Thread Standards and Inspection 151
Thread Standards and Inspection explains the various parts of threads and how to inspect them. Manufacturers inspect threads according to unified or ISO standards or using System 21, System 22, and System 23. Several features must be checked to make sure that threads meet specifications. Gaging inspection tools, or go/no-go gages, simply determine whether or not a part will fit. Variable thread inspection tools determine whether a thread falls within a specified tolerance range. Thread type and specifications affect the tools used to inspect threads.
Understanding the various components and classifications used to identify threads is critical for accurate inspection. After the class, the user will be able to explain how to measure common threaded features with internal and external thread gages and verify the features are within tolerance.
Number of Lessons 21
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- Introduction to Threads
- Anatomy of a Thread
- The Measured Parts of a Thread
- Types of Thread Forms
- Additional Thread Dimensions
- Finding Start, Lead, Pitch, and Threads per Inch
- Unified Threads
- ISO Metric Threads
- Thread System Requirements
- Visual Inspection
- Gaging Inspection Tools
- Matching Thread Labels
- Using the Mating Part as a Gage
- Inspecting with the Screw Pitch Gage
- Inspecting with the Thread Ring Gage
- Inspecting with the Thread Plug Gage
- Inspecting with the Thread Snap Gage
- Variable Thread Inspection Tools
- Inspecting with a Metal Rule
- Inspecting with the Indicating Thread Gage
- Final Review
- Define thread.
- Identify the basic parts of a thread. Identify the measured parts of a thread.
- Distinguish among different thread forms.
- Identify the common values needed to determine the travel distance of a thread.
- Identify the common values needed to determine the travel distance of a thread.
- Identify the parts of a standard unified thread specification.
- Identify the parts of a standard metric thread specification.
- Distinguish among the requirements of System 21, System 22, and System 23.
- Explain the advantages of visually inspecting threads before using inspection tools.
- Describe the characteristics of gaging inspection tools.
- Describe the disadvantages of using a mating part as a gage.
- Describe how to inspect threads using a screw pitch gage.
- Describe how to inspect threads using a thread ring gage.
- Describe how to inspect threads using a thread plug gage.
- Describe how to inspect threads using a thread snap gage.
- Describe the characteristics of variable inspection tools.
- Describe how to inspect threads using a metal rule.
- Describe the differences between using the three types of indicating thread gages.
A trapezoid-shaped thread form modified from the square thread to be easier to manufacture and adjust for wear. The Acme thread is used for power and motion transfer.
An allowed difference in dimension. On a threaded part, the allowance is a specific amount of clearance or interference between threads.
A fixed jaw against which an object to be measured is placed. On a thread snap gage, the anvils are segments of threaded cylinders that have been ground to the proper specifications.
Weaponry that fires ammunition across long distances. Artillery utilizes buttress threads in its manufacture.
To fasten tightly. A no-go gage should bind within one or two turns.
An externally threaded, cylindrical fastener with a head at one end and a threaded blunt end at the other. Bolts are designed to fit into non-threaded holes to join parts and are assembled with a nut.
A thread form with an asymmetrical ridge that has one straight and one angled flank. Buttress threads are used when the screw must withstand a great deal of pressure.
The device in a lathe or turning center that supports the end of a cylindrical workpiece opposite the spindle. When an object is between centers on a lathe, it is held between the spindle and the center on the tailstock.
An unwanted piece of metal that is removed from a workpiece. Chips are the by-product of machining.
class of fit
A system that designates how loose or tight a thread should be. Class of fit is used in unified thread specifications.
Inwardly curved in shape. A full form segment is a rectangular block with a concave, threaded side.
cone and vee rolls
The part of an indicating thread gage that measures pitch diameter. The vee measures the crest flanks and the cone measures the opposite root flanks.
A top or peak. On a thread, the crest is the high point where two flanks meet.
The top-to-bottom measurement of a recess or relief. For a thread, the depth is the distance between the root and the crest.
A measuring instrument with a contact point attached to a spindle and gears that move a pointer on the dial. Indicating thread gages have a dial indicator that tells whether or not the threads fall within the allowable tolerance range.
A thread consisting of two ridges wrapped around the cylinder. A double-start threaded fastener has a lead that equals two times its pitch.
A threading error in which the crest of a thread or threads is uneven. Drunken threads can be detected with visual inspection.
A type of thread that spirals around the exterior surface of a cylindrical component, such as bolts and screws. External threads are designated with an A in unified thread specifications and with a lower-case letter in metric thread specifications.
A unified thread specification that indicates the length of a fastener. Generally, fastener length is measured from the underside of the head to the tip of the fastener.
A device that holds two or more objects together. Common fasteners include screws and bolts.
A piece of hardware used to reinforce and secure ropes. Fittings use pipe threads.
A side. On a thread, the flank connects the root and the crest.
full form segments
A rectangular block on an indicating thread gage that has a concave, threaded side. Full form segments are used to measure functional diameter.
The smallest or largest pitch diameter that fits onto a screw or into a nut, including all of the form deviations such as lead, thread angle, taper, and roundness. System 21 requires that the largest example of a part's functional diameter must fit into its mating part.
A device that determines whether or not a part feature is within a specified limit, or tolerance. Most gages do not provide an actual measurement value.
gaging inspection tools
A device with an established standard size used in the physical inspection of part features. They are also referred to as go/no-go gages.
A gage on or in which a good part should fit easily. A go gage quickly checks a part's features without providing a measurement value.
A gage used to determine whether a part feature simply passes or fails inspection. When measuring with go/no-go gages, no effort is made to determine the exact degree of error.
The use of a gage to determine whether a part feature simply passes or fails inspection. In go/no-go gaging, no effort is made to determine the exact degree of error.
Inner diameter. The measurement of a hole's interior surface. Inner diameter extends the entire length of the hole.
The angle between two defined features. On a thread, the included angle is the angle created by two adjacent flanks.
indicating thread gages
A variable thread gage that has a dial or digital readout that tells where a measurement falls within the tolerance range. Indicating thread gages show not only pitch diameter and functional diameter, but also variation as the thread is turned along the gage.
ID. The measurement of a hole's interior surface. Inner diameter extends the entire length of the hole.
A measuring device used by an inspector to pass or reject finished parts. An inspection gage is set to a higher standard than a working gage.
A type of thread that spirals around the interior of a hole. Internal threads are designated with a B in unified thread specifications and with an upper-case letter in metric thread specifications.
ISO metric threads
A thread set to a measurement standard based on millimeters. ISO metric thread standards were established by the International Organization for Standardization (ISO), which develops standards, rules, and guidelines designed to ensure that products, processes, and services are fit for their purposes.
A tool used for lifting large or heavy objects. Jacks usually use Acme threads.
A machine tool used to create cylindrical parts. A lathe holds a cylindrical workpiece on one or both ends while a single-point cutting tool is gradually passed along the surface of the rotating part.
The distance a cylinder travels in one revolution. For a thread, the lead is the distance a threaded part advances in one revolution of the thread.
The long, threaded device that controls the precise movement of the components of a machine tool. Leadscrews have either square or buttress threads.
A single segment on a screw pitch gage that is used to inspect a thread. Each leaf of a screw pitch gage is cut into teeth that match a specific thread pitch.
A thread error where the lead is wider than it should be. A long lead can be detected with visual inspection.
A simple measuring instrument consisting of a long, thin, metal strip with a marked scale of unit divisions used by a skilled machine operator. A machinist's rule comes in many sizes and forms and can be rigid or flexible.
A lens held in front of the eye to enlarge an object for the viewer. Magnifying glasses can be used when conducting a visual inspection.
The largest diameter feature of a workpiece. On a thread, the major diameter is the diameter from crest to crest of an external thread or from root to root of an internal thread.
The object that a workpiece has been manufactured to fit. Nuts and bolts are mating parts.
maximum material limits
The most amount of material that an object may have and still pass inspection. System 21 requires that threads function under maximum material limits.
A simple measuring instrument consisting of a long, thin, metal strip with a marked scale of unit divisions. Also called a steel or machinist's rule, metal rules come in many sizes and forms and can be rigid or flexible.
The letter M that begins all metric thread designations. The metric designation distinguishes metric threads from other types of thread specification.
minimum material limits
The smallest amount of material that an object may have and still pass inspection. System 22 requires that threads function under minimum material limits.
The smallest diameter feature of a workpiece. On a thread, the minor diameter is the diameter from root to root of an external thread or from crest to crest of an internal thread.
A pair of cylinders on an indicating thread gage that measures functional diameter size. The rolls make full contact with the part's threads except for a small amount of clearance at the crests and roots.
A gage on or in which a good part should not fit. A no-go gage quickly checks a part's features without providing a measurement value.
The overall stated diameter of a part that might differ from the actual diameter. Nominal diameter is one of the parts of a metric thread specification.
The overall diameter of the fastener in inches. Nominal size is one of the parts of a unified thread specification.
Outer diameter. The measurement of a cylindrical object's exterior surface. Outer diameter extends the entire length of the cylinder.
OD. The measurement of a cylindrical object's exterior surface. Outer diameter extends the entire length of the cylinder.
Extending in the same direction and equidistant at all points. Parallel lines never intersect.
A V-shaped thread that wraps around a tapered cylinder. Pipe threads are used for plumbing.
The distance between a point on an individual groove to the corresponding point on the next groove. On a thread, the pitch is the distance between two adjacent crests.
The number of threads in a specified length along a workpiece. Pitch count is used to determine pitch.
The diameter of the part measured from the position on a thread where the distance between the flanks is equal in both the ridge and the groove. Pitch diameter is larger than the minor diameter but smaller than the major diameter.
The point in a thread at which the distance between the flanks is equal in both the ridge and the groove. Pitch points are used to calculate the pitch diameter.
progressive plug gage
A cylinder of heavy metal with two sets of threads on each end. For a part to pass inspection using a progressive plug gage, it should pass over the first set of go gage threads and bind on the no-go second set.
A measuring device used to check the accuracy of other gages. Also called master gages, reference gages are set to a higher standard than inspection gages.
roll thread snap gage
A type of snap gage that has free-turning, threaded rolls that wrap around the workpiece threads.
A base. On a thread, the root is the valley where two flanks meet.
An externally threaded, cylindrical fastener with a head and a threaded flat or pointed end opposite the head. Screws are designed either to fit into a threaded hole or form threads into material.
screw pitch gages
A device with a fan of individual gages, each of which has an edge of symmetrical points calibrated to a particular thread pitch. A screw pitch gage is used to check the thread pitch of a screw.
A thread error where the lead is narrower than it should be. A short lead can be detected with visual inspection.
A thread consisting of one ridge wrapped around the cylinder. A single-start threaded fastener has a lead that equals its pitch.
The thread form most able to carry heavy loads. A square thread has a box-like ridge whose crest meets its flank at a 90° angle.
An established rule or set of expectations. The American, British, and Canadian standard for threads describes unified threads.
For threaded parts, the start is the number of individual threads that wrap around the cylinder. A fastener can either have a single-start thread or a double-start thread.
The degree of roughness and variation on the surface of a part after it has been manufactured. Due to irregularities created when machining a part, surface finish cannot be perfectly smooth.
A quality in which all the features on either side of a point, line, or plane are identical. V-shaped symmetrical threads have symmetrical flanks inclined at equal angles with a vertical centerline running through the thread crest.
A thread tolerance specification stating that threads must have interchangeable assembly and pass a go-no go inspection. System 21 deals only with specifications for the functional diameter of a thread.
A thread tolerance specification stating that thread must have interchangeable assembly and size control at minimum and maximum material limits. System 22 includes specifications for both functional diameter and pitch diameter.
A safety critical thread tolerance specification stating that threads must conform with specifications for pitch diameter, functional diameter, major diameter, minor diameter, and surface finish. System 23 most often applies to aerospace parts.
A long, spiral ridge around the exterior or interior of a cylindrically shaped object. Threads are used to fasten or provide motion.
The specific shape or profile of a thread. Thread forms include V-shaped, square, Acme, buttress, and pipe.
A measuring device used to measure the accuracy of internal or external threaded surfaces. Thread gages are available in go and no go sizes.
thread plug gage
A cylinder of heavy metal with threaded ends use to check ID threads. A thread 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. A thread ring gage is screwed onto the part being inspected as though the two are mating parts.
A thread designation that identifies the coarseness or fineness of threads. Thread series is used in unified thread specifications.
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. Thread snap gages with dial indicators are variable inspection devices.
threads per inch
tpi. The number of threads in a linear inch along the length of a workpiece. Threads per inch is used in unified thread specifications.
The total amount a specific dimension is permitted to vary. The object will still meet specifications if it remains within its tolerance.
A number and letter combination that indicates the degree of fit and whether it is an internal or external thread. Tolerance class is used in metric thread specifications.
Threads per inch. The number of threads in a linear inch along the length of a workpiece. Tpi is used in unified thread specifications.
A four-sided shape with two parallel sides having the appearance of a triangle with a flattened top. The trapezoid shape is utilized in the creation of Acme threads.
type of thread
A thread designation that describes whether a thread is external or internal. Type of thread is used in unified thread specifications.
A thread set to a measurement standard based on inches. Unified threads conform to specifications of the American Society of Mechanical Engineers (ASME) and the American National Standards Institute (ANSI), both of which set standards in the United States.
A type of inspection that reveals the degree of variation from a given standard. Variable inspection gives the actual measurement of a part feature that can then be compared with its specification.
A workholding device with one fixed jaw and one moveable jaw. Vises use buttress threads.
V-shaped symmetrical thread
A type of thread form that has symmetrical flank angles with a 60° included angle. The V-shaped thread is the most common thread form for fasteners.
A measuring device, set to a working standard, that is used to inspect parts on the production floor. Working gages are usually used by machine operators during production.