Basic Measurement 101

The class “Basic Measurement” offers an overview of common gaging and variable inspection tools and methods. Variable inspection takes a specific measurement using common devices such as calipers and micrometers. The sensitivity of the instrument must be greater than the measurement being taken. Both calipers and micrometers are read by finding the alignments in lines on the devices. Gages, such as gage blocks, plug gages, ring gages, and thread gages, reveal whether a dimension is acceptable or unacceptable without a specific quantity. All inspection devices should be properly mastered and maintained to retain accuracy. One of the fundamental activities of any shop is the measurement of part features. Consistent measurement and inspection maintains standardization and ensures that out-of-tolerance parts do not reach customers. After taking this class, users should be able to describe the use and care of common inspection instruments and gages used in the production environment.

Class Details

Class Name:
Basic Measurement 101
Number of Lessons:
Related 1.0 Classes:
Basic Measurement 110, Linear Instrument Characteristics 115

Class Outline

  • The Importance of Measurements
  • Accuracy and Precision
  • Examples of Accuracy and Precision
  • Gaging and Variable Inspection
  • Examples of Gaging vs Variable Inspection
  • Sensitivity
  • Measurement
  • The Machinist’s Rule
  • Calipers
  • Vernier Calipers
  • Micrometers
  • Micrometer Basics
  • Vernier Micrometers
  • Reading a Micrometer with a Vernier Scale
  • The Vernier Scale
  • Other Micrometers
  • Gage Blocks
  • Inspecting with Gage Blocks
  • Plug Gages
  • Ring, Thread, and Snap Gages
  • Height Gages and Granite Plates
  • Mastering and Calibration of Instruments
  • Measurement Instruments


  • Define inspection and standardization.
  • Distinguish between accuracy and precision.
  • Distinguish between gaging and variable inspection.
  • Define sensitivity in measuring instruments.
  • Describe the steel rule.
  • Describe calipers.
  • Describe the micrometer.
  • Explain the vernier scale.
  • Identify commonly used micrometers.
  • Describe gage blocks.
  • Describe go/no-go gaging with plug gages.
  • Identify other commonly used gages.
  • Describe height gages and granite plates.
  • Recognize the importance of mastering and calibration.

Job Roles


  • CNC Lathe Operations
  • CNC Milling Operations
  • CNC Milling Programming, Setup, & Operation
  • CNC Turning Programming, Setup, & Operation
  • Drill Press I
  • Grinding I
  • Job Planning, Benchwork, & Layout I
  • Measurement, Materials, & Safety I
  • Milling I
  • Turning Operations: Turning Chucking Skills


Vocabulary Term Definition
accuracy The difference between a measurement reading and the true value of that measurement. The less error present in the measurement, the more accurate the results.
alloy steel Steel that contains added materials that change the property of the metal. Common alloy elements include chromium, manganese, molybdenum, and nickel.
blade micrometer A type of micrometer with flattened tips on the anvil and spindle. The blade micrometer is primarily used to measure the diameter of narrow grooves and slots.
calibration The comparison and adjustment of a device with unknown accuracy to a device with a known, accurate standard. Calibration eliminates any variation in the device being checked.
calipers A measuring instrument with a pair of jaws on one end and a long beam containing a marked scale of unit divisions. The jaws can measure both internal and external features.
carbide A compound developed by the combination of carbon, usually with chromium, tungsten, or titanium. Carbide materials are very hard and wear resistant.
depth micrometer A type of micrometer with a spindle perpendicular to a flat base. The depth micrometer is primarily used to measure the depth of holes.
English system A system of measurements based on the inch, pound, and degrees Fahrenheit primarily used in the United States and England. It is also known in the United States as the U.S. Customary System.
gage block A hardened steel block manufactured with highly accurate dimensions that is used to measure part dimensions after a part is made. Gage blocks are available in a set of standardized lengths.
gaging The physical inspection of part features using a device with an established standard size. Gaging results in a pass/fail decision.
go/no-go gaging The use of a gage to determine whether a part feature simply passes or fails inspection. No effort is made to determine the exact degree of error.
granite A dense, hard type of rock that exhibits excellent wear resistance, stability, and flatness. Granite tables and surface plates are used for various measuring applications such as inspection surfaces and for mounting devices.
groove micrometer A type of micrometer with a long stem and two small discs at the end. The groove micrometer is primarily used to measure the width and position of internal grooves.
height gage A measuring device with a column mounted on a base such as a granite surface plate, a unit that slides up and down, an indicator, and an arm that extends out. Height gages are used to measure vertical and other distances.
inspection The examination of a part during or after its creation to confirm that it adheres to specifications. During inspection, defects may be identified and corrected.
lapped Polished with an abrasive paste to remove the last bit of unwanted material. Gage blocks are lapped by hand or machine.
machinist’s rule A simple measuring instrument consisting of a long, thin metal strip with a marked scale of unit divisions used by a skilled machine operator. The steel rule comes in many sizes and forms and can be rigid or flexible.
mastering A quick check of a measuring device against a known standard reference, such as a gage block, to determine if the device is reading the measurement correctly. Instruments should be mastered on a regular basis.
Metric system An internationally recognized standard system of measurements based on the meter, kilogram, and degrees Celsius.
micrometer A U-shaped measuring instrument with a threaded spindle that slowly advances toward a small anvil. Micrometers are available in numerous types for measuring assorted dimensions and features.
pin gage A type of gage available in a set and used like a plug gage. Pin gages are available in different ranges from .028 inches to 1.000 inch sizes in increments of 0.001 inch and are useful when measuring maximum and minimum hole sizes.
plug gage A hardened, cylindrical gage used to inspect the size of a hole. Plug gages are available in standardized diameters.
precision The degree to which an instrument will repeat the same measurement over a period of time. Precision is also called repeatability, as it will show the same results under unchanged conditions.
ring gage A hardened, round gage with a hole used to inspect the size of cylindrical parts or features. Ring gages are often in go/no-go pairs.
rule of ten The inspection guideline that states that a measuring instrument must be ten times more precise than the acceptable tolerance of the inspected part feature. According to the rule of ten, a part with measurements to the thousandths place must be measured with an instrument sensitive to the ten-thousandths place, like a vernier micrometer.
sensitivity The smallest change in a measurement that an instrument is capable of detecting. Sensitivity is one of the qualities of measuring instruments.
snap gage A U-shaped gage with hardened, adjustable anvils on opposite ends used to inspect the length of part features. The snap gage is a form of go/no-go gage.
spherical ball micrometer A type of micrometer that is rounded for a precise measurement of any curved surface part. The spherical ball micrometer is more accurate than the standard flat pin micrometer and more durable than the point micrometer.
standardization The development of universally recognized units of measurement. Standardization ensures that parts of the same size are interchangeable.
steel rule A simple measuring instrument consisting of a long, thin metal strip with a marked scale of unit divisions. The steel rule comes in many sizes and forms and can be rigid or flexible.
stock Raw material that is used to make manufactured parts. Stock is available in standard shapes such as long bars, plates, or sheets.
surface plate A hard, flat surface, usually made of granite, which is used as a base for mounting gages. A height gage must be mounted on a surface plate.
thread A long, spiral ridge around the exterior or interior of a cylindrically shaped object. Threads are used to fasten or provide motion.
thread gage A measuring device that measures the accuracy of internal or external threaded surfaces. Thread gages are available in go and no go sizes.
tolerance An unwanted but acceptable variation or deviation from a desired dimension of a part. The part will still meet specifications.
variable inspection A type of inspection that reveals the degree of variation from a given standard. Unlike gaging, variable inspection does not result in a pass/fail decision, but instead gives the actual measurement of a part feature that can then be compared with its specification.
vernier scale A type of scale consisting of two opposing line markings with different divisions. Vernier scales appear on both manual and digital calipers and micrometers.
wear allowance The slight amount of material intentionally remaining on a gage to prevent the passing of defective parts. Calibration must correct for wear of a gage over time.
wring To twist and rub together so that the two surfaces cling to one another. Gage blocks are wrung together in various combinations to form any length.