|The internal combustion engine is made
up of many components which contribute to the successful operation
of the engine. The life of these components and the engine's
life is directly related to the preciseness of each component's
fit in its respective location.
The success of the automotive
mechanic and machinist is very much dependent upon his ability
to check out and fit components within the tolerances specified
by the engine manufacturer.
||The accuracy of these fits
is measured in thousandths and ten-thousandths utilizing micrometers
and/or precision gauges. The automotive machinist by necessity
is familiar with the use of "mikes", however many
skilled mechanics have to rely on calling in a machinist for
measuring crankshaft journals, cylinder bores, pistons, valve
stems, valve guides, etc.
A set of micrometers and the ability
to properly use them will greatly increase the mechanic's
speed and accuracy in performing his work.
consists of a highly accurate ground screw or spindle which
is rotated in a fixed nut, thus opening or closing the distance
between the measuring faces on the ends of the anvil and spindle.
A piece of work is measured by placing it between the anvil
and spindle faces and rotating the spindle by means of the
thimble until the anvil and spindle both contact the work.
The work dimension is then found from the micrometer reading
indicated by the graduations on the sleeve and thimble as
described in the following paragraphs.
Since the pitch of the screw thread on the spindle is 1/40
of an inch or 40 threads per inch in micrometers graduated
to measure in inches, one complete revolution of the thimble
advances to the spindle face toward or away from the anvil
face precisely 1/40 or .025 of an inch.
The longitudinal line on the sleeve is divided into 40 equal
parts by vertical lines that correspond to the number of threads
on the spindle.
Therefore each vertical line designates 1/40 or .025 of an
inch and every fourth line which is longer than the others
designates hundreds of thousandths.
For example the line marked one represents .100, line marked
two .200 and the line marked three represents .300, etc.
The beveled edge of the thimble is divided into 25 equal
parts with each line representing .001 of an inch and every
line numbered consecutively.
Rotating the thimble from one of these lines to the next,
moves the spindle longitudinally 1/25 of .025 or .001 of an
inch; rotating two divisions represents .002, etc.
Twenty-five divisions indicate a complete revolution, .025
or 1/40 of an inch.
||To read the
micrometer in thousandths, multiply the number of vertical divisions
visible on the sleeve by .025 and add to this the number of
thousandths indicated by the line on the thimble which coincides
with the longitudinal line on the sleeve.
An easy way to
remember is to think of the various units as if you were making
change from a ten dollar bill.
Count the figures on the sleeve as dollars, the vertical
lines on the sleeve as quarters, and the divisions on the
thimble as cents. Add up your change and put a decimal point
instead of a dollar sign in front of the figures.
The illustration is an example of a simulated micrometer
reading and how it is read:
The "1" line on sleeve is visible representing
100". There are 3 additional lines visible, each representing
.025"... 3 x .025 075". Line "3" on the
thimble coincides with the longitudinal line on the sleeve,
each line representing .001" . . . 3 x .001" = .003".
The micrometer reading is .178".
Micrometers are readily avail-able and in a wide variety
of prices and qualities and the average mechanic can avail
himself of a mike relatively inexpensively, and with an hours
practice can become familiar and handy with a set of micrometers.