BASICS OF SURFACE TEXT MEASUREMENT TECHNIQUES

BASICS OF SURFACE TEXT MEASUREMENT TECHNIQUES

• 1.
  SURFACE TEXTURE MEASUREMENT A greatadvancement in the present machine age has been possible due to the constant improvement of the surface texture. It is found that the fatigue life, bearing properties and wear qualities of any component of a machine have direct bearing with its surface texture.Failure due to fatigue always starts to occur at sharp corners.The irregularity acts as sharp corner &such part fails earlier. The action of cutting tool in machining never results in absolutely smooth &even surfaces of parts. It always bears some traces of unevenness, roughness, notches, scares, lines etc.Even the surfaces which seem perfectly smooth after lapping grinding etc have minor unevenness.
• 2.
  Difference between length &surface texturemeasurement Surface measurement is treated separately from length measurements because length measurement is concerned with the relationship of two surfaces on a workpiece where as surface measurement is concerned with the relationship of a surface on the workpiece to a reference which is not actually on the work.
• 3.
  Primary texture It iscaused due to the irregularities in the surface roughness which results from the inherent action of production process These are deemed to include transverse feed marks.
• 4.
  Secondary texture It resultsfrom the factors such as machine or work deflections, vibrations, chatter, heat treatment etc.
• 5.
  Lay The direction ofpredominant surface pattern ordinarily determined by the production method used.
• 6.
  Sampling length It isthe length of the profile necessary for the evaluation of the irregularities to be taken into account
• 7.
  Mean line ofthe profile It is the line having the form of the geometrical profile and dividing the effective profile so that within the sampling length the sum of square of distances (y ,y ,--- y )between effective profile points is a minimum
• 8.
  Centre line ofprofile It is the line parallel to the general direction of the profile for which the area embraced by the profile above and below the line are equal
• 9.
  METHODS Of MEASURING SURFACEFINISH There are two methods used for measuring the finish of a machined part. 1; surface inspection by comparison method. 2; direct instrument inspection. The various methods available under comparison methods are, 1. Touch inspection 2. Visual inspection 3. Scratch inspection 4. Microscopic inspection 5. Surface photograph
• 10.
  Touch: In thisdegree of surface roughness cannot be assessed Visual: Inspection by naked eye will not give good result . More accurate inspection can be done by using illuminated magnifier Scratch: softer materials like lead, babbitt or plastic is rubbed over the surface.
• 11.
  Direct Instrument Measurement These methodsenable to determine a numerical value of the surface finish of any surface. Nearly all instruments used are stylus probe type of instruments. This type of instrument consists of the following units, 1. 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 provides a datum for measurements. A line touching the crest of the profile is generally choosen as a datum line and this is obtained by using a skid of such a size which can span a large number of surface undulations.
• 12.
  2. A stylusor probe which moves over the surface with the skid. It moves vertically up and down relative to the skid movement due to the roughness of the surface. 3.An amplifying device for magnifying the stylus movement and an indicator. 4. A recording device to produce a trace or record of the surface profile 5. A means for analysing the trace obtained
• 13.
  PROFILOMETER This is adynamic 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 unit which is amplified further and operates an indicating device
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  Profilometer
• 15.
  TALYSURF The talysurf isan electronic instrument working on the principle of carrier modulation. The measuring head of this instrument consists of a diamond stylus of about 0.002mm tip radius and skid or shoe which is drawn across the surface by means of a motorised driving unit. The instrument gives a numerical value of the surface finish directly.It is also provided with a pen recorder to give a permanent trace of the variations measured.
• 16.
  On the twolegs of the E shaped stamping there are coils carrying an a.c current. These two coils with the 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 coil is modulated. This is further demodulated so that the current now is directly proportional to the vertical displacement of the stylus only. The demodulated out put is used to operate a pen recorder to produce a permanent record and meter to give a numerical assessment directly. Talysurf
• 18.
  TalyRond
• 19.
  Tomlinson surface meter Thediamond stylus of this instrument 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. The lapped cylinder is supported on one side by the stylus and on the other side by two fixed rollers. A 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.
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  ANALYSIS OF SURFACE TRACES
• 21.
  The maximum peakof valley height of roughness This is the most common measure of roughness but is not by any means of complete definition of roughness. This is relatively simple method of analysis.
• 22.
  Root Mean Square[R.M.S] Value It is defined as the square root of the squares of the ordinates of the surface measures from a mean line. In the figure selected length L is divided into n-equal parts ordinates are erected at points 1,2,3,4,.....n whose length are h1,h2,h3_,....hn Then hrms= %[(h15+h25+.....hn5)/n]
• 23.
  Center Line AverageMethod [CLA] This is defined as the average height from a mean line of all the surface regardless of the sign C.L.A =h1+h2+.....hn/n