Principle and Operation of Stylus and Probe Instruments

7/22/2019 Principle and Operation of Stylus and Probe Instruments

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LOVELY PROFESSIONAL UNIVERSITY

METROLOGY AND MEASUREMENT

TERM PAPER

ON

PRINCIPLE AND OPERATION OF

STYLUS PROBE INSTRUMENTS

SUBMITTED TO SUBMITTED BY

Mr. Rohit Sharma Satyam Kumar

School of Mechanical Reg. no.-111101473

Engineering sec-M3R28

Roll no.-A03


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INDEX Definition of stylus and probe

instruments

Introduction

Direct Instrument Measurement

Stylus and probe instrument

Profilometer

The Tomlinson Surface Meter

The Taylor-Hobson Talysyrf

Assessment of Surface

Roughness as Per Indian Standards

Information to be given in the

Statements of Surface Roughness


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DEFINITION OF STYLUS AND PROBE

INSTRUMENTS

A measuring instrument with a cone-shaped spherical tip connected to a probe.

The stylus contacts the part and traces its surface irregularities. These

instruments 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 stylusexploring 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 has the advantage that the

signal fed to the recorder depends only upon the position of the stylus.


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INTRODUCTION:

Stylus and probe instrument have been used in the assessment of

surface texture like flatness or roughness etc. Inspection and assessment

of surface roughness of machined work pieces can be carried out bymeans of different measurement techniques.

Direct Instrument Measurenment:

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, threes electrical instruments can

be of two kinds. In first type they operate on the carrier modulating

principle. The movements of the stylus expiring the surface are caused to

modulate a high frequency carrier current. The carrier modulated frequencytypes 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.


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STYLUS AND PROBE INSTRUMENT

This type of instrument generally consists of the

following units:

(i) A skid or shoe which is draw slowly over the surface either by

hand or by motor drive. This skid when moved over the surface

follows its general contours and provides a datum for themeasurements. 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 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.


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(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

20%. The stylus should be cone shaped with a spherical tip. This

records the micro-geometrical form surface.

(iii) 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 traceobtained.

(iv) A means for analyzing the trace is obtained. The analysis can be

done separately or some automatic device may be incorporated in

the instrument for analysis.


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STYLUS PROBE INSTRUMENTS:

Profilometer

The Tomlinson Surface Meter

The Taylor-Hobson Talysurf


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Profilometer :

This instrument is most commonly used in U.S.A for direct. 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.


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Tomlinson Surface Meter:

This instrument was designed by Dr. Tomlinson. This instrument usesmechanical-cum-optical means for magnification.

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


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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 motion except the vertical one by the tensions in coil and leaf

spring. The tensile forces in these two springs also keep the lapped steel

cylinder in position between the stylus and it carries at its tip a diamond

scriber which bears against a smoked glass.

The Taylor-Hobson Talysyrf:

The Talysyrf 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.


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The measuring head of this instrument consists of a diamond stylus of

about 0.002 mm tip radius an skid or shoe which is drawn across the

surface by means of a motorized driving unit (gearbox), which provides

three motorized speeds giving respectively 20 and 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.

On two legs of (outer pole pieces) the E-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.


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Most simple roughness measurements are made using a skidded probe.

Many different types of skidded probes are available. Most often, the

characteristics of the part will determine which probe is the best for the

application. For example, there are probes designed to measure.

The output of the bridge thus consists of modulation only as shown in

Fig. 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 operator a pen recorder to

produce a permanent record and a mere to give a numerical assessmentdirectly. In recorder of this instrument the marking medium is 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 is

strictly rectilinear one.


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Assessment of Surface Roughness

as Per Indian Standards:

There is various method of evaluating the surface roughness but the most

prominent and commonly used methods are the M (Centre Line Average

method (ClA) or the mean line system and the E (Envelope system) system.

The M system expresses the arithmetical average departure of the actual surface

both above and below a mean line, within a specified sampling length, and in a

plane substantially normal to the direction of surface. A similar system also

based on the mean line expresses the departure of the actual surface as a root

mean squared value. ClA system is more popular than RMS value method. To

eliminate the effects of secondary texture the profile is split into a number of

adjacent sampling lengths may not coincide even. The E system expresses the

arithmetical departure of a surface both above and below a "mean" vurve. This

mean curve is developed from a contacting mean curve are equal. The

contacting envelope referred to the contacting envelope referred to above is

obtained by displacing it to a position, where the areas enclosed by the profile

above and below the mean curve are equal. The contacting envelope referred to

above is obtained by rolling across the surface an amount equivalent to r.

The merits and demerits of the two systems (i.e., M and E) are given below.

The M system is designed to measure in this system. It is more useful and a

satisfactory means of controlling at the point of production, the consistency of

results from a process when the production parameters have been established.


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This system has the limitation that it is unable to control the functional

qualification of a surface whence associated with a machine process.

The E system, though, at present in its early stage of development is going

to be widely accepted in future because of various surfaces obtained by similar

operation; of course, it does not indicate the limits of irregularity R max and

Ra value and as such R max value alone can't be specified in drawings.

Information to be given in the

Statements of Surface Roughness:

The various roughness grade numbers N 1 to N12 in 5 groups are

specifically as under by 1s1. The relationship between the roughnesses by

symbol is given below.


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>>The values of the surface roughness expected from various manufacturing

processes are indicated below.

>> This is the direction of the predominant surface pattern and is determined

by the production method used. The surface roughness is generally measured

across the direction of the lay.

Preferred Values of Ra and Rz:

Preferred values for arithmetical mean deviation Ra in um are selected

from:0.025, 0.05, 0.1, 0.2, 0.4, 0.8

1.6, 3.2, 6.3, 12.5 and 5


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and the preferred values fro ten point height of irregularities Rz in um are

selected from:

0.05, 0.1, 0.2, 0.4, 0.8, 1.6, 3.2, 6.3, 12.5, 25, 50 and 100.

Selection of Sampling Length:

For measuring surface roughness, the value of the sampling length / is selected

from the following series depending upon the process of manufacturing:

0.08m 0.25, 0.8, 2.5, 8, 10 and 25 mm.

It is usual practice to choose shorter value of the finer and larger values for

the coarser grade of a given proves when more than one values are

given.

Measurement

It may by emphasized her again that the surface texture represents the combined. If

only the total height (i.e., the difference between the highest and lowest points on the

plot) is considered then idea of above three characteristics can be had by consideringdifferent sample length should be neither too big as to include the waviness, nor too

small as to ignore the occasional deep scratches. The best thing is, therefore, to limit the

measurement to a sufficiently short length of the surface and in order to take into

consideration the considerable variation of roughness from place to place, several

reading at various places are taken.


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Determination of Ra value:

The Ra value can be determined either by graphical methods or by direct

reading instruments. Whenever the surface texture has a directional quality, the

direction in which the measurement is made is approximately at right angles to

the lay. It is always preferable to compute the Ra value by taking mean result

from the measurement of several sampling lengths taken consecutively along

the profile so that it gives better indication and is the correct representation of

the whole of surface roughness.

Measurement of Surface Finish

Sampling length should bear some

relation to the type of profile. It is

found that the required length can

be related to the manufacturing

process. In a surface there are, different kinds of irregularities having differentfrequencies and it is never clear which has got more importance. Thus the best

thing would be to standardize some sampling length and that can be always

associated with the surface roughness value. To standardize the sampling

length is also not an easy task as the wavelengths vary to a considerable extent

according to the way in which the surface is produced.


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Further referring to Fig another problem is as to where the sample length must

be chosen, i.e. position of sample length on surface will also give differentresults. This effect is taken into account by taking the readings at several places

and taking the mean value. Sampling length is standardized for various

operations considering all

follows:above facts as

.

Process Symbol Sample length in mm

Milling M --- 0.75 2.5 7.5

Turning T 0.25 0.75 2.5 ---

Grinding G 0.25 0.75 2.5 ---

Planning P --- --- 2.5 ---

Lapping Lp 0.25 0.75 --- ---

Polishing Po --- --- 2.5 ---


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Bibliography

From internet

Lecture notes

Documents

Principle and Operation of Stylus and Probe Instruments