22MET42 MACHINING AND MEASUREMENT

Manufacturing

• Manufacturing can be defined as the process of converting raw materials (and information such as design specifications) into a usable form of products or goods for human needs.

(a) designing the product,

(b) selecting raw materials for the product

(c) deciding sequence of processes

• A manufacturing process is the activity (or a combination of activities) of transforming a given material into a product of different forms and sizes and with or without changing the physical and mechanical properties of the product material

PRODUCTION METHOD

• A production method comprises a set of various types of manufacturing processes carried in some sequence on a material to transform it into the final shape of a marketable commodity.
• Types of Production Method
• Piece or unit production
• producing one or several articles like prototypes, experimental products
• Lot or batch production
• identical shape and size are produced in a lot or batch
• Mass production
• large quantity of standard parts by using specialized workforce and incorporating the principles of interchangeable production

Broad classification of Engineering Manufacturing Processes. �

• Shaping or forming Manufacturing a solid product of definite size and shape from a given material taken in three possible states:
• in liquid or semi-liquid state – e.g., casting, injection moulding etc.
• in solid state – e.g., forging rolling, extrusion, drawing etc.
• in powder form – e.g., powder metallurgical process.

Joining process

• Welding, brazing, soldering etc.

Removal or Cutting process

• Machining (Traditional or Non-traditional), Grinding etc.

Regenerative manufacturing Process

• Production of solid products in layer by layer from raw materials in different form:
• liquid – e.g., stereo lithography
• powder – e.g., selective sintering
• sheet – e.g., LOM (laminated object manufacturing)
• wire – e.g., FDM. (Fused Deposition Modeling)

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Material Removal Processes – Metal Cutting Process�

• A family of shaping operations, the common feature of which is removal of material from a starting work part so the remaining part has the desired geometry
• Traditional Process (Machining) – Material removal by a sharp cutting tool, e.g., turning, milling, drilling
• Non traditional processes - Various energy forms other than sharp cutting tool to remove material. e.g., Laser and Electron Beam machining
• Abrasive processes – Material removal by hard, abrasive particles, e.g., grinding
• Most important machining operations
• Turning
• Drilling
• Milling

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Turning

Drilling�

Milling

Machining requirements

• Firmly holding the blank and the tool
• Transmit motions to the tool and the blank
• Provide power to the tool-work pair for the machining action.
• Control of the machining parameters, (speed, feed and depth of cut).

Classification of cutting tools

Single Point Cutting tool

• One dominant cutting edge
• Point is usually rounded to form a nose radius
• Turning uses single point tools

Multiple Point Cutting Edge Tools

• More than one cutting edge
• Motion relative to work achieved by rotating
• Drilling and milling use rotating multiple cutting edge tools

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Tool signature for single point cutting tool�

• Shank
• It is the main body of the tool
• Flank
• The surface of the tool adjacent to the cutting edge
• Face
• The surface on which the chip slides
• Nose
• It is the point where the side cutting edge and end cutting edge intersect
• Nose Radius
• Strengthens finishing point of tool
• Cutting Edge
• It is the edge on the face of the tool which removes the material from the work piece
• Side cutting edge angle
• Angle between side cutting edge and the side of the tool shank

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Single Point Cutting Tool Terminology

Cutting Tool Materials

• Carbon steels, High-speed steels
• Cast carbides, Cemented carbides, Coated carbides
• Cermets, Ceramic Tools
• Polycrystalline Cubic Boron Nitride (PCBN)
• Polycrystalline Diamond (PCD)

Properties of Cutting Tool Materials

• Harder than work piece.
• High toughness
• High thermal shock resistance
• Low adhesion to work piece material
• Low diffusivity to work piece material

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Theory of Metal Cutting

• Metal cutting or machining is the process of producing a work piece by removing unwanted material from a block of metal, in the form of chips.
• This process is most important since almost all the products get their final shape and size by metal removal, either directly or indirectly.

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Orthogonal and oblique cutting

Orthogonal cutting

• The cutting edge of the tool is straight and perpendicular to the direction of motion.

Oblique cutting

• The cutting edge of the tool is set at an angle to the direction of motion.

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Mechanics of Orthogonal Cutting

Orthogonal Cutting

• Ideal Orthogonal Cutting is when the cutting edge of the tool is straight and perpendicular to the direction of motion.
• During machining, the material is removed in form of chips, which are generated by shear deformation along a plane called the shear plane.
• The surface the chip flows across is called the face or rake face.
• The surface that forms the other boundary of the wedge is called the flank.
• The rake angle is the angle between the tool face and a line perpendicular to the cutting point of the work piece surface.

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Orthogonal cutting model:

• t₁ = un deformed chip thickness
• t₂ = deformed chip thickness (usually t₂ > t₁)
• α = rake angle
• If we are using a lathe, t₁ is the feed per revolution.

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Chip thickness ratio (or) cutting ratio

where

• r = chip thickness ratio or cutting ratio;
• t₁ = thickness of the chip prior to chip formation;
• t₂ = chip thickness after separation

Shear Plane Angle

Shear Plane Angle

• Based on the geometric parameters of the orthogonal model, the shear plane angle ө can be determined as:

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where

• r = chip thickness ratio or cutting ratio;
• α = Rake angle
• ө = Shear angle

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Chip formation

• Mechanics of metal cutting is greatly depend on the shape and size of the chips formed.

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Four Basic Type of Chips in Machining are�

• Discontinuous chip
• Continuous chip
• Continuous chip with Built-up Edge (BUE)
• Serrated chip

Discontinuous chip

• When brittle materials like cast iron are cut, the deformed material gets fractured very easily and thus the Chip produced is in the form of discontinuous segments

Reasons

• Brittle work materials
• Low cutting speeds
• Large feed and depth of cut
• High tool‑chip friction

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Continuous chip

• Continuous chips are normally produced when machining steel or ductile materials at high cutting speeds. The continuous chip which is like a ribbon flows along the rake face.

Reasons

• Ductile work materials
• High cutting speeds
• Small feeds and depths
• Sharp cutting edge
• Low tool‑chip friction

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Continuous chip with Built-up Edge (BUE)

• When the friction between tool and chip is high while machining ductile materials, some particles of chip adhere to the tool rake face near the tool tip. When such sizeable material piles upon the rake face, it acts as a cutting edge in place of the actual cutting edge is termed as built up edge (BUE). By virtue of work hardening, BUE is harder than the parent work material

Reasons

• Ductile materials
• Low‑to‑medium cutting speeds
• Tool-chip friction causes portions of

chip to adhere to rake face

• BUE forms, then breaks off, cyclically

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Serrated chip

• Semi Continuous ( saw tooth appearance) chips produced when machining tool steels or Harden materials at high cutting speeds.

Reasons

• Ductile materials
• Low‑to‑medium cutting speeds
• Tool-chip friction causes portions of

chip to adhere to rake face

• BUE forms, then breaks off, cyclically

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Chip Breakers

• Long continuous chip are undesirable
• Chip breaker is a piece of metal clamped to the rake surface of the tool which bends the chip and breaks it
• Chips can also be broken by changing the tool geometry, thereby controlling the chip flow

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Forces Acting on Chip

Forces Acting on Chip

• Friction force F and Normal force to friction N
• Shear force F_s and Normal force to shear F_n

Cutting Force and Thrust Force

• F, N, Fs and Fn cannot be measured directly, in order to measure these forces the forces acting on the tool to be measured initially
• Cutting force Fc and Thrust force Ft

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Resultant Forces

• Vector addition of F and N = resultant R
• Vector addition of F_s and F_n = resultant R'
• Forces acting on the chip must be in balance:
• R' must be equal in magnitude to R
• R’ must be opposite in direction to R
• R’ must be collinear with R

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Shear Stress

• Shear stress acting along the shear plane

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• where As = area of the shear plane

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• Shear stress = shear strength of work material during cutting

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Effect of Higher Shear Plane Angle

• Higher shear plane angle means smaller shear plane which means lower shear force, cutting forces, power, and temperature

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Force Calculations

• The forces and angles involved in cutting are drawn here,
• Having seen the vector based determination of the cutting forces, we can now look at equivalent calculations:

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Force Calculation

Where the Resultant force R is Given by

• We can write the cutting and thrust forces in terms of the shear force:

Velocity Calculations

• Having seen the vector based determination of the cutting forces, we can now look at equivalent calculations:
• V_c= Cutting velocity (ft/min) as set or measured on the machine
• V_s= Shearing velocity
• V_f= Frictional velocity

Cutting Force Vs Rake Angle α

• The effects of rake angle on cutting force are shown in the graph below,

The Merchant Equation

• To determine θ he assumed the minimum energy principle applied in metal cutting so that the deformation process adjusted itself to a minimum energy condition.
• Of all the possible angles at which shear deformation can occur, the work material will select a shear plane angle θ that minimizes energy, given by

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• Derived by Eugene Merchant

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What the Merchant Equation Tells Us

• To increase shear plane angle
• Increase the rake angle (α)
• Reduce the friction angle (β) or coefficient of friction

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Merchant's Force Circle

• Merchant's Force Circle is a method for calculating the various forces involved in the cutting process.
• Set up x-y axis labeled with forces, and the origin in the centre of the page. The scale should be enough to include both the measured forces. The cutting force (Fc) is drawn horizontally, and the tangential force (Ft) is drawn vertically. (These forces will all be in the lower left hand quadrant).

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Merchant's Force Circle

• Draw in the resultant (R) of Fc and Ft.
• Locate the centre of R, and draw a circle that encloses vector R. If done correctly, the heads and tails of all 3 vectors will lie on this circle.
• Draw in the cutting tool in the upper right hand quadrant, taking care to draw the correct rake angle (α) from the vertical axis.
• Extend the line that is the cutting face of the tool (at the same rake angle) through the circle. This now gives the friction vector (F).

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• A line can now be drawn from the head of the friction vector, to the head of the resultant vector (R). This gives the normal vector (N). Also add a friction angle (β) between vectors R and N. As a side note recall that any vector can be broken down into components. Therefore, mathematically, R = Fc + Ft = F + N.
• We next use the chip thickness, compared to the cut depth to find the shear force. To do this, the chip is drawn on before and after cut. Before drawing, select some magnification factor (e.g., 200 times) to multiply both values by. Draw a feed thickness line (t1) parallel to the horizontal axis. Next draw a chip thickness line parallel to the tool cutting face.

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• Draw a vector from the origin (tool point) towards the intersection of the two chip lines, stopping at the circle. The result will be a shear force vector (Fs). Also measure the shear force angle between Fs and Fc.
• Finally add the shear force normal (Fn) from the head of Fs to the head of R.
• Use a scale and protractor to measure off all distances (forces) and angles.

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Power and Energy Relationships

• There are a number of reasons for wanting to calculate the power consumed in cutting. These numbers can tell us how fast we can cut, or how large the motor on a machine must be. Having both the forces and velocities found with the Merchant for Circle, we are able to calculate the power,
• The power to perform machining can be computed from:

Pc = Fc . Vc in kw

Pc = Fc . Vc / 33,000 in HP

where

Pc = cutting power in KW

Fc = cutting force in KN

Vc = cutting speed in m/min

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Power and Energy Relationships

• Useful to convert power into power per unit volume rate of metal cut (power to cut one cubic inch per minute) Called unit power, Pu or unit horsepower, Hpu

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• where RMR = material removal rate
• Unit power is also known as the specific energy U

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Cutting Temperature

• Approximately 98% of the energy in machining is converted into heat
• This can cause temperatures to be very high at the tool‑chip
• The remaining energy (about 2%) is retained as elastic energy in the chip

High cutting temperatures

• Reduce tool life
• Produce hot chips that pose safety hazards to the machine operator
• Can cause inaccuracies in part dimensions due to thermal expansion of work material

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Process Parameters

• Speed (v), Feed (f), Depth of Cut (d)
• Material Removal Rate (MRR) = f x d x v

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Tool Wear

• Tools get worn out due to long term usage

Types of Tool Wear

Flank wear (VB)

• It occurs on the relief face of the tool and the side relief angle.

Crater wear (KT)

• It occurs on the rake face of the tool.

Notch wear or Chipping (VN)

• Breaking away of a small piece from the cutting edge of the tool

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Flank wear rate based on cutting speed

Tool Wear

Tool Life

• Tool life represents the useful life of the tool, expressed generally in time units from the start of cut to some end point defined by a failure criterion.

Tool Life Prediction

• Taylor’s tool life equation predicts tool failure based on flank wear of the tool

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where

• V is the cutting speed, t is the tool life,
• n is Taylor exponent.
• n=0.125 for HSS
• n=0.25 for Carbide
• n=0.5 for Coated Carbide/Ceramic
• C is a constant given for work piece material

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Machinability

• Machinability is a system property that indicates how easy a material can be machined at low cost.
• Good machinability may mean one or more of the following: cutting with minimum energy, minimum tool wear, good surface finish, etc.

Quantitative measures of machinability

• Machinability index: an average rating stated in comparison with reference materials. This measure can be misleading.
• Tool life: service time in minutes or seconds to total failure by chipping or cracking of the tool at certain cutting speed, or the volume of material removed before total failure.
• Surface finish produced at standardized cutting speeds and feeds.
• Others based on cutting force, power, temperature, or chip formation.

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Machinable Materials

Good machinable materials should have the following properties

• ductility, low strain-hardening exponent (n), low fracture toughness.
• Low shear strength (low TS), low hardness.
• A strong metallurgical bond (adhesion) between tool and work piece is undesirable when it weakens the tool material.
• Very hard compounds, such as some oxides, all carbides, many inter metallic compounds, and elements such as silicon, embedded in the work piece material accelerate tool wear, thus should be avoided.
• Inclusions that soften at high temperatures are beneficial.
• High thermal conductivity is helpful.

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Machinable Materials

Ferrous materials

• Carbon steels: annealed, heat-treated (spheroidized), cold worked
• Free-machining steels: special inclusions
• Alloy steels: hard
• Stainless steels: high strength, low thermal conductivity, high strain hardening rate
• Cast iron: white, gray, nodular cast iron

Non-ferrous materials

• Zinc, Magnesium, Aluminum alloys, Beryllium, Copper-based alloys, Nickel-based alloys and super alloys,
• Titanium, Plastics, composites.

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Factors Affecting Machining

Cutting Fluids

• a cutting fluid may be defined as any substance which is applied to a tool during cutting operation to facilitate removal of chip.

Function of cutting fluids

• to cool the cutting tool and the workpeice
• to lubricate the chip, tool and workpiece
• to help carry away the chips
• to lubricate some of the moving parts of the machine tools
• to improve surface finish
• to protect work against rusting

Types of cutting fluid

• Straight Oil (Petroleum based oils)
• Soluble Oil (water based oils)

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Requirement of a cutting fluid

• It should have long life, free of excessive oxide formation
• Suitable for variety of cutting tools and materials and cutting operations
• High lubricating qualities, high thermal conductivity and low viscocity to prevent easy flow and easy separation from impurities and chips and should not stick with workpeice or machine
• Transparent where high dimensional accuracy and fine finish are required in order to enable the operator to have clear view of tool and workpeice
• no fire or accident hazards or emit abnoxious odours or vapour harmful to operators

Cutting tool materials

Characteristics of an Ideal cutting-tool material

• remain harder than work material at elevated temperature
• material must withstand excessive wear even through relative hardness of tool-work material changes
• material must have sufficient strength and ductility to withstand shocks and vibrations and to prevent breakage
• coefficient of friction at the chip tool interface must remain low for minimum wear and reasonable surface finish
• cost and easeness of fabrication should be within reasonable limits

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Types of Tool Materials

• Carbon steels – low hardness- used for small work piece unto 12-15mm dia (200-250 C)
• Medium alloy steels – High carbon content with 5% alloy- (250-350 C)
• High speed steels – High red hardness- withstand upto 600-620 C
• Stellites – non ferrous alloy upto 1000 C & high cutting speed, very brittle – non metal cutting application
• Cemented carbides 1500 C;
• Tungsten-type cemented carbide
• Titanium-tungsten type carbide
• Ceramics
• Diamonds
• Abrasives

Nomenclature of Drill Bit

Nomenclature of Milling Cutter