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11.2 METHODS OF MEASURING SURFACE ROUGHNESS

•There are a number of useful techniques for measuring surface roughness,

-observation and touch - the human finger is very perceptive to surface roughness

-stylus based equipment - very common

-interferometry - uses light wave interference patterns (discussed later)

11.2.1 Observation Methods

•Human perception is highly relative. In other words, without something to compare to, you will not be certain about what you are feeling.

•To give the human tester a reference for what they are touching, commercial sets of standards are available.

•Comparison should be made against matched identical processes.

•One method of note is the finger nail assessment of roughness and touch method used for draw dies in the auto industry.

11.2.2 Stylus Equipment

•One example of this is the Brown & Sharpe Surfcom unit.

•Basically this technique uses a stylus that tracks small changes in surface height, and a skid that follows large changes in surface height. The use of the two together reduces the effects of nonflat surfaces on the surface roughness measurement. The relative motion between the skid and the stylus is measured with a magnetic circuit and induction coils.

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direction of travel over surface

magnetic core

induction coils

pivot

stylus

skid/shoe

work surface

•The actual apparatus uses the apparatus hooked to other instrumentation. The induction coils drive amplifiers, and other signal conditioning hardware. The then amplified signal is used to drive a recorder that shows stylus position, and a digital readout that displays the CLA/Ra value.

•The paper chart that is recorded is magnified in height by 100000:1, and in length by 82:1 to make the scale suitable to the human eye.

•The datum that the stylus position should be compared to can be one of three,

-Skid - can be used for regular frequency roughness

-Shoe - can be used for irregular frequency roughness

-Independent - can use an optical flat

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Skid - used for regular frequencies, and very common.

the height of the skid varies slightly, but effectively gives a datum

skid moves this way Skid

Flat Shoe: Used for surfaces with irregular frequencies

shoe

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Independent Datum - a separate datum is used for the reference datum.

This may be a good application for a laboratory.

optical flat

work piece

• Where the scan is stopped might influence the Ra value. This is especially true if the surface texture varies within a very small section of the surface. For example,

CASE 1: Measurement of l1, or l2 would yield the same Ra values, or very close.

l1

l2

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CASE 2: The datum changes when the longer sample is taken, thus changing the mean line, and the Ra value also.

l2

mean line for l2

l1 mean line for l1

CASE 3: The surface frequency.amplitude changes over the length of the surface

l1

l2

•In both cases 2 and 3 above, Ra would be higher over the longer sample (l2) than over the shorter sample (l1).

•The bearing surface that the skid/shoe runs on might also have an effected on the measurement.

Both of the two surface profiles shown below would result in the same Ra values