Surface tread measurement of screw thread

Introduction
• The surface texture greatly influences the
functioning of the machined parts.
• The properties such as appearance,
corrosion resistance, wear resistance,
fatigue resistance, lubrication, initial
tolerance, ability to hold pressure, load
carrying capacity, noise reduction in case
of gears, etc. are influenced by the surface
texture.

Introduction
• Whatever may be the manufacturing
process used, it is not possible to produce
perfectly smooth surface. The
imperfections and irregularities are bound
to occur.

Factors Affecting Surface Roughness
• Vibrations.
• Material of the workpiece.
• Type of machining.
• Rigidity of the system consisting of machine
tool, fixture, cutting tool and work.
• Type, form, material and sharpness of cutting
tool.
• Cutting conditions i.e. speed, feed and depth of
cut.
• Type of coolant used.

Reasons for controlling Surface Roughness
It is seen that different requirement demand different
surface texture.
• Heat exchanger tubes transfer heat better when
their surfaces are slightly rough rather than highly
finished.
• Brake drums and clutch plates, etc. work best with
some degree of surface roughness.
• The surface of the parts which are subjected to
high stresses and load reversals are finished highly
smooth.

Reasons for controlling Surface Roughness
The principal reasons for controlling the surface
texture are:
• To improve the service life of the components.
• To improve the fatigue resistance.
• To reduce initial wear of parts.
• To have a close dimensional tolerance on the parts.
• To reduce frictional wear.
• To reduce corrosion by minimizing depth of
irregularities.
• For good appearance.
• If the surface is not smooth enough, the moving
parts can heat up, blind and freeze.

Orders of Geometrical Irregularities
1. First Order: The irregularities caused by
inaccuracies in the machine tool itself.
e.g:
i) irregularities caused due to lack of straightness of
guideways on which the tool must move.
ii) surface irregularities arising due to deformation
of work under the action of cutting forces.
iii) due to weight of the material itself.

2. Second Order: The irregularities caused due to
vibrations of any kind.
e.g:
i) chatter marks on the surface of the parts.
3. Third Order: The irregularities caused by machining
due to characteristic of the process.
e.g:
i) feed mark of the cutting tool.
4. Fourth Order: The irregularities caused by the rupture
of the material during the separation of the chip.

Surface metrology (cont’)
• Three forms of asperity (un-evenness of surface,
roughness)
1. Roughness
2. Waviness
3. Error of form
• The fourth asperity is not distinguish by wavelength; it is
flaw
• Lay is the direction of the asperities which in most cases
means that roughness and waviness are perpendicular to
each other
Vary according to the length
of spacing or wavelength

Surface assessment
 Flaw (surface defect): random
irregularities such as
scratches, cracks, holes,
tears, inclusions, etc.
 Waviness (widely spaced
asperities): recurrent
deviation from a flat surface.
 Roughness (the finest of the
asperities): closely spaced
irregular deviations on a scale
smaller than that of waviness.

The irregularities on the surface of the part
produced can also be grouped into two
categories:
1. Primary Texture (Roughness) – Surface
irregularities of small wavelength. These
are caused by direct action of cutting
elements on the material. Includes
irregularities of third and fourth order.

The irregularities on the surface of the part
produced can also be grouped into two
categories:
2. Secondary Texture (Waviness) – Surface
irregularities of considerable wavelength
of a periodic character. These are caused
by inaccuracies of slides, wear of guides,
misalignment of centres, vibrations,
deformation of work, etc. Includes
irregularities of first and second order.

Primary Texture (Roughness) and Secondary Texture (Waviness)

Sampling Length and Lay
Sampling Length – It is the length of the
profile necessary for the evaluation of the
irregularities to be taken into account. It is
also known as ‘cut-off’ length.
It is related to the process employed for
finishing.
The standard lengths are 0.08, 0.25, 0.8, 2.5
and 25 mm.

Sampling Length and Lay
Lay – It is the direction of predominant
surface pattern produced by tool marks or
scratches. It is determined by the method
of production used.

Evaluation of Surface Finish
Three methods of evaluating primary
texture (roughness) of a surface:
1. Peak to valley height method
2. The average roughness
3. Form factor or bearing curve

The Average Roughness – For assessment of
average roughness, the following three
statistical criteria are used:
1. C.L.A. Method (Centre Line Average)
2. R.M.S. Method (Root Mean Square)
3. Ten point height Method (Rz)

1. C.L.A. Method (Centre Line Average)

2. R.M.S. Method (Root Mean Square)

3. Ten Point Height Method (Rz)
Rz

Statement of Surface Finish
1. Surface Roughness Value – It is expressed
as Ra value in microns (μm). If a single Ra
value is stated, it is understood that any
Ra value from zero to that stated is
acceptable.
2. Limiting Values – When both minimum
and maximum Ra values needed to be
specified, these shall be expressed as
follows:
Ra 8.0/16.0 or alternatively, Ra
8.0 – 16.0

Symbol to designate Surface finish on drawing
Machining Method
Sampling Length
Direction
of Lay
Machining
Allowance
Roughness
value in μm

Symbol to designate Surface finish on drawing
Milling
2.5
=
1.2
6.3

Symbols Used to Identify Surface Finishes and
Characteristics

Measurement of Surface Finish / Surface
Texture
1. Inspection by comparison (Qualitative
Analysis)
2. Direct Instrument Measurement
(Quantitative Analysis)

Texture
1. Inspection by comparison (Qualitative
Analysis)
i. Visual inspection
ii. Touch inspection
iii. Scratch inspection
iv. Microscopic inspection
v. Surface photographs
vi. Micro-Interferometer
vii. Wallace surface Dynamometer
viii.Reflected light intensity

Texture
2. Direct Instrument Measurement
(Quantitative Analysis)
Principle : Stylus probe type surface texture
measuring instruments – If a finely
pointed probe or stylus be moved over
the surface of a workpiece, the vertical
movement of the stylus caused due to
the irregularities in the surface texture
can be used to assess the surface finish
of the workpiece.

SURFACE EVALUATION
• Surface roughness comparator
• Microscope
• Stylus method

Surface roughness comparator
• The most common way to evaluate surface finish is to compare it
visually and by feel with roughness comparison specimens having
various surface finishes
• It consist of composite set of surface roughness specimen standard

Surface roughness comparator (cont’)

Microscope
• Examination of surfaces by
microscope can be informative
• But it does not usually allow the
heights of the asperities to be
determined without destroying
the test part by cutting a taper
through the surface

Stylus instrument
• The stylus instrument is a widely used technique for
measuring a surface profile.
• This technique uses a fine diamond stylus with tip size
approximately 0.1 to 10 µm to transverse the surface
• As the stylus tracks the surface peaks and valleys, its vertical
motion is converted to a time varying electrical signal that
represent surface profile
• Stylus instruments operate like a phonograph pickup: the
stylus is drawn across the surface and generates electrical
signals that are proportional to the changes in the surface
• The changes in height can be read directly with a meter or
on a printed chart

Two types of stylus instrument
1. True- datum or skidless instruments
2. Surface- datum or skid type instrument

True- datum instrument
• With this instrument, we draw across the surface in a very
precise, mechanically controlled movement
• Advantages
• The resulting graph is nearly a true representation of the surface
along that one line showing roughness, waviness, errors of form
and flaws
• Disadvantages
• Very difficult to set the instrument up; must precisely align the
surface being assessed with the path of the instrument

NUMERICAL VALUES FOR ASSESSMENT
• Arithmetic roughness average
The roughness average is the arithmetic average of the absolute
values of the deviation from the profile height measured from the
centerline along a specified sampling length.

Ra 
a bcd 
n
Rq 
a2
b2
c2
d2

n

NUMERICAL VALUES FOR ASSESSMENT
(cont’)
Sample Data
Either arithmetic average
roughness height (Ra) or
root mean square (Rq)

Other standardized assessment methods
1. Root-Means-Square roughness (Rq or RMS)
• Closely related to the roughness average (Ra)
• Square the distances, average them, and determine the square root of the
result
• The resulting value is the index for surface texture comparison
• Usually 11% higher than the Ra value
2. Maximum Peak-Valley Roughness (Rmax or Rt)
• Determine the distance between the lines that contact the extreme outer
and inner point of the profile
• Second most popular method in industry
• See figure A
3. Ten-Point Height (Rz)
• Averages the distance between the five peaks and five deepest valleys within
the sampling length
• See figure B

(cont’)
4. Average Peak-to-Valley Roughness (R or H or Hpl)
• Average the individual peak-to-valley heights
• See figure C
• Use the height between adjacent peaks and valleys, not measure from a
center line to peak valleys
5. Average Spacing of Roughness Peaks (Ar or AR)
• Average the distance between the peaks without regard to their height
• See figure D
6. Swedish Height of Irregularities (R or H)
• Also known as Profiljup methos
• Only standard in Sweden (H) and Denmark (R)
• It assume that, in wear situation, the peaks are affected by wear, but the
valleys are not.

(cont’)
7. Bearing Length Ration (Tp and others)
• Create a reference line through some of the peaks
• This line is at a predetermined height from the mean line, and you
have then divide the subtended length through the peaks by
sampling length to arrive at the assessment value
• See figure F
8. Leveling Depth (Rp and others)
• Measure the height between the highest peak and the mean line
• See figure G
9. Waviness Height (W)
• Assess the waviness without regard to roughness by determining the
peak-to-valley distance of the total profile within the sampling length

Surface tread measurement of screw thread

Surface tread measurement of screw thread

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Surface tread measurement of screw thread