Exam Questions Flashcards
(32 cards)
Describe, with a sketch graph, how the hardness of a steel workpiece would be expected to influence tool life. Suggest a processing sequence to achieve a rapid production route for machined steel components with a high final hardness.
- What the effect of the ratio of pearlite to ferrite shows is that there is a great advantage to machining the steel in the annealed condition
- This suggests that rapid production will be best achieved by beginning in an annealed condition (by taking the components through an annealing process if necessary) and then after machining taking the components through either a complete hardening process, if the whole body of the material needs to be in a hardened condition, or through a surface hardening process if it is only surface hardness that is needed, e.g. to resist wear.
Define ‘accuracy’ and ‘precision’. Explain how each is quantified in the context of machining operations.
Precision: Repeatability, exactness, and/or the ability to obtain the same quantity (of a dimension, say) over and over. Quantified as a function of standard deviation (statistical spread) of the quantity.
Accuracy: Being “on target” with a specification. Quantified by the bias, or difference, between the obtained result and the desired result.
Describe the factors that cause variation in workpiece dimensional accuracy during machining. Suggest, in each case, the measures which can be taken to minimise the variation.
- The factors that affect workpiece accuracy stem from the machine tool itself, the tool actions and the workpiece material. Sources of error from the machine design, environmental effects, machine workzone, the cutting tool and the workpiece are detailed in the figure. They can be broken down into static and dynamic effects. A good discussion would include the following issues:
Describe the internal and external sources of dynamic disturbances during machining operations and their effects. How can we identify whether a disturbance is caused by internal or external sources?
Explain how the effects of dynamic disturbances can be minimised.
- Dynamic disturbances are caused due to vibrations caused by external or internal processes. Machine tools are subject to three types of vibrations: free vibrations, self-excited vibrations and forced vibrations. Free vibration occurs in the absence of a long-term, external excitation force. This is not of much importance to us in this context.
- Self-excited vibrations (also known as “chatter”) occur because the dynamic cutting process forms a closed-loop system. Disturbances in the system (i.e., vibrations which affect cutting forces) are fed back into the system and over time, under appropriate conditions, may result in instability.
- Self-excited vibrations result from variations in chip thickness (recall earlier lectures), depth of cut, and cutting speed, which result in cutting force variations. Force variations can lead to machine vibration, which in turn can cause additional force fluctuations by inducing variations in the uncut chip thickness. The variations of the uncut chip thickness due to vibrations during the previous pass or previous tooth cause additional force fluctuations. When the dynamic cutting force is out of phase with the instantaneous relative motion between the tool and workpiece, this leads to the development of self-excited vibrations.
- Forced vibration takes place when a continuous, external periodic excitation produces a response with the same frequency as the forcing function (after the decay of initial transients). Two types of independent excitation exist:
- Harmonic
- Pulsating
- When the disturbance is harmonic in nature, the machine tends to vibrate at the forcing frequency. Examples of such disturbances are unbalanced rotating masses, bearing irregularity. However, when the disturbance is pulsating in nature, the machine tends to vibrate at its natural frequency. Examples of such disturbances are cutting forming forces on hammers, interrupted cuts on metal cutting machines, and vibrations from the floor. How can we identify whether a disturbance is caused by chatter or by external sources? Self-excited vibrations disappear when the cutting stops; forced vibrations will exist and will persist regardless of whether or not the tool is engaged. The characteristic features of chatter are: (i) the amplitude increases with time, until a stable limiting value is attained; (ii) the frequency of the vibration is equal to a natural frequency or critical frequency of the system; and (iii) the energy supporting the vibration is obtained from a steady internal source.
In machining operations, tool temperatures are low at low cutting speeds and high at high cutting speeds, but low again at even higher cutting speeds. Explain why.
At low cutting speeds, energy is dissipated in the shear plane and at the chip tool interface and conducted through the work piece and/or tool and eventually to the environment. At higher speeds, conduction cannot take place rapidly enough. At even higher speeds, the chips will carry the heat away, hence the workpiece will remain cooler. This is one of the main advantages of high speed machining.
Explain why negative rake angles are generally preferred for ceramic, diamond, and cubic boron nitride tools.
Although hard and strong in comrpession, these materials are brittle and weak in tension. Consequently, negative rake angles, which indicate larger included angle of the tool tip are preffered mainly because of the lower tendency to cause tensile stresses and chipping of the tools.
Explain why the cutting force, Fe, increases with depth of cut and decreasing rake angle.
Increasing the depth of cut means more material being removed per unit time. Thus, all other parameters remaining constant, the cutting force has to increase linearly because the energy requirement increases linearly.
As the rake angle decreases, the shear angle decreases and hence the shear strain increases. Therefore, the energy per unit volume of material removed increases, thus the cutting force has to increase.
With appropriate diagrams, show how the use of a cutting fluid can affect the magnitude ofthe thrust force, Ft, in orthogonal cutting.
The use of a cutting fluid will reduce the friction force, F, at the tool-chip interface. This, in turn, will change the force diagram, hence the magnitude of the thrust force, Ft.
Consider the sketch given below. The previous sketch shows cutting without an effective cutting fluid, so that the friction force, F is large compared to the normal force, N. The sketch below shows the effect if the friction force is a smaller fraction of the normal force because of the cutting fluid. As can be seen, the cutting force is reduced when using the fluid. The largest effect is on the thrust force, but there is also a noticeable effect on the cutting force, which becomes larger as the rake angle increases.
In the context of a milling tool explain what is meant by rake angle. Discuss the effects of rake angle on the cutting process, in terms of accuracy, cutting forces, tool life, surface finish and other relevant parameters.
There are two distinct tool geometries, positive and negative rake angles.
Positive is suitable for machining soft, ductile materials (like aluminum) and negative is for cutting hard materials, where the cutting forces are high (Hard material, high speed and feed). They offer the following characteristics:
- Makes the tool more sharp and pointed. This reduces the strength of the tool, as the small included angle in the tip may cause it to chip away.
- Reduce cutting forces and power requirements.
- Helps in the formation of continuous chips in ductile materials.
- Can help avoid the formation of a built-up edge.
Negative rake angles are often used with high strength cutters such as carbide inserts, they offer the following process characteristics
- Make the tool more blunt, increasing the strength of the cutting edge.
- Increase the cutting forces.
- Can increase friction, resulting in higher temperatures.
- Can improve surface finish.
Describe the main factors in machine tool design and operation that can lead to dimensional error in the machining of a part.
What are the consequences of designing a machine tool with low stiffness and how can these be addressed?
Low stiffness leads to a collection of vibrations in the machining system as a whole. There are two causes of this:
- Forced Vibrations. Caused by some periodic force present in the machine tool, such as from gear drives, imbalance of the machine tool components, misalignment, or motors and pumps.
- Self excited vibration or ‘chatter’. In machining, chatter is the vibration that feeds on itself as the tool moves across the part. The tool, tool-holder and spindle together will vibrate at some natural frequency—a frequency at which this assembly “naturally” wants to vibrate. In fact, the assembly is likely to vibrate at more than one such natural frequency at the same time. At the tool tip, this vibration leaves waves in the machined surface. The waviness can cause the next cutting edge to experience a variable load. When that happens, this variable load feeds the vibration that already exists, making it worse. “Self-excited vibration” is one term for this phenomenon. “Regenerative chatter” is another.
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Adverse Effects
- Poor surface finish
- Loss of dimensional accuracy
- Premature wear, chipping, and failure of the cutting tool
- Damage to the machine tool components from excessive vibration
- Objectionable acoustic emission, such as high frequency squeal
- A serious consequence relates to efficiency. Most shops deal with chatter by setting their machining parameters low. Therefore, instead of tool strength and spindle horsepower defining the metal removal rate, chatter becomes the limiting factor that keeps the process from reaching its potential.
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How can these be addressed?
- The basic solution to forced vibrations is to isolate or remove the forcing element. If this is not possible, the amplitude of the vibration can be reduced by increasing the stiffness or damping of the system.
- For self excited vibrations, Increasing rigidity is one option. Use a shorter tool or tool-holder, or switch to a tool-holder that clamps the tool more rigidly—these are examples of changes that might make the process less apt to vibrate. When milling at high spindle speeds, there is a potentially more promising option. Certain limited ranges of spindle speed may be stable zones. Within these ranges, the rate of cutting edge impacts synchronizes with a natural frequency of the system. The chip load becomes level, so the cut is smooth. The depth of cut can therefore be increased, and sometimes it can be dramatically increased. A stable value of spindle rpm applies only to a particular combination of spindle, tool, and tool-holder. These three factors make up a complete assembly. Every different assembly has to be evaluated separately; evaluating just the spindle is not enough. However, any particular spindle and tooling assembly is likely to have more than one stable zone. Therefore, finding just one stable speed zone might not be enough. Another, much faster speed may also be stable.
What properties of the work material have a significant influence on the success of a machining operation?
Material properties that have an influence on the ability of a material to be machined are: properties such has density, hardness, toughness, thermal conductivity, yield stress, chemical affitnity to the cutting tool material, coefficient of friction with the cutting tool material.
What criteria would you use to assess the success or otherwise of a particular machining operation for a specific workpiece material?
Various criteria are used to assess the success of a machining operation. The most important of which is (1) tool life because of its economic significance in a machining operation. Other criteria include (2) cutting forces (which determines the power of the machine), (3) accuracy and surface finish, and (5) ease of chip disposal. It should be recognized that machining performance depends on more than just material. The type of machining operation, tooling, and cutting conditions are also important factors. One work material may yield a longer tool life, whereas another material provides a better surface finish. All of these factors make evaluation of a material’s suitability for machining a complex
Discuss the characteristics of titanium that make it difficult to machine.
Titanium is a difficult material to machine and delivers relatively poor tool life, even at low cutting speeds. Answers to this question should discuss the following material aspects
- High chemical reactivity causes chips to gall and weld to cutting edges
- Low thermal conductivity increases cutting temperatures
- Usually produces abrasive, tough, and stringy chips
- Precautionary measures are needed since it is a reactive (combustable) metal
- Low elastic modulus easily causes deflection of work piece
- Easy work hardening.
Describe the composition and physical attributes of a cutting tool material that would be most suitable for machining titanium.
- An ideal cutting tool material for titanium would be a cemented carbide such as WC-Co, also useful for other nonferrous metals; In the non-steel-cutting grades, grain size and cobalt content are the factors that influence properties of the cemented carbide material. The typical grain size found in conventional cemented carbides ranges between 0.5 and 5 mm. As grain size is increased, hardness and hot hardness decrease, but transverse rupture strength increases.
- Cemented carbides with low percentages of cobalt content (3% to 6%) have high hardness and low TRS, whereas carbides with high Co (6% to 12%) have high TRS but lower hardness. Accordingly, cemented carbides with higher cobalt are used for roughing operations and interrupted cuts (such as milling), while carbides with lower cobalt (therefore, higher hardness and wear resistance) are used in finishing cuts.
Calculate the total machining time, material removal rate, and the power required where D1 = 100 mm, D2 = 94 mm, O = A = 5 mm, Lw = 80 mm, cutting velocity Vc = 500 mm s-1 , feed speed So = 1 mm rev-1 , and depth of cut per pass t = 1 mm. Assume the specific energy for cutting titanium alloy is 3.5 J mm-3 .
How might you improve the accuracy of your approach?
- This approach has limited accuracy due to the fact that the calculation of tangential cutting velocity Vc, is based on average Davg, or (D1+D2)/2. The error associated with this approach scales with the value of D1-D2. This is not a problem for finishing cuts, although large scale roughing cuts will see variations in machining times from those calculated as the tangential velocity reduces as D2 reduces, serving to increase machining times. In order to correct this it would be necessary to develop an equation for Vc as a function of radius (non-trivial), or to segment the calculations by developing solutions across a number of discrete steps when turning, thereby minimising the error in cutting velocity. In reality however, as the tangential velocity reduces, so does the work piece diameter and the cut path length. This has the effect of offsetting the error in Vc.
Why is it important to have knowledge of the force components in machining operations?
Cutting is a process producing considerable stresses and plastic deformations. The high compressive and frictional contact stresses on the tool face result in a substantial cutting forces.
Knowledge of the force components is essential for the following reasons:
- proper design of the cutting tools
- proper design of the fixtures used to hold the workpiece and cutting tool
- calculation of the machine tool power
- selection of the cutting conditions to avoid an excessive distortion of the workpiece
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Explain what is meant by the following terms: statistical process control; chance causes of variation; assignable causes of variation.
- Statistical Process Control. If a product is to meet or exceed customer requirements, it should be produced by a process that is stable or repeatable, i.e it should be produced with little variability around a target dimension or quality characteristic. Statistical Process Control or SPC is a collection of problem solving tools useful in achieving manufacturing process stability through the reduction of variability.
- Chance causes of variation are those that are produced by phenomena constantly active within a system that have predictable variation. They are said to be ‘in control’
- Assignable causes of variation are produced by new, unanticipated, emergent or previously neglected phenomena within the system. This variation is inherently unpredictable and is said to be ‘out of control’
Explain, with reasons, why control charts are widely used in the precision machining industry.
- Control charts are a proven technique for improving productivity. A successful control chart program will reduce scrap and re-work.
- Control charts are effective in defect prevention. The control chart helps to keep the process in control delivering a ‘right first time’ philosophy
- Control charts prevent unnecessary process adjustment. Adjusting processes based on tests unrelated to control charts, will often lead to an overreaction to the background noise of the process.
- Control charts provide background information. Frequently, the pattern of points will contain information of diagnostic value to an experienced operator.
- Control charts provide information about process capability. It provides information about the value of important parameters and their stability.
A precision machine shop wishes to establish a procedure for using a control chart during a turning operation for a particular part.
Describe the detailed steps to be taken when setting up and implementing a control chart for this operation, including any appropriate metrological methodologies.
The operator needs to know if the process meets the part specification by performing a capability study. In this case the machining operation is designed to establish particular dimension of the part, lets say diameter, and control limits need to be determined.
- If a process is in control, one could expect the control limits to be approximately 75% of the tolerance, centered within the tolerance band
- The process should be set-up for turning with a suitable tool material and standard process parameters for the tool-material combination
- The operator should decide how to measure the parts and how frequently these measurements need to be taken.
- Metrology. A measurement technique needs to be chosen that can measure the resulting part diameters to the level of precision required. This could be a direct contact gauge or a non-contact optical gauge. Given that in our case, the diameter is of interest, and is being processed with a high level of precision, there are a number of variations that can occur when taking these measurements. The diameter will vary both around the part and along the part, care needs to be taken to measure at a point along the part which is consistent from part to part, i.e avoid areas that are likely to taper such as at the end of the part, and to measure a number of diameters around the part in order to gain better insight into the process. One could also use this data to determine the level of roundness through determining the Range of the data.
- The operator should run the process to produce a large number of parts. Diameter data should be collected, with the mean and standard deviation determined.
- The mean and standard deviation can be used to determine the UCL and LCL limits (the limits that define variation due to natural causes).
- We now have a view of the limits of the machines normal capability, and can collect then plot data on the control chart to observe the process performance with time.
- If the measured data drifts outside of the control limits, we have part variations due to assignable causes, and action must be taken to correct the process (perhaps due to tool wear or machine variation).
- This acquisition and plotting process should continue at timed intervals throughout the production operation, taking a sample from time to time. The frequency of sampling can be altered if part variation is seen to increase, or decrease.
Explain what is meant by the terms surface roughness and waviness. Discuss how these can be influenced in machining operations, and why they are difficult to control.
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Surface roughness or texture is the measure of the finer surface irregularities in the surface and is composed of two main components: roughness; and waviness (form). These are the result of the manufacturing process employed to create the surface.
- The ability of a machining operation to produce a specific surface roughness depends on many factors. For example, in end mill cutting, the final surface depends on the rotational speed of the end mill cutter, the velocity of the traverse, the rate of feed, the amount and type of lubrication at the point of cutting, and the mechanical properties of the piece being machined.
- Waviness is most often the result of small fluctuations in process conditions such as changing distances between the cutting tool and the surface of the workpiece. These fluctuations may be caused by cutting tool wear or worn machine bearings, both of which generate unbalanced conditions, chatter, vibration and instability in the machining setup. It is important to eliminate sources of imprecision in machine tools in order to improve surface finish and form.
- The problem of erroneous movement that leads to roughness and waviness is very difficult to eliminate completely. Despite attempts to create optimised machine designs, there is a limit to the accuracy and surface finish that could be achieved. Errors induced by thermal deformation of the machine structure, cutting force deformation, or tool wear etc., cannot be completely eliminated by detailed machine designs. Even with the advent of ceramic materials technology within modern machine spindles and machine beds, these approaches are still subject to changes that occur in the process or in the machine shop environment on a day to day basis.
Describe methods by which cutting force can be measured in a turning operation. Explain how cutting force measurement provides information about machining performance
- Cutting forces can be directly measured with the use of an in-process force sensor or dynamometer, or indirectly measured from the use of currents drawn by servo motors within the machine tool. The error compensation system monitors the condition of the machine continuously and any error that may be generated is compensated for accordingly during the machining operation in order to effect greater control of surface finish and form.
- Cutting force is the most sensitive indicator of machining performance. It determines the requirements of the machining system to induce shearing along the cutting direction. It depends on the tool geometry, tool material, process settings such as rake angle, and coolant level. Cutting force can have both static and dynamic components depending on the stability of the process settings which are determined by the stability of the machine.
- Error compensation allows the traditional inaccuracies of machining operations to be overcome. Errors induced as a result of the cutting force variation due to the cutting action, caused either by excessive deformation at the tool/workpiece interface, or excessive deformation of the machine tool structure, lead to increased surface roughness or errors in the surface form (waviness) of the workpiece. Measurement of cutting forces during machining allows the machine to compensate for the errors caused by cutting force variations.
List the three classes of machine tool vibration and describe their origins
The machine, cutting tool, and the workpiece form a structural system that has complicated dynamic characteristics. Under certain conditions, vibrations of the structural system can occur which can be divided into three classes
- Free or transient vibration: resulting from impulses transferred to the structure through its foundation, from rapid motion of heavy masses such as machine tables, or the engagement of the cutting tool. The structure is deflected and oscillates at its natural frequency until the inherent damping causes this vibration to slowly fall away.
- Forced vibrations: resulting from periodic forces of the system such as imbalanced masses or periodic cutting actions as in multi-tip tools or vibration transmission from nearby machinery. An important consideration when choosing machine tool location in workshops. The machine tool will oscillate at the driving frequency and if this is close to the resonant frequency, the tool will vibrate in natural mode.
- Self excited vibration: resulting from a dynamic instability: usually resulting from a dynamic instability of the cutting process. This phenomenon is referred to as ‘chatter’ and operates at a natural mode of vibration.
