Measurement is the comparison of an unknown dimension to a known
standard. Good measuring instruments were a key to high volume
production. Without them, parts could not be built accurately enough to
be interchangeable. Each assembly had to be hand fitted together.
Today, measuring tools are essential for most machining operations from
initial part layout to final inspection.
The figure below depicts a caliper. It can measure lengths from 0 to
7.5 inches to a precision of one thousandth of an inch. One can measure
the outside of a part with the jaws, the inside of a hole or slot with
the nibs, or the depth of a hole or shoulder with the extension bar.
This particular one is has a vernier scale. It takes a little practice
to read it properly. Calipers often have a dial or digital readouts
A Vernier Caliper
To read a vernier caliper:
An example follows:
- Read the large number division first.
- Read the small number division.
- Read the number of smaller subdivisions. Each represents 0.025 inches
to be added to the measurement.
- Read which line on the vernier lines up with a line on the main beam.
For each line a thousanth must be added to the measurement.
Reading a Vernier Scale - An Example
A micrometer generally provides greater precision than a caliper, but
can measure a smaller range of lengths. A micrometer is depicted in the
Parts of a Micrometer
To use a micrometer, place the part in the opening. Next, turn the
thimble until the spindle contacts the work. To apply a consistent
pressure to the part, use the ratchet stop. Use the clamp ring to hold
the thimble in place while you read the micrometer. To read the
An example of reading the micrometer follows:
- Read the exposed number on the barrel.
- Read the number of divisions past the number. Each division represents
- Read the division on the spindle. These usually read to less than
thousandths of an inch.
Reading a Micrometer - An Example
On to part layout.
Return to the machine shop.
Return to the Mechanical Engineering Department.