measurement of surface finish

QIS COLLEGE OF ENGG. & TECH.:ONGOLE,A.P. METROLOGY LAB

Measurement o Surface Finish33

MEASUREMENT OF SURFACE FINISH


MEASUREMENT OF SURFACE FINISH

AIM: To measure the surface finish of the finished component using Talysurf

tester.

EQUIPMENT REQUIRED: Talysurf Tester, Surface plate, Adopter for vernier

height gauge.

THEORY:

On any finished surface, imperfections are bound to be there and these

take the form of a succession of hills and valleys which vary both in height and in

spacing and result in a kind of texture which in appearance or feel is often

characteristic of the machining process and accompanying defects. The several

kinds of departures are there on the surface and these are due to various

causes.

Methods of Measuring Surface Roughness :

Surface inspection of comparison methods

Direct instrument measurements

In comparative methods the surface texture is assessed by observation of the

surface. But these methods are not reliable as they can be misleading, if

comparison is not made with surfaces produced by same techniques. The

various methods available under comparison method are: (i) Touch Inspection (ii)

Scratch Inspection (iii) Microscopic Inspection (iv) Visual Inspection (v) Surface

Photographs (vi) Reflected Light Intensity

Direct Instrument Measurements 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.

Taylor – Hobson Talysurf:

Talysurf is an electronic instrument working on carrier modulating

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

about 0.002mm tip radius and skid or shoe which is drawn across the surface by



means of a motorized driving unit(gear box), which provides three motorized

speeds giving respectively X20 and X100 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. On two legs of the

E-shaped stamping there are coils carrying an a.c. current. These two coils with

two other 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. 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 record

of this statement the marking medium is an electric discharge through a specially

treated paper which blackens at the point of stylus so this has no distortion due

to drag and the record strictly rectilinear one.

Analysis of Surface Traces:

A numerical assessment is assigned to indicate the degree of smoothness

(roughness) in a number of ways. In practice three roughness measures have

shown themselves to be particularly useful.

1. Ra – Centre Line Average (CLA) or Arithmetic Average (AA ): This is most

widely used parameter for specifying surface roughness. It is the arithmetic

mean of the departures Y of the points on the profile from the mean line

shown in fig.

The mean line is first determined and the ordinates of the points from the

mean line are added without considering the sign(i.e. irrespective of whether

the points are above or below the mean line).

Where n is the number of ordinates in the sampling length L and Y is the

ordinate height. Ordinates are taken at equal intervals.

The CLA can also be calculated from the areas between the profile and the

mean line (shown in fig.)



CLA (Ra) in

Where P1, P2, P3 …… and Q1, Q2, Q3 …… are areas in mm2

L is the sampling length in mm

M is the vertical magnification of the recorded profile.

Talysurf has got built in arrangement fro integrating the areas and the

average value is directly given.

2. Roughness Average or Root Mean Square Average Height (RMS) – R q:

It is defined as the average root mean square deviation Y of the profile from

its mean length within the sampling length L.

Rq (RMS) =

Where n is the number of ordinates in the sampling length L.

3. R z – Ten Point Peak to Valley Average Height: Rz is the average difference

between the five heighest peaks and five deepest valleys within the sampling

length, the heights being measured from a line parallel to the mean line and

not crossing the profile ( shown in fig.)

Rz =



PROCEDURE:

The finished component is placed on the surface plate.

Talysurf tester is fixed to the vernier height gauge using adopter at a

convenient height.

Make sure that the stylus probe touches the work piece.

Fix the sampling length in the tester.

Then the power button is pressed so that the probe moves on the surface

to and fro.

Take the readings of the surface roughness directly from the instrument.

PRECAUTIONS:

The surface to be tested should be cleaned properly.

The tester should be fixed to the height gauge properly so that the

movement of the probe is exactly parallel to the surface of work.

Make sure that the probe gently touches the work.

RESULT:


measurement of surface finish

Documents