機械外文翻譯材料的機械加工性

1、The mach in ability of materialThe machinability of a material usually defined in terms of four factors:1Surface finish and integrity of the machined part;2Tool life obtained;3Force and power requirements;4Chip control.Thus, good mach in abilitygood surface finish and in tegrity, long toollife, and

2、low force and power requirements. As for chip control, long and thin (stringy)cured chips, if not broken up, can severely interferewiththe cutt ing operati on by being entan gled in the cutt ing zone.Because of the plex nature of cuttingoperations, it is difficult toestablish relati on ships that qu

3、a ntitativelydefi ne the mach in ability ofa material.In manufacturing plants, tool life and surface roughness are gen erally con sidered to be the most importa nt factors in mach in ability.Although not used much any more, approximate machi nability rati ngs are available in the example below.1. Ma

4、chinability Of SteelsBecause steels are among the most importa nt engin eeri ng materials , their mach in ability has bee n studied exte nsively. The mach in ability of steels has bee n mainly improved by add ing lead and sulfur to obta in so-called free-machi ning steels.Resulfurized and Rephosphor

5、ized steels.Sulfur in steels forms manganese sulfideinclusions(second-phase particles), which act asstress raisers in the primary shear zone. As a result, the chips produced break up easily and are small; this improves machi nability. The size, shape, distribution,and concentration of these inclusio

6、ns significantlyin flue nee mach in ability. Eleme nts such as tellurium and sele nium, 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, caus ing in creased hard ness. Harder steels

7、result in better chip formati on and surface finish. Note that soft steels can be difficult to machine, with built-up edge formatio n and poor surface fini sh. The sec ond effect is that in creased hard ness causes the formatio n of short chips in stead of continu ous stri ngy on es, thereby improvi

8、 ng mach in ability.Leaded Steels. A high percentage of leadin steels solidifies at thetip of manganesesulfide inclusions.In non-resulfurized grades of steel,lead takes the form of dispersed fine particles. Lead is in soluble in iron, copper, and aluminum and their alloys. Because of its low shear s

9、trength, therefore, lead acts as a solid lubricant and is smearedover the tool-chip in terfaceduri ng cutt in g.Thisbehavior has bee n verifiedby the prese neeof high concentrationsof lead on the tool-side face of chips whenmach ining leaded steels.When the temperature is sufficientlyhigh-forinstane

10、e,at highcutting speeds and feeds the lead melts directly in front of the tool, acting as a liquid lubricant.Inaddition to this effect, lead lowers theshear 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,

11、12xx, 41xx, etc. Leaded steels are identifiedby the letter L between thesec ond and third nu merals (for example, 10L45). (Note that in sta in less steels, similar use of the letter L means “ low carb on, " a con diti on that improves their corrosi on resista nee.)However, because lead is a wel

12、l-k nown tox in and a polluta nt, there are serious environmental concerns about its use in steels (estimated at 4500 tons of lead consumption every year in the production of steels). Con seque ntly, there is a continuing trend toward elim in ati ng the use of lead in steels (lead-free steels). Bism

13、uth and tin are now being in vestigated as possible substitutes for lead in steels.Calcium-Deoxidized Steels. Animporta ntdevelopme nt iscalcium-deoxidized steels, in which oxide flakes of calcium silicates (CaSo) are formed. These flakes, in turn, reduce the strength of thesec on dary shear zone, d

14、ecreas ing tool-chip in terface and wear. Temperature is correspondingly reduced. Consequently, these steels produce less crater wear, especially at high cutting speeds.Stai nless Steels.Auste nitic(300 series) steels are gen erallydifficult to machine. Chatter can be a problem, necessitating machin

15、e tools with high stiff ness. However, ferritic stai nless steels (also 300 series) have good machinability.Martensitic(400 series) steels areabrasive, tend to form a built-up edge, and require tool materials with high hot hard ness and crater-wear resista nce.Precipitati on-harde ning sta ini ess s

16、teels are strong and abrasive, requiri ng hard and abrasio n-resista nt tool materials.The Effects of Other Eleme nts in Steels on Mach in ability. The prese nee of aluminum and silicon in steels is always harmful because these elements bine with oxygen to form aluminum oxide and silicates, which ar

17、e hard and abrasive. These pounds in crease tool wear and reduce mach in ability. It is esse ntial to produce and use clea n steels.Carbon and mangan ese have various effects on the mach in ability of steels, depe nding on their positi on. Plain low-carb on steels (less tha n 0.15% C) can produce po

18、or surface finishby forming a built-up edge. Caststeels are more abrasive, although their machinability is similar to that of wrought steels. Tool and die steels are very difficult to mach ine and usually require ann eali ng prior to machi ning. Mach in ability of most steels is improved by cold wor

19、king, which hardens the material and reduces the tendency for built-up edge formati on.Other alloyi ng eleme nts, such as ni ckel, chromium, molybde num, and van adium, which improve the properties of steels, gen erally reduce machinability. The effect of boron is negligible. Gaseous elements such a

20、s hydroge n and n itroge n can have particularlydetrime ntaleffects on theproperties 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 mach in ability.In select

21、 ing various eleme nts to improve machi nability, we should consider the possibledetrimentaleffects of these elements on theproperties and stre ngth of the mach ined part in service. At elevated temperatures, for example, lead causes embrittleme nt of steels (liquid-metalembrittleme nt,hot short nes

22、s), although at roomtemperature it has no effect on mecha ni cal properties.Sulfur can severely reduce the hot workability of steels, because of the formati on of iron sulfide, uni ess sufficie ntmangan eseis prese nt topreve nt such formatio n. At room temperature, the mecha ni cal properties of re

23、sulfurized steels depend on the orientationof the deformed manganesesulfide inclusions (anisotropy). Rephosphorized steels are significantly less ductile, and are produced solely to improve mach in ability.2. Machi nability of Various Other MetalsAlumi numis gen erally very easy to mach ine, althoug

24、h the softer grades tend to form a built-up edge, resulting in poor surface finish. High cutt ing speeds, high rake an gles, and high relief an gles are reme nded. Wrought aluminum alloys with high siliconcontent and cast aluminum alloysmay be abrasive; they require harder tool materials. Dimensiona

25、l 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 mach ining in a con trolled en vir onment.Cast grayiron

26、s are gen erallymach in able but are abrasive.Freecarbides in casti ngs reduce their machi nability and cause tool chipp ing orfracture,necessitating toolswith high toughness. Nodularandmalleable irons are machi nable with hard tool materials.Cobalt-based alloys are abrasive and highly work-harde ni

27、ng. They require sharp, abrasion-resistanttool materials and low feedsandspeeds.Wrought copper can be difficult to machi ne because of built-up edgeformati on, although cast copper alloys are easy to mach ine. Brasses are easy tomachine,especially with the additionpf lead (leadedfree-machining brass

28、). 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 oxidati on and the dan ger of fire (the eleme nt is pyrophoric).Molybde num is ductile and work-harde

29、ning, so it can produce poor surface fini sh. Sharp tools are n ecessary.Nickel-based alloys are work-hardening, abrasive, and strong at high temperatures. Their mach in ability is similar to that of stai nless steels.Tantalum is very work-hardening,ductile, and soft. It produces a poorsurface fini

30、sh; tool wear is high.Tita nium and its alloys have poor thermal con ductivity (in deed, the lowest of all metals), causing significant temperature rise and built-up edge; they can be difficult to mach ine.Tun gste n is brittle,stro ng, and very abrasive, so its mach in abilityis low, although it gr

31、eatly improves at elevated temperatures.Zirconium has good machinability. It requires a coolant-type cutting fluid, however, because of the explosi on and fire.3. Machi nability of Various MaterialsGraphite is abrasive; it requires hard, abrasi on-resista nt, sharp tools.Thermoplastics gen erally ha

32、ve low thermal con ductivity,low elasticmodulus, and low softe ning temperature. Con seque ntly, mach ining 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

33、should be sharp.External cooling of the cutting zone maybe necessary to keep the chips from being “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 duringmachining. To relieve these stresses, machined

34、 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 uni formly to room temperature.Thermosetting plastics are brittle and sensitive to thermal gradients duri ng cutt ing. Their mach in ability is gen erally similarto th

35、at ofthermoplastics.Because of the fibers prese nt, rein forced plastics are very abrasive and are difficult to machine. Fiber tearing, pulling, and edge delam in ati on are sig ni fica nt problems; they can lead to severe reducti on in the load-carry ing capacity of the ponent. Furthermore, machi n

36、ing of these materials requires careful removal of mach ining debris to avoid con tact with and in hali ng of the fibers.The mach in abilityof ceramics has improved steadily with thedevelopme nt of nano ceramics and with the select ion of appropriate process ing parameters, such as ductile-regime cutt ing .Metal-matrix and ceramic-matrix posites can be difficult to machine, depe nding on the properties of the in dividual ponen ts, i.e., reinforcing or whiskers, as well as the matrix