MECHANICAL MEASUREMENT & METROLOGY

Metrology of Surface Finish

Branch :- Mechanical-4

Division :- A

Batch :- A3

Group No :- A18

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 & 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. = & also C.L.A. = =

where, … are heights of ordinates,…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, … are heights of ordinates, all parts are divided equally & L is the sampling length then,

R.M.S. = or =

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) =

& hence,

Degree of emptiness () =

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 MeaningBasic 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 direction of lay, it is specified by a symbol added to the surface texture symbol.

It represents the machining allowance, here its 2 mm.

Roughness value, = 4 .

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