chapter 7 measurement of surface finish

Dr. Dayananda Pai

MIT, Manipal

Contents Introduction

Definitions

Symbols used in surface finish

Analysis of traces

Taylor – Hobson “Talysurf”

simple numerical on surface roughness

Dayananda Pai, Aero & Auto Engg., Dept. MIT, Manipal

Introduction Functioning of machine parts, load carrying capacity,

tool life, fatigue life, bearing corrosion, and wear qualities of any component of a machine have direct bearing with its surface texture. Therefore, these effects made the control of surface texture very important


Introduction The root of any surface irregularity acts as sharp corner

and such part fails earlier.

Thus in order to increase the life of any part which is subjected to repeated reversals of stress, the working and non-working surfaces of that surface must be given very good finish.


Introduction Good bearing properties in any part are obtained when

the surface has large number of irregularities, i.e. a large number of hills and valleys. The rate of wear is proportional to the surface areas in contact and the load per unit area.

Thus it is seen that different requirements demand different types of surfaces. Therefore, it became essential to measure the surface texture quantitatively and methods were devised for this purpose


Introduction The departure from a truly smooth surface may arise

from a variety, of causes and may be of several kinds. The texture or roughness {succession of minute irregularities} on surface is influenced by the machining process employed.


Introduction Complete roughness is the resultant of irregularities of

various kinds.

If the hills and valleys on a surface are very close, then surface appears as rough. This is due to action of the cutting tool and is referred to as primary texture.

If the hills and valleys on a surface are far apart, it is due to imperfection in the machine tool and is referred to as secondary texture or waviness.


Introduction This distinction between primary and secondary

texture is due to difference in wavelength.

A surface actually is quite complex and consists of many different wavelengths caused due to feed of the tool, cutting action, vibrations, imperfections in machine tools, etc.


Meanings of Surface Texture and Some Definitions First Order. This includes the irregularities arising out of

inaccuracies in the machine tool itself e.g. lack of straightness of guide-ways on which tool post is moving.

Second Order. Some irregularities are caused due to vibrations of any kind such as chatter marks and are included in second order.

Third Order. Even if the machine were perfect and completely free of vibrations, some irregularities are caused by machining itself due to characteristic of the process.

Fourth Order. This includes the irregularities arising from the rupture of the material during the separation of the chip.


Meanings of Surface Texture and Some Definitions Further these irregularities of four orders can be

grouped under two groups:

First group includes irregularities of considerable wave-length of a periodic character resulting from mechanical disturbances in the generating set-up. These errors are termed as macro-geometrical errors and include irregularities of first and second order and are mainly due to misalignment of centres, lack of straightness of guide-ways and non-linear feed motion. These errors are also referred to as Waviness or Secondary Texture.


Meanings of Surface Texture and Some Definitions Second group includes irregularities of small

wavelength caused by the direct action of the cutting element on the material or by some other disturbance such as friction, wear, or corrosion. These errors are chiefly caused due to tool feed rate and due to tool chatter, i.e. it includes irregularities of third and fourth order and constitutes the micro geometrical errors. Errors in this group are referred to as Roughness or Primary Texture.


Surface Roughness The several kinds of departures are there on the

surface and these are due to various causes.

Roughness or texture in the form of a succession of minute irregularities is produced directly by the finishing process employed.

The characteristic roughness produced by the tool is not the only cause of roughness in case of machining operations, but the more openly spaced component or roughness are also produced from faults in the machining operation


Surface Roughness Thus for the complete study of the surface roughness,

it is essential that the measurement and analysis of all the component elements and an assessment of the effects of the resulting combined texture be made.

All this being very difficult and tedious job, in practice all that is essential is that a practical method of assessment be followed, the result of which can be readily compared with a specified requirement of quality, preferably on the numerical basis.


Surface Roughness The measurement of surface roughness poses a

problem in three dimensional geometry, but for simplification purposes, it is better to reduce it into two dimensional geometry by confining individual measurement to the profiles of plane sections taken through the surface. The direction of measurement is. usually perpendicular to the direction of the predominant surface markings or 'lay'.


Surface Roughness Surface roughness is concerned both with the size and

the shape of the irregularities e.g.. in certain profile the height of departure from the nominal profile may be same but the spacing of the irregularities may be wider or closer, or the space of the irregularities may be of various forms


Terminology Real Surface : is the surface limiting the body and

separating it from the surrounding surface.

Geometrical Surface: is the surface prescribed by the design or by the process of manufacture, neglecting the errors of form and surface roughness.

Effective Surface: is the close representation of real surface obtained by instrumental means.

Surface Texture: Repetitive or random deviations from the nominal surface which form the pattern of the surface. Surface texture includes roughness, waviness, lay and flaws.


Surface Roughness: It concerns all those irregularities which form surface relief and which are conventionally defined within the area where deviations of form and waviness are eliminated.

Primary Texture (Roughness) : It is caused due to the irregularities in the surface roughness which result from the inherent action of the production process. These are deemed to include transverse feed marks and the irregularities within them.

Secondary Texture (Waviness): It results from the factors such as machine or work deflections, vibrations, chatter, heat treatment or warping strains. Waviness is the component of surface roughness upon which roughness is superimposed.


Flaws: Flaws are irregularities which occur at one place or at relatively infrequent or widely varying intervals in a surface (like scratches, cracks, random blemishes, etc).

Centre line: The line about which roughness is measured.

Lay. It is the direction of the predominant surface pattern ordinarily determined by the method of production used


Traversing length is the length of the profile necessary for the evaluation of the surface roughness parameters. The traversing length may include one or more sampling lengths.

Sampling length (l) is the length of profile necessary for the evaluation of the irregularities to be taken into account. This is also known as the 'cut-off' length in regard to the measuring instruments.


Mean line of the profile is the line having the form of the geometrical profile and dividing the effective profile so that within the sampling length the sum of the squares of distances (Y1, y2, ... Yn) between effective points and the mean line is a minimum.

Centre line of profile is the line parallel to the general direction of the profile for which the areas embraced by the profile above and below the line are equal. When the waveform is repetitive, the mean line and the centre line are equivalent


Spacing of the irregularities is the mean distance between the more prominent irregularities of the effective profile, within the sampling length. This information is useful for measuring the wearing-in of surfaces in relative motion, and assessing the electrical and thermal conductivity between surfaces in contact. Irregularity spacing and height parameters used in combination are valuable for sheet-steel applications and for friction and lubrication studies


Arithmetical mean deviation from the mean line of profile (Ra) is defined as the average value of the ordinates (y1, y2,....yn) from the mean line.

The ordinates are summed-up without considering their algebraic signs i.e.


0

0

1| |

| |

l

a

n

i

a

R y dxn

y

Rn

n is the no. divisions over the length l

Ten point height of irregularities (Rz) is defined as the average difference between the five highest peaks and the five deepest valleys within the sampling length measured from a line, parallel to the mean line and not crossing the profile.


5

10864297531 RRRRRRRRRRRz

Maximum height of irregularity (Rmax) is defined as the distance between two lines parallel to the mean line and touching the profile at highest points within the sampling length.


Terms used to describe surface roughness Average peak to valley height Rz: Average of single peak

to valley heights from five adjoining sampling lengths.

Rt measurement. It is the maximum peak to valley height within the assessment length. This measurement is valuable for analysing finish to provide guidance for planning subsequent metal-cutting operations.

Average wavelength =2πRa/Mean slope.


Bearing area. (Bearing area fraction). This is the fraction of surface at a given height above or below the mean line.

Centre line. A line representing the form of the geometrical profile and parallel to the general direction of the profile throughout the sampling length, such that the sums of the areas contained between it and those parts of the profile which lie on either side of it are equal


Depth of surface smoothness.


max

0

1( )

L

pR h h dxL

Least-squares mean line. A reference line representing the form of the geometrical profile within the limits of the sampling length, and so placed that within the sampling length the sum of the squares of the deviations of the profile from the mean line is a minimum.

Measuring traversing length. The length of the modified profile used for measurement of surface roughness parameters. It is usual for the measuring length to contain several sampling lengths.

Waviness height. Separation of highest peak and lowest valley of waviness over a waviness sampling length, corrected for roughness.


Methods of Measuring Surface Finish There are two methods used for measuring the finish of

machined part :

1. Surface Inspection by Comparison Methods.

(i)Touch Inspection, (ii) Visual Inspection, (iii) Scratch Inspection, (iv) Microscopic Inspection, (v) Surface Photographs, (vi) Micro-Interferometer, (vii) Reflected Light Intensity


Touch Inspection This method can simply tell which surface is more

rough. In this method, the finger-tip is moved along the surface at a speed of about 25 mm per second and the irregularities as small as 0.01 mm can be easily detected.

A modification of it is possible by using a table tennis ball, which is rubbed over the surface and vibrations from the ball transmitted to hand and surface roughness judged thereby.


Visual Inspection Visual inspection by naked eye is always likely to be

misleading particularly when surfaces having high degree of finish are inspected. The method is, therefore, limited to rougher surfaces and results vary from person to person. More accurate inspection can be done by using illuminated magnifiers.


Scratch Inspection In this method, a softer material like lead babbit or

plastic is rubbed over the surface to be inspected. By doing so it carries the impression of the scratches on the surfaces which can be easily visualised.


Microscopic Inspection In this method, a master finished surface is placed

under the microscope and compared with the surface under inspection.

This is probably the best method for examining the surface finish but suffers due to limitation that only a small portion of the surface can be inspected at a time. Thus several readings are required to get an average value.


Surface Photographs In this method magnified photographs of the surface

are taken with different types of illumination.

In case we use vertical illumination, then defects like irregularities and scratches appear as dark spots and flat portion of the surface appears as bright area. In case of oblique illumination, reverse is the case. Photographs with different illumination are compared and the results assessed.


Micro interferometer In this method, an optical flat is placed on the surface

to be inspected and illuminated by a monochromatic source of light. Interference bands are studied through a microscope.

Defects, i.e. scratches in the surface appear as interference lines extending from the dark bands into the bright bands. The depth of the defect is measured in terms of the fraction of the interference band.


Reflected light intensity In this method a beam of light of know quantity is

projected upon the surface., This light is reflected in several directions as beams of lesser intensity and the change in light intensity in different directions is measured by a photocell. The measured intensity changes are already calibrated by means of reading taken from surface of known roughness by some other suitable method.


Direct Instrument Measurements These methods enable to determine a numerical value of

the surface finish of any surface. Nearly all instruments used are stylus probe type of instruments

Stylus probe Instrument

(i) Profilometer

(ii) Tomlinson surface meter

(iii) Taylor Hobson talysurf


Stylus probe Instrument i. This type of instrument generally consists of the

following units

ii. A skid or shoe which is drawn slowly over the surface either by hand or by motor drive

iii. A stylus or probe which moves over the surface with the skid

iv. An amplifying device for magnifying the stylus movement

v. A recording device to produce a trace

vi. A means for analysing the trace


Skid A skid or shoe which is

drawn slowly over the surface either by hand or by motor drive. The

skid when moved over the surface, follows its general contours and provides a datum for the measurements.


Stylus probe A stylus or probe which

moves over the surface with the skid. The stylus for Ra measurement on new instrument can have a radius of 10 microns ±30%. Stylus should be cone shaped with a spherical tip.


Amplifying An amplifying device for

magnifying the stylus movement and an indicator.

Electronic or optical magnification is employed.


Recording A recording device to

produce a trace or record of the surface profile. Usually the vertical movement is magnified more in comparison to horizontal movement, thus the record will not Stylus give the actual picture of surface roughness but a distorted trace obtained.


Profilometer This is a dynamic instrument similar in principle to a

gramophone pick-up. A finely pointed stylus mounted in the pick-up unit is traversed across the surface either by hand or by motor drive. The instrument records the rectified output from the pick-up which is amplified further and operates an indicating device


Tomlinson Surface meter


The diamond stylus on the surface finish recorder is held by spring pressure against the surface of a lapped steel cylinder. 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 conveniently. The stylus is restrained from all motios except the vertical one by the tensions in coil and leaf spring. A light spring steel arm is attached to the horizontal lapped steel cylinder and it carries at its tip a diamond scriber which bears against a smoked glass.


When measuring surface finish, body is traversed across the surface by a screw rotated by a synchronous motor. Any vertical movement of the stylus caused by the surface irregularities, causes the horizontal lapped steel cylinder to roll.


By its rolling, the light arm attached to its end provides a magnified movement on a smoked glass plate. This vertical movement coupled with the horizontal movement produces a trace on the glass magnified in vertical direction and there being no magnification in horizontal direction.


Taylor-Hobson Talysurf


This instrument also gives the same information as the previous instrument, but much more rapidly and accurately.

This instrument also as the previous one records the static displacement of the stylus and is dynamic instrument like profilometer.


The measuring head of this instrument consists of a diamond stylus of about 0.002 mm tip radius and skid or shoe which is drawn across the surface by means of a motorised driving unit. A neutral position in which the pick-up can be traversed manually is also provided.


The arm carrying the stylus forms an armature which pivots about the centre piece of E-shaped stamping. On two legs of (outer pole pieces) the E-shaped stamping there are coils carrying an a.c. current.


These two coils with other two resistances form an oscillator. The amplitude of the original a.c. current flowing in the coils is modulated because of air gap between the armature and E-shaped stamping. This is further demodulated so that the current now is directly proportional to the vertical displacement of the stylus only.


The demodulated output is caused to operate a pen recorder to produce a permanent record and a meter to give a numerical assessment directly. In recorder of this instrument the marking medium is an electric discharge through a specially treated paper which blackens at the point of the stylus.


Analysis of Surface Traces Maximum Peak to

Valley Height of Roughness.

This is the most common measure of roughness but is not by any means a complete definition of roughness. But, since this is a relatively simple method of analysis.


Analysis of Surface Traces Root Mean Square

(R.M.S) Value

R.M.S. value is defined as the square root of the mean of the squares of the ordinates of the surface measured from a mean line.


2 2 21 2 ......... n

rms

h h hh

n

Analysis of Surface Traces Centre Line Average

(C.L.A) value.

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


1 2 ........ . nh h h

C L An

Analysis of Surface Traces Things can be much

simplified by using a planimeter which can find out the area of any curve. Then C.L.A. value


1 2 ........ . nA A A

C L AL

ionmagnificatVertical

1

Calculate the CLA (Ra) value of a surface for which the sampling length was 0.8 mm. The graph was drawn to a vertical magnification of 10000 and a horizontal magnification of 100. and the areas above and below the datum line were :


Above(mm2) 180 90 155 55

Below(mm2) 70 90 170 150

Solution:


In the measurement of surface roughness, heights of 20 successive peaks and troughs were measured from a datum and were 35, 25, 40, 22, 35, 18, 42, 25, 35, 22, 36, 18, 42, 22, 32, 21, 37, 18, 35, 20 microns.

If these measurements were obtained over a length of 20 mm, determine the C.L.A. (Ra) and R.M.S value of the rough surface.


chapter 7 measurement of surface finish

Engineering

dayananda pai mit

surface areas

smooth surface

control of surface texture

meanings of surface

aero auto engg

important dayananda

purpose dayananda pai