle and sensitive to thermal gradients during

cutting. Their machinability is generally similar to that of thermoplastics.

Because of the fibers present, reinforced plastics are very abrasive and are

difficult to machine. Fiber tearing, pulling, and edge delamination are significant

problems; they can lead to severe reduction in the load-carrying capacity of the

component. Furthermore, machining of these materials requires careful removal of

machining debris to avoid contact with and inhaling of the fibers.

The machinability of ceramics has improved steadily with the development of

nanoceramics (Section 8.2.5) and with the selection of appropriate processing

parameters, such as ductile-regime cutting (Section 22.4.2).