Engineering Metrology

Transcription

1 Albaha University Faculty of Engineering Mechanical Engineering Department Engineering Metrology Lecture 06: Roughness Measurements Ossama Abouelatta Mechanical Engineering Department Faculty of Engineering Albaha University Aims This lecture aims: to identify factors affecting surface finish. to identify measuring techniques and instruments To differentiate between mechanical, electrical, optical, pneumatic, electronic instruments. to explore in recent trends in surface finish measurement. Mechanical Engineering Department Lecture (6): Roughness Measurements (2)

2 Outlines Measuring Techniques and Instruments Mechanical instruments Electrical instruments Optical instruments Pneumatic instruments Electronic instruments Tactile methods Surface replicas Recent trends in surface finish measurement Mechanical Engineering Department Lecture (6): Roughness Measurements (3) Measuring Techniques and Instruments A number of instruments and techniques are in use for the purpose of surface finish assessment, but the following are probably the most important from the point of view of the engineer: Mechanical instruments Electrical instruments Optical instruments Pneumatic instruments Electronic instruments Tactile methods Surface replicas Recent trends in surface finish measurement Mechanical Engineering Department Lecture (6): Roughness Measurements (4)

3 Mechanical Instruments Dial depth gauge It is useful to define coarser grades of roughness as those above 50 μm R a. The dial depth gauge has been developed primarily to measure rougher surfaces, such as rough turned, shaped, milled, grit and shot blasted, spark eroded or cast surfaces, etc. Mechanical Engineering Department Lecture (6): Roughness Measurements (5) Mechanical Instruments Johansson Mikrokator instrument In the simplest form of mechanical instrument for measuring surface finish, a vertical Johansson Mikrokator comparator is used. The stylus is traversed manually over the lateral range of the instrument very slowly, and observations are made at regular intervals. This instrument is mostly used for measuring the surface roughness of bearings, crank shafts, cylinder bores, cams, etc.a graph. Johansson portable instrument This is another model of the Johansson instrument, and it is a handy and portable type. The diamond stylus operates on the test surface through a hole in a skid which also provides support for the instrument. A mechanical feeding device can be fitted for plotting the movement of a graph. Mechanical Engineering Department Lecture (6): Roughness Measurements (6)

4 Mechanical Instruments Tomlinson surface finish recorder It has the following features: It is a 100% mechanical instrument. It is not affected by electrical variations. It provides high accuracy. No scratch is developed on the surface under test, due to very light pressure on it. The inertia effects of moving parts are minimized. Mechanical Engineering Department Lecture (6): Roughness Measurements (7) Electrical Instruments A stylus instrument is the basic part of electrical instruments for measuring surface finish. The magnification is obtained by some electrical method only. The output of the instrument may be in the form of some value in which averaging is obtained automatically, or in the form of a graphical representation of the surface contours. The important merits of such instruments are: A high magnification is possible. The response is very quick. The magnification can be changed. Mechanical Engineering Department Lecture (6): Roughness Measurements (8)

5 Electrical Instruments Talysurf surface measuring instrument This instrument measures the texture of a surface by means of a sharply pointed stylus, whichistraversed over the surface. The movements of the stylus are amplified electrically. The position of the stylus, therefore, controls the current in these coils, which are arranged in such a way that only the changes in the current (which are proportional to the air gap) are amplified and recorded. The pick-up is traversed across the surface by a motorized gearbox driving unit. The results of the test may be in the form either of a graph or of an average number shown on an indicator. The measuring head, as on the Tomlinson, consists of a stylus and shoe (T) which are drawn across the surface under test by an electric motor and gearbox. In this case the arm carrying the stylus forms an armature which pivots about the center piece of a stack of E-shaped stampings around each of the outer pole pieces of which is a coil carrying an AC current as in the figure. Mechanical Engineering Department Lecture (6): Roughness Measurements (9) Electrical Instruments Talysurf surface measuring instrument As the armature pivots about point M it causes the air gaps to vary and thus modulates the amplitude of the original AC current flowing in the coils. As these form part of a bridge circuit the output consists of the modulation only. This is fed to an amplifier and caused to operate a pen recorder to produce a permanent record, and to a meter to give a numerical assessment direct. The pen recorder is of interest in that the trace is produced on carbon-backed paper by a pointer whose tip arcs electrically across through the paper. This produces a much finer line than ink type pen recorders, with no distortion due to drag. Mechanical Engineering Department Lecture (6): Roughness Measurements (10)

6 Optical Instruments Light interference microscopes The advantage of the interference microscope is that it allows an area of the surface to be examined, and the more expensive versions provide a range of magnifications and allow for a camera, so that a permanent record of the fringe pattern may be obtained if desired. Very high resolution is possible with these instruments and scratches spaced 0.5 μm apart are fairly easily seen. This order of resolution compares favorably with the normal electronic stylus instrument, where the response begins to fall off at scratch widths below 2.5 μm. The main advantages are as follows: 1. The area sample obtained allows surface details to be seen as a composite in the field of view, whereas stylus exploration is very selective. 2. The fringes can be thought of as a series of cross-sectional planes, and their direction can be adjusted to provide the most informative pattern. 3. The method is non-destructive, as there is no contact with the surface. The surfaces of soft or thin materials, which normally cannot effectively be explored with a stylus, can be examined. 4. No instrument recalibration is required. Mechanical Engineering Department Lecture (6): Roughness Measurements (11) Optical Instruments Hilger & Watts surface finish microscope This instrument uses interference fringes to examine metallic surfaces, and is designed for workshop use. It can be used for examining flat, spherical or cylindrical workpieces by employing standard comparison flat or concave work surfaces, using the comparison hemisphere or cylinder. The magnification is x125, which is convenient for workshop purposes, since this instrument is designed for workshop use. The measurement of surface texture in terms of light waves is absolute. However, for convenience, sample standards can be provided to enable comparisons to be made with specimens of known CLA or RMS values. A photographic attachment also can be employed for record purposes. Mechanical Engineering Department Lecture (6): Roughness Measurements (12)

7 Optical Instruments Hilger & Watts surface finish microscope Illumination is provided by the miniature, high intensity mercury vapor lamp A. The light passes through the green filter B and adjustable iris diaphragm C. It is then focused by condensers D, after which it is reflected at a 50% reflecting surface E on the back focal plane of the objective F, which is especially designed to work with collimated light. In front of the objective is the spring-loaded reference flat G coated with a special surface film to render it highly reflecting and, at the same time, non-absorbing, so that none of the incidence light is wasted. The surface of this reference flat, which is rendered durable by a scratch-resisting protective coating, is preadjusted to be in focus when viewed through the eyepiece H. When the tube is lowered, so that the flat rests on the work, the interference fringes are immediately visible. By rotating a knob, the flat can be given a very slight tilt, in order to vary the spacing between the fringes. Mechanical Engineering Department Lecture (6): Roughness Measurements (13) Optical Instruments Hilger & Watts surface finish microscope Half inch diameter ball-bearing x150 Lapped surface x75 Highly reflecting chromium plating Polished steel surface x75 Steel roller x75 Fine ground steel surface x150 Mechanical Engineering Department Lecture (6): Roughness Measurements (14)

8 Optical Instruments Interferometer Mechanical Engineering Department Lecture (6): Roughness Measurements (15) Pneumatic Instruments System of pneumatic amplification Principle of pneumatic instruments Use of Solex surface gauge Principle of pneumatic amplification Mechanical Engineering Department Lecture (6): Roughness Measurements (16)

9 Electronic Instruments This method is accurate and gives an idea of the average variation in height of the crests and roots. An electronic unit E is held in an annular member A having a knife-edge rim, which contacts the surface under test and forms the second electrode of the system. First the head is placed on an optically flat surface. In such a case the lower end of E will be at certain distance from standard surface. Next, the head is placed on the surface under test. The distance between the lower surface of the electrode unit E from the average line between peak and valley now changes and provides a reading that is a measure of the roughness. Cornelius surface measurement head Mechanical Engineering Department Lecture (6): Roughness Measurements (17) Tactile Method Mechanical roughness instrument (Mecrin) This method was developed on the principle that any thin metallic blade will begin to buckle when moved along a surface at a certain angle to it. The higher the roughness, the lesser will be the angle of attack. A development from the tactile method which provides Ra values is the mechanical roughness indicator or Mecrin gauge. The gauge is suited only to irregular surfaces, where the frictional properties measured compare with those provided by electronic stylus instruments. On regular surfaces of rougher values, the instrument can be used as a comparator against surface roughness blocks, but the readings then bear no relation to Ra values. The Mecrin instrument assesses the surface through its frictional properties and the average slope of the irregularities. A thin metallic blade when pushed against a surface at a certain angle will either slide or buckle, depending on two factors: The surface roughness. The angle of attack, i.e., the angle of the blade to the surface. Mechanical Engineering Department Lecture (6): Roughness Measurements (18)

10 Surface Replicas There are cases where the surface texture of a component is required but the surface is not readily accessible to the probe, and is not reflective enough for interference methods. It is often possible to obtain a trace by making a replica of the surface. This was originally done by damping a piece of cellulose-acetate film in acetone to soften it, and pressing against the surface until it hardened. By this method a reproduction fidelity of about 80% could be obtained. Modern techniques use epoxy and other resins, and approach 100% fidelity. This is a simple but effective method of surface texture measurement, where a surface is inaccessible to instruments or where the part is too large to mount or bring to an instrument. Essentially, the technique involves either pressing a piece of softened plastic on to the surface or placing a plastic wall or metal ring round the specimen area and pouring in a plastic fluid. In either case, after a suitable setting time the replica may be removed and an instrument reading taken using a lightly loaded stylus. The bores and other inaccessible parts of workpieces may be examined by using a replica technique. A plastic blank is warmed and pressed against the surface to be replicated, on which a small amount of the liquid plastic has been painted previously. It takes only a few minutes for the replica to dry, after which it is ready for examination. It will now be a convex negative which is easily examined by using the comparison flat. Mechanical Engineering Department Lecture (6): Roughness Measurements (19) Recent Trends in Surface Finish Measurement Many new methods are being developed for surface finish measurement. The trend is toward accurate and fast methods. The rate of measurement and accuracy are to be optimized to suit a particular purpose. A method found more accurate for research work may not be suitable in industry, if it happens to be complicated. Some of the new methods are briefly stated here. Ultrasonic methods Ultrasonic methods of surface finish measurement are still at the laboratory stage. The method can measure even very small undulations, but minute details about surface characteristics are not known completely. Field emission method This method can detect surface irregularities even on a submicroscopic scale approaching the dimensions of the molecules. The principle of this method is field emission from metal surfaces. If the surface is rough, the role of the field emission is increased. Thus, by comparing the actual rate of the field emission with the theoretical rate (determined mathematically), an idea of the surface roughness can be obtained. Vision method In this technique, a light source is used to illuminate the surface with a digital system to view the surface, and the data is sent to a computer to be analyzed. The digitized data is then used with a correlation chart to get the actual roughness values. Mechanical Engineering Department Lecture (6): Roughness Measurements (20)

11 Recent Trends in Surface Finish Measurement Mechanical Engineering Department Lecture (6): Roughness Measurements (21)