6/13/2014 1 Lab#5: Surface Roughness Test. To understand surface roughness terminology and its parameters. To be able to use surface roughness measurement

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Lab#5: Surface Roughness Test

To understand surface roughness terminology and its parameters.

To be able to use surface roughness measurement instrument.

To know how PERTHOMETER M1 can be used to measure surface roughness

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Surface finish is a characteristic of any machined surface. It is sometimes called surface texture or roughness.

The design engineer is usually the person that decides what the surface finish of a work piece should be. They base their reasoning on what the

work piece is supposed to do.

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There are two principal reasons for surface control: To reduce friction: When a film of lubricant

must be maintained between two moving parts, the surface irregularities must be small enough so they will not penetrate the oil film under the most severe operating conditions.

To control wear: Surface finish is also important to the wear service of certain parts that are subject to dry friction, such as machine-tools bits, threading dies, stamping dies, rolls, clutch plates, and brake drums.

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Roughness is a measurement of the small-scale variations in the height of a physical surface. It consists of surface irregularities which result from the various machining process. These irregularities combine to form surface texture.

This is in contrast to large-scale variations, which may be either part of the geometry of the surface or unwanted 'waviness'.

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Roughness is sometimes: An undesirable property, as it may cause

friction, wear, drag and fatigue, A beneficial property, as it allows surfaces

to trap lubricants and prevents them from welding together

Roughness Height: It is the height of the irregularities with respect to a reference line. It is measured in millimeters or microns or micro inches. It is also known as the height of unevenness.

Roughness Width: The roughness width is the distance parallel to the nominal surface between successive peaks or ridges which constitute the predominate pattern of the roughness.

Roughness Width Cut Off: The greatest spacing of respective surface irregularities to be included in the measurement of the

average roughness height. It should always be greater than the roughness width in order to obtain the total roughness height

rating. It is used to differentiate between roughness and waviness. The cut-off length should be at least 2.5 times the peak-to-peak

spacing of the profile roughness. This means that at least two peaks and valleys are found in each cut-off length.

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Lay: the direction of predominant surface pattern produced and it reflects the machining operation used to produce it. Roughness is measured perpendicular to the lay.

Waviness: the irregularities which are outside the roughness width

cut off values. Waviness is the widely spaced component of the surface texture. This may be the result of work piece or tool deflection during machining, vibrations or tool runout.

Waviness Height : the peak to valley distance of the surface profile, measured in millimeters.

Waviness Width: the peak to peak distance of the surface profile, measured in millimeters.

Arithmetic Average (AA): A close approximation of the arithmetic

average roughness-height can be calculated from the profile chart of the surface. Averaging from a mean centerline may also be automatically performed by electronic instruments using appropriate circuitry through a meter or chart recorder.

Root Mean Square (RMS or Rq ) :can be calculated as shown

below. Its numerical value is about 11% higher than that of AA.


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H: Swedish height. The roughness between two predefined reference lines.

ISO Flatness: Areal flatness deviation. The measure of surface deviation from perfectly flat. It is the distance between two parallel planes obtained by applying a Chebychev fit to the surface data.

Pt ISO: Total peak-to-valley profile height. The distance between the highest peak and the deepest valley over the entire evaluation length. The profile data has form removed but is unfiltered.


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R3z: Base roughness depth. The distance between the third highest peak and the third lowest valley. A peak is a portion of the surface above the mean line and between center line crossings.

R3z ISO: Base roughness profile depth. The height of the 3rd highest peak from the 3rd lowest valley per sampling length. The base roughness depth is found in five sampling lengths and then averaged.

Rmax ISO: Maximum peak-to-valley profile height. The greatest peak-to-valley distance within any one sampling length.


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Ra: Arithmetical mean deviation. The average roughness or deviation of all points from a plane fit to the test part surface. The average roughness is the area between the roughness profile and its center line divided by the evaluation length. This is the parameter that has been used universally for many years and the European and ISO standards now more generally use Rz

Rq (RMS): Root-mean-square (RMS) roughness. The average of the measured height deviations taken within the evaluation length or area and measured from the mean linear surface


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Rv (Valley): Lowest valley. The maximum distance between the mean line and the lowest point within the sample. It is the maximum data point height below the mean line through the entire data set.

Rp (Peak): Highest peak. The maximum distance between the mean line and the highest point within the sample. It is the maximum data point height above the mean line through the entire data set.

Rt (PV): Maximum peak-to-valley height. The absolute value between the highest and lowest peaks.


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vpt RRR

Rtm: Mean peak-to-valley roughness. It is determined by the difference between the highest peak and the lowest valley within multiple samples in the evaluation area. For profile data it is based on five sample lengths. ISO, n=5

Ry (Rmax): Maximum peak-to-valley roughness. The vertical distance between the top of the highest peak and the bottom of the deepest valley within the sampling length. It is the maximum of all the peak-to-valley values.


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n

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Rz: Ten-point height. The average absolute value of the five highest

peaks and the five lowest valleys over the evaluation length.

Rz ISO: Average peak-to-valley profile roughness. The average peak-to-valley roughness based on one peak and one valley per sampling length. The single largest deviation is found in five sampling lengths and then averaged


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PerthometerIt is characterized by a multitude of functions. After carrying out a measurement, periodic and non-periodic profiles can be identified and the cutoff set according to standards automatically, such that unintentional non-standard measurements are excluded


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Measuring range up to 150 µm (6000 µin) Measuring units µm/µin selectable Standards: DIN/ISO/JIS Tracing lengths 1,75 mm; 5,6 mm; 17,5 mm (0,07 in; 0,22 in; 0,7

in) Cutoff 0,25 mm/0,8 mm/2,5 mm (0,01 in/0,032 in/0,100 in) Reduced cutoff selectable Number of sampling lengths selectable from 1 to 5 Automatic selection of filter and tracing length confirming to

standards Phase-corrected profile filter as per DIN EN ISO 11562 Parameters as per DIN/ISO/SEP: Ra, Rz, Rmax, RPc and JIS: Ra, Rz Automatic scaling according to the profile amplitude Printing of roughness profile and measuring record Dynamic pick-up calibration Blocking of instrument settings to prevent unintentional

modifications


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Parameter Value

Total length( Lt) 5.6 mm

Cut-off length (Lc) 0.8mm

Measured Length (Lm) 5.6-(2*0.8)=4mm

Average Roughness (Ra) 0.937µm

Average Roughness Depth (Rz) 6.26 µm

Maximum peak-to-valley (Rmax) 6.89 µm

RPc (0.2,-0.2) 90/c

Vertical Scale Division 2.5 µm


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Parameter Value

Total length( Lt) 1.75 mmCut-off length (Lc) 0.25mmMeasured Length (Lm) (1.75 – 2 (0.25)) = 1.25mmAverage Roughness (Ra) 0.9555µm Average Roughness Depth (Rz) 6.22 µmMaximum peak-to-valley (Rmax) 9.14 µmRPc (2.5,-2.5) 16 / c Vertical Scale Division 5 µm


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6/13/2014 1 Lab#5: Surface Roughness Test. To understand surface roughness terminology and its parameters. To be able to use surface roughness measurement