SURFACE TEXTURE

With the more precise demands of modern engineering products, the control of surface texture together with dimensional accuracy has become more important.

Surface texture greatly influences the functioning of the machined parts.

Properties influenced by surface texture:Appearance, corrosion resistance, wear resistance, fatigue resistance, lubrication, ability to hold pressure, load carrying capacity, noise reduction in case of gears

If the surface finish is not smooth, moving parts can heat up, bind and freeze. Shafts and bearings with less surface finish say in electrical household appliances require more power.

Surface texture depends upon the specific application of the part.

Surface finish should not be high in1.Heat exchanger tubes2.Break drums and clutch plates

The imperfections and irregularities are bound to occur. The irregularities on the surface are in the form of succession of hills and valleys varying in height and spacing.

These irregularities are usually termed as surface roughness, surface finish, surface texture or surface quality.

Machining processes like turning, grinding, honing, lapping, shaping etc. are employed to finish the surface.

It is not possible to produce perfectly smooth surface .

Lapping and honing: irregular texture and multidirectional

Grinding: irregular texture with unidirectional-widely used

Turning, boring, shaping : evenly spaced and unidirectional

Schematic illustration of a cross-section of the surface structure of metals. The thickness of the individual layers is dependent on processing conditions and processing environment.

Factors affecting surface roughness

1.Vibrations2.Material of the workpiece3.Type of machining4.Rigidity of the system consisting of machine tool, fixture cutting tool and work5.Type, form, material and sharpness of cutting tool6.Cutting conditions i.e feed, speed, and depth of cut7.Type of coolant used

The irregularities on the surface of the part produced can also be produced into two categories:

1.Roughness or primary texture2.Waviness or secondary texture

1. Primary texture(roughness)-micro geometrical errors

The surface irregularities of small wavelength are called primary texture or roughness. These are caused by direct action of the cutting elements on the material i.e., cutting tool shape, tool feed rate or by some other disturbances such as friction, wear or corrosion.

l/h ration denoting micro-errors is less than 50

Where l= length along the surfaceh= deviation of the surface from the ideal one.

2. Secondary Texture(waviness): -macro geometrical error.

The surface irregularities of considerable wavelength of a periodic character are called secondary texture or waviness.These irregularities are due to inaccuracies of slides, wear of guides, misalignment of centres, non-linear feed motion, deformation of work under the action of cutting forces, vibrations of any kind etc.

The ratio l/h is more than 50

Any finished surface can be considered as the combination of two forms of wavelength (large wavelength for waviness and smaller wavelength for roughness) superimposed upon each other. These two forms of irregularities superimposed on each other tends to form a pattern or texture on the surface.

Standard terminology and symbols to describe surface finish.

Surface characteristics and terminology

Standard Lay Symbols for Engineering Surfaces

Drawing symbols

Center Line Average Method (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.First establish a mean line.1.Draw a general line X-X in the general direction of the surface and for convenience touching the deepest valley.2.Select a suitable sampling length L such that if the surface has a distinguishable waveform a whole number of waveforms are enclosed3.Find the area A under the curve. The height of the centre line C-C may be found from H= A/L Where H is the perpendicular height from X-X to C-C4.The trace is now divided into two halves so that the sum of the enclosed areas above the line equates the sum of the enclosed areas below the line.5. Ra = (sum of areas above the line C-C + sum of the areas below the line C-C)/L

Root Mean Square (RMS) roughness (Rq) is the root mean square average of the roughness profile ordinates.Note: Rq is also called RMS.Mean Roughness (Roughness Average Ra) is the arithmetic average of the absolute values of the roughness profile ordinates.

Ra is one of the most effective surface roughness measures commonly adopted in general engineering practice. It gives a good general description of the height variations in the surface. The units of Ra are micrometres or microinches.Note: Ra is also called CLA.

Direct instrument measurement:

These are quantitative. These enable to determine numerical value of the surface finish of any surface by using instruments of stylus probe type operating on electrical principles. output has to be amplified and the amplified output is used to operate recording or indicating instrument.

Principle: If a finely pointed probe or stylus be moved over the surface of a workpiece, the vertical movement of the stylus caused due to the irregularities in the surface texture can be used to assess the surface finish of the workpiece.

Stylus type instruments generally consist of the following units:

1.Skid or shoe2.Finely pointed stylus or probe3.An amplifying device for magnifying the stylus movement and indicator4.Recording device to produce a trace 5.Means for analysing the trace.

Stylus is a fine point made of diamond or any such hard material is drawn over the surface to be tested. The movements of the stylus are used to modulate a high frequency carrier current or to generate a voltage signal. The output is then amplified by suitable means and used to operate a recording or indicating instrument.

Profilometer: Profilometer is an indicating and recording instrument used to measure roughness in microns.

The principle of the instrument is similar to gramophone pick up. It consists of two principle units. a tracer and an amplifier.

Tracer is a finely pointed stylus(diamond tip of radius 12 microns). Stylus carries at its upper end a small induction coil located in a radial permanent magnet.

When the tracer is moved across the surface to be tested, it is displaced vertically up and down due to the surface irregularities.

This causes the induction coil to move in the field of the permanent magnet and induces a current. The induced current is amplified and recorded.

Roughness describes the short cycles of unevenness.

Waviness describes larger-scale unevenness.

(a) Measuring surface roughness with a stylus. The rider supports the stylus and guards against damage. (c) Path of stylus in surface roughness measurements (broken line) compared to actual roughness profile. Note that the profile of the stylus path is smoother than that of the actual surface.

Taylor-Hobson Talysurf: is a stylus and skid type of instrument working on carrier modulating principle

Its response is more rapid and accurate. The measuring head of this instrument consists of a sharply pointed diamond stylus of about 0.002 mm tip radius and skid or shoe which is drawn across the surface by means of a motorized driving unit.

In this instrument the stylus is made to trace the profile of the surface irregularities, and the oscillatory movement of the stylus is converted into changes in electric current by the arrangement as shown in fig.

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. 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 only as shown in figure. 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 the meter to give a numerical assessment directly.

Demodulation is the act of extracting the original information-bearing signal from a modulatedcarrier wave. A demodulator is an electronic circuit (or computer program in a software-defined radio) that is used to recover the information content from the modulated carrier wave.[1]

These terms are traditionally used in connection with radio receivers, but many other systems use many kinds of demodulators. Another common one is in a modem, which is a contraction of the terms modulator/demodulator.

Modulation is the addition of information (or the signal) to an electronic or optical signalcarrier. Modulation can be applied to direct current (mainly by turning it on and off), to alternating current, and to optical signals. One can think of blanket waving as a form of modulation used in smoke signal transmission (the carrier being a steady stream of smoke).Morse code, invented for telegraphy and still used in amateur radio, uses a binary (two-state) digital code similar to the code used by modern computers. For most of radio and telecommunication today, the carrier is alternating current (AC) in a given range of frequencies. Common modulation methods include:Amplitude modulation (AM), in which the voltage applied to the carrier is varied over timeFrequency modulation (FM), in which the frequency of the carrier waveform is varied in small but meaningful amountsPhase modulation (PM), in which the natural flow of the alternating current waveform is delayed temporarily

A computer with an online or Internet connection that connects over a regular analog phone line includes a modem. This term is derived by combining beginning letters from the words modulator and demodulator. In a modem, the modulation process involves the conversion of the digital computer signals (high and low, or logic 1 and 0 states) to analog audio-frequency (AF)tones.

Schematic diagram of Stylus instrument

Coordinates used for surface-roughness measurement

Typical surface profiles produced by various machining and surface-finishing processes. Note the difference between the vertical and horizontal scales.

A highly polished silicon surface measured in an atomic force microscope. The surface roughness is Rq = 0.134 nm.

Three-Dimensional Surface Measurement