Fundamentals of machining chapter 21

Name the factors that contribute to the formation of discontinuous chips

- chip produced during metal cutting - broken off into smaller pieces due to shear force of the cutting piece - smaller pieces stacked on top of each other - brittle materials due to low shearing capacity - materials with hard inclusions and impurities such as gray cast iron - large depth of cut, low rake angle, ineffective cutting fluid and low stiffness of machine tool

Explain why continuous chips are not necessarily desirable.

- metal (usually ductile material) cutting - as a result of internal shearing: 1. cutting force exerted by cutting tool in primary shear zone 2. frictional force of surface of cutting tool where secondary shear zone develops - smooth surface finish - chips entangle in the tool holder and workpiece and can also interfere with chip removal system. Can be adjusted by varying speed, orientation or cutting fluid

What is the cutting ratio? Is it always less than 1? Explain.

- ratio between the depth of cut (to) to the chip thickness (tc) - used to calculate rake angle (alpha) or shear angle (theta) - less than because chip thickness is always larger than depth of cut Reason 1: chip thickness is the measure of a shearing cut along shear angle with diagonal orientation and depth of cut is perpendicular to the surface of the piece to be cut Reason 2: added chip thickness due to deformation of chip during shearing

Material for the cutting tool

1. Carbides / cermets. Optimal balance between hot hardness, wear resistance, strength and toughness. Mainly tungsten and titanium carbides. 2. High Speed Steel. Machine faster than previously thought with temperatures between 500 and 600. Inexpensive and can be coated for optimal performance. Positive rake angles and machines with low stiffness.

Given your understanding of the basic metal-cutting process, what are the important physical and chemical properties of a cutting tool?

1. High hardness and strength so the tool can cut through the workpiece without breaking or chopping off. 2. High wear resistance in order to prevent the variation to the pattern it creates and optimize dimensional accuracy. 3. High resistance to thermal expansion so it doesn't change shape during a high temperature cutting operation. Also, shouldn't combust 4. Chemically inert so it doesn't react with the metallic workpieces.

List the factors that contribute to poor surface finish in cutting.

1. Poor surface integrity. Affects the end product of cutting due to nature of the surface after being processed. E.g. metals that are too conjugated, they produce a rough or porous surface. 2. Low tool life. Cutting tool deteriorates quickly upon constant usage, especially the edge. Affects tool's effectiveness to cut and surface finish 3. High power and energy requirement to cut workpiece. 4. Poor management of chips. Accumulate and impede cutting operation.

Explain the difference between discontinuous chips and segmented chips.

1. Segmented chips for materials with low thermal conductivity, discontinuous chips for brittle materials 2. Segmented chips don't fully separate from each other but are connected by a small part. Shear localization develops large areas of low shear stress and small areas of high shear stress depending on the distance from the shearing point. 3. Continuous chips are loose from each other and produce fully serrated chips. Brittle materials don't have a tendency to elongate significantly and tend to break off.

Comment on the role and importance of the relief angle.

Angular distance between the cutting tool and the surface of the tool being cut, about 5 degrees on average Prevents rubbing between the cutting tool and the work piece If it's too large, tool tip can fail and break If it's too small, flank wear may be excessive

What is a BUE? Why does it form?

Built up edge Layer of chip builds up in front of the workpiece until it accumulates, becomes unstable and breaks off More obstruction in front of cutting piece, possibility of dulling the cutting edge will increase along with adverse effects on surface finish Thin BUE accumulation is desirable because its flakes serve as protection against tool wearing

Nose wear

Cutting edge loses its sharpness and becomes round Reduces effectiveness of the cut of the tool Increases heat developed due to the friction on the edge

Flank wear

Due to friction between the flank surface and the machined surface Looks like a portion of the cutting piece is scratched off the edge of the tool, decreases effectiveness of cut and sharpness of the tool

What is the function of chip breakers? How do they function? Do you need a chip breaker to eliminate continuous chips in oblique cutting? Explain.

Feature of cutting tool that prevent build up of continuous chips. Mounted on top surface of the cutting tool, where chip build up usually takes place Directs chip into a much steeper angle from the surface until it eventually breaks off and discontinues. Oblique cutting doesn't require this as chips are usually discarded to the side and rarely build up on the surface of cutting tool due to the angle of approach to the blade

Explain the characteristics of different types of tool wear.

Flank wear Crater wear Nose wear Notching Plastic deformation Chipping Gross fracture

Factors that affect the choice of the cutting tool material

Fracture toughness, wear resistance, thermal resistance, chemical stability, fatigue resistance, price, machinability, available shape and dimensions

Why are tool temperatures low at low cutting speeds and high at high cutting speeds?

High level temperatures are attributed to internal residual stresses due to shear chips and friction at the tool chip interface. Heat is initially contained in the chip, then the workpiece, the cutting tool and eventually released to the surroundings. Low cutting speed means the production rate of heat is low, so the conducted heat of the system can easily be dissipated to the environment. High cutting speed means heat production rate is high and the system doesn't have sufficient time to dissipate the heat. Heat accumulates and increases the temperature of the system.

Why does the temperature in cutting depend on the cutting speed, feed, and depth of cut? Explain in terms of the relevant process variables.

Higher cutting speed and feed rate mean more frequent contact time and less downtime for the machine. More contact time means more heat produced due to friction. Less downtime prevents the heat dissipation of the operation to the environment, thereby further building up the heat during operations. Higher depth of cut, more shear force supplied, more chips produced, more friction

Brittle materials rake angle

Higher shear strain in segmented chip scenarios so a negative rake angle should be used to create more plastic deformation for the chips using compressive forces Hard and brittle materials cut better for under compressive forces. Mostly used for cast iron and polymers

The cutting force increases with the depth of cut and decreasing rake angle. Explain why.

Increased depth of cut, more material present in the form of chips to impede the cutting tool, more force required to overcome this. Decreasing rake angle, wider angle of cutting tool and blade, making cut much wider, chip production angled more upwards and forced into the phase of the tool. Friction and wide cut cause higher cutting forces

Why is it not always advisable to increase the cutting speed in order to increase the production rate?

Increasing cutting speed decreases tool life so it's not advisable An optimal cutting speed needs to be chosen so it has a minimum effect on tool life and maximum machining rate

The Taylor tool-life equation is directly applicable to flank wear. Explain whether or not it can be used to model tool life if other forms of wear are dominant.

It is applicable to all types of wear because all types of wear are affected by the cutting speed and therefore affect tool life.

Explain whether it is desirable to have a high or low (a) n value and (b) C value in the Taylor tool-life equation.

Large C + Small n increases the tool life Small C + large n decreases tool life C - constant attributed to workpiece material n - constant attributed to tool material

As shown in Fig. 21.14, the percentage of the total cutting energy carried away by the chip increases with increasing cutting speed. Why?

Less time for chip to transfer heat energy to other parts of the cutting operation such as the workpiece and the cutting tool

Tool life can be almost infinite at low cutting speeds. Would you then recommend that all machining be done at low speeds? Explain.

Low cutting speed, production rate also reduced, tool will be used for longer so it is unnecessary. This is impractical and expensive for an actual business because more resources such as energy and manpower would be used to produce a smaller output in the same time

Identify the forces involved in a cutting operation. Which of these forces contribute to the power required?

Main forces that contribute to the cutting power are the cutting force and the thrust force. Cutting force provides energy required to cut the workpiece. Thrust force provides the energy required to keep the cutting tool in contact with the workpiece. Resultant force of cutting and thrust force. Frictional force is the contact friction force between the chip and the cutting tool Normal force is the counterforce between the chip and the tool surface

Is material ductility important for machinability? Explain

Mostly with ferrous metal which is a ductile material Chips more likely to accumulate and produce BUE If thin, can protect tool from wear Harmful if BUE has accumulated so much it impedes the path of the cutting tool and causes a poor surface finish

Notching

Phenomenon where the depth of cut is not uniform throughout the machined surface Noticeably uneven

Chipping

Portion of cutting edge breaks off due to wear or brittle material properties

Explain the difference between positive and negative rake angles. What is the importance of the rake angle?

Positive rake angle has a sharp edge and is directs chips away from the movement of the cutting tool.It requires less force to cut and prevent accumulation of chips. Negative rake angle has a blunt edge and directs chips towards the movement of the cutting tool. It requires a stronger cutting edge and cutting force. It produces a smooth surface finish. However, it also has a tendency to be subjected to high friction forces, temperatures and chip accumulation in front of the cutting tool. Rake angle affects the shear angle and the thickness of the chip developed.

Crater wear

Repeated contact and friction between the rake tool and the machined surface. Wearing develops on the rake face of the tool Normal force applied by the chip to the rake face contributes to the crater wear since this force increases friction between tool and chip

Explain why studying the types of chips produced is important in understanding cutting operations.

To understand how the cutting process will affect the workpiece. Chipping type directly affects the machinability of the workpiece, namely surface finish and power required to process it. E.g. increasing built up edge affects surface finish and power increasingly. More chip build up in front of the tool, more power required to pass through the impeding chips

Gross fracture

Type of chipping where chipped of piece of cutting tool is so big that it changes the tools shape. Mechanical shock because force is too large for the edge Thermal fatigue due to subjection of high and low temperatures.

Tool holder has inserts for ...

chip breaking

Coating increased tool life by

creating low friction coefficient higher hot hardness wear resistance chemical inertness

Lubricants and their objective

e.g. oil, emulsion, synthetic reduces friction and wear, cool down cutting zone, flush away chips, protect cutting zone from environmental corrosion

Plastic deformation

heat developed, mostly due to friction, becomes high enough for the material to soften and deform significantly

Larger relief angle means ...

lower friction resistance between workpiece and cutting edge

Heat generated due to ...

plastic deformation caused by shear stresses and friction at the tool chip interface

BUE

plastically deform a material so much that it remains on the cutting edge, compresses and attaches to the tool

Cutting

process of removal of chips using shear forces

Cutting edge

sides of the cutting tool

Why cutting edge tool angle has to be as big as possible?

stronger tool more dissipation of heat