A vernier caliper
A
caliper (
British
spelling also
calliper) is a device used to
measure the distance between two
symmetrically opposing sides. A caliper can be as simple as a
compass with inward or
outward-facing points. The tips of the caliper are adjusted to fit
across the points to be measured, the caliper is then removed and
the distance read by measuring between the tips with a measuring
tool, such as a
ruler.
They are used in many fields such as
metalworking,
mechanical engineering,
gunsmithing,
handloading,
woodworking,
woodturning and in
medicine.
Nomenclature variants (colloquial only)
A
plurale tantum sense of the word
"calipers" coexists in natural usage with the regular noun sense of
"caliper". That is, sometimes a caliper is treated cognitively like
a pair of glasses or a pair of scissors, resulting in a phrase such
as "hand me those calipers" or "those calipers are mine" in
reference to one unit. However, this usage is mostly
colloquial and the regular noun sense of
caliper usually dominates, especially in writing. There is no
rigorous grammatical logic to defend the plurale tantum sense; it
is just the way language sometimes naturally flows
colloquially.
Also existing colloquially but not in formal usage is referring to
a vernier caliper as a "vernier".
History
The
earliest caliper has been found in the Greek Giglio
wreck near
the Italian
coast. The ship find dates to the 6th century BC. The wooden
piece already featured a fixed and a movable jaw. Although rare
finds, caliper remained in use by the Greeks and
Romans.
By the
Han Dynasty (202 BC – 220 AD), the
Chinese
also used the sliding caliper, which they made of
bronze and manufactured each tool with an inscription of the day,
month, and year it was made (according to Chinese era names and their lunar calendar).
The modern vernier caliper, reading to thousandths of an inch, was
invented by American Joseph R. Brown in 1851. His Brown and Sharpe
company inaugurated true precision manufacture in the United
States. It was the first practical tool for exact measurements that
could be sold at a price within the reach of ordinary
machinists.
Types
Inside caliper
Two inside calipers
The
inside calipers are used to measure the
internal size of an object.
- The upper caliper in the image (at the right) requires manual
adjustment prior to fitting, fine setting of this caliper type is
performed by tapping the caliper legs lightly on a handy surface
until they will almost pass over the object. A light push
against the resistance of the central pivot screw then spreads the
legs to the correct dimension and provides the required, consistent
feel that ensures a repeatable measurement.
- The lower caliper in the image has an adjusting screw that
permits it to be carefully adjusted without removal of the tool
from the workpiece.
Outside caliper
Three outside calipers.
Outside calipers are used to measure the external
size of an object.
The same observations and technique apply to this type of caliper,
as for the above inside caliper. With some understanding of their
limitations and usage these instruments can provide a high degree
of accuracy and repeatability. They are especially useful when
measuring over very large distances, consider if the calipers are
used to measure a large diameter pipe. A vernier caliper does not
have the depth capacity to straddle this large diameter while at
the same time reach the outermost points of the pipe's
diameter.
Divider caliper
A pair of dividers
In the metalworking field
divider calipers are
used in the process of
marking out
suitable workpieces. The points are sharpened so that they act as
scribers, one leg can then be placed in the dimple created by a
center or prick punch and the
other leg pivoted so that it scribes a line on the workpiece's
surface, thus forming an arc or circle.
A divider caliper is also used to measure a distance between two
points on a
map. The two caliper's ends are
brought to the two points whose distance is being measured. The
caliper's opening is then either measured on a separate ruler and
then converted to the actual distance, or it is measured directly
on a
scale drawn on the map. On a
nautical chart the distance is often
measured on the
latitude scale appearing on
the sides of the map: one minute of arc of latitude is
approximately one
nautical mile or
1852
metres.
Dividers are also used in the medical profession. They are used to
measure electrocardiogram (ECG) lines. This instrument is called an
ECG caliper or
EKG caliper. These calipers have
changed through the years, and there are even pocket calipers,
invented 20 years ago by cardiologist Robert A. Mackin.
Oddleg caliper
Odd leg calipers
Oddleg calipers,
Hermaphrodite
calipers or
Oddleg jennys, or just plain
Ol' Jennys, as pictured on the left, are generally used to scribe a
line a set distance from the edge of workpiece. The bent leg is
used to run along the workpiece edge while the scriber makes its
mark at a predetermined distance, this ensures a line parallel to
the edge.
In the diagram at left, the uppermost caliper has a slight shoulder
in the bent leg allowing it to sit on the edge more securely, the
lower caliper lacks this feature but has a renewable scriber that
can be adjusted for wear, as well as being replaced when
excessively worn.
Vernier caliper
[[Image:Vernier caliper.svg|thumb|right|400px|Parts of a vernier
caliper:
- Outside jaws: used to measure external
diameter or width of an object
- Inside jaws: used to measure internal diameter
of an object
- Depth probe: used to measure depths of an
object or a hole
- Main scale: gives measurements of up to one
decimal place(in cm).
- Main scale: gives measurements in fraction(in
inch)
- Vernier gives measurements up to two decimal
places(in cm)
- Vernier gives measurements in fraction(in
inch)
- Retainer: used to block movable part to allow
the easy transferring a measurement
- ]]
A variation to the more traditional caliper is the inclusion of a
vernier scale; this makes it possible
to directly obtain a more precise measurement.
Vernier calipers can measure internal dimensions (using the
uppermost jaws in the picture at right), external dimensions using
the pictured lower jaws, and depending on the manufacturer, depth
measurements by the use of a probe that is attached to the movable
head and slides along the centre of the body. This probe is slender
and can get into deep grooves that may prove difficult for other
measuring tools.
The vernier scales may include both metric and
inch measurements on the upper and lower part
of the scale.
Vernier calipers commonly used in industry provide a precision to a
hundredth of a millimetre (10 micrometres), or one thousandth of an inch.
A more precise instrument used for the same purpose is the micrometer.
Dial caliper
A further refinement to the vernier caliper is the
dial
caliper.
In this instrument, a small
gear rack drives a
pointer on a circular
dial.
Typically, the pointer rotates once every inch, tenth of an inch,
or 1 millimeter, allowing for a direct reading without the need to
read a vernier scale (although one still needs to add the basic
inches or tens of millimeters value read from the slide of the
caliper). The dial is usually arranged to be rotatable beneath the
pointer, allowing for "differential" measurements (the measuring of
the difference in size between two objects, or the setting of the
dial using a master object and subsequently being able to read
directly the plus-or-minus variance in size of subsequent objects
relative to the master object).
The slide of a dial caliper can usually be locked at a setting
using a small
lever or screw; this allows
simple
go/no-go checks of part
sizes.
Digital caliper
Digital caliper
A refinement now popular is the replacement of the analog dial with
an
electronic digital display on which the reading is
displayed. Some digital calipers can be switched between metric and
inch units. All provide for zeroing the display at any point along
the slide, allowing the same sort of differential measurements as
with the dial caliper but without the need to read numbers that may
be upside down. Digital calipers may contain some sort of "reading
hold" feature, allowing the reading of dimensions even in awkward
locations where the display cannot be seen.
With all of these benefits, digital calipers have by no means
replaced the dial caliper. Digital calipers typically do not have
the beam structure of a dial or vernier caliper and therefore do
not have the repeatability or accuracy to an amateur user.
Ordinary 6-in/150-mm digital calipers are made of stainless steel,
have a rated accuracy of .001" (.02mm) and resolution of .0005"
(.01mm).
The same technology is used to make longer 8-in and 12-in calipers;
the accuracy for bigger measurements declines to .001" (.03mm) for
100-200mm and .0015" (.04mm) for 200-300mm.
Increasingly, digital calipers offer a serial data output to allow
them to be interfaced with a
personal
computer. This means measurements can be taken and instantly
stored in a
spreadsheet or similar piece
of software, significantly decreasing the time taken to take and
record a series of measurements. The output of non-name brand
calipers is usually 24 bit 90 kHz synchronous. A suitable interface
to convert the output to
RS-232 levels and
format can be built or purchased.
Like dial calipers, the slide of a digital caliper can usually be
locked using a lever or thumb-screw.
Digital calipers contain a linear
encoder. A
pattern of bars is etched directly on the
Printed circuit board in the slider.
Under the scale of the caliper another printed circuit board also
contains an etched pattern of lines. The combination of these
printed circuit boards forms two variable
Capacitors. As the slider moves the capacitance
changes in a linear fashion and in a repeating pattern. The two
capacitances are out of phase. The circuitry built into the slider
counts the bars as the slider moves and does a linear interpolation
based on the magnitudes of the capacitors to find the precise
position of the slider.
Vernier, dial and digital calipers can be used with accessories
that extend their usefulness. Examples are a base that extends
their usefulness as a depth gauge and a jaw attachment that allows
measuring the center distance between holes.
Each of the above types of calipers have their relative merits and
faults.
Vernier calipers are rugged and have long lasting accuracy,are
coolant proof, resist magnetic fields and shock proof. Can have
both metric and imperial scales. However they require good eyesight
or a magnifiing glass to read and are hard to read from a distance
or at awkward angles.
Dial calipers are easy to read especially when seeking exact center
by rocking and observing the needle movement. They can be set to 0
at any point for comparisons. Cons, only moderately shockproof.
Very prone to dirt in the gearing causing errors.
Digital calipers switch easily between metric and imperial systems.
Can be set to 0 easily at any point with full count in either
direction. Can take measurments even if the display is completely
hidden by setting to 0 in place then withdrawing and closing
completely which shows the measurement in negative.Cons,
mechanically fragile, electronically fragile. Most require
batteries. Most do not resist coolant well. Only moderate shock
resistance. vulnerable to dirt.
The accuracy of all these types is basically the same at +/- .001"
for 6" calipers.
Micrometer caliper
A caliper with a
micrometer
built in is called a
micrometer caliper or, more often,
simply a
micrometer. (Sometimes the term
caliper,
referring to any other type in this article, is held in
contradistinction to
micrometer.)
Use
Using the vernier caliper
A caliper must be properly applied against the part in order to
take the desired measurement. For example, when measuring the
thickness of a plate a vernier caliper must be held at right angles
to the piece. Some practice may be needed to measure round or
irregular objects correctly.
Accuracy of measurement when using a caliper is highly dependent on
the skill of the operator. Regardless of type, a caliper's jaws
must be forced into contact with the part being measured. As both
part and caliper are always to some extent
elastic, the amount of force used
affects the indication. A consistent, firm touch is correct. Too
much force results in an underindication as part and tool distort;
too little force gives insufficient contact and an overindication.
This is a greater problem with a caliper incorporating a wheel,
which lends
mechanical
advantage. This is especially the case with digital calipers,
calipers out of adjustment, or calipers with a poor quality
beam.
Simple calipers are uncalibrated; the measurement taken must be
compared against a scale. Whether the scale is part of the caliper
or not, all analog calipers—verniers and dials—require good
eyesight in order to achieve the highest precision. Digital
calipers have the advantage in this area.
Calibrated calipers may be mishandled, leading to loss of
zero. When a calipers' jaws are fully closed, it
should of course indicate zero. If it does not, it must be
recalibrated or repaired. It might seem that a vernier caliper
cannot get out of calibration but a drop or knock can be enough.
Digital calipers have zero set buttons.
Since the 1970s a clever modification of the moveable jaw on the
back side of any caliper allows for "step"-measurements. For
example: the distance from the side of a screw head to the edge of
a surface.
See also
References
- Mensun Bound: The Giglio wreck: a wreck of the Archaic
period (c. 600 BC) off the Tuscany island of Giglio, Hellenic
Institute of Marine Archaeology, Athens 1991, p.27 & 31
(Fig.65)
- Roger B. Ulrich: Roman woodworking, Yale University
Press, New Haven, Conn., 2007, ISBN 0-300-10341-7, p.52f.
- "hand tool." Encyclopædia Britannica from Encyclopædia
Britannica 2006 Ultimate Reference Suite DVD .[Accessed July 29,
2008]
- Temple, Robert. (1986). The Genius of China: 3,000 Years of
Science, Discovery, and Invention. With a forward by Joseph
Needham. New York: Simon and Schuster, Inc. ISBN 0671620282. Page
86–87.
- Joseph Wickham Roe, English and American tool builders (1916)
p. 203
- www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=47257
6" Digital Caliper
- http://www.msi-viking.com/digital_caliper/
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