© Su-Jin Kim GNU

Grinding, EDM

Grinding, EDM

Grinding, Electric discharge machining

Professor Su-Jin Kim

School of Mechanical EngineeringGyeongsang National University

© Su-Jin Kim GNU

Grinding, EDM

Contents

1. Abrasive and mechanics2. Grinding

3. EDM (Electrical discharge machining)4. Water Jet Cutting

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Grinding, EDM

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 ±2.5 μm.

©김태용, GNUHow to grind (9 min) : https://youtu.be/7FwL55ErfDI

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Grinding, EDM

Abrasives (연삭입자)

• Abrasives are harder than conventional cutting‐tool materials.

1. Aluminium oxide, Al2O3

2. Silicon Carbide, SiC3. CBN (Cubic Boron Nitride)4. PCD (Poly crystal diamond)• Grit number (입도) indicates the size which is a the 

function of a sieve(체) size. (10: very rough, 100: fine, 500 very fine) Surface area / 1g

• Mesh(체) No. wire # / inch

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Grinding, EDM

Grinding wheel (연삭숫돌)

• Abrasive grains are held together by a bonding materialto achieve a high material removal rate.

• Bond types (결합제) : ceramic bond, plastic based bond, rubber, metal bonds

Cheil grinding wheel (8min): https://youtu.be/KZ27UaSFduw

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Grinding, EDM

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)

ф

Chip

Workpiece

Grain

vα

Material Specific energy (GPa, J/mm3)

Aluminum 7~27

Cat iron 12‐60

Low carbon steel 14‐68

Tool steel 18‐82(10 X machining)

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Grinding, EDM

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.

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Grinding Wheel Wear (연삭숫돌마멸)

Wear Grain fracture

Bond fracture

Porosity

Grain

Micro crack

Bond

• 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.

• Bond fracture(결합재파괴): if bond is too strong, dull grains cannot be dislodged

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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

CNC grinding, Dressing, Fluid (SME): https://youtu.be/Nn8j3hp_z6M

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Grinding, EDM

Standard marking system

• Standard marking system of grinding wheel ANSI standard B74. 13‐1970 (연삭숫돌표시기호)

Abrasive Grain size Grade Structure Bond Manuf

A 46 L 5 V 23Al2O3SiCCBNPCD

8‐24 Rough30‐60 Mid100 Fine400 Very Fi

A Soft|Z Hard

1 Dense|16 Open

VitrifiedB ResinSilicateRubber

Manufacture’s private

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Grinding, EDM

Surface grinding (평면연삭)

• Surface grinding involves grinding flat surfaces.

http://ma.gnu.ac.kr/vod/Machining/Grinding_Machine.MP4 Surface grinding machine

Workpiece 0.2~1 m/s

Grinding wheel25~50 m/s

Small depth0.01~0.05 mm

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Cylindrical grinding (원통연삭)

• Cylindrical grinding, internal grinding and thread grinding are done on cylindrical grinders or centerless grinders

Workpiece

Grinding wheel

TiC Internal grinding (3min): https://youtu.be/FVLhyg-_ch8Cylindrical grinding machine (6min): https://youtu.be/NHqMc4zXph8

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Grinding, EDM

Centerless & Creep‐feed grinding

• Centerless grinding

• Creep‐feed grindinglow feed, large depth

Grinding wheel

Regulating wheel

Work-rest blade

Low speed0.1~1 m/min

Large depth1~6 mm

Centerless grinding: https://youtu.be/8P4MlqID8yo

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Grinding, EDM

Surface grinding, low-carbon steel workpieceWheel diameter D = 250 mm, width of cut w = 25 mm, depth of cut d = 0.05 mm Spindle speed S = 4000 rpm, Feed rate F = 1500 mm/minCalculate cutting force Fc ?

Material removal rate MRR = d w F (mm3/min)Power consumed P = u MRR (W) = T ω (Torque x angular speed)Torque T = Fc r (Cutting force x radius)

Ex) Forces in surface grinding

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Grinding, EDM

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 material‐removal process.

3. Lapping & polishing: Two surfaces are rubbed together with an abrasive between them to creates a smooth and shiny surface

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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

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Electrical Discharge Machining (EDM)

Electrical discharge machining (EDM 방전가공) : Material is removed by electric discharges (spark), between electrode and workpiece, separated by dielectric fluid(oil).

Sinker(Ram) EDM , Wire EDM

https://youtu.be/q4FinKsDfww Ram vs. Wire EDM 4min

Electrode

Workpiece

Spark

Time (ms)

Cur

rent

(A) -

+

5

50

80 V

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Grinding, EDM

Sinker EDM

• Shape of soft electrode (copper or graphite) is copied to hard workpiece (mold steel, tool steel).

https://youtu.be/L1D5DLWWMp8 SME 1minhttp://ma.gnu.ac.kr/vod/Machining/EDM.WMV Machine

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Grinding, EDM

EDM Condition

• Current I, On‐time ton, Off‐time toff, Melting temp Tw• Material removal rate: MRR  V I ton / (ton + toff ) / Tw• Roughness Ra  V I ton

On-

time

(ms)

Current (A)

MRR

Ra

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Grinding, EDM

Wire EDM

http://www.youtube.com/watch?v=pBueWfzb7P0 WireEDM

http://ma.gnu.ac.kr/vod/Machining/WireEDM.wmv WireEDM

- Wire electrode

+ Workpiece

• 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.

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Small Hole EDM

• EDM drill rows of holes into the leading and trailing edges of turbine blades used in jet engines.

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Grinding, EDM

ECM (Electrochemical Machining, 전해가공)

http://www.youtube.com/watch?v=Z_U_ZZty5Ns

• High rate of electrolyte movement washes metal ions away from the workpiece.

• It is used for machining extremely hard conductive materials with soft tool.

Workpiece

Insulating coating

Tool electrode

Pump for circulating electrolyteDC

powersupply

Tank

Electrolyte

(-)

(+)

©갈창우, GNU

© Su-Jin Kim GNU

Grinding, EDM

Electrochemical Grinding (전해연삭)

• Combines the processes of electrochemical machining and conventional grinding.

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Grinding, EDM

Laser‐beam Cutting

http://ma.gnu.ac.kr/vod/Machining/Laser_beam_machining.MP4

• High power (kW) laser melts and evaporates material.• CO2 & Nd:YAG lasers are used for industrial cutting.

Mirror

XenonFlashlamp

Nd:YAGcrystal

Laser

Mirror

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Grinding, EDM

Laser‐beam Cutting

• Standard roughness ∝material thickness / laser power, cutting speed : r ∝ t0.5 / (P0.5 v0.3)

• Cutting rate ≈ laser power / evaporation energy per volume, thickness, width of laser:  v ≈ P / (e t w)

Laser beam

Focusing lens

Gas

Material

wt

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Grinding, EDM

Laser‐beam welding

• Uses a high‐power laser beam as a heat source to produce a fusion weld.

• Has a high energy density and a deep penetrating capability.

http://www.youtube.com/watch?v=51d6gQxg1KI

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Laser‐beam welding

• Laser welding of diff‐case.

Laser welded

Bolted Press 10 ton + Laser welding

Section

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Laser Welding Machine

• Multi axis machine used for laser cutting, welding, marking.

http://ma.gnu.ac.kr/vod/Joining/Laser_Welding.wmv Machine

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Laser drill

• Non‐contact, a wide range of materials, accuracy and consistency, as small as 10 – 20 microns.

Aerospace part: http://www.youtube.com/watch?v=83OQTZZ4ML8

Glass: http://www.youtube.com/watch?v=CS60VzAkhxk

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Water‐Jet Cutting

• Water jet acts like a saw and cuts the material.• Advantages: start at any location, no heat, no deflection, 

environmentally friendly.

5-axis: http://www.youtube.com/watch?v=2jm4_HikMqk&p=9B6D9EAE75875D9D

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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.

Abrasive waterjet: http://www.youtube.com/watch?v=GCBsTumaIhs

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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.

http://ma.gnu.ac.kr/vod/machining/Ultrasonic_Machining.MP4 Ultrasonic Knife

Workpiece

Power supply Transducer

Abras slurryTool

©갈창우, GNU

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Grinding, EDM

Reference

• CHEIL GRINDING WHEEL IND. CO. (숫돌) http://www.grinding.co.kr

• 유일연삭기: http://www.youilmc.com (연삭기)• 대한EDM:  http://www.daehanedm.com (EDM)• 원일정기: http://www.avg.co.kr/wire‐edm

• 우진기공: http://www.wjmc.kr• 국일공작소: http://kookil.com