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7/21/2019 MECHANICAL INSTRUMENTS FOR MEASUREMENT
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1.( )irect Length Meaure!ent *ith Mechanical Appliance
1.(.1 Uni+eral ,ernier Caliper -age
,ernier calipergage is an instrument used in making measurements of length dimensions
on physical ob'ects. The main elements in its construction are shown in (ig.&.&:
(ig.&.& )niversal vernier caliper gage with knife !'aws" edges for internal measurements and tongue !stem" for depth
measurements.
In (ig.&.*, the vernier scale !main scale" is divided into &+ e%ual divisions and thus the leat
count of the instrument is .1mm. /oth the !ain scale and the +ernier scale readings are taken
into account while making a measurement. The !ainscale reading is the #irtreading on the main
scale i!!ediatel" to the le#t of the 0ero of the vernier scale ! mm", while the +ernier scale
reading is the mark on the vernier scale which exactly coincidewith a !ar on the main scale !+.-
mm". The reading is therefore .- mm.
(ig.&.* The reading here is .- mm.
*
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The sliding 'aw indicationcan be on a +ernier !(ig.&.a", on a round dial !(ig.&.b", or
with digital readouts !(ig.&.c". ith digital readouts is eaierto read and le sub'ect to human
error than reading verniers.
a" b" c"
(ig.&. #liding 'aw indication: a" on a vernier/ b" on a round dial/ c" with digital readouts.
0ccording to the highetlimit of the length dimension that can be measured, #T0# &+-1
23 establishes as follows limit dimensions: &4+, *++, ++, 4++, 2++, &+++, &4++, *+++ mm.
The clai#ication of the vernier caliper gages is made according to the next criteria:
a2 destination, where can be for external and internal measurements, for depth measurements,
for gear tooth and for grooves/
$2 vernier division value, which are with a resolution of +,& mm, +,+4 mm and +,+* mm.
1.(.( Caliper height gage
The$aicdesign principles of vernier caliper gages are applied in caliper height gageas
well. The primary use of caliper height gages is in the field of surfaceplatework as a layout tool,
for marking +ertical distances and for measuring height di##erence between tep at various
levels.
In distinction to the caliper gages, the vernier height gages !(ig.&.3,", have a ingle'aw only,
because the surface plateon which the instrument base rests is functioning as the re#erenceplane.
The vernier height gages are made with *idebases and with bars of trongcross section which
carry the same kind of graduations found on the beams of vernier calipers.
These gages usually have offset 'aws whose contact surfaces can be brought to coincide with
the reference plane when the slide position indicates $ero height
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a" b" c"
(ig.&.3 5aliper height gages: a" 6ernier hight gage/ b" 7ial hight gage/ c" 7igital height gage.
(or !aring purposes, cri$er can be attached to the contact 'aw. These scribers are
designed to have the edge substantially at e%ual level with the contact surface of 'aw, in order to
make the height of the scribed line coincident with the dimension indicated by the gage.
1.(.3 -ear Tooth ,ernier Caliper
0 further application of the vernier caliper principle is found in the gear tooth caliper
!#T0# &-841-". These measuring tools are used to check the pitchlinethicneof gearteeth
by measuring the tooth chordat specific ditance!chordal addendum" from the top of the gear
tooth. The gage consists of t*o independently actuated vernier calipers, each having its own
movable slide, but the beams and the stationary 'aws are made of a common single piece. One of
the slides has the form of a plate, called the tongueof the instrument, which contacts the topof the
gear tooth. 9y moving this slide, the gage can be ad'usted to operate at the desired addendum
distance. The econd slide, integral with the movable 'aw, carries out the actual chordal thickness
measurement at the pitch line.
a" b"
(ig.&.4 a" 6ernier gear1tooth caliper with two vernier scales/ b" Measuring the thickness of gear teeth.
1.(.3 ,ernier )epth -auge
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perating on the principles of vernier caliper gages, +ernier depth gage!(ig.&.;" differ in
the design application of the basic concept, resulting in the reversal of the usual process:
a"b"
(ig.&.; a" 6ernier depth gauge/ b" 7ial depth gauge.
&. The lide is connected with the cross beam of the instrument which, by contacting a
reference plane on the ob'ect surface, establishes the datu! of the measurement/
*. The end #aceof the beam takes over the function of the movable 'aw, in contacting the
ob'ect element whose distance from the datum is to be measured.
The use of the vernier depth gage is not limited to actual depth measurements, although these
constitute the ma'or application for the instrument. )itancewhich are referenced from, and are
nor!al to a flat surface of the ob'ect, can also be measured conveniently with the aid of vernier
depth gages.
1.(.4 Micro!eter
The eential element of measuring instruments operating on the micrometer principle is a
cre* with preciel" controlled lead, having a pitch of usually & 4 mm. The screw of themicrometer is integral with the measuring pindle&whose face establishes the measuring contact
with the ob'ect. The distance of that contact face from a fixed datum constitutes the measuring
length, which is then displayed by the scale graduations of the micrometer.
The !icro!eter !(ig.&.
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(ig.&.- 0 micrometer screw gauge.
The#irtsignificant #igure is taken from the lat graduation showing on the lee+edirectly to
the le#t of the re+ol+ing thimble. =ote that an additional half cale division !+.4 mm" must be
included if the mark $elo* the main scale is +ii$le between the thi!$le and the !ain scale
division on the lee+e.The remaining t*o significant figures !hundredths of a millimeter" are taken
directl"from the thi!$le opposite the main scale.
a" b"
(ig.&.2 a" The reading is -.2 mm/ b" The reading is -.-* mm.
In (ig.&.2a the lat graduation visible to the le#t of the thi!$le is - mm and the thi!$le
lines upwith the !ain scale at 2 hundredths of a millimeter !+.2 mm"/ therefore the reading is
-.2 mm. In (ig.&.2b the lat graduation visible to the le#t of the thi!$le is -.4 mm/ therefore the
reading is -.4 mm plus the thimble reading of +.** mm, giving -.-* mm.
1.(.6 Micro!eter #or E5ternal Meaure!ent
;
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The !ot common application for micrometers is the measurement of length dimensions
between t*oparallel end surfaces on theouterside of an o$7ector feature. This is currently known
as e5ternal or outidemeasurement!(ig.&.
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Disc micrometermeasuresforming tools,cutting edges,
narrowslots.
The disc micrometer is also called aflange micrometer.
This micrometer finds its application inmeasuring flanges and hard to reach areas.
The 60 degreecomparatormicrometer isusually called ascrewthreadcomparator
micrometer orpitchmicrometer.
Measuring the pitch diameter of a screwthread.
The pitch diameter, the most importantmeasurement of a screw thread.
1.(.8 Micro!eter #or Inide Meaure!ent
The micrometer type instruments have many useful applications for inide measurements as
well. These comprise the !eaure!ent of an ob'ect feature $ounded by *all having parallel
elements in the gaging plane, such as the dia!eterof a cylindrical$oreor the*idthof a parallel
> sided groo+e.
Internalmeasurements, particularly of $ore diameters, are affected by more +aria$le than
the measurement of length between outside surfaces. S!allbore si$es pre+ent the introduction of
inide micrometers. The depth of the $ore&when its diameter must be measured at a greater
distance from theopenend, cancauedifficulties for micrometer applications.
(ig.&.&+ Inside micrometer set with interchangeable
measuring rods and handle for inserting the instrumentinto deeper holes.
The inide !icro!eter consists of a
headpart comprising the micrometer cre*&and
of independent measuringrod !(ig.&.&+". The
rod are !ade in different length, in tepof
(4mm, can be ae!$led with the headpart by
means of a threaded connection and are
accurately positioned on a shoulder.
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The !allet bore which can be measured with this type of micrometer is 4 mm in
dia!eter&and the !a5i!u! diameter depends on the available rods, a practical upper limit being
about 9 mm. 0 handle can be attached to the head to permit measurement at greaterdepths
inside a bore.
(ig.&.&& #mall hole gage for transfer of actual bore
dimension which is then mesurable with a regular
outside micrometer.
/ore dia!eter !eaure!ent $" tran#er.
S!all bores !?+,4 > ?&* mm" cannot be
measured directly because of pace limitations.
It is possible, however, to tran#er the inside
dimension to be measured by uing an
appropriate !ean which will represent the
re+ere replica of the inside length. The
resulting physical outide length can then bemeasured with a standard outidemicrometer.
The !all hole gage!(ig.&.&&", consisting of a plit $allwhich can be e5pandedto the i0e
of the dia!eterof the$oreto be measured. Rotating the knurled knob of the handle ad+ancethe
inide screw whose conical end causes the ball hal+e to eparate to the re'uired extend. The
actual pread of the balls is!all, in the order of3!!&and therefore these gages are upplied in
et encompassing a *ider range of dimensions.
The telecoping gage!(ig.&.&*" has a tu$ularmember to which a handle is attached at
right angle position.
(ig.&.&* The use of a telescoping gage in taking
the si$e of a bore for ultimate measuring by an
outside micrometer.
The tu$ular member has either one or t*o
plunger, which are under pring pressure and
telecope into the #i5ed tube. The #ree ends of the
plungers or, in the case of a single plunger, the closed
end of the fixed member too, have pherical forms andare hardened to serve as contact elements. hen
introduced into the hole to be mesured, the previously
retracted plungers are releaed to extend to a length
e'ual to the dia!eterof the ob'ect.
#ubse%uently, the thus #i5ed length of the
telescoping rode can be !eaured with a regular
outide micrometer.
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(ig.&.&& Three1point contact, internal micrometer with
self1centering properties. !@eft" general view. !Aight"
Bartial section showing the mechanism for the
controlled expansion of the contact plungers.
Three % point contact internal !icro!eter.
#everal of the di##icultie connected with bore
diameter mesurements by a micrometer can be
reduced when using the three:point internal
micrometer, shown in (ig.&.&&. The el# %
aligning property of this instrument is
particularly useful when measuring deepbores,
for which purpose an e5tenion part can be
attachedto the $aictool.
Ad+ancing the cone along its axis pread the engaged measuring points radiall",resulting in a larger envelope circle&while the cone movement in the inverse sense causes the
spring loaded contact members to retract. The cone is attachedto a pindlewhose a5ialposition is
ho*n on the !icro!eter sleeve and thi!$le.
The three1point internal micrometers are a+aila$le in i0e from 6 to 3 !! bore
diameters. It is advisable to recali$rate these instruments periodicall", using a standard ringgage.
)sually these micrometers are procured in et, to cover a more extended range of measurable bore
diameters.
1.(.9 Special Micro!eter Intru!ent
The following examples of measuring instruments, based on the application of the
micrometer screw as the measuring member, should illustrate the wide uses of the micrometer
principle.
(ig.&.&* Indicator micrometer with ad'ustable
tolerance marks.
Indicator !icro!eter !(ig.&.&*". The narro*
range indicator, whose mechanism is coupled with
the !o+a$le anvil of the !icro!eter frame, has
graduation in &( or &1 !! increments. The
!alletgraduation of the thi!$leis in thouandth
and the lee+e can have or notvernier graduation.
hen is making measurements with the instrument without vernier the !icro!eter screw is
advanced to the thouandth mark nearet to the #inal si$e as ignaledby the !o+e!ent of the
&+
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indicator pointer from its rest position. 0t this point the !eured si$e is readby combining the
i0e shown on the thi!$leand the poitionof theindicatorpointer.
The built1in indicator improves the repeat accuracy of the micrometer measurements and
adds to the versatility of the instrument applications since:
a" The contant measuring force, as checedby the pointer movement, is particularly valuable
for co!parati+e and repetiti+e mesurements/
b" Meauringerrors caused by !itae in e+aluating the +ernierpositing are eliminated/
c" The micrometer can be ued as an ad'ustable napgagewith added indicating potential,
when the range of si$e variations does not exceed the measuring spread of the indicator. The
ad7uta$le indexes on the indicator sector can be etto the li!it si$es of the o$7ect/
)epth !icro!eter are used to measure the ditance of an ob'ect #eaturefrom a #lat referencesurface !(ig.&.&".
(ig.&.& 7epth micrometer for measuring the
distance of protruding features from a flat reference
surface.
Cxamples of application are the mesurement of
tep&the depth of flanges or the$otto!surface in a
$ore, and the heightof an ob'ect feature in relation to
a re#erence surface, when accesibility permits it to be
i!ultaneoul" contacted by the $ae member and
the pindle face.
/ench !icro!eter !(ig.&.&3" can substantially improve the precision of micrometer
mesurements, particularly when the ob'ects to be measured are !all.
(ig.&.&3 9ench micrometer for the sensitive
measuremenof small parts.
The ta$leposition of the instrument during the
!eure!ent, permits a more precie locating of the
work and the hea+"base adds to the rigidit" e%uipped
with largediameterpindleand thi!$le, permitting a
finer pitch thread for higher eniti+it", and direct
reading in ten:thouandth, or smaller.
&&
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Micro!eter head without a #ra!e as an integral part, have wide applications in the
instrument $uildingfield. ne or the more#re'uentuses is the controlleddisplacement of lide,
either ingle or cross lide!(ig.&.&4".
(ig.&.&4 Micrometer heads as independent units and
mounted on an instrument slide.
5ommonly, the micrometer will !o+e
the lide against a !oderate spring preure,
which is applied to aure a positive contact
point on the lide face. Spring exerting a
uni#or! force along the entire slide traverse
are preferred.
&*
Recommended