Micrometers (Fig. 2.113) (Metrology)

2.37.
The micrometer screw gauge essentially consists of an accurate screw having about 10 or 20
threads per cm and revolves in a fixed nut. The end of the screw forms one measuring tip and the
other measuring tip is constituted by a stationary anvil in the base of the frame. The screw is
threaded for certain length and is plain afterwards. The plain portion is called sleeve and its end
is the measuring surface. The
spindle is advanced or retracted by
turning a thimble connected to the
spindle. The spindle is a slide fit
over the barrel and barrel is the
fixed part attached with the frame.
The barrel is graduated in unit of
0.05 cm. i.e. 20 divisions per cm,
which is the lead of the screw for
one complete revolution. The
thimble has got 25 divisions Fig. 2.113
around its periphery on circular portion. Thus it sub-divides each revolution of the screw in 25 equal
parts, i.e. each division corresponds to 0.002 cm.
A lock nut is provided for locking a dimension by preventing motion of the spindle.
Ratchet stop is provided at the end of the thimble cap to maintain sufficient and uniform
measuring pressure so that standard conditions of measurement are attained. Ratchet stop consists
of an overriding clutch held by a weak spring. When the spindle is brought into contact with the
work at the correct measuring pressure, the clutch starts slipping and no further movement of the
spindle takes place by the rotation of ratchet. In the backward movement it is positive due to shape
of ratchet.
2.37.1.


Description of various parts and their specifications

Frame. The frame of micrometer is so shaped as to permit measurements of the cylinder of
diameter equal to the measuring range of micrometer, and the stiffness of the frame should be such
that a test load of 1 kg weight does not alter the distance between them by more than 1.5 um for
range 0 to 25 mm, 2 um for range 25 to 50 mm and so on. It should be neatly and evenly black
enamelled or treated by other means to prevent corrosion and rusting. The frame is generally made
of steel, cast steel, malleable cast iron or light alloy. The use of light alloys is recommended, for
sizes over 300 mm in order to facilitate the handling of large micrometers. But it has to be understood
that coefficient of expansion of light alloys is appreciably greater than that of steel or iron and thus
the temperature changes during handling of the micrometer frame may result in appreciable
changes in the zero reading. It is, therefore desirable in such cases that initially micrometer be
checked against a standard length bar or setting gauge and appropriate correction made to the
reading. It is desirable that the frame of the micrometers be provided with conveniently placed
finger grips of heat insulating material.
Anvil and Spindle. The fixed anvil of micrometer screw gauge should protrude at least 3 mm
from the frame in order to permit the attachment of measuring wire support. The measuring faces
of these are hardened to about 800 HV (62 HRC approx.) and aged. In some cases, faces may also
be tipped with tungsten carbide or some other suitable material. The anvil should be accurately
ground and lapped with its measuring face flat and parallel to the measuring face of the spindle.
The diameter of the anvil should be equal to the diameter of spindle within 0.04 mm and axes of
both in exact alignment.
Th» front parallel portion of the spindle be a good free-turning fit in its bush, without
perceptible shake. The spindle and screw are initially lubricated with a thin, light non-corrosive oil
and in that condition the spindle should run freely and smoothly throughout the length of its travel.
tmp3-5_thumb
There should be no perceptible backlash between the spindle screw and nut. There should be full
engagement of nut and the micrometer screw when the micrometer is at its maximum reading. The
design of the spindle clamp should be such that it effectively locks the spindle without altering the
distance between the measuring faces by more than 0.003 mm. It is generally desirable that
diamond knurled spindle lock-nut be provided with micrometer.
Ratchet Driver. The micrometer should be provided with a ratchet of friction stop sufficiently
diamond knurled to enable satisfactory operation. The torsional moment of the ratchet drive should
be so regulated that the force exerted between the measuring faces is between 0.5 to 1 kgf.
The material used for thimble, barrel, ratchet and all other locking and clamping devices for
all sizes of micrometers should be suitable quality wear resistant steel.
Thimble and barrel. All graduation lines on the barrel should be clearly engraved and for
ease of reading the surface of the thimble and barrel should have a dull finish and the graduation
lines should be blackened.
Adjusting nut. Micrometer screw gauges are generally provided with a friction or adjusting
nut to compensate for wear between the screw portion of the spindle and nut. They are also provided
with means of adjustment to compensate for wear on the measuring faces and for adjusting the zero
setting. The adjustments are carried out by suitable spanners and keys which are provided with
micrometer for this purpose. The means of adjustment should be such that after resetting, the parts
are secured and the original accuracy of the instrument is not impaired.
2.37.2.

Important Terms.

The various important terms used in connection with the
micrometer screw gauges are defined below :
Backlash. It is the lack of motion or lost motion of the spindle when the rotation of thumble
is changed in direction.
Measuring range. It is the total travel of the measuring spindle for a given micrometer.
Total error. It corresponds to the maximum difference of ordinates of the cumulative error.
Cumulative error. It is the deviation of measurement from the nominal dimension deter-
mined at any optional point of the measuring range. It includes the effect of all possible individual
errors such as errors of the thread, errors of measuring faces etc. It can be determined by some
tests with slip gauges.
2.37.3.

Indian Standard Specifications.

As per IS: 2967—1964 (Specification for external
micrometers); the micrometer screw should be of (M 10 x 1.5). It is recommended that the threads
of the screw and nut be truncated so as to confine contact to the flanks of the thread.
The diameter of the graduated surface of the barrel should not be less than 10 mm. The angle
on the bevel (angle a) at the graduated end of the thimble should not exceed 20° as measured from
the barrel. The thimble should be sufficiently diamond knurled to facilitate ease of rotation. The
distance from the barrel to the reading end of graduations (dimension B in Fig. 2.114) should not
exceed 0.5 mm and should remain substantially constant for appropriately spaced measurements
within the range of thimble movement.
The thimble should be graduated with 50
divisions, each representing 0.01 mm and numbered 0,
5, 10, 15…45. The barrel should bear longitudinal
reference line parallel to the axis of the spindle. The
graduations of barrel have to be provided in two parts,
viz., one above the reference line and other below. The
graduations above the reference line are graduated at
1 mm intervals and the first and every fifth graduation
are marked long and numbered 0, 5,10,15, 20 and 25.
 Details of Barrel and thimble of screw gauge.
Fig. 2.114. Details of Barrel and thimble
of screw gauge.
The lower graduations are graduated at 1 mm intervals but each graduation shall be placed at the
middle of the two successive upper graduations to be read 0.5 mm. These lines are not numbered
and distinguished from both of these by their greater length. The thickness of the graduations
should be between 0.15 to 0.20 mm.
When tested at 20°C, the total error should not exceed the following values :
for grade 1, total error = 14 + j um and for grade 2, total error = 110 + ^ j (xm
where L = upper limit of the measuring range in mm.
The micrometer must be so adjusted that the cumulative error at the lower and upper limit
of the measuring range does not exceed half the total error.
2.37.4.

Adjusting for wear of threads, wear of measuring surfaces, spindle locking

arrangement and ratchet stop mechanism.

In the external micrometer shown in Fig. 2.115,
an abutment or anvil is provided which occupies a fixed position in relation to the main nut, in
which the spindle moves. The anvil and the main nut are carried by a bow-shaped frame. The main
Part-sectional view of an external micrometer showing wear adjustmentspindle locking arrangement and the mechanism of ratchet stop Micrometer Range 25 to 50 mm).
Fig. 2.115. Part-sectional view of an external micrometer showing wear adjustment,
spindle locking arrangement and the mechanism of ratchet stop
(Micrometer Range 25 to 50 mm).
nut is fitted tightly into the sleeve or barrel which is integral with the frame. The thimble is
permanently secured to the screw and is knurled on the outer surface. The spindle having the
measuring face at its end is tightly fitted into the screw. The spindle is thus rotated by rotating the
thimble. It will be observed that the thimble extends over the end of the barrel so that threaded
portions of the screw and the main nut are all the time completely enclosed. The nut and the thread
portion of the screw are so proportioned as to ensure full length engagement in all positions of
spindle. The fixed index line is marked upon the barrel and the angular graduations around the
left hand chamfered end of the thimble. It will be noted that in addition to the main nut there is a
shorter nut also by its side with which the screw also engages. The adjacent faces of the main nut
and the shorter nut are provided with small V-shaped teeth which form a clutch and prevent the
shorter nut from rotating with the screw. In the recess between these two nuts is housed a light
coil spring, the tendency of which is to force the two nuts apart and in this way any slight backlash
between the threads in the nuts and the screw is automatically taken up.
Adjustment for wear of threads. In addition to forming an abutment for the coil spring, the
shorter nut also provides for adjustment in the event of wear on the threads. When the two nuts
are originally assembled on the screw, the clutch teeth are meshed in a certain position and marks
are provided on the outer surfaces to facilitate replacement in the event of the instrument being
dismantled. The wear on the threads is compensated by withdrawing the screw and turning the
smaller nut through one, two or more clutch tooth spaces from its original angular position and
then bringing it in mesh with the main nut. In this way the threads on the smaller nut are slightly
displaced. At the time of reassembling, care must be taken to see that the clutch teeth on the two
nuts are meshing properly and that the small nut is held in position until the screw has entered
the main nut.
In another most commonly used method of compensating for wear on the thread, provision
of an external tapered thread is made on one end of the main nut. The tapered thread is engaged
by an adjusting nut having a parallel thread. When sufficient wear occurs between the two threaded
portions, the adjusting screw is advanced so as to contact the bore of the main nut slightly and thus
making it take up the clearance between the threads of the nut and the screw.
2.37.5.

Adjustment for wear of measuring surfaces.

It will be appreciated that wear is
bound to occur between the measuring surfaces of fixed anvil and the spindle and that this wear
will result in errors being introduced. One method of compensating for such wear (shown in Fig.
2.115) is to accommodate the fixed anvil in a reamed hole in the frame and provide a thread extension
which passes through a tapped hole of slightly smaller diameter than the reamed locating hole.
Thus on rotating the anvil by means of a slot at the threaded end, it can be advanced to compensate
for the wear which has taken place. After adjustment the anvil is locked by a screw in a transverse
hole in the frame. The head of the screw is radiused to correspond with the radius of the anvil shank
thereby obtaining a secure locking action.
In another design the main nut instead of being press fit inside the barrel is threaded into
it. The adjustment for taking up wear in such a case can be easily made by screwing the main nut
farther into the barrel and thus advancing the spindle without changing the reading on the thimble.
In yet another design shown in Fig. 2.116 the adjustment for wear of the measuring faces is
carried out by rotating the thimble relative to the screw. Thus in this method, first the two
A typical method of adjusting for wear of the measuring faces.
Fig. 2.116. A typical method of adjusting for wear of the measuring faces.
measuring surfaces are brought into contact with each other and the thimble is turned until the
zero line marked on it coincides with the index of datum line on the sleeve. Referring to Fig. 2.116,
it will be seen that a plain extension is provided at the right-hand end of the spindle beyond the
threaded portion on which is mounted a sleeve having a slight external taper. The bore of the thimble
is also finished to a corresponding taper. The thimble and the spindle at the tapered surface are
held in secure engagement by a screw at the end of the thimble. The thimble is released from the
spindle by slackening this screw and pulling thimble out. It is then turned independently of the
spindle to obtain the desired setting and the screw is again tightened to secure thimble to the
spindle.
In another design which is reverse of this method, a sleeve is fitted over the barrel and is
retained in position by friction. The datum line is marked on this sleeve instead of marking on the
barrel. This sleeve can be turned in relation to the barrel by means of a small spanner in order to
bring the datum line opposite to the zero line on the thimble and thus compensate for wear on
measuring faces.
2.37.6.

Spindle locking arrangement.

In order that any setting of the micrometer be
definitely retained, it is desirable to have features of locking the spindle in any position temporarily.
One way of achieving this is shown in Fig. 2.115, in which a sliding pin is located in a hole in the
Micrometer spindle locking arrangement.
Fig. 2.117. Micrometer spindle locking arrangement.
frame and is pressed against the side of the spindle by a cam. The cam is fitted with dished and
knurled heads on both sides of the frame. The shape of the cam is such that when a slight turning
movement is imparted to it by gripping the dished and knurled heads on both sides of the frame
between finger and thumb, the pin is forced against the spindle and a rigid lock is obtained.
In another design shown in Fig. 2.117, the micrometer is slotted to receive a split ring which
surrounds the spindle and is prevented from rotating. The split ring is surrounded by a knurled
outer ring. A cam slot is provided on the periphery of the split ring in which a roller moves up and
down.
When the outer ring is turned in one direction the roller rides up and cam surface thereby
tending to close the split ring and securely clamping the spindle. The grip is released on turning
the outer ring in opposite direction.
2.37.7.

Ratchet stop mechanism.

The object of the ratchet stop is to ensure that a certain
maximum torque on the spindle is not exceeded and the sense of the feel of operator is eliminated
and consistent readings are obtained. By providing this arrangement, when the spindle has engaged
the work with a certain pressure* further rotation causes the ratchet merely to slip, no additional
movement being imparted to the spindle. The ratchet stop mechanism is incorporated in the knurled
extension provided at the end of the thimble. The mechanism is shown in Fig. 2.115 and is illustrated
below. The knurled extension is free to rotate on its retaining stud. The inner face of this is provided
with fine ratchet teeth, and through these teeth and the spring loaded pawl (shown in Fig. 2.115)
the movement is transmitted to the spindle. As soon as the resistance to the motion of the latter
reaches a certain value, the pawl is forced back against the pressure of the spring and ratchet slips.
2.37.8.

Reading a micrometer.

In order to make it possible to read upto 0.0001 inch in
micrometer screw gauge a vernier scale is generally made on the barrel. The vernier scale has 10
straight lines on barrel and these coincide with exact 9 divisions on the thimble. Thus one small
division in thimble is further sub-divided into 10 parts and for taking the reading one has to see
which of the vernier scale division coincides with a division _
of the thimble. Accordingly the reading for the given arran-
gement in Fig. 2.118 will be
on main barrel : 0.120″
on thimble : 0.014″
on vernier scale : 0.0002″
.-. Total reading = 0.1342″.
Reading a micrometer
Fig. 2.118. Reading a micrometer.
Before taking readings, anvil and spindle must be brought together carefully and the initial
reading noted down. Its calibration must be checked by using standard gauge blocks.
In metric micrometers, the pitch of the screw thread is 0.5 mm so that one revolution of the
screw moves it axially by 0.5 mm. Main scale on barrel has least divisions of 0.5 mm. The thimble
has 50 divisions on its circumference.
0 5
.-. One division on the thimble = -zV mm = 0.01 mm.
50
If vernier scale is also incorporated then sub-divisions
on thimble can be estimated upto an accuracy of 0.001 mm.
Reading of micrometer in Fig. 2.119 is 3.5 mm on barrel
and 7 divisions on thimble
= 3.5 + 7 x 0.01 = 3.5 + 0.07 = 3.57 mm.
Reading a micrometer.
Fig. 2.119. Reading a micrometer.
2.37.9.

Cleaning the micrometer.

Micrometer screw gauge should be wiped free from oil,
dirt, dust and grit. When micrometer feels gummy and dust ridden and the thimble fails to turn
freely, it should never be bodily dunked in kerosene or solvent because just soaking the assembled
micrometer fails to float the dirt away. Further it must be remembered that the apparent stickiness
of the micrometer may not be due to grit and gum but to a damaged thread or to a warped and
sprung frame or spindle.
Everytime the micrometer is used, the measuring surface, the anvil and spindle should be
cleaned. Screw the spindle lightly but firmly down on to a clean piece of paper held between spindle
and anvil. Pull the piece of paper out from between the measuring surface. Then unscrew the spindle
a few turns and blow out any fuzz or particles of papers that may have clung to sharp edges of anvil
and spindle.
2.37.10.

Precautions in using micrometer.

In order to get good results out of the use of
micrometer screw gauge, the inspection of parts must be made as follows : Micrometer should be
cleaned of any dust and spindle should move freely.
The part whose dimension is to be measured must be held in left hand and the micrometer
in right hand. The way for holding the micrometer is to place the small finger and adjoining finger
in the U-shaped frame. The forefinger and thumb are placed near the thimble to rotate it and the
middle finger supports the micrometer holding it firmly.
Then the micrometer dimension is set slightly larger than the size of the part and the part
is slide over the contact surfaces of micrometer gently. After it, the thimble is turned till the
measuring tip just touches the part and the final movement given by ratchet so that uniform
measuring pressure is applied. In case of circular parts, the micrometer must be moved carefully
over representative arc so as to note maximum dimension only. Then the micrometer reading is
taken.
The micrometers are available in various sizes and ranges, and the corresponding
micrometer should be chosen depending upon the dimension.
Errors in reading may occur due to lack of flatness of anvils, lock of parallelism of the anvils
at part of the scale or throughout, inaccurate setting of zero reading, etc. Various tests to ensure
these conditions should be carried out from time to time.
2.37.11.

Tests.

The cumulative errors in the micrometer are determined by means of slip
guages of Grade I accuracy. The tests are carried out over the entire measuring range, preferably
in steps of 0.5 mm, as well as near the maximum and minimum points of the curve of errors in steps
of 0.1 mm over a length of 0.5 mm towards the left and right of these points. If the cumulative errors
are plotted for various lengths on a graph paper then the total error is the distance between the
ordinates of the highest and lowest points determined. The total error of the micrometer is not
Cumulative Error Curve.
Fig. 2.120. Cumulative Error Curve.
considered as positive or negative deviation from the zero point, but as maximum difference of
ordinates of the curve of cumulative errors.
Enlargement of curve A Enlargement of curve B
Enlargement of curve A Enlargement of curve B
Fig. 2.121. Error curve enlargement.
Alternative method of calibrating micrometer screws. The accuracy of the readings of a
micrometer is usually checked by taking readings on a series of slip gauges. The sizes of these gauges
are chosen in such a way as to test the micrometer not only at complete turns of its thimble but also
at intermediate positions. This is required as a check on the accuracy of the graduations round the
thimble. The following series of readings serve for testing both progressive and periodic errors
throughout its range : 2.5, 5.1, 7.7, 10.3, 12.9,15.0, 17.6, 20.2, 22.8 and 25.0 mm. This series gives
readings which work round the thimble twice over and so provide a double check on any periodic
error which may be present.
Accuracy of Measuring Faces. Deviations of flatness of the measuring faces are tested with
an optical flat moved in such a manner that the number of interference fringes are reduced to a
minimum or closed curves are produced (Refer Chapter 6). The number of interference fringes
observed on each of the two measuring faces should not exceed 2 for practically all the ranges of
grade I micrometer.
The errors in parallelism of the measuring faces of micrometers of various ranges upto 100
mm are tested by observation of interference fringes produced between a flat parallel plate and the
measuring faces. The sum of the interference fringes observed on the two measuring faces should
not exceed 6,8 and 10 for ranges 0 to 25,25 to 75 and 75 to 100 mm in respect of grade I micrometers.
Another suitable method for this test is by using auto-collimator.
Bending. The degree of bending of the frame is tested by suspending the frame directly at
the spindle and loading with a weight of about 5 to 10 kg directly at the anvil. When the load is
removed then the frame should return to its original position. The difference between the measure-
ments of micrometer in unloaded and loaded conditions should not exceed 1.5 um per kg for
micrometer of range 0 to 25 mm. The corresponding values for other ranges are 2, 2.5, 3 yon for 25
to 50, 50 to 75, 75 to 100 mm range.
2.37.12.

Accuracy with micrometer measurements.

The accuracy of a micrometer
depends on (i) the degree of calibration of the spindle movement, which is affected by the lead errors
of the screw. This effect is usually cumulative and increases with the length of the spindle travel.
This type of inaccuracy can be reduced by balanced calibration, i.e. by adjusting the thimble to
produce error-free reading in the middle of full travel, or of the most frequently used section of the
spindle traverse ; (») the linearity of the spindle movement. For this, any fractional rotation of the
screw should result in a proportional advance of the measuring spindle. Drunken threads, or
stick-and slip condition of the screw in the nut adds to this inaccuracy. The accuracy of the
micrometer indication can be assessed by the calibration process. The accuracy can be improved by
manufacturing the spindle of well stabilised material, precisely grinding the screw thread after
hardening, using lapped nuts, etc.
The errors recorded on calibration chart will comprise the combined effect of flatness and
parallelism of the measuring surfaces, deflection of frame by the applied measuring force, process
errors (caused by head transfer while holding the micrometer), reading errors, inadequate align-
ment or stability in mutual positioning of object and measuring tool, wear, etc.
Following precautions in use of micrometer screw gauge will be of great help in reducing
errors:
— Employing plastic insulating grips on the frame to reduce heat transfer.
— Reading errors can be minimised by providing satin chrome finish to eliminate glare,
distinct graduation lines applied on a bevelled thimble surface, to facilitate reading with
a minimum of parallax error, use of larger diameter thimbles.
— Using a stand which rests on the bench to clamp the hand micrometer instead of holding
it in hand, thereby improving alignment and holding stability.
— Applying uniform and minimum measuring force.
— Taking care of wear after some use.


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