MICROMETER MEASURING INSTRUMENTS
Of all measuring instruments used in the shop intended for accurate
measurements, those working on the principle of the ordinary micrometer
calipers are the most common. In the present chapter we shall describe and
illustrate a number of different designs of these tools, intended to be
used for various purposes. The instruments shown in Figs. 19 to 23 were
built, in leisure hours, by Mr. A. L. Monrad, of East Hartford, Conn.
Micrometer for Snap Gages
[Illustration: Fig. 19. Micrometer for Snap Gages]
Fig. 19 shows a form of micrometer that has proved very handy for measuring
snap gages, and thicknesses, and can also be used as a small height gage to
measure the distance from a shoulder to the base, as shown in Fig. 20. In
measuring snap gages or thicknesses, the outside and inside of the
measuring disks are used, respectively. This instrument may also come in
very handy when setting tools on the planer or shaper. As will be seen in
the engraving, there are two sets of graduations on the sleeve _A_, thus
enabling the operator to tell at a glance what measurement is obtained from
the outside or the inside of the measuring disks. Each of the disks is
0.100 inch thick, so that the range of the micrometer is 0.800 and 1.000
inch for the outside and inside, respectively. The details of the
instrument are as follows:
The sleeve _A_ is composed of the inside measuring disk, the graduated
sleeve, and the micrometer nut combined. On the disk are two projections
_KK_, which are knurled, thus providing a grip when operating the tool. The
sleeve is threaded on the inside of one end, which acts as a micrometer
nut, and the outside of this same end is threaded to receive the adjusting
nut _D_. The sleeve has two slots, each placed 90 degrees from the
graduations, and these provide for compensation for wear. The disk part is
hardened by heating in a lead bath, and is finished by grinding and
lapping. The barrel _B_ is the same as a regular micrometer barrel, and is
graduated with 25 divisions. Spindle _E_ consists of the outside disk and
the micrometer screw, and the barrel _B_ fits on its end, which is tapped
out to receive the speeder _C_, which serves to hold the barrel in
position. The thread is ¼ inch, 40 pitch, and the disk and unthreaded parts
are hardened, ground and lapped. To adjust this, instrument, loosen the
speeder _C_ and turn the barrel until the proper adjustment is obtained.
Then lock the barrel by tightening the speeder again.[9]
[Illustration: Fig. 20. Micrometer in Fig. 19 used as Height Gage]
Micrometer Caliper Square
Fig. 21 shows an assembled view and the details of a micrometer caliper
square which, if accurately made, is equal and often preferable to the
vernier caliper now so generally used. One of its advantages over the
vernier is that when the measurement is taken, it can be readily discerned
without straining the eyes, and this instrument is as easy to manipulate as
the regular micrometer.
In the details, part _A_, which is the main body of the instrument, is made
of tool steel, the forward or jaw end being solid with the body. This end
is hardened, and the jaw ground and lapped. The body is bored out and two
flats milled on the outside, which lighten it up and make it neat in
appearance. The jaw end is counterbored out with a 45-degree counterbore to
form a bearing for the forward end of the micrometer screw. A slot, 1/8
inch in width, extends from the fixed jaw to the other end, and in this
slides the movable jaw _C_. There are 44 divisions along the side of this
slot, each division being 0.050 inch apart, giving the tool a range of
2.000 inches for outside and 2.200 inches for inside measurements. The
screw _B_ is the most essential part of this tool, its construction
requiring great accuracy. Its diameter is 3/8 inch and it is cut with 20
threads per inch. On its forward end fits the cone _F_, which is hardened
and ground, the round part acting as the forward bearing of the screw and
fitting in the 45-degree counterbored hole in the body _A_. On its other
end fits the graduated barrel _D_ and also the speeder _G_.
[Illustration: Fig. 21. Micrometer Caliper Square]
The barrel is graduated in fifty divisions, each division equaling 0.001
inch. On the inside of the barrel is a 45-degree bearing which rides on the
cone _M_, the cone being held stationary on the end of the body. Thus it
will be seen that both front and back ends of the micrometer screw are
carried in cone bearings, which give a very small point of contact, thereby
causing but little friction and preventing any danger of gumming up so as
to run hard. The sliding jaw _C_ is made of tool steel, hardened, ground
and lapped, and combined with it is the micrometer nut which is drawn to a
spring temper. This nut is split and adjusted by two screws to compensate
for wear. On this jaw are the two zero marks that tell at a glance the
outside or inside measurements taken. The screw and washer, marked _H_ and
_I_, go onto the end of the micrometer screw and take up the end play. To
make a neat appearance, the cap _E_ is placed in the forward counterbored
hole, being held in place by a tight fit. The adjustment of the tool is
accomplished by loosening the speeder _G_ and turning the barrel on the
screw; when the adjustment is made, the speeder is again tightened down and
the barrel locked.[10]
Micrometer Depth Gage
The depth gage, shown in Fig. 22, has a ½-inch movement of the rod, and may
be used with rods of any desired length. These have small
45-degree-on-a-side grooves cut into them at intervals of ½ inch. A small
spiral spring, marked _I_, gives the rod a constant downward pressure, so
that, when taking a measurement, the base of the tool is placed on the
piece of work, and the rod always finds the bottom of the hole; then, by
tightening the knurled screw _F_ the rod is clamped in position and the
tool may be picked up and its measurement read from the dial. The
graduations on this instrument are similar to those of the vernier caliper,
only they are much plainer, as a half-inch movement of the rod turns the
dial one complete revolution. The figures on the dial denote tenths of an
inch, and those on the body of the tool thousandths; each graduation on the
dial is therefore equal to 0.010, so that to show the depth of a hole to be
0.373 the dial would be revolved around so that the seventh division beyond
the 3 mark would be near to 0, and then by looking from the 0 mark toward
the left, the third graduation on the body and one on the dial would be in
line, thus denoting 0.373.
[Illustration: Fig. 22. Micrometer Depth Gage]
The most essential part of this tool is the threaded screw _B_, which acts
as a rack, and the worm-wheel, solid with the dial _C_. The upper end of
the screw forms a split chuck which grips the measuring rods, while the
part marked _R_ is flatted off, and against this portion bears a threaded
sleeve _G_, which acts as a key to keep the screw in position. This sleeve
is threaded, both inside and outside, and screws into the body of the tool,
while the binding screw _F_ fits into it and binds against a small piece of
copper, marked _H_, which in turn holds the screw in position. The thread
on _B_ is 0.245 inch in diameter and is cut with 40 threads per inch. The
worm-wheel which meshes into this screw is solid with the dial, as shown at
_C_. It is 0.18 inch in diameter, and requires great accuracy in cutting;
it is not hobbed, but the teeth, of which there are twenty, are milled with
a circular cutter of the same diameter as the screw _B_ plus 0.002 inch.
The little studs, marked _EE_, on the dial and on the body _K_, hold the
coiled spring in position. Very great accuracy must be attained when
locating the holes in _K_ that are to receive the screw and dial _B_ and
_C_. The screw marked _J_ fits into the dial, where it serves as a bearing
and also holds the dial in position. The knurled cap _D_ tightens the split
chuck in order to hold the measuring rod firmly.[11]
Indicator for Accuracy of Lead-screws
[Illustration: Fig. 23. Indicator for Accuracy of Lead-screws]
All of the tools that have been described require an accurately cut screw,
and, as very few lathes are capable of producing this, it may be well to
illustrate an indicator for testing the accuracy of the lead-screw, and to
explain the method by which it is used. This instrument is shown in Fig.
23, where it is applied to a test screw _K_. It consists of a body _A_ on
one end of which is a projection _L_ serving as the upper bearing for the
pivoted lever _D_. This lever swings about a small steel pivot which can be
adjusted by the screw _E_. The rear end of the lever is forked, and between
the prongs is passed a thread making a double turn about the pivot _F_ that
carries the pointer _J_. Any movement of this lever will, therefore, cause
this pointer to revolve about the dial _C_. This dial has 20 divisions,
each indicating one-half thousandth of an inch movement of the front end of
the lever, so that a total revolution of the pointer about the dial would
indicate a movement of the front end of the lever of 0.020 inch. The screws
_I_ serve to hold the dial in place on the body of the indicator, while the
spring _M_ keeps the pointer normally at the zero mark. The indicator is
held in the toolpost by the arm _G_, which can be set at any angle and
firmly clamped by the screw _H_.
To use the indicator, remove the screw from a micrometer which is known to
be accurate, and, with the aid of a brass bushing, chuck it in the lathe so
that the thread end will project. Now gear the lathe to cut 40 threads per
inch and apply the indicator. When the lathe is started, the point of the
indicator follows along in the thread of the micrometer screw, and any
variation in the lead will be noted by a movement of the pointer over the
dial. If, on the other hand, no movement takes place, it is an indication
that the pitch of the lead-screw is correct.[12]
Micrometer Attachment for Reading Ten-thousandths of an Inch
[Illustration: Fig. 24. Micrometer with Attachment for Reading
Ten-thousandths of an Inch]
Fig. 24 shows an attachment for micrometers designed and made for readings
in tenths of thousandths of an inch. With very little fitting it is
interchangeable for 1-, 2-, or 3-inch B. & S. micrometers. The idea is
simple, as can be seen by the illustration. The diameter of the thimble is
increased 3 to 1 by a disk which is graduated with 250 lines instead of 25,
making each line represent 0.0001 inch instead of 0.001 inch. A piece of
steel is then turned up and bored and cut away so as to form the index
blade and a shell to clasp the micrometer frame, the whole thing being made
in one piece. The thimble disk being just a good wringing fit, it can be
easily adjusted 0 to 0. The attachment can be removed when fine measuring
is not required.[13]
Special Micrometer for Large Dimensions
Fig. 25 shows a 6-inch micrometer caliper designed for measuring from 0 to
6 inches by half-thousandths. The sliding micrometer head travels on a
cylinder barrel through which a hole is accurately bored to suit three
plugs, one, two, and three inches long, as shown in the engraving. These
plugs serve to locate the traveling head at fixed distances one inch apart.
The micrometer screw itself has a travel of one inch, like any standard
micrometer. A locknut is used to hold the screw in any desired position. A
thumb screw at the end of the barrel bears against the end plug, and zero
marks are provided to bring the screw against the plug with the same degree
of pressure at each setting. When the head is clamped by means of the
locking nut, it is as rigid as though it were solid with the barrel, and
the faces of the measuring points are thus always parallel.
[Illustration: Fig. 25. Special Micrometer for Large Dimensions]
Combination Micrometer
[Illustration: Fig. 26. Combined One- and Two-inch Micrometer]
A combined one- and two-inch micrometer is shown in Fig. 26. One side
records measurements up to one inch, and the other side up to two inches. A
single knurled sleeve or nut serves to move the double-ended measuring
piece one way or the other as desired, this piece having a travel of one
inch. The spindle is non-rotating, so that the faces of the screw and anvil
are always parallel. A locking device holds the screw in any position. This
tool is convenient for use both in measuring and as a gage, since it can be
conveniently held by the finger ring appearing at the back.
Micrometer Stop for the Lathe
Most micrometer lathe stops are limited in their use to work where only a
stationary height is required. It is, however, often necessary to use the
stop at different heights, to accommodate different lathes; then again, we
wish to use it on the right-hand side as well as the left. The form of
holder shown in Fig. 27 can be used either right or left, and for various
heights, and, by simply taking out the screw _A_, the micrometer may be
removed and used in any other form of holder desired.
[Illustration: Fig. 27. Micrometer Stop for the Lathe]
Both an assembled view and details of the holder are shown in the
engraving, so that it can be easily constructed by any one desiring to do
so. The micrometer and barrel may be procured from any of the manufacturers
of measuring instruments. The swivel _C_ is bored out so that the axis of
the micrometer screw will be parallel to the body of the holder when it is
in place. The swivel is made of tool steel and is fastened to the holder by
the screw _A_. It is hardened and lapped to a true bearing surface on the
sides and bottom, and so adjusted that it will turn to either side and
remain in the desired position without moving the screw. The holder _B_ is
milled through its entire length with a 90-degree cutter so that it will
fit along the ways of the lathe, and the bottom is lapped to a true
surface. For a neat appearance, the tool should be color hardened. On top
the holder is spotted or countersunk with a drill to form a recess for the
C-clamp. A knurled ring _D_ is driven onto the micrometer sleeve so that it
can be turned around to bring the graduations uppermost when the position
of the barrel is changed.[14]
Micrometer Surface and Height Gage
[Illustration: Fig. 28. Micrometer Surface and Height Gage]
Fig. 28 shows a form of surface gage that has proved very handy, and which
can be used also as a height gage for measuring distances from shoulders to
the base. If accurately made it is equal, and often preferable, to the
vernier or slide caliper now so generally used with an attachment to the
sliding jaw. One of its advantages over the vernier is the readiness with
which the graduations are discerned, and it is as easy to manipulate as the
ordinary micrometer. The part _B_, which forms the main body of the
instrument, is made of tool steel, and one end is fitted into the base
where it is held in position by the screw _D_. The remainder is milled to a
thickness of 1/8 inch and has graduations of 0.025 inch for a distance of
three inches. The screw _A_ is the most essential part of the tool, and its
construction requires great accuracy. Its diameter is ½ inch, and it is cut
with 20 threads per inch. In the upper end of the screw is driven the ball
_H_ for the sake of giving a neat appearance. The top of the thread is
turned off 0.010 inch to allow the scriber _F_ to slide freely on the
screw. The barrel _I_ is used for raising and lowering the slide, but
instead of having the graduations placed directly upon it, they are made
upon the sleeve _C_, which fits over a shoulder on the barrel. This allows
more easy means of adjustment than would be possible were the graduations
placed on the barrel itself. The sleeve is graduated with fifty divisions
each equaling a movement of the scriber of 0.001 inch. This sleeve may be
turned by means of a small spanner wrench so as to bring the zero line into
correct position to compensate for wear. A knurled locking nut is also
provided for holding the scriber in any fixed position. The scriber itself
is hardened and lapped to a finished surface, the tail end being slotted
and provided with two screws to compensate for wear. On the scriber is
placed the zero mark which shows at a glance the measurement that is being
taken. The block _K_ is three inches in height, and by using this block and
placing the gage on its top, the range of the gage is increased to six
inches. The screw _E_ is used for fastening the gage to the top of the
block. The center of the block is drilled out and slots cut through the
sides in order to make it light and neat in appearance.[15]
Micrometer of from One- to Five-inch Capacity
[Illustration: Fig. 29. Micrometer of from One- to Five-inch Capacity]
Fig. 29 shows a very simple and light five-inch micrometer that can be
quickly set to exact position from one to five inches. The round beam is
graduated by a series of angular grooves, 1 inch apart, which are of such a
form and depth that the clamping fingers at the end of part _A_ spring in,
allowing one inch adjustment of the beam to be quickly and positively made.
The sleeve _K_ is of tool steel, being counterbored from the forward end
for all but one-half inch of its length. For this half inch it is threaded
on the inside and acts as a micrometer nut. The outside of the same end is
threaded to receive the adjusting nut _F_, and two slots are cut in the
sleeve, at 90 degrees with the graduations. These slots, by a movement of
the nut _F_, provide a means for compensating for wear. The bushing _E_ is
hardened and lapped, and fitted tightly in the forward counterbore of this
sleeve, where it acts as a guide for the front end of the micrometer screw.
The barrel _J_ is the same as that of a regular micrometer, and is
graduated in 0.025 inch divisions.
The most essential part of the tool is the threaded screw _I_, over the end
of which fits the barrel _J_. The end is tapped out to receive the speeder
_H_, which serves to hold the barrel in position. The thread is 5/16 inch
in diameter, with 40 threads per inch, while the unthreaded part is
hardened, ground and lapped. To adjust the instrument, loosen the speeder
_H_ and turn the barrel until the proper adjustment is obtained; lock the
barrel by again tightening the speeder. The beam _C_ has a ¼-inch hole
drilled throughout its entire length in order to make it light. Small
90-degree grooves are cut into it at intervals of 1 inch, and a 1/8-inch
slot is milled through one side to within 1¼ inch of the forward end. The
back end of part _A_ forms a spring-tempered split chuck, which grips the
beam and holds _A_ in position, while the exterior is threaded to receive
the knurled cap _B_ by which the chuck is tightened firmly to the beam.
From the front end, toward the split chuck, the body is counterbored 5/8
inch and the bushing _D_ driven in tight. This bushing has a key _G_ fitted
into it, which slides in the slot of the beam and prevents the arm from
turning. The projecting arm is bored and tapped to receive the sleeve _K_.
This gage must be carefully and accurately made to be of value.[16]
Inside Micrometer for Setting Calipers
[Illustration: Fig. 30. Method of Setting Calipers from Inside Micrometers]
Fig. 30 shows an application of inside micrometers which is very handy. The
hole for the scriber in the scriber clamp of a surface gage is reamed out
to fit the rods used with inside micrometers. This forms a convenient
holder for the micrometer when used for setting outside calipers to it. The
calipers can be set easily and accurately at the same time, and where
extreme accuracy is not necessary this arrangement is more handy than that
of using large-sized micrometers.
With care and practice an accuracy of within one-quarter of 0.001 inch is
obtainable in this way. Mistakes, in fact, are more easily guarded against
than is the case when using the micrometers directly.
Micrometer Frame
[Illustration: Fig. 31. Useful and Handy Micrometer Frame]
Fig. 31 shows a micrometer frame used some years ago at the Westinghouse
works. The frame is an aluminum casting, and the anvil is simply a
tool-steel pin, which fits well in the hole into which it is inserted, and
can be clamped anywhere within the limits of its length. The micrometer end
of the frame is supplied with an inside micrometer head. The tool is
adjusted to a gage, either to a standard pin gage, or to an inside
micrometer gage. The capacities of three of these micrometers in a set may
be from about 3½ to 7 inches, 6 to 11 inches, and 10 to 15 inches. When the
head is turned outward, as shown in the lower view in the cut, the tool is
very handy around a horizontal boring machine where a pin gage cannot be
used without removing the boring bar.
Micrometer Stop for the Lathe
[Illustration: Fig. 32. Micrometer Stop for the Lathe]
The simple micrometer stop shown in Fig. 32 is used on the engine lathe for
obtaining accurate movements of the lathe carriage. It consists of a
micrometer head, which can be purchased from any micrometer manufacturer,
and a machine steel body which is bored to fit the micrometer head. This
tool is clamped on the front way of the lathe bed, and when the jaw of the
micrometer is against the lathe carriage, it can easily be adjusted to a
thousandth of an inch. Of course, care should be taken not to bump the
carriage against the micrometer.[17]
Use of Micrometer for Internal Thread Cutting
[Illustration: Fig. 33. Method of using Micrometer for Internal Thread
Cutting]
Fig. 33 illustrates a means of determining the size of internally threaded
work. The work shown is intended for a lathe chuck. The outside diameter of
the hub on the work is turned to the same size as the hubs on small
faceplates which are furnished with all new lathes. The threaded size is
then taken and transferred with a micrometer, over the anvil of which is
fitted a 60-degree point as shown enlarged at _A_. In connection with a
graduated cross-feed screw this greatly facilitates the work over the usual
cut-and-try method.[18]
Inside Micrometer
The inside micrometer shown in sections in Figs. 34 and 35 is adapted to
measuring, by use of extension rods, from 2 inches up to any size of hole,
and has one inch adjustment of the measuring screw.
[Illustration: 35]
Referring to the section shown in Fig. 35, the measuring screw _S_ is
secured to the thimble _B_ with the screw _D_, the head of which is
hardened and forms the anvil. By loosening this screw _D_, the thimble can
be rotated to compensate for wear. The wear of the measuring screw and nut
is taken up by screwing the bushing _A_ into the frame with the wrench
shown in Fig. 37. This bushing is split in three sections for about
two-thirds of its length on the threaded end. The three small lugs on the
wrench fit into these slots. The handle end of the wrench is a screw driver
which is used for manipulating the set screw _C_. The bushing is made an
easy fit in the frame on its plain end and tapered, as shown, on its
outside threaded part. This thread being the same pitch as the measuring
screw, adjustment for wear does not affect the reading of the micrometer.
This manner of adjustment brings the nut squarely down on the measuring
screw for its whole length, presenting the same amount of wearing surface
after adjustment as when new.
[Illustration: Fig. 36. Handle for Inside Micrometer]
[Illustration: Fig. 37. Wrench used with Inside Micrometer]
The point _F_, which is hardened on its outer end, screws into the frame,
and is secured by the taper-headed screw _O_, which screws into and expands
the split and threaded end of the point _F_. The handle, Fig. 36, clamps
over the knurled part of the frame for use in small, deep holes. The rods,
six in number, running from 1 to 6 inches inclusive, are made by screwing a
sleeve onto a rod with a hardened point and locking it with a taper-headed
screw on its threaded and split end, the same as in the point _F_. The
extension pieces, Fig. 38, are adjustable, on their socketed ends, in the
same way, and run in lengths of 6, 12, 18 inches, etc.[19]
[Illustration: Fig. 38. Adjustable Extension Pieces for Inside Micrometer]
Direct Fractional-reading Micrometer
[Illustration: Fig. 39. Direct Fractional-reading Micrometer]
The direct fractional-reading micrometer shown in Fig. 39 is the result of
talks with many mechanics in which all agreed that such a feature added to
a micrometer would, by making it both a fractional and decimal gage, more
than double its practical value. While approximate readings in 64ths, etc.,
may be obtained by the graduations on the barrel _B_ as on an ordinary inch
scale, the exact readings of 64th, etc., may be obtained only by reference
to graduations on the movable thimble _A_. There are but eight places on
_A_ which coincide with the long graduation line on _B_ when any 64th, 32d,
16th, or 8th is being measured, and each of these eight places is marked
with a line, and the 64th, 32d, 16th, or 8th for which that line should be
used is marked thereon. (See _a_ and _b_, Fig. 40.) The line _a_ would be
used for 3/32, 7/32, 11/32, etc., and the line _b_ for 1/64, 9/64, 17/64,
etc. Now suppose we wish to accurately measure 15/32 inch. We first roughly
read it off the inch scale on sleeve _B_ by turning out thimble _A_. Having
secured it closely by drawing edge of _A_ over that graduation, we find
that the line _a_ (Fig. 40) on the movable thimble very nearly or exactly
coincides with the long graduation line on _B_. When these lines coincide,
we have the exact measurement of 15/32 inch without reference to how many
thousandths may be contained in the fraction. Thus all through the scale
any fraction may be found instantly. There is no mental arithmetic, use of
tables, or memory work in using the tool. The new graduations are
independent of the old, and may be used equally well with or without them.
[Illustration: Fig. 40. Graduations on the Fractional-reading Micrometer]
Micrometers may also be graduated as in Fig. 41. Instead of using the zero
line on _A_ as a base line, a point is taken one-fifth of a turn around
_A_, and the graduated scale on _B_ is placed to correspond, as shown in
the engraving; also, instead of making lines _a_, _b_, etc., on _A_, full
length, they are made about half an inch long, and the numerators are
entirely omitted and the denominators placed at the end instead of under
the line. To the ordinary user of the tool, this is all that is necessary
for a perfectly clear reading of the fractions.[20]
[Illustration: Fig. 41. Another Method of Graduating for Fractional
Reading]
Sensitive Attachment for Measuring Instruments
No matter how finely and accurately micrometers and verniers may be made,
dependence must in all cases be placed on the sensitiveness of a man's hand
to obtain the exact dimensions of the piece to be measured. In order to
overcome this difficulty and eliminate the personal equation in the
manufacture of duplicate and interchangeable parts, the sensitive
attachment to the micrometer shown in Fig. 42 may be used, and will be
found of much value.
[Illustration: Fig. 42. Sensitive Micrometer Attachment]
The auxiliary barrel _A_ is held to the anvil of the micrometer by means of
a thumb screw _B_. At the inside end of the barrel is a secondary anvil
_C_, the base of which bears against the short arm of the indicating lever
_D_. The action will be clearly seen by reference to the engraving. The
micrometer is so set that when a gage, _G_, of exact size, is placed
between the measuring points, the long arm of the indicator stands at the 0
mark. If the pieces being calipered vary in the least from the standard
size it will be readily noted by the movement of the pointer. Hard rubber
shapes turned from rough casting often vary from 0.003 to 0.005 inch after
having passed the inspector's test with an ordinary micrometer. With this
attachment the inspector's helper can detect very minute variations from
the limit size. Anything within the limits of the micrometer can be made to
show to the naked eye variations as small as a ten-thousandth inch.[21]
Another Sensitive Micrometer Attachment
[Illustration: Fig. 43. Another Sensitive Micrometer Attachment]
When testing the diameters of pieces that are handled in great quantities
and are all supposed to be within certain close limits of a standard
dimension, the ordinary micrometer presents the difficulty of having to be
moved for each piece, and small variations in diameters have to be
carefully read off from the graduations on the barrel. Not only does this
take a comparatively long time, but it also easily happens that the
differences from the standard diameter are not carefully noted, and pieces
are liable to pass inspection that would not pass if a convenient
arrangement for reading off the differences were at hand. Fig. 43 shows a
regular Brown & Sharpe micrometer fitted with a sensitive arrangement for
testing and inspecting the diameters of pieces which must be within certain
close limits of variation. The addition to the ordinary micrometer is all
at the anvil end of the instrument. The anvil itself is loose and consists
of a plunger _B_, held in place by a small pin _A_. The pin has freedom to
move in a slot in the micrometer body, as shown in the enlarged view in the
cut. A spring _C_ holds the plunger _B_ up against the work to be measured,
and a screw _D_ is provided for obtaining the proper tension in the spring.
The screw and the spring are contained in an extension _E_ screwed and
doweled to the body of the micrometer. A pointer or indicator is provided
which is pivoted at _F_ and has one extensional arm resting against the pin
_A_, which is pointed in order to secure a line contact. At the end of the
indicator a small scale is graduated with the zero mark in the center, and
as the indicator swings to one side or the other the variations in the size
of the piece measured are easily determined. A small spring _G_ is provided
for holding the pointer up against the pin _A_. The case _H_ simply serves
the purpose of protecting the spring mentioned. As the plunger _B_ takes up
more space than the regular anvil, the readings of the micrometer cannot be
direct. The plunger _B_ can be made of such dimensions, however, that 0.100
inch deducted from the barrel and thimble reading will give the actual
dimension. Such a deduction is easily done in all cases. In other words,
the reading of the micrometer should be 0.100 when the face of the
measuring screw is in contact with the face of the plunger; the 0.100 inch
mark is thus the zero line of this measuring tool.
When desiring to measure a number of pieces, a standard size piece or gage
is placed between the plunger _B_ and the face _L_ of the micrometer screw,
and the instrument is adjusted until the indicator points exactly to zero
on the small scale provided on the body of the micrometer. After this the
micrometer is locked, and the pieces to be measured are pushed one after
another between the face _L_ and the plunger _B_, the indications of the
pointer _M_ being meanwhile observed. Whenever the pointer shows too great
a difference, the piece, of course, does not pass inspection. All
deviations are easily detected, and any person of ordinary common sense can
be employed for inspecting the work.
Micrometer Scale
[Illustration: Fig. 44. Micrometer Mounted on Machinist's Scale]
A micrometer, mounted as shown in Fig. 44 is very handy. The micrometer may
be used in combination with a 4-, 6-, 9-, or 12-inch scale. It can be
adjusted on standard plugs, or one can make a set of gages up to 12 inches,
out of 3/16-inch round tool steel wire, and use these for setting. In
mounting the micrometer, before cutting it apart, mill the shoulders shown
at _A_, and in milling the bottom pieces _B_, use a piece of machine steel
long enough for both, cutting the piece in half after milling the slots. In
this way one obtains perfect alignment. In a shop where a set of large
micrometers is not kept, this arrangement is very useful.[22]
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