Hole Standards and Inspection 141

The class “Hole Standards and Inspection” provides a comprehensive introduction to hole inspection using contact instruments. Hole inspection ensures that a hole will meet its proper job specifications, including fit, diameter, roundness, and condition. Gaging instruments, like pin and plug gages, determine fit. Variable instruments determine size and must make three points of contact to find out-of-round conditions. Variable instruments may be mechanical, electronic, optical, or pneumatic. More complex handheld devices include telescoping gages, split-ball gages, calipers, inside micrometers, and bore gages. Job specifications, environmental concerns, and economic issues all determine which hole inspection device to use. Choosing the wrong tool could result in an out-of-tolerance hole passing inspection. After taking this class, users should be able to explain how to measure common hole features with plug gages, pin gages, and calipers and verify they are within tolerance.

Class Details

Class Name:
Hole Standards and Inspection 141
Number of Lessons:

Class Outline

  • Hole Inspection
  • Common Hole Irregularities
  • Points of Contact
  • Best Practices
  • Hole Inspection Facts
  • Pin Gages and Plug Gages
  • Pin and Plug Gage Basics
  • Calipers
  • Inside and Depth Micrometers
  • Hole Inspection with Micrometers
  • Telescoping Gages
  • Telescoping Gage Hole Inspection
  • Hole Inspection Instrument Review
  • Split-Ball Gages
  • Small Hole Gage Inspection
  • Bore Gages
  • Hole Inspection Devices


  • Explain the purpose of hole inspection.
  • Identify common hole irregularities.
  • Explain how the amount of contact influences measurement.
  • Identify the best practices for hole inspection.
  • Describe pin and plug gages and how they inspect holes.
  • Describe the characteristics of calipers.
  • Describe inside and depth micrometers and how the inspect holes.
  • Describe a telescoping gage and how it inspects holes.
  • Describe the characteristics of split-ball gages and how they inspect holes.
  • Describe the characteristics of bore gages and how they inspect holes.

Job Roles


  • CNC Milling Operations
  • CNC Milling Programming, Setup, & Operation
  • CNC Milling: Operations-FastTrack
  • CNC Milling: Programming, Setup, and Operations-FastTrack
  • Job Planning, Benchwork, & Layout I
  • Job Planning, Benchwork, and Layout-FastTrack
  • Manual Milling Skills-FastTrack
  • Milling I


Vocabulary Term Definition
bore gage A mechanical or electronic contact variable measuring instrument that is handheld and provides very accurate readings of hole sizes. A bore gage makes three points of contact within the hole.
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.
chatter The development of surface imperfections on the workpiece caused by vibrations of the cutting tool if it is applied in an incorrect manner. Chatter can be the result of minor out-of-round conditions in a hole.
concavity A condition in which the surface of an object curves inward. Concavity can affect the dimensions of a hole.
contact instrument A measuring device that touches the part in order to obtain its measured value. A variable instrument must have three points of contact to detect out-of-round conditions in a hole.
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.
dial indicator A measuring instrument with a contact point attached to a spindle and gears that move a pointer on the dial. Dial indicators have graduations that show different measurement values.
extension rod A piece that is added to the head of an inside micrometer to expand it to the width of the hole. An extension rod allows the inside micrometer to measure wide holes.
go/no-go gage An instrument with an established standard size that determines whether a part feature passes or fails inspection. Go/no-go instruments do not determine the degree of variation.
inside diameter ID. The interior surface of a cylindrical workpiece or round hole. A contact instrument would physically touch this surface to inspect and measure it.
inside micrometer A mechanical or electronic, variable, handheld contact instrument used to measure the inside diameter of larger holes. An inside micrometer is usually cylindrically shaped.
linear scale A series of parallel lines that represent a measurement standard. A steel rule contains a linear scale.
linearity The amount of error change throughout an instrument's measurement range. Linearity is also the amount of deviation from an instrument's ideal straight-line performance.
lobing A condition in which the manufacturing process creates a rounded projection from what would otherwise be a circular hole. A hole may have more than one lobe.
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 before use.
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.
micrometer head The main component of an inside micrometer that includes the scale and indicating device. Extension rods are added to the micrometer head.
out-of-round An irregularly shaped circle that is not totally round. Many out-of-round conditions, such as ovality, are caused by lobing.
ovality A condition in which a hole that should be round has two opposing lobes, resulting in an elongated or egg-shaped hole. Ovality is an example of lobing.
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.
radiused Rounding on the end of a part, piece, or jaw. Telescope gages have telescoping plungers with radiused ends.
ring gages 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.
small hole gage A cylindrical device with an expanding, flat-ended ball on one end and a locking device on the other. Also called a split-ball gage, the small hole gage is is a handheld, mechanical contact instrument that is used for comparison measurement of small hole inside diameters.
split-ball gage A cylindrical device with an expanding, flat-ended ball on one end and a locking device on the other. Also called a small hole gage, a split-ball gage is a handheld, mechanical contact instrument that is used for comparison measurement of small hole inside diameters.
taper A gradual narrowing of the inside or outside surface. Taper in a hole results in a gradual decrease in diameter/width from one end to another.
telescoping gage A T-shaped measuring device that has two measuring arms and a lock in the base. A telescoping gage is a handheld, mechanical contact instrument used for comparison measurement.
thimble A ring or cylinder that fits around the spindle of a micrometer. To advance the spindle, the thimble must be turned.
tolerance range The expected range of measurements produced by a given operation. The tolerance range is also known as a tolerance zone.
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 and instead gives the actual measurement of a part feature that can then be compared with its specifications.