MECHANICAL MEASUREMENT AND METROLOGY

Metrology of Surface FinishGuided By : D. K. PATEL

CONTENT:-

Concepts of Surface Metrology

Introduction to Surface Texture

Terminology of Surface Texture

Analysis of Surface Traces

1) Peak to valley height of roughness

2) The average roughness

3) Form factor and Bearing curve

Specification of Surface Texture Characteristics

Concepts of Surface Metrology:-

Surface metrology is the measurement of small-scale features on surfaces, and is a branch of metrology. Surface primary form, surface waviness and surface roughness are the parameters most commonly associated with the field.

The various manufacturing processes applied in industry produce the desired shapes in the components within the prescribed dimensional tolerances and surface quality requirements.

The properties such as appearance, corrosion resistance, wear resistance, fatigue resistance, lubrication, initial tolerance, ability to hold pressure, load carrying capacity, noise reduction etc. are influenced by surface finish.

Basic industrial requirements with surface finish can be listed as,

Low manufacturing cost

Better properties

Good appearance

Quality of component/product

Satisfactory performance etc.

Now, the factors which affect the above requirements are,

> Material of the workpiece

> Type of machining process

> Vibrations of machines

> Cutting tool properties i. e. material, properties, sharpness, etc.

> Cutting conditions i. e. speed, feed, depth of cut.

> Type of coolant used.

Introduction to Surface Texture:-

The characteristic quality of an actual surface due to small departures from its general geometrical form which, occurring at regular or irregular intervals, tend to form a pattern or texture on the surface

Surface texture is a foremost characteristic among the surface integrity magnitudes and properties imparted by the tools used in the processes, machining mostly.

The characterization and evaluation of engineering surface texture has constituted a challenging metrological problem that has remained open so far, especially when high-precision and/or functional performance requirements exist.

This fact is attributed to the usually complicated form of surface textures and the need to obtain a satisfying description globally, as well as at various levels.

Each manufacturing process produces a surface texture. The process is usually optimized to ensure that the resulting texture is usable. If necessary, an additional process will be added to modify the initial texture. i. e. grinding (abrasive cutting), polishing, lapping, abrasive blasting, honing, electrical discharge machining (EDM), milling, lithography, industrial etching/chemical milling, laser texturing etc.

These differences in surface textures are apparent by many methods, i. e. Touch Inspection, Visual Inspection, Scratch Inspection, Microscopic Inspection, Surface Photographs, Micro-Interferometer, Reflected Light Intensity.

Textures on the surfaces may be regular or irregular in character and may be directional or non-directional.

The geometrical irregularities can be classifies as follows,

Order Irregularities arise due to…

First Inaccuracies in machine tool.i.e. straightness in guideways, deformation of work, weight of material etc.

Second Vibrations in machine.i.e. due to cutting forces

Third Human errors in machining.i.e. imperfect speed/feed/depth of cut.

Fourth Rupture of the material during separation from already finished surface of the workpiece.

Terminology of Surface Texture:-

Surface :- It is the surface limiting the body & separating it from surrounding.

Actual surface :- It is the surface prescribed by the design or by the process of manufacture.

Nominal surface :- It is the surface prescribed by an average of irregularities superimposed on it.

Form error :- These are very widely spaced repetitive irregularities occurring over the full length of the work surface.

Roughness :-

The surface irregularities o finest or short wavelength are known as roughness or primary texture.

It can also be said that closely spaced irregular deviations on a scale smaller than that of waviness

Basically, these irregularities are caused by direct action of the cutting element on the material or by other disturbance such as friction, wear or corrosion.

It includes irregularities of first and third order.

Waviness :-

The surface irregularities of considerable wavelength of a periodic character are known as waviness or secondary texture.

These irregularities are caused due to misalignment of centres, lack of straightness of guideways and non-linear feed motion.

These includes irregularities of first and second order.

Waviness errors are intermediate in wavelength between roughness and form error.

Flaws :- These are the surface irregularities which occur at on place or at

relatively infrequent or widely varying intervals on a surface. It includes random irregularities such as scratches, cracks, holes, tears, inclusions, etc.

Centre line :- It is an imaginary line about which the roughness is measured.

Roundness :- It is the radial uniformity of a work surface measured from the centre line of the workpiece.

Lay :- It is the direction of predominant surface pattern. It is produced by tool marks or scratches. Symbols used to indicate lay direction are as shown.

Profile :- It is defined as the contour of any section through a surface.

Effective profile :- It is the real contour of a surface obtained by using

instrument.

Sampling length :- It is the length of the profile necessary for the evaluation of

the irregularities to be taken into account. It is also known as cut-off length.

Mean line profile :- It is the line having the form of the geometrical profile and dividing the effective profile.

Analysis of Surface Traces:-

To indicate degree of surface smoothness and roughness, a numerical assessment may be assigned by different methods.

a. Peak to valley height method roughness,

b. The average roughness,

c. Form factor and Bearing curve.

Peak to valley height method 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.

In this method, the maximum depth is accepted as the measure of roughness. The disadvantage of this method is that it may be read the same h for two largely different texture.

To overcome this lack of representation, the ten point height average is used. This is determined by drawing the line parallel to general lay of trace.

The average roughness:-

Centre line average method:-

It is the average height from the mean line of all ordinates of the surface.

Surface roughness in this method can be determined by the average deviation from the nominal surface.

Centre line average value is given by,

C.L.A. = ℎ1+ ℎ2+ ℎ3+⋯+ ℎ𝑛

𝑛

&also C.L.A. = 𝐴1+ 𝐴2+ 𝐴3+⋯+ 𝐴𝑛

𝐿= Σ𝐴

𝐿

where, ℎ1 , ℎ2 , ℎ3 , …ℎ𝑛 are heights of ordinates,

𝐴1 , 𝐴2 , 𝐴3 , …𝐴𝑛 are enclosed areas,& L is the sampling length.

Root mean square value method:-

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.

It is the geometrical average of ordinates of profile about the mean line.

Average areas above the mean line is approximately equal to areas below it.

If we consider, ℎ1 , ℎ2 , ℎ3 , …ℎ𝑛 are heights of ordinates, all parts are divided equally & L is the sampling length then,

R.M.S. = ℎ12+ℎ2

2+ℎ32+⋯+ℎ𝑛

2

𝑛or ℎ𝑟𝑚𝑠 =

1

𝐿 0𝐿ℎ2. 𝑑𝐿

Generally R.M.S. value is greater than C.L.A. value for same profile about 1.11 times.

Form Factor:-

The load carrying area of every surface is often much less than might be thought. This is shown by reference to form factor. The form factor is obtained by measuring the area of material above the arbitrarily chosen base line in the section and the area of the enveloping rectangle. Then,

Degree of fullness (K) = 𝐴𝑟𝑒𝑎 𝑜𝑓 𝑚𝑒𝑡𝑎𝑙

𝐴𝑟𝑒𝑎 𝑜𝑓 𝑒𝑛𝑣𝑒𝑙𝑜𝑝𝑖𝑛𝑔 𝑡𝑟𝑖𝑎𝑛𝑔𝑙𝑒

& hence,

Degree of emptiness (𝐾𝑝) = 1 − 𝐾

Bearing curve:-

The bearing area curve is also called as Abbot's bearing curve. This is determined by adding the lengths a, b, c etc. at depths x, y, z etc. below the reference, line and indicates the percentage bearing area which becomes available as the crest area worn away. Fig. indicates the method of determining the bearing curve.

Specification of Surface Texture Characteristics:-

As per IS: 696 surface texture specified by indicating the following

Roughness value i.e., Ra value in mm

Machining allowance in mm.

Sampling length or instrument cut-off length in mm.

Machining production method, and

Direction of lay in the symbol form as = ⊥, X, M, C, R

Symbol Meaning

Basic symbol, which is only be used when its meaning is explained by a note.

If the removal of material by machining is required, a bar is added to the basic symbol.

If the removal of material is not permitted,a circle is added to the basic symbol.

It shows the production method used. Here, milled.

It shows the sampling length, here it is 2.5 mm.

If it is necessary to control the directionof lay, it is specified by a symbol added to the surface texture symbol.

It represents the machining allowance, here its 2 mm.

Roughness value, 𝑅𝑎 = 4 𝜇𝑚.

If the machining method used is milling, sampling length is 0.25 mm with roughness value 2.5 𝜇𝑚, direction of lay is perpendicular and machining allowance is 5 mm with 12.5 roughness grade then it can be shown as figure below.

METHODS OF MEASURING SURFACE FINISH

The surface finish of machined part can be measured by the following two methods:

(i) Surface Inspection of Comparison Methods.

(ii) Direct Instrument Measurements.

In comparative methods, the surface texture is assessed by observation of the surface. But these methods are not reliable as they can be misleading if comparison is not made with surfaces produced by same techniques. The various methods available under comparison method are :(i) Touch Inspection, (ii) Visual Inspection, (iii) Scratch Inspection, (iv) Microscopic Inspection, (v) Surface Photographs, (vi) Micro-Interferometer, (vii) Wallace Surface Dynamometer and (viii) Reflected Light Intensity.

Touch Inspection.

The main limitation of this method is that the degree of surface roughness can’t be assessed. Also the minute flaws can’t be detected. 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.

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.

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. In this method, a master finished surface is placed under the microscopic and compared with the surface under inspection. In another method a straight edge is placed on the surface to be inspected and abeam of light projected at about 60° to the work. Thus the shadows cast into the surface scratches are magnified and the surface irregularities can be studied.

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. This has been explained in detail in the chapter of ‘Measurement by Light Wave Interference’.

Wallace Surface Dynamometer.

This is a sort of friction meter and consists of a pendulum in which the testing shoes are clamped to a bearing surface and a predetermined spring pressure can be applied. In this method the pendulum is lifted to its initial starting position and allowed to swing over the surface to the tested. If the surface is smooth, then there will be less friction and pendulum swings for a longer period. Thus time of swing is a direct measure of surface finish.

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 MEASUREMENT METHOD

The following methods are the direct instrument measurement methods of surface roughness:

1. Profilometer

2. Tomlinson surface tester

3. The Taylor – Hobson – Talysurf surface roughness tester

Profilometer. This instrument is most commonly used in U.S.A. for direct

measurement of surface quality. 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. Thus this records the average height of the surface roughness. In this instrument, roughness together with waviness and flaws comprises the irregularities found on the surface. An indication is obtained onlywhen the pick-up is moving. This instrument is best suited for measuring surface finish of deep bores.

The Tomlinson Surface Meter.

This instrument was designed by Dr. Tomlinson. This instrument uses mechanical-cum-optical means for magnification . 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 lapped cylinder is supported on one side by the stylus and on the other side by two fixed rollers . The stylus is restrained from all motions except the vertical one by the tensions in coil and leaf spring. The tensile forces in these two springs also keep the lapped steel cylinder in position between the stylus and a pair of fixed rollers. 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. The smoked glass trace is then, further projected at x 50 or x 100 magnification for examination. This instrument is comparatively cheap one and gives reliable results.

The Taylor-Hobson Talysurf.

The Talysurf is an electronic instrument working on carrier modulating principle. This instrument also gives the same information as the previous instrument, but much more rapidly and accurately. This instrument as also 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 (gearbox), which provides three motorised speeds giving respectively x 20 and x 100 horizontal magnification and a speed suitable for average reading. A neutral position in which the pick-up can be traversed manually is also provided. In this case 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 J5-shaped stamping there are coils carrying an a.c. current. These two coils with other two resistances form an oscillator. As the armature is pivoted about the central leg, any movement of the stylus causes the air gap to vary and thus the amplitude of the original a.c. current flowing in the coils is modulated. The output of the bridge thus consists of modulation.

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 statement the marking medium is an electric discharge through a specially treated paper which blackens at the point of the stylus, so this has no distortion due to drag and the record strictly rectilinear one.Now-a-days microprocessors have made available complete statistical multi-trace systems measuring several places over a given area and can provide standard deviations and average over area-type readings and define complete surface characterization. These systems lend themselves to research applications where specialized programming can achieve autocorrelation, power spectrum analysis and peak curvature.

ADVERSE EFFECTS OF POOR SURFACE FINISH

Surface finish is important not only as a matter of appearance or expert workmanship but , in case of mating surfaces has a positive and prolonged effect on following properties such as :

1. Fatigue strength

2. Wear resistance

3. Corrosion resistance

4. Strength of interference fits

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