FIELD OF INVENTION
The present invention relates to an improved micrometer device for accurate
measurement of residual stress in thin walled stainless steel tubes.
More particularly the invention relates to an improved micrometer device for accurate
measurement of outside diameter of thin walled stainless tubes for the purpose of
measuring residual stress.
BACKGROUND OF INVENTION
Thin walled (between 0.5-0.9 mm thickness) welded stainless steel tubes for heat
exchange applications according to the specification, should have residual stresses less
than ± 4 Kg/mm2
The residual stress of the tubes is measured as per guidelines given in ASTM A1928.
According to the existing practice, a 100 mm length of tube is taken and the outside
diameter (d0) is measured This tube is then slit along its length at 180° to the weld
seam (Fig. 1). The diameter of the tube (di) at 90° to the slit, is then measured. From
these values of d0 and di, using a formula given in the ASTM standard, the residual
stress in the tube is measured The measurement of the residual stress of tubes
involves accurate measurements of outside diameter of tubes before and after slitting
of tubes in a longitudinal direction.
The problem encountered in respect of the prior art is that thickness and the resilience
of the tubes being low, the force exerted by conventional micrometers cause the tube
to 'sink' during measurement resulting in low diameter values, which translates to low
residual stresses determined.

Also since the tube is very thin, the micrometer ratchet cannot be used to ensure
positive contact as it exerts too much force, and contact has to be ensured by feel only
And, this varies from person to person, introducing a further subjectivity and error in
the results It has been determined that an error of 0.01 mm translates to an error of
0 76 Kg/mm2, when residual stress of 16mm, 0.9 wall thickness is measured.
No commercial equipment (other than the very costly laser base devices) are available
which can accurately and reproducibly measure the diameters of thin walled tubes.
OBJECT OF THE INVENTION
An object of the invention is to propose an improved micrometer device for accurate
measurement of outside diameter of thin walled stainless tubes for the purpose of
measuring residual stress.
Another object of the invention is to propose an improved micrometer device for
accurate measurement of outside diameter of thin walled stainless tubes for the
purpose of measuring residual stress, which eliminates the subjectivity and perusal bias.
A further object of the invention is to propose an improved micrometer device for
accurate measurement of outside diameter of thin walled stainless tubes for the
purpose of measuring residual stress, which is easy to fabricate.
A still further object of the invention is to propose an improved micrometer device for
accurate measurement of outside diameter of thin walled stainless tubes for the
purpose of measuring residual stress, which is cost effective.

SUMMARY OF THE INVENTION
Accordingly, there is provided an improved micrometer device for accurate
measurement of residual stress in thin-walled stainless steel tubes, the residual
stress measurement involving twin measurement of outside diameter of the tube
before and after slitting of the tube along its length at 180° to the weld seam,
the twin measurement values of the outside diameter (d0, di) being applied to a
formula provided in the ASTM A 1928 standard which output the value of the
residual stress, the improvement is characterized in that a v-anvil type
micrometer is mounted on a micrometer stand to provide rigidity of the device;
the width of the anvil of the v-anvil micrometer is extended on one side
compared to the normal width so that the extension is enabled to support the
full-length of the tube; the v-anvil is electrically insulated including provision of
an electrical circuit which ensures a contact signal between the micrometer and
the tube via sounding a buzzer, wherein the extension to the width of the v-anvil
of the micrometer is configured through fixing of at least two pieces of Perspex
sheet having thickness and width corresponding to those of the v-anvil, and
wherein the top surfaces of the Perspex sheets are disposed at the same planes
identical to that of respective two planes of the v-anvil of the micrometer.
For measurements of residual stress, the outside diameter of a tube is to be
measured before and after the longitudinal slitting. A micrometer is mounted on
a micrometer stand to provide stability to the whole device. The V anvil of the
micrometer is enlarged to provide a support to the full length of the tube. The V
anvil of the micrometer is electrically insulated.

An electrical circuit, is configured with the known components in such a way that
when the shaft of the micrometer, during measurement of the outside diameter
of the tube just touches the tube, a buzzer sounds indicating the contact. The
exact diameter of the tube is measured at the time of said contact.
Brief description of the accompanying drawings
Figure 1 . shows a sample of a tube having longitudinal slit
Figure 2 : shows a V anvil type micrometer as per prior art
Figure 3 : shows a V anvil type micrometer with extensions according to the
invention
Figure 4 : shows a complete assembly of the micrometer device according to the
invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
A tube (1) on which measurements are to be made is shown in figure 1
Figure 4 shows a conventional V anvil type micrometer (2) is mounted on a
micrometer stand (3). (Fig. 4)
As the width of the 'V anvil of the micrometer (2) is only 20 mm, to ensure a
positive support to the tube (1) along its length, it is necessary to extend this

width. As shown in figure - 3, this extension is done by fixing on one side of the
micrometer (2), at least two pieces of Perspex sheet (4) having the same
thickness and width of the 'V of the micrometer (2) which ensures that the tops
of these Perspex sheet (4) are at the same plane as that of the V of the
micrometer. This extension by using the Perspex (4) is necessary on one side
only
The 'V anvil is electrically insulated.
An electrical circuit, is configured in such a way that when the shaft of the -
micrometer (2) just touches the tube (1), a buzzer (5) sounds indicating the
contact between the shaft of the micrometer (2) and the tube (1).
With this device, it has been found possible to discriminate between the states of
contact/ no-contact of the micrometer (2) with the tubes (1) within less than half
of the least count of the micrometer, (2), i.e. 0.005 mm. This makes it possible
to read the exact micrometer position, when the micrometer (2) is just touching
the tube (1)
The modified device is calibrated satisfactorily (Fig. 4)
The device has demonstrated excellent repeatability of results. The OD
measurements carried out by different persons does not show any difference
which ensures eliminates of human errors.

WE CLAIM
1. An improved micrometer device for accurate measurement of outside diameter of
thin walled stainless tubes for the purpose of measuring residual stress involving
twin measurement of outside diameter of the tube before and after slitting of the
tube along its length at 180° to the weld seam, the twin measurement values of the
outside diameter (do, d1) being applied to a formula provided in the ASTM A 1928
standard which output the value of the residual stress, comprising and characterized
by
- a V-anvil type micrometer (2) is disposed on a micrometer stand (3) to provide
rigidity of the device wherein the width of the anvil of the V-anvil micrometer (2) is
extended on one side compared to the normal width so that the extension is
enabled to support the full-length of the tube (1);
- the V-anvil is electrically insulated including provision of an electrical circuit which
ensures a contact signal between the micrometer and the tube (1) via sounding a
buzzer (5).
2. the device as claimed in claim 1, wherein the extension to the width of the V-anvil of
the micrometer (2) is configured through fixing of at least two pieces of Perspex sheet
(4) having thickness and width corresponding to those of the V-anvil, and wherein the
top portions of the Perspex sheets (4) are disposed at a plane identical to that of the V-
anvil of the micrometer (2).
3. The device as claimed in claim 1, wherein a discrimination between the states of
contact and no-contact of the tube (1) with the micrometer (2) within less than half of
the least count of the micrometer is achievable.

ABSTRACT

TITLE "AN IMPROVED MICROMETER DEVICE FOR ACCURATE
MEASUREMENT OF RESIDUAL STRESS IN THIN-WALLED STAINLESS
TUBES"
The present invention relates to an improved micrometer device for accurate
measurement of residual stress in thin-walled stainless tubes, the residual stress
measurement involving twin measurement of outside diameter of the tube before
and after slitting of the tube along its length at 180° to the weld seam, the twin
measurement values of the outside diameter (d0, d1) being applied to a formula
provided in the ASTM A 1928 standard which output the value of the residual
stress, a V-anvil type micrometer (2) is disposed on a micrometer stand (3) to
provide rigidity of the device, the width of the anvil of the V-anvil micrometer (2)
is extended on one side compared to the normal width so that the extension is
enabled to support the full-length of the tube (1); the V-anvil is electrically
insulated including provision of an electrical circuit which ensures a contact signal
between the micrometer and the tube (1) via sounding a buzzer (5).