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Manual Machining

Threading on the Engine Lathe 301

The class Threading on the Engine Lathe explains threads and how to make them using a manual lathe. Engine lathes can create both external (OD) and internal (ID) threads classified according to either Unified or ISO metric standards. To manually machine a thread a machinist must know how to set up and use a threading tool, compound rest, and quick-change gearbox before engaging the threading feed by moving the half-nut lever to the proper line on the threading dial. Machinists should also know how to perform initial operations such as chamfering and making an undercut. In order to cut threads successfully on the lathe, an operator must understand the various lathe components and processes used for threading. After taking this class, users should be able to describe best practices for external and internal threading on an engine lathe.

Difficulty Advanced
Format Online
Number of Lessons 16
Language English

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Threading on the Engine Lathe
Thread Parts
Review: Thread Parts
Thread Variations
Unified vs. ISO threads
Lathe Threading Components
Review: Thread Variations and Lathe Components
Determining Thread Depth
Pre-Machining for External Threads
External Threading Setup
Cutting External Threads
Inspecting External Threads
Review: External Threading
Setup for Internal Threads
Cutting Internal Threads
Cumulative Review

Describe threading.
Identify the basic parts of a thread.
Describe common thread variations.
Distinguish between Unified and ISO threads.
Identify engine lathe components used for threading.
Demonstrate the steps necessary to determine thread depth.
Describe necessary operations prior to external threading.
Describe setup for external threading on the lathe.
Describe steps for cutting external threads on a part.
Describe instruments commonly used to inspect external threads.
Describe setup prior to internal threading.
Describe steps for cutting internal threads on a part.

Vocabulary Term

Definition

allowance

An allowed difference in dimension. On a threaded part, the allowance is a specific amount of clearance or interference between threads.

anvil

A fixed jaw against which an object to be measured is placed. On a micrometer, the anvil is an immobile block from which the measurement is taken.

blade micrometer

A type of micrometer with flattened tips on the anvil and spindle that is primarily used to measure narrow external grooves. A blade micrometer is often used to measure an undercut.

carriage

A component of the lathe that slides along the ways parallel to the spindle axis. Carriages support the cross slide, compound rest, and tool post.

center gage

An instrument that checks the angle of a cutting tool to a workpiece. Center gages help ensure the cutting tool is perpendicular to the workpiece for threading operations on a lathe.

chamfer

A small angled surface added to the end of a shaft, around the opening of a hole, or along an edge. A chamfer eliminates a sharp corner, removes burrs, and facilitates assembly.

chase

Threading over existing threads. Chasing occurs to deepen existing threads or repair damaged threads.

class of fit

A system that designates how loose or tight a thread should be. Class of fit is used in Unified thread specifications.

constant value

A number or variable that does not change. A series of constant values are used to determine thread depth.

crest

The top or peak of an object. On a thread, the crest is the highest point where two flanks meet.

cross feed

Movement of the cutting tool perpendicular to the axis of the spindle. Cross feed on the lathe is controlled by the cross slide.

depth

The top-to-bottom measurement of a recess or relief. For a thread, the depth is the distance between the root and the crest.

end gears

A set of controls on the quick-change gearbox that transmit power from the lathe spindle to the leadscrew. End gear position determines the ratio between the spindle speed and the speed of the leadscrew.

engine lathe

The original and most basic type of manual lathe. An engine lathe uses a single-point cutting tool to remove material from a rotating cylindrical workpiece.

external thread

A type of thread that spirals around the exterior surface of a cylindrical component, such as bolts and screws. An external thread is designated with an A in Unified thread specifications and with a lowercase letter in metric thread specifications.

facing

A lathe cutting operation that feeds a cutting tool across an end of a cylindrical workpiece to create a flat surface and shorten its length. Facing is an outer diameter operation.

fastener length

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.

feed change lever

A lever that is used to select one of three types of feed: longitudinal feed, cross feed, or threading feed. The feed change lever is located on the apron of the lathe.

flank

The side of an object. On a thread, the flank is the angled surface that connects the root and the crest.

go gage

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.

go/no-go gages

An instrument with an established standard size that determines whether a part feature passes or fails inspection. Go/no-go gages do not determine the degree of variation.

grooving

A lathe cutting operation that cuts a narrow channel into a rotating cylindrical workpiece. Grooving can be an inner or outer diameter operation.

half-nut lever

A lever that is engaged when the line on the threading dial matches up with the proper reference mark. The half-nut lever is attached to the side of the apron of the lathe and is only used in threading operations.

helical

Winding in the shape of a spiral or coil. Helical grooves, called threads, appear on screws and other fasteners.

included angle

The angle between two defined features. On a thread, the included angle is the angle created by two adjacent flanks.

inner diameter

ID. The diameter measurement of a hole’s interior surface. An inner diameter measurement extends the entire length of the hole.

internal thread

A type of thread that spirals around the interior of a hole. An internal thread is designated with a B in Unified thread specifications and with an uppercase 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.

lead-in

An angled surface on the end of a cylindrical workpiece. Lead-ins prevent tool and workpiece damage when a cutting tool first engages the workpiece during cutting.

leadscrew

The long, threaded device that controls the precise movement of the carriage of the lathe. The ratio between the spindle speed and the speed of the leadscrew determines the threads-per-inch or pitch of a thread.

left-hand thread

A type of thread that is turned in a counterclockwise direction. Left-hand threads are rare.

longitudinal feed

Movement of the cutting tool parallel to the axis of the spindle. Longitudinal feed on the lathe is controlled by the carriage.

major diameter

The largest diameter feature of a workpiece. Major diameter is the diameter from crest to crest of an external thread or root to root of an internal thread.

major diameter

The largest diameter of a thread. Major diameter measures the distance from crest to crest of an external thread or root to root of an internal thread.

metric designation

The letter M that begins all metric thread designations. The metric designation distinguishes metric threads from other types of thread specification.

minor diameter

The smallest diameter feature of a workpiece. Minor diameter is the diameter from root to root of an external thread or from crest to crest of an internal thread.

no-go gage

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.

nominal size

The overall diameter of an object. Nominal size is one of the parts of a ISO metric thread specification, measured in millimeters.

outer diameter

OD. The diameter measurement of a cylindrical object’s exterior surface. The measurement of the outer diameter extends the entire length of the cylinder.

pipe threads

A V-shaped thread that wraps around a tapered cylinder. Pipe threads are used for plumbing.

pitch

The distance between a point on an individual groove to the corresponding point on the next groove. Pitch describes the distance between threads.

pitch diameter

An identifying dimension of threaded objects or components. Pitch diameter is a theoretical dimension halfway between major and minor diameter.

quick-change gearbox

The location of the gears that control the power being transferred from the lathe motor to the spindle. The quick-change gearbox allows the spindle speed and rotation to be adjusted quickly.

right-hand thread

A type of thread that is turned in a clockwise direction. Most fasteners have right-hand threads.

root

The base or bottom of an object. On a thread, the root is the lowest point where two flanks meet.

specifications

The design parameters that set the limits of acceptable deviation for a part’s intended application. Specifications are established either in a blueprint or directly in CAD/CAM software.

square

A 90° orientation to another object. A cutting tool that is square to a workpiece is perpendicular to the workpiece axis or surface.

thread micrometer

A U-shaped measuring instrument with a V-shaped anvil and a cone-shaped spindle. A thread micrometer is used to measure a thread's pitch diameter.

thread plug gage

A hardened, cylindrical go/no-go gage used to inspect the fit of threads in a hole. Thread plug gages are available in standardized sizes.

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 series

A thread designation that identifies the coarseness or fineness of threads. Thread series is used in Unified thread specifications.

threading

The process of cutting a long, spiraling groove into a cylindrical workpiece with a single-point tool. Threading can be an inner or outer diameter operation.

threading dial

A rotating dial with lines and numbers that is used to precisely set the leadscrew and carriage positions in relationship to the workpiece. The threading dial allows the lathe to repeatedly return to the same position.

threading tool

A tool designed to cut threads into the interior or exterior of a workpiece. Threading tools can be used to make fasteners such as bolts or screws.

threading tool

A type of single-point tool fitted with a tool insert used for threading and/or grooving operations. The shape of a threading tool depends on the type of thread being machined.

threads

A long, spiral ridge around the exterior or interior of a cylindrical object. Threads are used to fasten or provide motion.

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.

tolerance class

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.

turning

A machining operation that rotates a cylindrical workpiece while a single-point tool is guided along the length of the part. Turning is performed on a lathe.

type of thread

A thread designation that describes whether a thread is external or internal. Type of thread is used in Unified thread specifications.

undercut

A recessed diameter machined at the end of a thread. Undercut is a visual reference that indicates the end of the thread.

Unified threads

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.

V-shaped symmetrical threads

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.

zero

To establish a starting point on a workpiece or machine by setting the coordinate values to zero. To zero, or set zero, an operator positions the tool in a location and then resets the feed dials or the digital readout (DRO) display to zero.