铣床电气控制系统的创新设计及PLC改造文献翻译.docVIP

1 MACHINABILITY The 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 stringyaon