Principle and operation of stylus probe instruments
Direct Instrument Measurements (Metrology):
These methods enable to determine a numerical value of the surface finish of any surface. Nearly all instruments used are stylus probe type of instruments. These operate on electrical principles. Further, these electrical instruments can be of two kinds. In first type, they operate on the carrier-modulating principle. The movements of the stylus exploring the surface are caused to modulate a high frequency carrier current. The second type includes those operating on voltage-generating principle. In these the movements of the stylus are caused to generate a voltage signal. In both these types the output has to be amplified and the amplified output is used to operate a recording or indicating instrument .The carrier modulated frequency type of instruments have the advantage that the signal fed to the recorder depends only upon the position of the stylus. While in the voltage generating type, when the oscillatory movement of the stylus stops, the output falls to zero no matter where the stylus may be.Some pneumatic instruments are also used for measuring surface finish.
Stylus Probe Instruments:
This type of instrument generally consists of the following units:
(i) 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 follows its general contours and provides a datum for the measurements. In case a skid is not used and only a probe is used then probe will trace the actual profile, but upward and downward movement of probe will be dependent upon the setting of the work under probe. But since the roughness of the surface does not depend upon the position of the work, it will be necessary to choose a datum from which the measurement is to be taken. A line touching the crest of the profile, that is, the envelope line which defines the macro-geometrical form is generally chosen as datum line and this is obtained by using a skid of such a size which can span a large number of surface undulations. A lot of work has been done on shape of skid, and different types of skids are available for different purposes by which the true macro-geometrical form may be obtained.
(ii) A stylus or probe which moves over the surface with the skid. The stylus for Ra measurement on new instrument can have a radius of 10 microns ± 30%. When in use, tip radius is allowed to vary + 50% (5 to 10 u.m). However for calibration purposes, stylus should have 10 \im nominal size radius ± 20%. The stylus should be cone shaped with a spherical tip. This records the micro-geometrical form of the surface. It moves vertically up and down relative to skid movement due to roughens of the surface. Generally it is desired that if the skid is moving up then the stylus must also be moving up. But when the pitch between the skid and stylus is comparable to that of half the wavelength of surface, then record will not be desirable one, e.g., in the skid and stylus position shown in Fig. 11.7 skid is moving up, while stylus is giving downward indication.
(iii) An amplifying device for magnifying the stylus movement and an indicator.
(iv) A recording device to produce a trace or record of the surface profile. Usually the vertical movement is magnified more in comparison to horizontal movement, thus the record will not give the actual picture of surface roughness but a distorted trace obtained.
(v) A means for analysing the trace is obtained. The analysis can be done separately or some automatic device may be incorporated in the instrument for analysis.
This instrument is most commonly used in U.S.A. for direct measurement of surface quality. This is a dynamic 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 which is amplified further and operates an indicating device. Thus this records the average height of the surface roughness. In this instrument, roughness together with waviness and flaws comprises the irregularities found on the surface. An indication ‘is obtained only when the pick-up is moving. This instrument is best suited for measuring surface finish of deep bores.
The Tomlinson Surface Meter:
This instrument was designed by Dr.Tomlinson. This instrument uses mechanical-cum-optical means for magnification (Fig. 11.8). The diamond stylus on the surface finish recorder 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 and its height is adjustable to enable the diamond to be positioned conveniently. The lapped cylinder is supported on one side by the stylus and on the other side by two fixed rollers as shown in Fig. 11.8. The stylus is restrained from all motions except the vertical one by the tensions in coil and leaf spring.
The tensile forces in these two
Fig.11.8. Tomlinson Surface Meter.
Springs also keep the lapped steel cylinder in position between the stylus and a pair of fixed rollers. A light 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. When measuring surface finish, body is traversed across the surface by a screw rotated by a synchronous motor. Any vertical movement of the stylus caused by the surface irregularities causes the horizontal lapped steel cylinder to roll. By its rolling, the light arm attached to its end provides a magnified movement on a smoked glass plate. This vertical movement coupled with the horizontal movement produces a trace on the glass magnified in vertical direction and there being no magnification in horizontal direction. The smoked glass trace is then, further projected at x 50 or x 100 magnification for examination. This instrument is comparatively cheap one and gives reliable results.
The Taylor-Hobson Talysurf:
The Talysurf is an electronic instrument working on carrier modulating principle. This instrument also gives the same information as the previous instrument, but much more rapidly and accurately. This instrument as also the previous one records the static displacement of the stylus and is dynamic instrument like profilometer. The measuring head of this instrument consists of a diamond stylus of about 0.002 mm tip radius and skid or shoe which is drawn across the surface by means of a motorised driving unit (gearbox), which provides three motorised speeds giving respectively x 20, and x 100 horizontal magnification and a speed suitable for average reading. A neutral position in which the pick-up can be traversed manually is also provided. In this case the arm carrying the stylus forms an armature which pivots about the centre piece of E-shaped stamping as shown in Fig. 11.9. On two legs of (outer pole pieces) the J5-shaped stamping there are coils carrying an a.c. current. These two coils with 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 coils is modulated. The output of the bridge thus consists of modulation only as shown in Fig
Fig. 11.9. Schematic Layout of Talysurf.
11.9. This is further demodulated so that the current now is directly proportional to the vertical displacement of the stylus only.
The demodulated output is caused to operate a pen recorder to produce a permanent record and a meter to give a numerical assessment directly. In recorder of this statement the marking medium is an electric discharge through a specially treated paper which blackens at the point of the stylus, so this has no distortion due to drag and the record strictly rectilinear one. Now-a-days microprocessors have made available complete statistical multi-trace systems measuring several places over a given area and can provide standard deviations and average over area-type readings and define complete surface characterisation. These systems lend themselves to research applications where specialised programming can achieve autocorrelation, power spectrum analysis and peak curvature.
Phonograph needles, though used in some cases are found to be too large and too heavily loaded. It also causes damage. Diamond styli are used universally. Some of them are cones of 90° include dangle and tip radius 4-12 urn. A popular stylus with truncated pyramid is shown in Fig. 11.10. The angle between the faces is 90°. The short edge is parallel to the direction of motion. Thus this stylus cannot resolve a wavelength shorter than 6 \xm, and integrates over a narrow strip of surface 8 \im wide.
It may be noted that this pick up has finite dimensions, and it is constrained to move in a nearly vertical plane, relative to the moving pickup. Thus the stylus cannot record re-entrant features, an unimportant drawback for engineering investigations as re-entrant structures are absent on most machined surfaces. This stylus will fail to follow peaks and valley faithfully and produces a distorted record of the surface.
Since the dimensions of the stylus are finite, so also is the load on it. The load is of the order of 70 mg force. But as the area of contact is too small, the local pressure may be sufficiently high to cause significant local elastic downward deformation of the surface under examination.