Machinability PDF

Machinability 11/12/2023 Hassan El-Hofy 1  1. Definition 2. Judging Machinability 3. Relative Machinability 4. Factors Affecting Machinability A. Condition of Work Material B....

Machinability 11/12/2023 Hassan El-Hofy 1  1. Definition 2. Judging Machinability 3. Relative Machinability 4. Factors Affecting Machinability A. Condition of Work Material B. Physical Properties of Work Materials C. Machining Parameters 12 November 2023 Hassan El-Hofy 2 1- Introduction Machinability: Refers to the ease with which a metal can be machined to an acceptable surface finish. 1. Materials with good machinability:  Require little power to remove material  Achieve cutting at high speed  Easily obtain a good finish  Do not cause tool wear (long tool life). 3. Machinability is difficult to predict because machining has so many variables 4. The machinability is not a material characteristic. 5. A material that is machinable by a certain process may not be machinable by another process. 6. A particular machining process found suitable under given conditions may not be equally efficient for machining the same material under other conditions. 12 November 2023 Hassan El-Hofy 3 Judging Machinability Hassan El-Hofy 4 12 November 2023 Judging Machinability 1-Tool life  Materials cut without rapid tool wear (longer tool life) are generally machinable  A workpiece material having small hard inclusions has lower machinability because of its rapid tool wear (short tool life).  One problem of using tool life as a machinability index is its sensitivity to tool material. 2. Chip formation  Chip formation, friction at the tool/chip interface, and BUE are determinant to machinability.  Chip formation is a function of the machine variables and workpiece material, and the machinability ratings obtained by this method could be changed by provision of a suitable chip breaker.  Long thin curled ribbon chips, can interfere with the operation leading to hazardous cutting area (low machinability)  A ductile material that has a tendency to adhere to the tool face or to form BUE (low-carbon steel, pure aluminum, Cu, and stainless steel). is likely to produce a poor finish (low machinability) 12 November 2023 Hassan El-Hofy 5 Judging Machinability 3. Surface finish The quality of the workpiece machined surface is sometimes useful in determining the machinability of a material.  Stainless steels, gas turbine alloy, and other metals with high strain-hardening ability also tend to machine with BUEs indicating poor machinability.  Materials, which machine with high shear angles, (aluminum alloys, cold-worked steels, free- machining steels, and brass and titanium alloys) tend to minimize BUE effects have high machinability.  In roughing operations, no attention to surface finish which is a meaningless criterion of judging machinability  Machinability ratings based on surface finish do not agree with those obtained by cutting force, cutting power, and tool life method.  Titanium alloys have a high rating by finish measurements, low by tool life tests, and intermediate by cutting force and power measurements Hassan El-Hofy 6 12 November 2023 Judging Machinability 4. Cutting forces and power consumption  A metal through which the cutting forces are low has a good machinability.  Net power consumption as an index of the machinability is similar to the cutting force.  Machinability in terms of specific energy (power/VRR, high specific energy indicates less machinability.  The metal removal factor (reciprocal of the specific energy).(high metal removal factor could be said to have high machinability). Relative Importance of Machinability Criterion in Roughing and Finishing Order of Machinability Criterion Rough Cut Finish Cut 1 Tool life Surface finish 2 Power consumption Tool life 3 Surface finish Power consumption 12 November 2023 Hassan El-Hofy 7 Relative Machinability  Free-cutting steel (AISI B1112) - a steel having 0.08%–0.13% C, 0.60%–0.90% Mn, 0.09%– 0.13%,Ph and 0.16%–0.23% S and a hardness of 160 BHN is a reference material of a machinability rating/index of 1.0.  A material having an index 1, it is comparatively easier to machine that material.  The machinability of the reference material can be expressed in terms of cutting speed V60 for a tool life T = 60 min for a given tool material. Relative machinability R m of a material. 𝑉60 𝑜𝑓 𝑡ℎ𝑒 𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙 𝑅𝑚 = 𝑉60 𝑜𝑓 𝑟𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒 𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙 8 12 November 2023 Hassan El-Hofy Relative Machinability Machinability Rating Materials Excellent rating Mg-alloys, Al-alloys, duralumin Good rating Zn-alloys, gunmetal, grey CI, brass, free cutting steel Fair rating Low carbon steel, cast Cu, annealed Ni, low alloy steel Poor rating Ingot iron, free cutting 18-8 stainless steel Very poor rating HSS, 18-8 stainless steel, Monel metal Not machinable White CI, Stellite, carbides, ceramics If different tool materials are used to assess relative machinability, different ratings may occur 12 November 2023 9 Hassan El-Hofy Factors Affecting Machinability Condition of Work Material Heat treatment Fabrication Yield strength Work material Grain size Microstructure conditions Chemical composition Hardness Tensile strength 12 November 2023 10 Hassan El-Hofy Factors Affecting Machinability Physical Properties of Work Materials Thermal conductivity Physical Work hardening Thermal expansion properties Modulus of elasticity 11 12 November 2023 Hassan El-Hofy Factors Affecting Machinability Machining Parameters Tool material Tool geometry Cutting speed Machining Parameters Cutting fluid Rigidity of the machine tool Machining operation 12 12 November 2023 Hassan El-Hofy 12 November 2023 Hassan El-Hofy