Surface Roughness MCQ Quiz in मल्याळम - Objective Question with Answer for Surface Roughness - സൗജന്യ PDF ഡൗൺലോഡ് ചെയ്യുക

Concept:

Surface texture is the degree of finish conveyed to the machinist by a system of symbols devised by a Standards Association. These symbols provide a standard system of determining and indicating the surface finish.

Surface terms:

Flaws: Flaws are random irregularities or scratches, holes depression, seams, tear, or inclusions. These are defects caused during the machining operation.

Lay: It is the direction of the surface pattern caused by the machining and is visible to the naked eye.

Roughness: Roughness is the irregular deviation of the surface.

Roughness height: Roughness height, Ra is the deviation of the center line.

Roughness width: Roughness width is the distance between successive roughness peaks parallel to the nominal surface.

Waviness: Waviness is the recurrent deviation form a flat surface, it is measured and described in terms of the surface between adjacent crests of the waves (waviness width) and height between the crests and valleys of the waves (waviness height).

Standard symbol to represent surface texture:

Explanation:

• a = Waviness height
• b = Waviness depth
• c = Sample length
• d = Maximum roughness
• e = Directional lay
• f = Allowance
• g = Roughness width

Explanation:

Surface roughness

• 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 surface roughness are:

• Root mean square value and the arithmetic average or centre line 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).

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RMS value was a popular choice for quantifying surface roughness; however, this has been superseded by the centre line average value.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.

\({h_{rms}} = \frac{{\sqrt {h_1^2 + h_2^2 + \ldots + h_n^2} }}{n}\)

Centre Line Average Value

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

\(Ra = \frac{{{A_1} + {A_2} + \ldots + {A_n}}}{L}\)

Explanation:

Mecrin Instrument

• The Mecrin instrument assesses surface irregularities through frictional properties and the average slope of the irregularities.
• This gauge is suited for surfaces manufactured by processes such as grinding, honing, and lapping, which have low Ra values in the range 3–5 µm.
• Figure illustrates the working principle of this instrument.
• A thin metallic blade is pushed against the workpiece surface at a certain angle.
• The blade may slide or buckle, depending on the surface roughness and the angle of attack.
• At lower angles of attack, the blade tip will slide over the surface of the workpiece.
• As the angle of attack is increased, a critical value is reached at which the blade starts to buckle.
• This critical angle is a measure of the degree of roughness of the surface.
• The instrument is provided with additional features for easier handling.
• A graduated dial will directly give the reading of roughness value.

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)

Centre Line Average Value

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

\(Ra = \frac{{{A_1} + {A_2} + \ldots + {A_n}}}{L}\)

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.

So, Roughness values (0.2 to 0.8 ) microns are indicated by which symbol Three triangles.

Explanation:

There are basically two approaches for measuring surface finish:

• Inspection by comparison (Qualitative Analysis): In these methods, the surface texture is assessed by observation of the surface.
• Direct Instrument Measurement (Quantitative Analysis): Direct methods enable to determination a numerical value of the surface finish of any surface. It enables a numerical value to be assigned to the surface finish

The various methods which are used for comparison are:

• Touch Inspection
• Visual Inspection
• Microscopic Inspection (Master finished surface is placed under the microscope and is compared)
• Scratch Inspection
• Micro Interferometer
• Surface photographs
• Reflected Light Intensity
• Wallace Surface Dynamometer
• Diffraction technique

Some of the direct measurement instruments are:

• Tomlinson surface meter
• Profilometer
• Taylor-Hobson Talysurf

Stylus probe-type instrument:

Principle:

• When the stylus is moved over the surface which is to be measured, the irregularities in the surface texture are measured and it is used to assess the surface finish of the workpiece.

Working

• The stylus type instruments consist of a skid, stylus, amplifying device and recording device.
• The skid is slowly moved over the surface by hand or by motor drive.
• The skid follows the irregularities of the surface and the stylus moves along with the skid.
• When the stylus moves vertically up and down and the stylus movements are magnified, amplified and recorded to produce a trace. Then it is analyzed by an automatic device.

Advantage

• Any desired roughness parameter can be recorded.

Disadvantages

• Fragile material cannot be measured.
• High Initial cost.
• Skilled operators are needed to operate

Tomlinson Surface meter:

This instrument uses mechanical-cum-optical means for magnification.

Construction

• In this the diamond stylus on the surface finish recorder is held by spring pressure against the surface of a lapped cylinder.
• The lapped cylinder is supported one side by probe and other side by rollers.
• The stylus is also attached to the body of the instrument by a leaf spring and its height is adjustable to enable the diamond to be positioned and the light spring steel arm is attached to the lapped cylinder.
• The spring arm has a diamond scriber at the end and smoked glass is rest  on the arm.
• When measuring surface finish the body of the instrument is moved across the surface by a screw rotation.
• The vertical movement of the probe caused by the surface irregularities makes the horizontal lapped cylinder to roll.
• This rolling of lapped cylinder causes the movement of the arm. So this movement is induces the diamond scriber on smoked glass.
• Finally the movement of scriber together with horizontal movement produces a trace on the smoked glass plate and this trace is magnified by an optical projector.

Profilometer:

• It is an indicating and recording instrument to measure roughness in microns.
• The main parts of the instrument are tracer and an amplifier.
• The stylus is mounted in the pickup and it consists of induction oil located in the magnet.
• When the stylus is moved on the surface to be tested, it is displaced up and down due to irregularities in the surface.
• This movement induces the induction coil to move in the direction of permanent magnet and produces a voltage. This is amplified and recorded.

Taylor-Hobson-Talysurf:

• It is working a carrier modulating principle and it is an accurate method comparing with the other methods.
• The main parts of this instrument is diamond stylus (0.002mm radius) and skid.

Principle:

• The irregularities of the surface are traced by the stylus and the movement of the stylus is converted into changes in electric current.

Working:

• On two legs of the E-shaped stamping there are coils for carrying an A.C. current and these coils form an oscillator.
• As the armature is pivoted about the central leg the movement of the stylus causes the air gap to vary and thus the amplitude is modulated.
• This modulation is again demodulated for the vertical displacement of the stylus.
• So this demodulated output is move the pen recorder to produce a numerical record and to make a direct numerical assessment.

Concept:

Nose radius is the radius at the tip of the tool between the major and minor cutting edges.

It is provided to obtain a better surface finish and strength of the cutting tip of the tool.

The surface finish (R) can be calculated from the following relation,

\(h = \frac{{{f^2}}}{{8R}}\)

where f - feed rate; r - nose radius; h = surface roughness

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)

Centre Line Average Value

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

\(Ra = \frac{{{A_1} + {A_2} + \ldots + {A_n}}}{L}\)

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 surface roughness are:

• Root mean square value and the arithmetic average or centre line 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).

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.

\({h_{rms}} = \frac{{\sqrt {h_1^2 + h_2^2 + \ldots + h_n^2} }}{n}\)

Centre Line Average Value

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

\(Ra = \frac{{{A_1} + {A_2} + \ldots + {A_n}}}{L}\)

Explanation:

• Metallographic polishing, just like metallographic grinding, is the final stage in the sample preparation process of metals for subsequent analysis. Its purpose is to rectify the deformations caused by previous work steps (during sectioning and cutting). 
• Metallographic polishing consists of several steps, with each step using a finer abrasive than the previous one. Typically, a distinction between three operations is made: Pre-, intermediate, and final polishing.​

Metallographic Polishing in Three steps:

• The general aim of the first process step is to achieve the best possible material removal in the shortest amount of time.
• A good flatness of the surface after the grinding process can only be maintained by ensuring that the diamond is always in motion, always rolling, if possible. This rolling motion causes the necessary removal of of excess material.
• Consumables used: Diamond 15 µm to 9 µm / Hard-woven or hard-pressed cloths

• In the intermediate step, deformation and smear layers are removed.
• If the material is very hard, several intermediate steps may be necessary.
• Consumables used: Diamond 9 µm to 3 µm / Hard-woven or lightly pile cloths

• The last step is intended to remove deformations and especially smearing on the surface. This step is not easy but possible. However, a final-polished finish is not necessary for every task.
• Consumables used: Diamond 3 µm to 0.5 µm / Oxide polishing 0.1 µm to 0.06 µm / Pile or flocked cloths, foamed for oxide suspensions