Abstract :
This report is to obtain the value of surface roughness of the piston cylinder. In the order to gather necessary information about this laboratory report, some procedures were carried out at the Metrology laboratory. The result of the surface roughness is obtained by the Centre Line Average (CLA) or Roughness Average (Ra) method using a computerize Roughness Measuring Machine.
Introduction :
The development of modern technologies has called not only for improved control of dimensional accuracy but also of the texture and geometric from of both working and non-working surfaces of components. There are three main factors which have made the control of surface texture important: fatigue life, bearing properties and wear. Most bearing are rotary in nature and it is obvious that roundness is an important geometric property of the component parts and roundness can be controlled only by measurement.
Types of Surfaces
Surface
A surface is a boundary that separates an object from another object or substance.
Nominal Surface
A nominal surface is the intended surface. The shape and extent of a nominal surface are usually shown and dimensioned on a drawing. The nominal surface does not include intended surface roughness.
Real Surface
A real surface is the actual boundary of an object. It deviates from the nominal surface as a result of the process that created the surface. The deviation also depends on the properties, composition, and structure of the material the object is made of.
Measured Surface
A measured surface is a representation of the real surface obtained with some measuring instrument. This distinction is made because no measurement will give the exact real surface. Later portions of this manual describe many different types of measuring instruments.
Objective : To obtain the value of surface roughness of a component by Centre Line
Average (CLA/Ra) method using Computerized Roughness Machine.
Apparatus :
Roughness Measuring Machine Specimen (cuboids block)
System.
- A Roughness Measuring Machine System (Surface GV-600; Mitoyo Auto Leveling Table.
- A Specimen-Piston.
- A Planimeter.
Theory :
A surface can never be perfectly smooth and will always have two components of surface texture namely roughness and waviness as shown in Figure 1. They may vary from fine to coarse according to the machining.
Surface texture includes roughness and waviness. Many surfaces have lay: directional striations across the surface.
Roughness
Roughness includes the finest (shortest wavelength) irregularities of a surface. Roughness generally results from a particular production process or material condition.
Waviness
Waviness includes the more widely spaced (longer wavelength) deviations of a surface from its nominal shape. Waviness errors are intermediate in wavelength between roughness and form error. Note that the distinction between waviness and form error is not always made in practice, and it is not always clear how to make it. New standards are emerging that define this distinction more rigorously as developed in later sections.
Lay
Lay refers to the predominant direction of the surface texture. Ordinarily lay is determined by the particular production method and geometry used.
Turning, milling, drilling, grinding, and other cutting tool machining processes usually produce a surface that has lay: striations or peaks and valleys in the direction that the tool was drawn across the surface. The shape of the lay can take one of several forms as shown below. Other processes produce surfaces with no characteristic direction: sand casting, peening, and grit blasting. Sometimes these surfaces are said to have a non-directional, particulate, or protuberant lay.
A means of measuring surface finish that is normally used by ISO is the CLA method. It measures the average values of the departures of both what are above and below the centre line of the surface through a prescribed sample length L. The mean result of several sampling length taken consecutively will give the actual roughness value of the surface of the component.
Center Line
The center line of a profile is the line drawn through a segment (usually a sample length) of the profile such that the total areas between the line and the profile are the same above and below the line.
This concept is little used in modern instruments; it mainly served as a graphical method for drawing a mean line on the output of a profile recording instrument with no built-in parameter processing.
Formula used
=Sum of Area above and below centerline
L = Sample Length
Vertical Magnification.
Procedure :
1. Select a cut off wavelength of 0.8mm on the machine. Set the machine to a Vertical Magnification of 2000 and Horizontal Magnification of 50.
2. Place the component on the Auto Leveling Table at a suitable position such that when the pick-up head is lowered, its stylus contacts the surface of the specimen.
3. Set the machine ready to record the measurement.
4. Obtain the roughness profile graph for the specimen.
5. Manual process:
I. Draw the centerline on the profile graph.
II. Measure the areas above and below the centerline using a digital planimeter. Take at least two measurements.
III. Calculate the CLA value.
6. Compare the computerized with the manual the calculated result.
Result :
Result of value of Roughness Average (Ra) using a computerized Roughness Measuring Machine:
1. Speed = 0.500mm/s
2. Length (L) = 0.8mm
3. Range = 600µm
4. Pre-travel length = 0.400mm
5. Amplitude transmittance = 50%
6. Low-band cut off = 0.800mm
7. Filter = Gauss
8. Ra = 3.40µm
Result of Roughness Average (Ra) using by the Centre Line Average (CLA)
| Aa (cm2) | Ab (cm2) |
| 5.2 | 5.3 |
| 5.3 | 4.5 |
| 4.7 | 5.0 |
Length = 14cm
For Aa = 5.2 and Ab = 5.3,
Center Line Average = ∑A / (L × MV)
= (5.2 + 5.3) / (14 × 2000)
= 3.75 × 10-4 cm
= 3.75µm
For Aa = 5.3 and Ab = 4.5,
Center Line Average = ∑A / (L × MV)
= (5.3 + 4.5) / (14 × 2000)
= 3.5 × 10-4 cm
= 3.5µm
For Aa = 4.7 and Ab = 5.0,
Center Line Average = ∑A / (L × MV)
= (4.7 + 5.0) / (14 × 2000)
= 3.46 × 10-4 cm
= 3.46µm
Discussion :
1. Comment on any vibrations in the results for the measured profile?
The roughness profile includes only the shortest wavelength deviations of the measured profile from the nominal profile. The roughness profile is the modified profile obtained by filtering a measured profile to attenuate the longer wavelengths associated with waviness and form. Optionally, the roughness may also exclude (by filtering) the very shortest wavelengths of the measured profile which are considered noise or features smaller than those of interest.
- What are the possible errors involved?
Errors result from large scale problems in the experimental such as errors in machine tool ways, guides, or spindles, insecure clamping, inaccurate alignment of a work piece, or uneven wear in machining equipment. Error is on the dividing line in size scale between geometric errors and finish errors.
- What are the necessary precautions?
- During the experimental some precautions must be taking such as setting up the specimen from moving during the experiment.
- Make sure the trace profile just nice on the line of the specimen surface if not this will affect the resultant or may damage the trace profiling instrument.
- By using this Computerized Roughness Measuring Machine the operator should knows to operate this machine to make this experiment easier.
Conclusion :
Roughness is of significant interest in manufacturing because it is the roughness of a surface (given reasonable waviness and form error) that determines its friction in contact with another surface. The roughness of a surface defines how that surfaces feels, how it looks, how it behaves in a contact with another surface, and how it behaves for coating or sealing. For moving parts the roughness determines how the surface will wear, how well it will retain lubricant, and how well it will hold a load.
References :
- Metrology of Engineers; J.F.W.Galver & C.r shotbolt.
- Industrial Metrology: Surfaces and Roundness. G.T. Smith. Springer-Verlag, 2002.
- Manufacturing Engineering and Technology: Serope Kalpakjian.
- Metrology and Properties of Engineering Surfaces : Evaristus Mainsah, Jim A. Greenwood, Derek G. Chetwynd - Technology & Engineering - Kluwer Academic Publishers (1998)
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