Surface Roughness MCQ Quiz - Objective Question with Answer for Surface Roughness - Download Free PDF

Explanation:

Light Interference Microscope:

• A Light Interference Microscope is an advanced optical instrument that uses the principles of light interference to measure surface roughness with high precision. This technique is particularly beneficial for examining surfaces where traditional contact-based methods may cause damage or fail to achieve accurate results. The microscope relies on the interference patterns created by the interaction of light waves reflected from the surface being measured and a reference surface. These patterns provide detailed information about the surface texture, enabling precise measurement of roughness.

Soft or thin surfaces are best suited for measurement using a Light Interference Microscope due to several reasons:

• Non-contact Measurement: Light Interference Microscopes do not physically touch the surface being analyzed. This is crucial for soft or thin surfaces, as contact-based methods could deform or damage them, leading to inaccurate results.
• High Sensitivity: These microscopes are highly sensitive and can capture minute details of surface texture, which is particularly important for thin or delicate materials where even small imperfections can impact performance or aesthetics.
• Precision in Measurement: The interference patterns provide extremely precise data about surface roughness, making the method ideal for applications requiring detailed analysis of soft or thin surfaces.
• Versatility: Light Interference Microscopes can adapt to various material types without causing harm, making them suitable for thin films, soft coatings, or delicate substrates commonly found in industries like electronics, optics, and biotechnology.

Explanation:

Stylus Instruments

• Surface finish is an essential attribute in manufacturing processes, as it directly influences the functionality, aesthetics, and performance of components. To measure surface finish accurately, stylus instruments are widely used. These instruments are classified into two main types: contact and non-contact instruments. Let us delve into the details of these types and understand why they are the correct choice for measuring surface finish.

Contact Stylus Instruments:

• Contact stylus instruments work by physically touching the surface being measured. A stylus, usually made of a hard material like diamond, is drawn across the surface of the component. As the stylus moves, it traces the surface profile and records the variations in height and texture. These variations are then converted into electrical signals, which are processed to produce a graphical representation of the surface finish.

The key features of contact stylus instruments include:

• High Accuracy: Contact stylus instruments provide highly accurate measurements, making them ideal for applications requiring precise surface finish characterization.
• Wide Applicability: They are suitable for measuring various types of surfaces, including flat, curved, and intricate geometries.
• Detailed Analysis: Contact instruments can measure parameters such as roughness, waviness, and form, providing a comprehensive surface profile.
• Limitations: Despite their accuracy, contact stylus instruments may not be suitable for delicate or soft surfaces, as the physical contact can alter or damage the surface.

Non-contact Stylus Instruments:

• Non-contact stylus instruments measure surface finish without physically touching the surface. These instruments use optical or laser-based technologies to scan the surface and capture its profile.

The key features of non-contact stylus instruments include:

• No Physical Contact: Since there is no physical contact, non-contact instruments are ideal for measuring delicate, soft, or sensitive surfaces.
• High Speed: Non-contact instruments can measure surface finish quickly, making them suitable for high-throughput manufacturing environments.
• Versatility: They can measure surfaces with complex geometries, including hard-to-reach areas.
• Limitations: Non-contact instruments may struggle with highly reflective or transparent surfaces, as optical interference can affect measurement accuracy.

Explanation:

The Ra value is used to measure the roughness of a surface. Surface roughness is a critical parameter in engineering and manufacturing, as it affects the performance, durability, and appearance of a component. The Ra value, or the arithmetic average roughness, is a widely used metric for quantifying surface roughness. It is defined as the average deviation of the surface profile from the mean line, calculated over a specified length. This measurement provides a single value that represents the overall texture of the surface.

Importance of Surface Roughness Measurement:

Surface roughness measurement is essential in various industries, including automotive, aerospace, medical devices, and consumer electronics. The roughness of a surface can influence several factors:

• Friction and Wear: Rough surfaces tend to have higher friction and wear rates, which can affect the efficiency and lifespan of moving parts.
• Lubrication: Surface roughness impacts the ability of a surface to retain lubricants, which is crucial for reducing friction and wear in mechanical systems.
• Fatigue Strength: Surface irregularities can act as stress concentrators, reducing the fatigue strength of a material.
• Aesthetic Appearance: In consumer products, surface roughness can affect the visual and tactile qualities, influencing customer perception and satisfaction.
• Sealing and Adhesion: The roughness of a surface can impact the effectiveness of seals and the adhesion of coatings or adhesives.

Measurement Techniques:

Several techniques are used to measure surface roughness, including:

• Contact Profilometers: These devices use a stylus that physically traces the surface profile, recording the deviations to calculate the Ra value.
• Optical Methods: Techniques such as laser scanning and interferometry use light to measure surface roughness without physical contact.
• Atomic Force Microscopy (AFM): AFM provides high-resolution measurements by scanning the surface with a fine probe.

Calculation of Ra Value:

The Ra value is calculated using the formula:

Ra = (1/n) × Σ |Zi|

Where:

• n = Number of measured points
• Zi = Deviation of each point from the mean line

This formula gives the arithmetic average of the absolute deviations from the mean line, providing a single value that represents the surface roughness.

Applications:

Surface roughness measurements are used in various applications, including:

• Quality Control: Ensuring components meet specified roughness criteria for optimal performance.
• Process Optimization: Adjusting manufacturing processes to achieve desired surface finishes.
• Research and Development: Studying the effects of surface roughness on material properties and performance.

Understanding and controlling surface roughness is crucial for producing high-quality components that meet the requirements of various applications.

Analysis of Other Options:

Option 1: Flatness

Flatness is a measure of how much a surface deviates from a perfect plane. It is an important parameter in machining and assembly processes, where components must fit together precisely. Unlike roughness, flatness is concerned with the overall shape of the surface rather than the texture.

Option 3: Roundness

Roundness is a measure of how closely the shape of an object approaches a perfect circle. It is commonly used in the inspection of cylindrical parts such as shafts and bearings. Roundness is critical for ensuring the proper function of rotating components and minimizing vibration and wear.

Option 4: Straightness

Straightness refers to the extent to which a surface or edge deviates from a straight line. It is an important parameter in the production of linear components such as rails, beams, and shafts. Straightness ensures the proper alignment and function of mechanical systems.

In summary, while flatness, roundness, and straightness are important geometric parameters, they differ from roughness, which specifically measures the texture of a surface. The Ra value is a key metric for quantifying surface roughness, providing essential information for optimizing performance and quality in various applications.

Explanation:

Surface Finish:

• Surface finish refers to the texture and smoothness of a machined surface, which is influenced by the machining parameters and the material properties. Achieving a good surface finish is critical in manufacturing, as it impacts the functional performance, aesthetic appeal, and fatigue strength of the component.
• During machining, the interaction between the cutting tool and the workpiece material results in the removal of material in the form of chips. The surface finish is influenced by several factors, such as cutting speed, feed rate, depth of cut, tool geometry, and the formation of built-up edge (BUE). By optimizing these factors, manufacturers can achieve the desired surface quality.

Why Increased Cutting Speed Improves Surface Finish:

• At higher cutting speeds, the cutting tool interacts with the workpiece material at a faster rate, leading to reduced friction and heat generation at the cutting zone.
• Higher cutting speeds reduce the tendency for the formation of a built-up edge (BUE) on the cutting tool. The BUE is a common phenomenon in machining, where a layer of workpiece material adheres to the cutting edge of the tool. This adversely affects surface finish by creating irregularities and roughness on the machined surface.
• When the cutting speed is increased, the material removal process becomes smoother and more uniform, resulting in a finer and more consistent surface texture.
• Increased cutting speed enhances the shearing action and reduces the tearing effect on the material, thereby improving the quality of the machined surface.
• For most materials, there exists an optimal range of cutting speeds that minimizes tool wear while achieving the best surface finish. Operating within this range ensures efficient machining and superior surface quality.

Limitations of Excessive Cutting Speed:

• Excessive cutting speed can lead to increased tool wear and reduced tool life.
• It may cause excessive heat generation, which can affect the metallurgical properties of the machined surface and lead to thermal damage.
• High cutting speeds require advanced cutting tools and machine tools capable of operating at such speeds, which may increase operational costs.

Concept:

Surface roughness is a measure of the texture of a surface, characterized by the presence of peaks and valleys. It is an important parameter in quality control and can significantly affect the performance and longevity of mechanical components. Measuring surface roughness accurately is crucial for ensuring that components meet the required specifications.

There are various instruments available for measuring surface roughness, each with its own advantages and limitations. The choice of instrument depends on factors such as the required accuracy, the nature of the surface, and the specific application.

Explanation:

Let's analyze the given options to determine which instrument is most appropriate for measuring the surface roughness of a finely machined component:

Option 1: Optical Profilometer

An optical profilometer is a non-contact measurement device that uses light to create a three-dimensional map of a surface. It is highly accurate and can measure surface roughness at a very fine scale. This instrument is especially useful for measuring delicate or finely machined surfaces where contact methods might cause damage. Optical profilometers can measure a wide range of surface roughness parameters, including Ra (average roughness), Rz (average maximum height of the profile), and more. Given its precision and non-contact nature, an optical profilometer is ideal for measuring the surface roughness of finely machined components.

Option 2: Micrometer

A micrometer is a mechanical instrument used to measure small distances or thicknesses with high accuracy. While a micrometer is excellent for measuring the dimensions of a component (such as diameter or thickness), it is not designed to measure surface roughness. Surface roughness involves the measurement of micro-scale variations on the surface, which a micrometer cannot accurately capture. Therefore, a micrometer is not appropriate for measuring surface roughness.

Option 3: Dial Indicator

A dial indicator is a precision instrument used to measure small linear distances and deviations in the alignment of mechanical components. It is commonly used for tasks such as checking the flatness of a surface or the concentricity of a shaft. However, like the micrometer, a dial indicator is not suitable for measuring surface roughness. It does not have the capability to measure the fine-scale variations that characterize surface roughness.

Option 4: Vernier Caliper

A vernier caliper is a versatile measuring tool used to measure the dimensions of an object, such as its length, width, and depth. It is widely used in engineering and manufacturing for its accuracy and ease of use. However, vernier calipers are not designed to measure surface roughness. They are used for measuring macroscopic dimensions, not the microscopic variations that define surface roughness.

Conclusion:

Based on the analysis of the given options, the most appropriate instrument for measuring the surface roughness of a finely machined component is the Optical Profilometer. This instrument provides the required precision and non-contact measurement capability, making it ideal for accurately assessing the surface roughness of finely machined surfaces.

The correct answer is option 1: Optical Profilometer.

Explanation:-

The surface roughness on a drawing is represented by triangles.

Surface texture or roughness representation

The basic symbol consists of two legs of unequal length inclined at approximately 60° to the line representing the considered surface.

The symbol must be represented by a thin line.

The value of roughness is added to the symbols.

1. Roughness ‘a’ obtained by any production process.

2. Roughness ‘a’ obtained by machining.

3. Roughness ‘a’ obtained without removal of material.

If it is necessary to impose maximum and minimum limits of surface roughness, both values are indicated. a1 = Maximum limit; a2 = Minimum limit

Concept:

Center line average is given by

Where ∑a → sum of height in +y direction, ∑b → sum of height in –y direction,

n → number of division

Calculation:

∴ R_a = 1

Explanation:

The surface roughness on a drawing is represented by inverted triangles.

The basic symbol consists of two legs of unequal length inclined at approximately 60° to the line representing the considered surface.

The symbol must be represented by a thin line.

The value of roughness is added to the symbols.

1. Roughness ‘a’ obtained by any production process.

2. Roughness ‘a’ obtained by machining.

3. Roughness ‘a’ obtained without removal of material.

If it is necessary to impose maximum and minimum limits of surface roughness, both values are indicated. a₁ = maximum limit, a₂ = minimum limit.

Explanation:

The machined surface is having two types of irregularities:

1. Waviness: Large wavelength deviations are called waviness secondary texture is being produced by machine vibrations chatter errors in guideways etc. 
2. Roughness: Small wavelength fluctuations are called roughness, it appears due to improper selection of cutting fluid development of temperature on the rake face.

Asperities

• Asperities on the hard surfaces act as indenter that results in developing scratches on the soft surfaces due to the relative movement between the mating surfaces under abrasive wear conditions.

Surface roughness

• Whatever may be the manufacturing process, an absolutely smooth and flat surface cannot be obtained.
• The peak and valley 
• The machine element or parts retain the surface irregularities left after manufacturing.
• The surface irregularities are usually understood in terms of surface finish, surface roughness, surface texture or surface quality.

Evaluation of surface roughness:

1. Root mean square value: r.m.s. value
2. Centreline average (CLA) or arithmetic mean deviation (Ra)
3. Maximum peak to valley height (R_t or R_max)
4. The average of five highest peaks and five deepest valleys in the sample (R_z)
5. The average or levelling depth of the profile (R_p)

Root Mean Square Value

RMS value was a popular choice for quantifying surface roughness; however, this has been superseded by the centre line average value. The RMS value is defined as the square root of the mean of squares of the ordinates of the surface measured from a mean line.

Centre Line Average Value

It is defined as the average height from a mean line of all ordinates of the surface, regardless of sign.

Explanation

Surface roughness is represented by the triangle.

Explanation:

Roughness:

• The irregularities in the surface texture result from the inherent action of the production process.
• These will include traverse feed marks and irregularities within them.
• The roughness value of grade N1 is 0.025 microns.
• When components are produced either by machining or by hand processes, the movement of the cutting tool leaves certain lines or patterns on the work surface.
• This is known as surface texture.
• These are, in fact, irregularities, caused by the production process with a regular or irregular spacing which tends to form a pattern on the workpiece. 

Explanation:

Whatever may be the manufacturing process, an absolutely smooth and flat surface cannot be obtained. The machine element or parts retain the surface irregularities left after manufacturing. The surface irregularities are usually understood in terms of surface finish, surface roughness, surface texture or surface quality.

Evaluation of surface roughness:

1. Root mean square value: r.m.s. value
2. Centreline average (CLA) or arithmetic mean deviation (Ra)
3. Maximum peak to valley height (Rt or Rmax)
4. The average of five highest peaks and five deepest valleys in the sample (Rz)
5. The average or levelling depth of the profile (Rp)

The two most accepted methods of assessing the surface roughness are: Root mean square value and the arithmetic average or Centreline average value.

In both the methods, the surface roughness is measured as the average deviation from a nominal surface.

The value of surface roughness is expressed in micrometres (μm).

Important Points

Surface roughness is represented by the triangle.

Explanation:

Magnetic particle testing:

(i) The magnetic particle test is conducted to check for very small voids and cracks at or just below the surface of a casting of a ferromagnetic material.

(ii) The test involves inducing a magnetic field through the section under inspection.

(iii) The powdered ferromagnetic material is spread out onto the surface. The presence of voids or cracks in the section results in an abrupt change in the permeability of the surface; this, in turn, causes leakage in the magnetic field. The powdered particles offer a low resistance path to leakage. Thus, the particles accumulate on the disrupted magnetic field, outlining the boundary of a discontinuity. 

Magnetic particle technique:

• Yoke technique
• Prods technique
• Coil technique
• Central conductor

Explanation:

The surface roughness has a vital role in influencing functional characteristics like wear resistance, fatigue strength, corrosion resistance and power loss due to friction. Unfortunately, normal machining methods like turning, milling or even classical grinding cannot meet this stringent requirement.

Therefore, superfinishing processes like lapping, honing, polishing, burnishing are being employed to achieve and improve the above-mentioned functional properties in the machine component.

Lapping Process:

In the lapping process, small amounts of material are removed by rubbing the work against a lap charged with a lapping compound. Lapping is a precision finishing operation carried out using fine abrasive materials. 

Filing:

Filing is one of the methods of removing small amounts of material from the surface of a metal part. A file is a hardened steel tool, having slant parallel rows of cutting edges or teeth on its surfaces. On the faces the teeth are usually diagonal to the edge. One end of the file is shaped to fit into a wooden handle. Figure shows the parts of a hand file.

Milling:

Milling is a process of producing flat and complex shapes with the use of multi-point (or multi-tooth) cutting tool. The axis of rotation of the cutting tool is perpendicular to the direction of feed, either parallel or perpendicular to the machined surface.

Explanation:

Stylus based measurement:

• The stylus system of measurement is the most popular method to measure surface finish.
• The operation of stylus instruments is quite similar to a phonograph pickup.
• In most stylus-based instruments, a stylus drawn across the surface of a component being inspected generates electrical signals that are proportional to the changes in the surface asperities.
• An electrical means of amplifying signals, rather than a purely mechanical one, minimizes the pressure of the stylus on the component.
• Changes in the height of asperities may be directly read by a meter or a chart.
• Most instruments provide a graph of the stylus path along the surface. Examples:
• Tomlinson Surface Meter
• Taylor-Hobson Talysurf
• Profilometer

Tomlinson Surface Meter

• This is a mechanical-optical instrument.

• The sensing element is the stylus, which moves up and down depending on the irregularities of the workpiece surface.
• The stylus is constrained to move only in the vertical direction because of a leaf spring and a coil spring.
• The translatory motion of the stylus causes rotation of the cross roller about point A, which in turn is converted to a magnified motion of the diamond point.
• The diamond tip traces the profile of the workpiece on a smoked glass sheet. The glass sheet is transferred to an optical projector and magnified further.
• Typically, a magnification of the order of 50-100 is easily achieved in this instrument.