Overview of Threads 150

This class describes the various parts of a screw thread, common thread standards and tolerances, and the various tools used to inspect them.

Class Details

Class Name:
Overview of Threads 150
This class describes the various parts of a screw thread, common thread standards and tolerances, and the various tools used to inspect them.
Number of Lessons:
Additional Language:
Related 2.0 Class:
Thread Standards and Inspection 151

Class Outline

  • Objectives
  • What Is a Thread?
  • Types of Threaded Parts
  • Anatomy of a Thread
  • Measured Parts of a Thread
  • Start, Pitch, and Lead
  • Types of Thread Forms
  • Thread Manufacturing Processes
  • Thread Cutting
  • Thread Rolling
  • Thread Tapping
  • Thread Grinding
  • History of Thread Standards
  • Thread Standards and Tolerance Systems
  • Thread Measurement and Inspection Devices
  • Gaging Thread Inspection Tools
  • Variable Thread Inspection Tools
  • Summary


  • Define thread.
  • Describe the purposes of threads.
  • Identify the basic parts of a thread.
  • Identify the measured parts of a thread.
  • Identify the common values needed to determine the travel distance of a thread.
  • Describe how thread form influences its purpose.
  • Identify major thread manufacturing processes.
  • Describe characteristics of a thread made by cutting.
  • Describe characteristics of a thread made by rolling.
  • Describe characteristics of a thread made by tapping.
  • Describe characteristics of a thread made by grinding.
  • Describe the factors that necessitated thread standards.
  • Identify the major thread standards.
  • Distinguish between variable inspection and go-no go gaging.
  • List gaging thread inspection tools.
  • List variable thread inspection tools.

Job Roles


  • Metalforming I
  • MSSC Quality Practices and Measurement


Vocabulary Term Definition
Acme thread A trapezoid-shaped thread used for power and motion transfer.
auger A hand-powered tool that drives a drill bit used to make holes in the ground.
blank Generally, a piece of metal with a basic geometric form that is machined into a workpiece. Screws start as blank cylinders.
bolt A cylindrically shaped, threaded device used for fastening parts. Bolts usually have blunt ends and mate with a nut.
buttress thread A thread with an asymmetrical ridge that has one straight and one angled flank.
casting The process of pouring a liquid material into a mold until it cools or dries into a solid form.
center A hardened, pointed, cylindrical component used to hold the end of a workpiece by inserting its tip into a matching hole in the part.
centerless grinding A type of grinding in which cylindrical parts are not held between centers but are supported on a work rest and rotated.
chip An unwanted piece of metal that is removed from a workpiece. Chips are formed when a tool cuts or grinds metal.
chuck A device that holds a workpiece in place as it rotates. The chuck commonly has three or four jaws that can be adjusted to fit various sizes.
cold forming The shaping of a metal at room temperature.
crest The top where two flanks meet on a thread.
depth The length of the space measured from the base of the root to an imaginary line spanning the crests of a thread.
die A component with specific dimensions used to shape metal through forming or cutting when a part is forced through or against it.
double-start thread Having two threads wrapped around the cylinder.
fatigue resistance Opposition to breaking down or failing from use.
flank An angled side of the body of a thread that connects the root to the crest.
gaging inspection The physical inspection of part features using a device with an established standard size. Gaging results in a pass/fail or go-no go decision.
go-no go 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.
grinding wheel A grinding tool made by bonding abrasive grits together and forming them into a circular shape.
helical Winding around like a spiral, spring, or coil.
included angle The angle created by two adjacent flanks.
inside diameter The interior surface of a spherical or cylindrical object.
International Organization for Standardization An organization based in Switzerland that develops and publishes standards for its international membership base. It is often referred to as ISO.
ISO metric thread A standard thread form based on millimeters that was developed for use in Europe.
lapping An abrasive process that uses an abrasive paste to produce refined surfaces.
lathe A machine tool commonly used to create cylindrical forms. A lathe holds a cylindrical workpiece on one or both ends while the cutting tool is gradually passed along the surface of the rotating part.
lead The distance that a screw travels in one revolution.
major diameter The boundary of the crests of an OD thread or the roots of an ID thread. Also referred to as the major cylinder.
metal rule A very precise metal ruler with an inch scale on one side and a metric scale on the other. Metal rules can be used to inspect OD threads.
metric thread A thread made to a scale that is based on millimeters.
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.
mill A multi-point cutting tool that is used to remove metal from the surface of a workpiece.
minor diameter The boundary of the roots of an OD thread or the crests of an ID thread. Also called the minor cylinder.
multi-point tool A cutting tool with more than one cutting edge used to remove material.
multi-rib grinding wheel An abrasive tool with a tooth-like profile, giving it multiple surfaces on the outer diameter with which to grind multiple grooves at a time.
nut A block, usually of metal, with a threaded hole that mates with a bolt. The outer shape of a nut is often six-sided.
optical comparator A sophisticated measuring instrument that projects an image of a part onto a screen to compare the shape, size, and location of its features.
outer diameter The exterior surface of a spherical or cylindrical object.
pass The single movement of a cutting tool across the workpiece, from one end to another.
pipe thread A V-shaped thread that wraps around a tapered pipe. Pipe threads are used for plumbing.
pitch The distance from one thread groove to the next measured from crest to crest.
pitch diameter The measured distance between the pitch points taken in the groove between the threads. Pitch diameter is one of the most important dimensions in thread inspection.
pitch point The position on the thread where an equal amount of distance exists between the flanks on the solid part of the thread and in the space between the threads.
power screw A threaded device used to transfer power or motion. Power screw threads often have a square or trapezoidal profile.
regulating wheel A wheel, usually made of plastic or rubber bond, used during centerless grinding to rotate the workpiece and pull it through the operation.
root The base where two flanks meet on a thread.
screw A threaded device used for fastening parts or transferring motion. Screws usually have pointed ends.
screw pitch gage A fan-like device comparable to a set of keys, each of which has an edge of symmetrical points calibrated to a particular thread pitch.
single-point tool A tool with just one cutting edge.
single-rib grinding wheel An abrasive tool that has just one surface on its outer diameter with which to grind one groove at a time.
single-start thread Having a single thread wrapped around the cylinder.
square thread A thread with box-like ridges. The crests are at 90° angles from the flanks.
start The number of individual threads on a device.
tap A cylindrically shaped, threaded device that either cuts or presses threads into a pre-drilled hole.
thread A raised, helical rib or ridge around the exterior of a cylindrically shaped object or the interior of a hole. Threads are found on screws, nuts, and bolts.
thread blank A cylinder of smooth metal into which threads are cut, rolled, pressed, or ground.
thread cutting A method of producing screw threads that uses a single-point tool to cut a blank or workpiece as it rotates on a lathe.
thread form The shape or profile of a thread. Thread forms include V-shaped threads or square threads.
thread grinding A precise method of producing threads that uses an abrasive wheel to either cut threads into solid metal or to refine threads that have been produced through other means.
thread micrometer A U-shaped measuring instrument with a V-shaped anvil and a cone-shaped spindle used to measure threads.
thread milling A method of producing screw threads that uses a rotating, multi-point tool to cut a blank or workpiece.
thread plug gage A cylinder of heavy metal with threaded ends used to check ID threads. A plug gage is also used to calibrate thread ring gages.
thread ring gage A disk of heavy metal with a central, threaded hole made to match a particular OD thread. The gage is screwed onto the part being inspected as though the two are mating parts.
thread rolling A cold-forming process that uses hard dies to press threads into a solid blank or workpiece.
thread snap gage A C-shaped device designed with either an upper and lower roll wiggled into place or one stationary roll and two go and no-go rolls, respectively. Those with dial indicators are variable inspection devices.
thread tapping A method of producing ID threads using a threaded tool to either cut or form threads in a pre-drilled hole.
three-wire measurement A thread inspection method that uses three wires of equal diameter placed at three different points on a thread. The inspector places a micrometer in simultaneous contact with the wires and takes measurements.
tolerance An unwanted but acceptable deviation from a given dimension. Tolerances indicate the allowable difference between a physical feature and its intended design.
trapezoid shape A four-sided shape with two parallel sides having the appearance of a triangle with a flattened top.
Unified screw thread A standard thread form based on inches that was developed by the U.S., Canada, and Great Britain.
variable inspection The inspection of part features using an instrument calibrated in standard measurement units. Variable inspection reveals the degree of variation from a given standard.
work harden To strengthen metal through repeated bending and shaping.
work rest A part of a centerless grinding machine that supports the workpiece as it is ground.
workpiece A part that is being machined or otherwise worked on.