Metrology & Quality Control

Fall 1433H (2012G)

Saturday, Monday & Wednesday 11:00am -

11:50am and Saturday 13:00am - 14:50pm

MENG 436 Class FA

Dr. Walid A. AissaDr. Walid A. Aissa

Associate Professor, Mech. Engg. Dept.

Faculty of Engineering at Rabigh, KAU, KSA

Chapter #5

September XX, 2012

Announcements:

Dr. Walid’s e-mail and Office Hours

walid_aniss@yahoo.com

Office hours for Metrology & Quality Control will be

every Sunday and Tuesday from 11:00 – 13:00 am in Dr.

Walid’s office (Room 5-109).

Text book:Text book:

1-Metrology for Engineers, J.F.W. Galyer & C. R.

Shotbolt, 4th Edition, Cassell Ltd., London, ISBN-0-

304 30612 6, 1980.

2-Essentials of Quality With Cases and

Experimental Exercises, Victor E. Sower, John Wiley

& Sons Inc., London, ISBN-978-0-470-50959-3, 2011.

Chapter 5Surface Texture

MEASUREMENTSMEASUREMENTS

Objectives of CH5: To

• Recognize

-Why we measure surface roughness?.

-Tracing Surface Irregularities.-Different Parameters for Surface Roughness

.Interpretation.

-Surface Roughness Value Conversions.

- Surface Roughness symbol in drawing.

5. SURFACE TEXTURE (TOPOGRAPHY)

MEASUREMENTS

Chapter 5

A surface is a boundary that separates an objectfrom another object or substance .Surface finish is

one of the most common measures of surface

quality and a wide variety of methods and

parameters have been developed to measure it.

5.1. Introduction:

parameters have been developed to measure it.

Methods for their measurement range from naked

eye viewing and rubbing with a thumb to the use

of ellipsometers and sophisticated atomic force

microscopes.

5.2. Why we measure surface roughness?

Figure 1

Figure 2

Figure 3

5.3. General:

Surface finish affects how a part will fit, wear,

reflect light, transmit heat, distribute lubrication

and accept coatings.

The finish should be determined by the part's

function: you want a surface that fulfills the function: you want a surface that fulfills the

engineering requirements of the application,

without wasting time and effort on a higher

quality finish than is necessary.

(In fact, many applications do better with a certain

amount of "texture," and too fine a finish can be as

bad as too coarse.).

Thirty years ago, most dimensional tolerances were

measured in thousandths of an inch.

Now that tolerances of "tenths" or even tens of

millionths are commonplace.

As shown in Figure 4, surface finish -- also known

as profile -- is composed of two components:

waviness and roughness.

5.4.Tracing Surface Irregularities:

Figure 4

Conical

diamond

stylus

• Traces are typically done 90° to “lay,” with a

conical diamond stylus.

• To separate surface wavelengths, a filter is applied

to the profile data.

Figure 5

Waviness, is the more widely spaced

component of surface texture. Roughness may

be considered superposed on a “wavy” surface

or longer-wavelength variation, is caused by

macro-type influences, like worn spindle

bearings, or vibration from other equipment on the

shop floor.shop floor.

Roughness – the short-wavelength pattern of tool

marks from grinding, milling or other machining

processes --is influenced by the condition and

quality of the tooling.

Both can be influenced by the operator's choice

of feed rate and depth of cut.

Few of the most popular methods of surface

roughness plot interpretation are:

� Distance between peak to centerline.

There can be many ways, through which the surface

roughness plot can be interpreted.

5.5. Different Parameters for Surface Roughness

Interpretation:

� Distance between peak to centerline.

�Average distance between peaks and valleys for

sample length.

� Sum of average of first 5 peaks and valleys.

�Maximum peak to valley height.

�Maximum peak height.

� RMS of peak and valley average.

Because of this reason only, there are varieties of

methods prevailing in the engineering world for

indicating surface roughness values in the drawing.

As per the data, worldwide more than 128 different

formulae with which to calculate surface finish

parameters from the same measurement data.

The most common and popular method amongst all is

interpreting thro average roughness indication. This is

known as Ra – in which, R stands for “Roughness”

and “a” stands for ‟average‟. The other methods are

Rz, Rt, Rmax etc. Below is the list of popular

parameters:

Parameter Name

Ra Roughness Average (Ra)

Rq Root Mean Square (RMS) Roughness

Rt Maximum Height of the Profile

Rv, Rm Maximum Profile Valley Depth

Rp Maximum Profile Peak Height

Rpm Average Maximum Profile Peak Height

Rz Average Maximum Height of the Profile

Rmax Maximum Roughness Depth

Rc Mean Height of Profile Irregularities

Rz(iso) Roughness Height

Ry Maximum Height of the Profile

Ra – Average Roughness

The average roughness is the area between the

roughness profile and its mean line, or the integral of

the absolute value of the roughness profile height

over the evaluation length.

Average Roughness; Ra

(Mean line)

Figure 6

where, ι is the sampling length and y is the ordinate

of the profile.

Graphically, the average roughness is the area

(1)

Graphically, the average roughness is the area

between the roughness profile and its center line

divided by the evaluation length (normally five

sample lengths with each sample length equal to one

cutoff).

You might have seen the symbols like – Ra 0.8 or

Ra 25 or N9 on various features of component.

Many of these (about half-dozen parameters) are

very application-specific and are used to describe

the measurements. In almost all cases,

measurements are presented in microinch or micron measurements are presented in microinch or micron

units.

Ra , or average roughness, is the most commonly

specified parameter for surface finish measurements,

because it provides an arithmetic average of surface

irregularities measured from a mean line that lies

somewhere between the highest and lowest points on

a given cut-off length. Ra provides a good general

guide for part performance over a wide range of guide for part performance over a wide range of

applications.

See Appendix C & D.

Figure 7

When performing an Ra measurement, it is essential

to choose a cutoff length appropriate to the process.

The cutoff length should be short enough so that the

measurement will not be influenced by waviness. On

the other hand, it must not be so short that only a

portion of a tool mark is measured, as shown in

Figure 8. Figure 8.

Figure 8

Although Ra is universally accepted as a means of

expressing surface parameters can often be

misleading. It is possible that two surfaces having

A cutoff long enough to include five complete sets

of tool marks is desirable to obtain a good average

roughness measurement.

misleading. It is possible that two surfaces having

widely different profiles could have the same, but

perform quite differently.

Ra is not the end of the story

Ra does not tell the whole story about a surface. For

example, here three surfaces that all have the same

Ra, but you need no more than your eyes to know

that they are quite different surfaces. In some

applications they will perform very; differently as

well. well.

Figure 9: Surfaces with same Ra, but a lot of

difference.

Hence, Ra has numerous limitations when

applications are highly specific, or when small details

of surface finish can make a big difference in part

performance.

The value of Ra is normally measured in the

microinches or in microns in the metric system.microinches or in microns in the metric system.

If we want to distinguish between surfaces that differ

in shape or spacing, we need to calculate other

parameters for a surface that measure peaks and

valleys and profile shape and spacing.

The more complicated the shape of the surface, we

want and the more critical the function of the want and the more critical the function of the

surface, the more sophisticated we need to be in

measuring parameters beyond Ra.

The roughness parameter, Rq, represent the root

mean square (RMS) of the peak heights and is more

sensitive to the occurrence of occasional high and

low points. The RMS is the geometric average

height of roughness irregularities over the sampling

length. RMS is expressed in microns or microinches.

Figure 10

Parameter Rv – Maximum depth

Rv is the maximum distance between valley and

mean line in the limits of a sampling length L.

Parameter Rp – Maximum height

Rp is the maximum distance between peak and

mean line in the limits of a sampling length L.

mean line in the limits of a sampling length L.

Parameter Rt – Total height

Rt is the maximum distance between valley and

peak in the limits of a sampling length L.

The maximum peak-to-valley height on a surface;

Rt is usually four or five times greater than the

average surface finish, as measured by the Ra

method. A part with an Ra value of 16µµµµ",

therefore, probably has a peak-to-valley height ;

Rt of 64 µµµµ " or greater.

Also useful is Rz -- the "ten-point height" parameter

-- which calculates the average of ten maximum

peak-to-valley differences within the sampling

range.

Parameter Rz

The Rz value is known as the ISO 10-point height The Rz value is known as the ISO 10-point height

parameter. It is measures over a single sampling

length and is itself an average of positive and

negative peak values. It is useful parameter when

only a short length of the surface is available for

assessment.

Figure 11 shows how Rz values are determined

graphically. The five highest peaks and the five

deepest valleys are conveniently measured from an

arbitrary datum line A … B drawn parallel to the

centre line AB for a chosen length L, so that:

Rz = [(P1+P2+……+P5)/5-(V1+V2+….+V5)/5] (2)

Figure 11

In Europe, the more common parameter for

roughness is Mean Roughness depth (Rz).

Rz is calculated by measuring the vertical distance

from the highest peak to the lowest valley within

five sampling lengths, then averaging these

distances. Rz averages only the five highest peaks

and the five deepest valleys — therefore extremesand the five deepest valleys — therefore extremes

have a much greater influence on the final value.

Over the years the method of calculating Rz has

changed, but the symbol Rz has not. As a result,

there are three different Rz calculations still in use,

and it is very important to know which calculation

is being defined before making the measurement

Different machining processes naturally generate

different tool patterns. The roughness produced by

grinding, for example, is generally of a shorter

wavelength than that left by turning.

Milling leaves even longer wavelength patterns,

though not as long as those produced by single point

boring. (Note that the wavelength we're referring toboring. (Note that the wavelength we're referring to

here is the spacing of the individual tool marks, not

the waviness component of surface profile.)

Example 5-1: if a surface has heights and depths as

illustrated below, evaluate Ra, Rp, Rv, Rt , Rz , Rq

values (in units of height):

Solution:

Ra = Average (1,4,6,8,5,2,1,4,1,2,1,4,7,4,1,2,5,8,5,2,1,4,1,1)

(in units of height) = 3.333 (in units of height)

n = 24

Figure 12

Rp (maximum distance between peak and mean line )

= 8.0 (in units of height).

Rv (maximum distance between valley and mean

line) = 7.0 (in units of height).

Rt (maximum distance between valley and peak) = Rt (maximum distance between valley and peak) =

15.0 (in units of height).

As illustrated in Slide 33; Rt is [15.0/3.333 = 4.5]

four or five times greater than the average

surface finish, as measured by the Ra

P1

P2 P3

P4

P5

V1 V2 V3 V4 V5

Evaluation of Rz

P1

p1

v1

V1

A

Figure 13

Rz = [(P1+P2+……+P5)/5-(V1+V2+….+V5)/5]

where, P1……P5 are the five highest peaks, and ,

V1……V5 are the five deepest valleys measured

= [(P1-V1)+(P2-V2)+(P3-V3)+(P4-V4)+(P5-V5)]/5

(2′′′′)

But, from figure 13,

from an arbitrary datum line.

Pi =pi+A, Vi=A-vi, i = 1-5.

Hence, Pi-Vi=pi+vi (#-1)

Hence, from Eqs. (2′′′′) and (#-1),

Rz = [(p1+v1)+(p2+v2)+(p3+v3)+(p4+v4)+(p5+v5)]/5

Rz = [(p1+p2+p3+p4+p5)+(v1+v2+v3+v4+v5)]/5

i.e.,

(2′′′′′′′′)(2′′′′′′′′)

Rz = [(8+8+6+5+5)+(7+4+4+4+4)]/5 = 11

Hence,

(in units of height)

Rq =

√(12+42+62+82+52+22+12+42+12+22+12+42+72+42+12+22+52

+82+52+22+12+42+12+12)/24 = 4.041 (in units of height)

5.6. Surface Roughness Value Conversions:

5.7. Surface Roughness symbol in drawing:

Surface roughness symbol is given to convey

manufacturing process related information only.

Unless written specifically on the symbol, they do

not carry the surface texture type (i.e. plated / milled

/ cold drawn). These symbols are given irrespective/ cold drawn). These symbols are given irrespective

of material and its surface condition.

Though the surface roughness symbol is capable of

conveying lot much information (as mentioned later

in this article), presently it is mentioned to convey

only manufacturing process related information. Also,

note that the raw material type is not controlled

through the surface roughness symbol given in the

drawing. Presently, either of following information isdrawing. Presently, either of following information is

conveyed through symbols:

�We do not need to remove material.

�We need machining operation.

�We need to manufacture by any process.

It is required to give surface finish to each feature

– irrespective of its importance and manufacturing

process. And, as in tolerance, designer should

select the most “Rough” surface permissible for the

performance. However, unlike to tolerances where we

have a concept of “Open Tolerance”, we have to

specify each surface roughness in the drawing. specify each surface roughness in the drawing.

5.8. Indian context and practices of using Ra:

Surface texture obtained by any

manufacturing process (e.g., turning,

grinding, plating, bending)

Surface texture obtained by material

removal by machining Operation (e.g.,

turning, drilling, Milling, slotting) turning, drilling, Milling, slotting)

Surface texture obtained by WITHOUT

removal of material (e.g., casting

surfaces, welding faces, Procurement

size surface)

Excepting the symbol of “Without removal of

material”, both the other symbols are defined with

value of Ra required for a particular manufacturing

process.

a = Roughness value Ra in

micrometer or grade number.

b = Production method, treatment

or coating.

Meaning of complete surface finish symbol is as below:

c = Sampling length.

d = Direction of lay.

e = Machining allowance.

f = Other roughness value than Ra.

5.9. MEASURING INSTRUMENTS

A variety of instruments are available for

measuring surface roughness. The majority of

these devices employ a diamond stylus, which is

moved at a constant rate across the surface,

perpendicular to the lay pattern. The rise and fall of

the stylus is detected electronically, amplified andthe stylus is detected electronically, amplified and

recorded on a strip chart, or processed

electronically to produce readings for a meter (see

figure 14).

Figure 14 Electronic equipment for measuring surface

roughness

Problem 5–1: if a surface has heights and depths as

illustrated below, evaluate Ra, Rp, Rv, Rt , Rz , Rq values

(in units of height).

Sheet # 5

6

8

7

8

11

22

4

11

1

4

1

4

6

5

2

2

4

55

1

4

1

Problem 5–2: if a surface has heights and depths as

illustrated below, evaluate Ra, Rp, Rv, Rt , Rz , Rq values

(in units of height).

44

2

11

2

11

1

2

3

1

2

3

2 2

11

1111

1

2

11

1 1

2

1