Transcript Grinding
Manufacturing Processes Grinding(연삭), EDM(방전) Associate Professor Su-Jin Kim Grinding, EDM Manufacturing Processes © 2012 Su-Jin Kim GNU School of Mechanical Engineering Gyeongsang National University Contents 1. Abrasive and mechanics 2. Grinding 3. EDM (Electrical discharge machining) 4. Water Jet Cutting Grinding, EDM Manufacturing Processes © 2012 Su-Jin Kim GNU Grinding (연삭) • In grinding, an abrasive material rubs against the metal part and removes a small amount of material. • Reasons for grinding are: The material is too hard to be machined. Tolerance is less than ±0.0025 mm. Grinding, EDM Manufacturing Processes © 2012 Su-Jin Kim GNU ©김태용, GNU Abrasives (연삭입자) • 1. 2. 3. 4. • Abrasives are harder than conventional cutting-tool materials. Aluminium oxide, Al2O3 Silicon carbide, SiC CBN (Cubic boron nitride) PCD (Poly crystal diamond) Grit number (입도지수) indicates the size which is a the function of a sieve(체) size. (10: very rough, 100: fine, 500 very fine) Grinding, EDM Manufacturing Processes © 2012 Su-Jin Kim GNU Grinding wheel (연삭 숫돌) • • Abrasive grains are held together by a bonding material to achieve a high material removal rate. Bond types (결합제) : ceramic bond, plastic based bond, rubber, metal bonds Grinding, EDM Manufacturing Processes © 2012 Su-Jin Kim GNU Mechanics of Grinding (연삭이론) • • • • Cutting tool is an individual irregular abrasive grain The average rake angle of the grains is highly negative Cutting speeds of grinding wheels are very high Power = Specific energy x MRR (material removal rate) Grain Chip ф Workpiece Grinding, EDM Manufacturing Processes © 2012 Su-Jin Kim GNU α v Material Specific energy (GPa, J/mm3) Aluminum 7~27 Cat iron 12-60 Low carbon steel 14-68 Tool steel 18-82 Temperature • • • Most of the cutting power transfers into heat. Temperature rise affects the surface properties and causes residual stress on the workpiece. Use fluid to prevent excessive temperature rise and to improve the surface finish and dimensional accuracy. Grinding, EDM Manufacturing Processes © 2012 Su-Jin Kim GNU http://www.youtube.com/watch?v=Nn8j3hp_z6M CNC grinding, Dressing, Coolant Grinding Wheel Wear (연삭숫돌마멸) • Attritious wear(소모마멸): caused by interaction of grain and workpiece • Grain fracture(입자파괴): caused by excessive attritious wear, Friability is the ability of an abrasive grain to fracture into smaller pieces. Grain • Bond fracture(결합재파괴): if bond is too strong, dull grains cannot be dislodged Bond Porosity Bond fracture Micro crack Wear Grinding, EDM Manufacturing Processes © 2012 Su-Jin Kim GNU Grain fracture Dressing • Once the sharp edges of the grinding wheel are worn out, they need to be dressed. • Grinding dressers are used for making different profile on grinding wheel. The dressing of a wheel in order to return the wheel to its original shape. vitrified bonded dressing tools single grit diamond dressing tools Grinding, EDM http://www.ehow.com/video_4418709_dressing-grinding-wheel-sharpening-tools.html Dressing Manufacturing Processes © 2012 Su-Jin Kim GNU Surface grinding (평면연삭) • Surface grinding involves grinding flat surfaces. Grinding wheel 25~50 m/s Workpiece 0.2~1 m/s Small depth 0.01~0.05 mm Grinding, EDM http://ma.gnu.ac.kr/vod/Machining/Grinding_Machine.MP4 Surface grinding machine Manufacturing Processes © 2012 Su-Jin Kim GNU Cylindrical grinding (원통연삭) • Cylindrical grinding, internal grinding and thread grinding are done on cylindrical grinders or centerless grinders Grinding wheel Workpiece Grinding, EDM Manufacturing Processes © 2012 Su-Jin Kim GNU Centerless & Creep-feed grinding • Centerless grinding Grinding wheel Regulating wheel Work-rest blade • Creep-feed grinding : low feed, large depth Large depth 1~6 mm Grinding, EDM Manufacturing Processes © 2012 Su-Jin Kim GNU Low speed 0.1~1 m/min Ex) Forces in surface grinding Assume that you are performing a surface-grinding operation on a low-carbon steel workpiece using a wheel of diameter D = 25.4 cm that rotates at N = 4000 rpm. The width of cut is w = 2.54 cm, depth of cut is d = 0.00508 cm, and the feed rate of the workpiece is v = 152.4 cm/min. Calculate Fc and Fn. Solution Material removal rate is MRR dwv 0.005082.54152.4 1.966 cm3/min Power consumed is Power u MMR 682.5811.966 1342W 6.1183kg - m/min 8210.76 m - kg/min Grinding, EDM Manufacturing Processes © 2012 Su-Jin Kim GNU Ex) Forces in surface grinding Solution Since power is defined as Power T 25.4 8210.76 Fc 2 4000 2 Fc 25.8 kg Since thrust force is about 30% higher than the cutting force, Fn 1.325.8 33.54 kg Grinding, EDM Manufacturing Processes © 2012 Su-Jin Kim GNU Finishing Operations (마무리작업) 1. Coated abrasives (Sandpaper) is used in finishing flat or curved surfaces of metallic and non-metallic parts. Surface finish depends primarily on grain size. 2. Wire brushing produces a fine surface texture and serves as a light materialremoval process. 3. Lapping & polishing: Two surfaces are rubbed together with an abrasive between them to creates a smooth and shiny surface Grinding, EDM Manufacturing Processes © 2012 Su-Jin Kim GNU Finishing Operations 4. Honing: Abrasive stone scrubs metal woekpiece to improve a surface texture. http://www.youtube.com/watch?v=f6CEkkG-j8s&p=9B6D9EAE75875D9D Honing engine block Grinding, EDM Manufacturing Processes © 2012 Su-Jin Kim GNU Ultrasonic Machining (초음파가공) • Material is removed from the workpiece surface by the process of microchipping with abrasive particles with vibrations between the tool and the surface of the workpiece. Power supply Transducer Abras slurry Tool Workpiece Grinding, EDM Manufacturing Processes © 2012 Su-Jin Kim GNU ©갈창우, GNU http://ma.gnu.ac.kr/vod/machining/Ultrasonic_Machining.MP4 Ultrasonic Knife ECM (Electrochemical Machining, 전해가공) • • High rate of electrolyte movement washes metal ions away from the workpiece. It is used for machining extremely hard conductive materials with soft tool. DC power supply Insulating coating (-) Pump for circulating electrolyte Tool electrode (+) Tank Workpiece Electrolyte ©갈창우, GNU Grinding, EDM Manufacturing Processes © 2012 Su-Jin Kim GNU http://www.youtube.com/watch?v=Z_U_ZZty5Ns Electrochemical Grinding (전해연삭) • Combines the processes of electrochemical machining and conventional grinding. Grinding, EDM Manufacturing Processes © 2012 Su-Jin Kim GNU EDM (Electrical-Discharge Machining, 방전가공) • • Based on erosion of metals by spark discharges. EDM system consists of electrodes(전극) connected to a DC and placed in a dielectric fluid(방전액). Electrode Spark Workpiece Grinding, EDM Manufacturing Processes © 2012 Su-Jin Kim GNU http://www.youtube.com/watch?v=q4FinKsDfww EDM EDM (Electrical-Discharge Machining) • 50~380 V, 0.1~500 A, 50~500 kHz, 1~100 μm3/Hz • Metal removal rate (mm3/min) = Exp const x Current (A) / Meting temperature (°C): MRR = 4x104 I Tw-1.23 1st RC EDM Grinding, EDM Manufacturing Processes © 2012 Su-Jin Kim GNU EDM (Electrical-Discharge Machining) • Used to machine high hardness metals like as die by copper of graphite electrode. Grinding, EDM Manufacturing Processes © 2012 Su-Jin Kim GNU http://ma.gnu.ac.kr/vod/Machining/EDM.WMV Machine Wire EDM( Wire Electric Discharge Machining) • • A brass wire is fed through the workpiece submerged in a tank of dielectric fluid. It is used to cut plates as thick as 0.3 m and to make punches, tools, and dies from hard metals. - Wire electrode + Workpiece Grinding, EDM Manufacturing Processes © 2012 Su-Jin Kim GNU http://www.youtube.com/watch?v=pBueWfzb7P0 WireEDM http://ma.gnu.ac.kr/vod/Machining/WireEDM.wmv WireEDM Small Hole EDM • EDM drill rows of holes into the leading and trailing edges of turbine blades used in jet engines. Grinding, EDM Manufacturing Processes © 2012 Su-Jin Kim GNU Water-Jet Cutting • • Water jet acts like a saw and cuts the material. Advantages: start at any location, no heat, no deflection, environmentally friendly. Grinding, EDM Manufacturing Processes © 2012 Su-Jin Kim GNU 5-axis: http://www.youtube.com/watch?v=2jm4_HikMqk&p=9B6D9EAE75875D9D Abrasive-Jet Cutting Abrasive water-jet cutting • The water jet contains abrasive particles and increases the material removal rate. • Various thicknesses can be cut in single or multiple layers. Abrasive-jet cutting • High-velocity jet is aimed at a surface under controlled conditions. Grinding, EDM Manufacturing Processes © 2012 Su-Jin Kim GNU Abrasive waterjet: http://www.youtube.com/watch?v=GCBsTumaIhs Reference • • • • • 제일연마: http://www.grinding.co.kr (숫돌) 진영정기: http://www.kojin.co.kr (연삭기) 우진기공: http://www.wjmc.kr 국일공작소: http://kookil.com 두산인프라코어: http://www.doosan.com/doosaninfracoremachinetool (방전가공기) Grinding, EDM Manufacturing Processes © 2012 Su-Jin Kim GNU