khon kaen university thailand
department of mechanical engineering faculty of engineering
Content Cutting Processes Theories Common processes Common tools Tool wear Machining economics
Grinding Honing & superfinishing Lapping Ultrasonic machining (UM)
Advanced Machining Processes Electrochemical machining (ECM) Electrical-discharge machining (EDM) Wire EDM Water-jet machining (WJM)
Machining describes a group of processes that consist of material removal on a workpiece and modification of its surfaces. The major advantages of machining processes are their ability to produce flexible geometry and high precision parts. However, these processes usually give high material waste, require a lot of energy, and need extremely careful process control. Three major machining processes will be described here: cutting processes, abrasive processes, and advanced machining processes.
Sigma CNC Milling Machine Reference: www.qrbiz.com
Mechanics of Chip Formation Machining processes remove material from the workpiece surface by producing chips as shown on the left image. The model of chip formation called Merchant model is illustrated on the right. In this model, known as orthogonal cutting, the thickness of the chip, tc, can be determined through the cutting ratio, r. Note that chip thickness is always greater than the dept of cut. = r
to sin ϕ = <1 tc cos (ϕ − α )
tan φ =
r cos α 1 − r sin α
During cutting, the material also undergoes shear strain and can be expressed as: γ=
AB AO OB = + = cot φ + tan (φ − α ) OC OC OC
V sin φ cos (φ − α )
Note that high shear strains are associated with low shear angles.
(a) Schematic illustration of the basic mechanism of chip formation in metal cutting (b) Velocity diagram in the cutting zone
Cutting Forces & Power Requirements The knowledge of cutting forces and power requirements are essential to proper design machine tools and avoid excessive distortion of the workpiece. Here are the steps to calculate cutting forces and power requirements:
Shear force causing shear defor mation to occur in shear plane
F+N = R
Normal force to shear force
Friction force between the tool and chip along the rake surface
Normal force to friction
Fc + Ft = R
F = tan β N
Fs + Fn = R
Resultant force Coefficient of friction
F Cutting force in the direction of c cutting Forces acting on a cutting tool in two-dimensional cutting. Note that the resultant force, R, must be colinear to balance the forces.
Note: Fc and dynamometer
can be measured by
Cutting Forces & Power Requirements Shear force
= Fs Fc cos ϕ − Ft sin ϕ Fn Fc sin ϕ + Ft cos ϕ =
Coefficient of friction
Forces acting on a cutting tool in two-dimensional cutting. Note that the resultant force, R, must be colinear to balance the forces.
α β ϕ = 45 + − 2 2 o
Davg Average diameter of workpiece
Ft + Fc tan α Fc − Ft tan α
MRR Material removal rate
F Ft cos α + Fc sin α = N Fc cos α − Ft sin α
MRR = π Davg dfN
ut ( MRR )
Dept of cut
Feed (distance per revolution)
Workpiece rotational speed
1. Slab milling In this operation, the axis of cutter rotation is parallel to the surface of the workpiece to be machined as shown on the right. Two types of milling methods can also be used 1.1 Conventional milling (up milling) is the milling direction where cutting ends at the thickest location of the chip. This method provides high clamping forces. However, it also gives poor surface finish, a tendency for the workpiece to be pulled upward. 1.2 Climb milling (down milling) is the milling direction where the cutting starts at the thickest location of the chip. This method
offers low clamping force, smooth cut, nice surface finish, and less tool wear. However, this method is not recommended for hard material and requires a rigid setup.
2. Face milling
3. End milling
In this operation, the cutter is mounted on a spindle with an axis of rotation perpendicular to the workpiece surface and removes material.
In this operation, the cutter usually rotates on an axis perpendicular to the workpiece, although it can be tilted to produce tapered surfaces as well.
Turning Turning processes produce parts that are basically round in shape. Various types of turning processes are shown here:
Schematic illustration of a turning operation showing depth of cut, d, and feed, f. Cutting speed is the surface speed of the workpiece at the Fc, is the cutting force, Ft is the thrust or feed force in the direction of feed, Fr is the radial force that tends to push the tool away from the workpiece being machined.
Tool wear Tool-life curves can be approximated by the following equations: 1. Taylorâ€™s tool life equation
VT n = C V
T n Tool life in minutes
2. Modified Taylorâ€™s tool life equation
VT n d x f y = C d
Dept of cut (mm)
Feed rate (mm/rev)
n values for various cutting tools:
Here are the range of Most cutting tools are subjected to high forces, elevated temperatures, and sliding. As a result, tool wear usually occur as shown above.
(a) Flank wear (b) Thermal cutting on rake face (c) Crater wear (d) Catastrophic failure (fracture) (e) Chipped cutting edge
High-speed steels 0.08-0.20 Cast alloys 0.10-0.15 Carbides 0.20-0.50 Ceramics 0.50-0.70 Note: The values of x and y must be obtained from the experiments.
C p = Cm + Cs + Cl + Ct Cp Cost-per-piece Cm Machining cost Cs Setup cost
Vo _ min =
Cl Loading cost Ct Tooling cost
Whether or not machining processes should be selected, the economics of machining must be considered. The most two important parameters are the minimum cost per part, and the maximum production rate.
C ( Lm + Bm )
1 − 1 Tc ( Lm + Bm ) + Tg ( Lg + Bg ) + Dc n
Maximum production Vo _ max =
C 1 − 1 Tc n
Lm Labor cost per hour Bm Overhead charge of the machine Time to change tool Tc Tg
Time to grind tool
Labor cost for grinding the tool
Depreciation of the tool in dollars Dc per grind
Grinding Sometimes the workpiece material is either too hard or too brittle, or its shape is difficult to produce with sufficient dimensional accuracy. Using abrasives can be an alternative because they are capable of removing small amount of material from a surface. Here are some examples of abrasive processes:
Grinding is basically a chip removal process in which the cutting tool is an individual abrasive agent.
The types of workpieces and operations typical of grinding: (a) cylindrical surfaces, (b) conical surfaces, (c) fillets on a shaft, (d) helical profiles, (e) concave shape, (f) cutting off or slotting with thin wheels, and (g) internal grinding.
Honing & superfinishing
Honing & Superfinishing
Schematic illustration of a honing tool used to improve the surface finish of bored or ground holes.
Honing is an operation used primarily to give holes a fine surface finish
Schematic illustrations of the superfinishing process for a cylindrical part. (a) Cylindrical mircohoning, (b) Centerless microhoning.
(a) Schematic illustration of the lapping process. (b) Production lapping on flat surfaces. (c) Production lapping on cylindrical surfaces.
Lapping is a finishing operation on flat or cylindrical surfaces A double lapping polishing process was used to give the phone the highest level of polish possible. These images are from www. aesir-copenhagen.com
Ultrasonic Machining In ultrasonic machining, material is removed from a workpiece surface by the mechanism of microchipping or erosion with abrasive particles. The picture on the right is a glass ultrasonic machine sample (Ref: http://kikkawashoji.com.) (a) Schematic illustration of the ultrasonic machining process. (b) and (c) Types of parts made by this process. Note the small size of holes produced.
ULTRASONIC hard machining of a fully automatic dental crown made of lithium disilicate (glass ceramic) with a thinning margin (Ref: http://ids-cologne.de)
ADVANCED MACHINING PROCESSES
Electrochemical Machining (ECM) In electrochemical machining, an electrolyte acts as current carrier, and the high rate of electrolyte movement in the toolworkpiece gap washes metal ions away from the workpiece (anode) before they have a chance to plate onto the tool (cathode).
The middle picture is the SEM image of electrochemically machined Au-pillars, 4 Âľm high (Ref: http://www.cfn.kit.edu) The bottom right picture is the chematic illustration of the electrochemicalmachining process.
ADVANCED MACHINING PROCESSES
Electrical-Discharge Machining (EDM) In electrical-discharge machining, when two current-conducting wires are touched, an arc is produced eroding away a small portion of the metal. The bottom left picutre shows the schematic illustration of the electrical-discharge machining process.
The left picture shows the cavities produced by the EDM process, using shaped electrodes. The bottom right picture shows A spiral cavity produced by EDM using a slowly rotating electrode, similar to a screw thread.
ADVANCED MACHINING PROCESSES
Wire Electrical-Discharge Machining (Wire EDM) www.microwireedm.com
This process is a variation of EDM where a slowly moving wire travels along a precribed path, cutting the workpiece with the discharge sparks. The top left picture is the schematic illustration of the
EDM process. As much as 50 hours of machining can be performed with one reel of wire, which is then discarded. The top right image shows the sample parts of collets and internal splines.
ADVANCED MACHINING PROCESSES
Water-Jet Machining (WJM) In this process, the force from the jet is utilized in cutting and deburing. The water acts like a saw and cuts a narrow groove in the material.
The top left image is the schematic illustration of water-jet machining. The bottom left image is the examples of various nonmetallic parts produced by the water-jet cutting process. The right image shows the water-jet nozzle.
References - S. Kalpakjian, S. R. S. (2003). Manufacturing Engineering and Technology. New Jersey, Pearson Prentice Hall. - Groover, M. P. (2010). Principles of Modern Manufacturing: Materials, Processes, and Systems, John Wiley & Sons Ltd.