黄河科技学院毕业设计（论文）黄河科技学院毕业设计（论文） 第第 1 页页1 MACHINABILITYThe machinability of a material usually defined in terms of four factors: 1、Surface finish and integrity of the machined part; 2 、Tool life obtained; 3 、Force and power requirements; 4 、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 opeation by becoming entangled in the cutting zone. because of the complex nature of cutting operations,it is difficult to establish relationship that quantitatively define the 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 Streels because steels are among the most important engineering materials(as noted in chapter5),their machinability has been studied extensively.The machinability of steels has mainly improved by adding lead and sulfur to obtain so-called free-machining steels. Resufurized and Rephoshorized steels in forms manganese sulfide inclusion(second-phase- particles),which act stress raisers in the primary shear zone.As a result,the chips produced break up easily and are small;this improves machinability.This size,shape distribution and concentration of these inclusion 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 mmajor 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 bulit-up edge formation and poor surface finish.The second effect is that increased hardness causes the formation of short chips instead of continuous stringyaones,thereby improving machinability. Leaded Streels.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 a solid lubricant(section32.11)and is smeared over the tool-chip interface during cutting.This behavior has been werified 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 (section20.6)-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 forces and power consumption.Lead can be used in every grade of 2 黄河科技学院毕业设计（论文）黄河科技学院毕业设计（论文） 第第 2 页页steel,such as 10xx,11xx,12xx,41xx,ect.Leaded steels are identfied by the letter L between the second and numerals(For example,10L45).(Note that in stainless steels,similar use of the letted L means“low carbon,“a condition that improves their corrosion resistance.) However,because lead is a well-know toxin and a pollutant,there are serious environmental concerons 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 Streels.An important development is calcium-deoxidized steels,in which oxide flakes of calcium silicates(CaSo)are formed.These flakes,inturn,reduce the strength of the secondary shear zon,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 machin.Chatter can be a 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 forma bulit-up edge,and require tool materials with high hot hardness and crater-wear tesistance.Precipitation- hardening stainless steels are strong and abrasive,requiring hard and abrasion-resistant 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 steel