Thread cutting
Encyclopedia
Threading is the process of creating a screw thread
. More screw threads are produced each year than any other machine element
. There are many methods of generating threads, including subtractive methods (many kinds of thread cutting and grinding, as detailed below); deformative or transformative methods (rolling and forming; molding and casting); additive
methods (such as 3D printing
); or combinations thereof.
In general, certain thread-generating processes tend to fall along certain portions of the spectrum from toolroom
-made parts to mass-produced parts, although there can be considerable overlap. For example, thread lapping following thread grinding would fall only on the extreme toolroom end of the spectrum, while thread rolling is a large and diverse area of practice that is used for everything from microlathe leadscrews (somewhat pricey and very precise) to the cheapest deck screws (very affordable and with precision to spare).
Threads of metal fasteners are usually created on a thread rolling machine. They may also be cut with a lathe
, tap or die. Rolled threads are stronger than cut threads, with increases of 10% to 20% in tensile strength and possibly more in fatigue resistance and wear resistance.
s, hand taps do not automatically remove the chips
they create. A hand tap cannot cut its threads in a single rotation because it creates long chips which quickly jam the tap (an effect known as "crowding"), possibly breaking it. Therefore, in manual thread cutting, normal wrench usage is to cut the threads 1/2 to 2/3 of a turn (180 to 240 degree rotation), then reverse the tap for about 1/6 of a turn (60 degrees) until the chips are broken by the back edges of the cutters. It may be necessary to periodically remove the tap from the hole to clear the chips, especially when a blind hole is threaded.
For continuous tapping operations (i.e., power tapping) specialized spiral point or "gun" taps are used to eject the chips and prevent crowding.
to produce a thread form on a cylinder or cone. The tool moves linearly while the precise rotation of the workpiece determines the lead of the thread. The process can be done to create external or internal threads (male or female). In external thread cutting, the piece can either be held in a chuck
or mounted between two centers
. With internal thread cutting, the piece is held in a chuck. The tool moves across the piece linearly, taking chips off the workpiece with each pass. Usually 5 to 7 light cuts create the correct depth of the thread.
The coordination of various machine elements including leadscrew
, slide rest, and change gears was the technological advance that allowed the invention of the screw-cutting lathe
, which was the origin of single-point threading as we know it today.
Today engine lathes and CNC
lathes are the commonly used machines for single-point threading. On CNC machines, the process is quick and easy (relative to manual control) due to the machine's ability to constantly track the relationship of the tool position and spindle position (called "spindle synchronization"). CNC software includes "canned cycles", that is, preprogrammed subroutines, that obviate the manual programming of a single-point threading cycle. Parameters are entered (e.g., thread size, tool offset, length of thread), and the machine does the rest.
All threading could feasibly be done using a single-point tool, but because of the high speed and thus low unit cost of other methods (e.g., tapping, die threading, and thread rolling and forming), single-point threading is usually only used when other factors of the manufacturing process happen to favor it (e.g., if only a few threads need to be made, if an unusual or unique thread is required, or if there is a need for very high concentric
ity with other part features machined during the same setup).
with a rotating milling cutter
if the correct helical
toolpath can be arranged. This has been possible mechanically since the early nineteenth century, but it was never a commonplace method of threading until the widespread dissemination of affordable, fast, precise CNC. Since that development, internal and external threads are often milled. Some advantages of thread milling, as compared to single-point cutting, are a better surface finish
; improved concentricity in some cases; and that a left- or right-hand thread can be created with the same tool. Additionally, for large, awkward workpieces (such as a fire hydrant
casting), it is simply easier to let the workpiece sit stationary on a table while all needed machining operations are performed on it with rotating tools, as opposed to rigging it up for rotation around the axis of each set of threads (that is, for the "arms" and "mouth" of the hydrant).
There are various types of thread milling, including several variants of form-milling and a combination of drilling and threading with one cutter, called thrilling.
Form-milling uses either a single- or multiple-form cutter. In one variant of form-milling, the single-form cutter is tilted to the helix angle
of the thread and then fed radially into the blank. The blank is then slowly rotated as the cutter is precisely moved along the axis of the blank, which cuts the thread into the blank. This can be done in one pass, if the cutter is fed to the full thread depth, or in two passes, with the first not being to the full thread depth. This process is mainly used on threads larger than 1.5 in (3.8 cm). It is commonly used to cut large-lead
or multiple-lead threads. A similar variant using a multiple-form cutter exists, in which the process completes the thread in one revolution around the blank. The cutter must be longer than the desired thread length. Using a multiple-form cutter is faster than using a single-form cutter but it is limited to threads with a helix angle less than 3°. It is also limited to blanks of a substantial diameter and no longer than 2 in (5.1 cm).
Another variant of form-milling involves holding the cutter's axis orthogonally (no canting to the thread's helix angle) and feeding the cutter in a toolpath that will generate the thread. The part is usually a stationary workpiece, such as a boss
on a valve body (in external thread milling) or a hole in a plate or block (in internal thread milling). This type of thread milling uses essentially the same concept as contouring with an endmill or ball-nose mill, but the cutter and toolpath are arranged specifically to define the "contour" of a thread. The toolpath is achieved either using helical interpolation (which is circular interpolation in one plane [typically XY] with simultaneous linear interpolation along a third axis [typically Z]; the CNC control model must be one that supports using the third axis) or a simulation of it using extremely small increments of 3-axes linear interpolation (which is not practical to program manually but can be programmed easily with CAD/CAM software). The cutter geometry reflects the thread pitch but not its lead; the lead (thread helix angle) is determined by the toolpath. Tapered threads can be cut either with a tapered multiple-form cutter that completes the thread in one revolution using helical interpolation, or with a straight or tapered cutter (of single- or multiple-form) whose toolpath is one or more revolutions but cannot use helical interpolation and must use CAD/CAM software to generate a contour-like simulation of helical interpolation.
The tooling used for thread milling can be solid or indexable. For internal threads, solid cutters are generally limited to holes larger than 6 mm (0.236220472440945 in), and indexable internal thread cutting tools are limited to holes larger than 12 mm (0.47244094488189 in). The advantage is that when the insert wears out it is easily and more cost effectively replaced. The disadvantage is the cycle time is generally longer than solid tools. Note that solid multiple-form thread cutting tools look similar to taps, but they differ in that the cutting tool does not have a backtaper and there is not a lead-in chamfer. This lack of a lead-in chamfer allows the threads to be formed within one pitch length of the bottom of a blind hole.
Thrilling is the process of drilling and threading internal threads using a specialized cutting tool on a CNC mill. The cutting tool tip is shaped like a drill, while the shank has a thread shaped form. The cutter first plunges to drill the hole. Then the thread is circularly interpolated just like the multiple-form cutter described above. The advantage is this process eliminates a tool, tool holder, and tool change. The disadvantage is that the process is limited to hole depth no greater than three times the diameter of the tool.
using specially dressed grinding wheel
s matching the shape of the threads. The process is usually used to produce accurate threads or threads in hard materials; a common application is ball screw mechanisms. There are three types: center-type grinding with axial feed, center-type infeed thread grinding and centerless thread grinding. Center-type grinding with an axial feed is the most common of the three. It is similar to cutting a thread on a lathe with a single-point cutting tool, except the cutting tool is replaced with a grinding wheel. Usually a single ribbed wheel is used, although multiple ribbed wheels are also available. To complete the thread multiple passes are commonly required. Center-type infeed thread grinding use a grinding wheel with multiple ribs that is longer than the length of the desired thread. First, the grinding wheel is fed into the blank to the full thread depth. Then the blank is slowly rotated through approximately 1.5 turns while axially advancing through one pitch per revolution. Finally, the centerless thread grinding process is used to make head-less set screw
s in a similar method as centerless grinding
. The blanks are hopper-fed to the grinding wheels, where the thread is fully formed. Common centerless thread grinding production rates are 60 to 70 pieces per minute for a 0.5 in (1.3 cm) long set screw.
in order to achieve the highest precision and surface finish achievable. This is a toolroom
practice when the highest precision is required, rarely employed except for the leadscrews
or ballscrews
of high-end machine tools.
(EDM) into hard materials using a sinker style machine.
. These processes are used for large production runs because typical production rates are around one piece per second. Forming and rolling produce no swarf
and less material is required because the blank size starts smaller than a blank required for cutting threads; there is typically a 15 to 20% material savings in the blank, by weight. A rolled thread can often be easily recognized because the thread has a larger diameter than the blank rod from which it has been made; however, necks and undercuts
can be cut or rolled onto blanks with threads that are not rolled. Also, the end of the screw usually looks a bit different from the end of a cut-thread screw.
Materials are limited to ductile materials because the threads are cold formed. However, this increases the thread's yield strength, surface finish, hardness, and wear resistance. Also, materials with good deformation characteristics are necessary for rolling; these materials include softer (more ductile) metals and exclude brittle
materials, such as cast iron
. Tolerances are typically ±0.001 in. (±0.025 mm), but tolerances as tight as ±0.0006 in (±0.015 mm) are achievable. Surface finishes range from 6 to 32 micro-inches.
There are four main types of thread rolling, named after the configuration of the dies
: flat dies, two-die cylindrical, three-die cylindrical, and planetary dies. The flat die system has two flat dies, the bottom one is held stationary and the other slides. The blank is placed on one end of the stationary die and then the moving die slides over the blank, which causes the blank to roll between the two dies forming the threads. Before the moving die reaches the end of its stroke the blank rolls off the stationary die in a finished form. The two-die cylindrical process is used to produce threads up to 6 in (15.2 cm) in diameter and 20 in (50.8 cm) in length. There are two types of three-die processes; the first has the three dies move radially out from the center to let the blank enter the dies and then closes and rotates to roll the threads. This type of process is commonly employed on turret lathe
s and screw machine
s. The second type takes the form of a self-opening die head
. This type is more common than the former, but is limited by not being able form the last 1.5 to 2 threads against shoulders. Planetary dies are used to mass produce threads up to 1 in (2.5 cm) in diameter.
Thread forming is performed using a , or , which closely resembles a cutting tap without the flutes. There are lobes periodically spaces around the tap that actually do the thread forming as the tap is advanced into a properly sized hole. Since the tap does not produce chips, there is no need to periodically back out the tap to clear away chips, which, in a cutting tap, can jam and break the tap. Thus thread forming is particularly suited to tapping blind holes, which are tougher to tap with a cutting tap due to the chip build-up in the hole. Note that the tap drill size differs from that used for a cutting tap and that an accurate hole size is required because a slightly undersized hole can break the tap. Proper lubrication is essential because of the frictional forces involved, therefore a lubricating oil
is used instead of cutting oil
.
When considering the blank diameter tolerance, a change in blank diameter will affect the major diameter by an approximate ratio of 3 to 1. Production rates are usually three to five times faster than thread cutting.
and molding
the threads are directly formed by the geometry of the mold cavity in the mold
or die
. When the material freezes in the mold, it retains the shape after the mold is removed. The material is heated to a liquid, or mixed with a liquid that will either dry or cure (such as plaster or cement). Alternately, the material may be forced into a mold as a powder and compressed into a solid, as with graphite
.
Although the first thoughts that come to mind for most machinists regarding threading are of thread cutting processes (such as tapping, single-pointing, or helical milling), Smid points out that, when plastic bottles for food, beverages, personal care products, and other consumer products are considered, it is actually plastic molding that is the principal method (by sheer volume) of thread generation in manufacturing today. Of course, this fact highlights the importance of the moldmaker
s getting the mold just right (in preparation for millions of cycles, usually at high speed).
Cast threads in metal parts may be finished by machining, or may be left in the as-cast state. (The same can be said of cast gear
teeth.) Whether or not to bother with the additional expense of a machining operation depends on the application. For parts where the extra precision and surface finish is not strictly necessary (although it might be nice), the machining is forgone in order to achieve a lower cost. With sand casting parts this means a rather rough finish; but with molded plastic or die-cast metal, the threads can be very nice indeed straight from the mold or die. A common example of molded plastic threads is on soda (pop) bottles. A common example of die-cast threads is on cable gland
s (connectors/fittings).
, of which there are many variants, including fused deposition modeling
, direct metal laser sintering
, 3D printing
, solid freeform fabrication, layered object manufacturing, and stereolithography
. Most additive technologies are still on the laboratory end of their historical development, but further commercialization
is picking up speed. Additive methods today generally produce a rough surface finish
, which suggests that their earliest commercial wins will be in parts that don't require secondary finishing by subtractive methods.
, desktop manufacturing, direct manufacturing
, direct digital manufacturing, digital fabrication, instant manufacturing, or on-demand manufacturing.
of the finished screw threads can be achieved in various ways, with the expense of the method tailored to the requirements of the product application. Shop-floor inspection of a thread is often as simple as running a nut
onto it (for male threads) or a bolt into it (for female threads). This is plenty good enough for many applications (e.g., MRO
or hobbyist work), although it is not good enough for most commercial manufacturing. Higher-precision methods are discussed below.
Commercial-grade inspection of screw threads can involve most of the same inspection methods and tools used to inspect other manufactured products, such as micrometer
s; vernier or dial calipers
; surface plate
s and height gauge
s; gauge blocks; optical comparator
s; white light scanner
s; and coordinate-measuring machine
s (CMMs). Even industrial radiography
(including industrial CT scanning
) can be used, for example, to inspect internal thread geometry in the way that an optical comparator can inspect external thread geometry.
Conical micrometer anvils, specifically suited to resting on the sides of the thread, are made for various thread angle
s, with 60° being the most common. Mics with such anvils are usually called "thread mics". Users who lack thread mics rely instead on the "3-wire method", which involves placing 3 short pieces of wire (or gauge pins) of known diameter into the valleys of the thread and then measuring from wire to wire with standard (flat) anvils. A conversion factor
(produced by a straightforward trigonometric calculation) is then multiplied with the measured value to infer a measurement of the thread's pitch diameter. Tables of these conversion factors were established many decades ago for all standard thread sizes, so today a user need only take the measurement and then perform the table lookup (as opposed to recalculating each time). The 3-wire method is also used when high precision is needed to inspect a specific diameter, commonly the pitch diameter, or on specialty threads such as multi-start or when the thread angle is not 60°. Ball-shaped micrometer anvils can be used in similar fashion (same trigonometric relationship, less cumbersome to use). Digital calipers and micrometers can send each measurement datum as it occurs through an interface (commonly RS-232
) to storage and as input to software, in which case the table lookup is done in an automated
way, and quality assurance
and quality control
can be achieved using statistical process control
.
Screw thread
A screw thread, often shortened to thread, is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter called a tapered thread...
. More screw threads are produced each year than any other machine element
Machine element
Machine elements are basic mechanical parts used as the building blocks of most machines. Most are standardized to common sizes, but customs are also common for specialized applications.-Types:*Shafts**Couplings*Keys*Splines*Bearings**Roller bearings...
. There are many methods of generating threads, including subtractive methods (many kinds of thread cutting and grinding, as detailed below); deformative or transformative methods (rolling and forming; molding and casting); additive
Additive manufacturing
Additive manufacturing is defined by ASTM as the "process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies, such as traditional machining...
methods (such as 3D printing
3D printing
3D printing is a form of additive manufacturing technology where a three dimensional object is created by laying down successive layers of material. 3D printers are generally faster, more affordable, and easier to use than other additive manufacturing technologies. However, the term 3D printing is...
); or combinations thereof.
Overview of methods (comparison, selection, etc)
There are various methods for generating screw threads. The method chosen for any one application is chosen based on constraints—time; money; degree of precision needed (or not needed); what equipment is already available; what equipment purchases could be justified based on resulting unit price of the threaded part (which depends on how many parts are planned); etc.In general, certain thread-generating processes tend to fall along certain portions of the spectrum from toolroom
Toolroom
The term toolroom can refer to three related concepts. The concepts have evolved over the past two centuries as technology itself has evolved.- Storing tools :...
-made parts to mass-produced parts, although there can be considerable overlap. For example, thread lapping following thread grinding would fall only on the extreme toolroom end of the spectrum, while thread rolling is a large and diverse area of practice that is used for everything from microlathe leadscrews (somewhat pricey and very precise) to the cheapest deck screws (very affordable and with precision to spare).
Threads of metal fasteners are usually created on a thread rolling machine. They may also be cut with a lathe
Lathe (metal)
A metal lathe or metalworking lathe is a large class of lathes designed for precisely machining relatively hard materials. They were originally designed to machine metals; however, with the advent of plastics and other materials, and with their inherent versatility, they are used in a wide range of...
, tap or die. Rolled threads are stronger than cut threads, with increases of 10% to 20% in tensile strength and possibly more in fatigue resistance and wear resistance.
Thread cutting
Thread cutting, as compared to thread forming and rolling, is used when full thread depth is required, when the quantity is small, when the blank is not very accurate, when threading up to a shoulder is required, when threading a tapered thread, or when the material is brittle.Taps and dies
A common method of threading is cutting with taps and dies. Unlike drill bitDrill bit
Drill bits are cutting tools used to create cylindrical holes. Bits are held in a tool called a drill, which rotates them and provides torque and axial force to create the hole. Specialized bits are also available for non-cylindrical-shaped holes....
s, hand taps do not automatically remove the chips
Swarf
Swarf, also known as turnings, chips, or filings, are shavings and chippings of metal — the debris or waste resulting from metalworking operations including milling and grinding. It can usually be recycled, and this is the preferred method of disposal due to the environmental concerns regarding...
they create. A hand tap cannot cut its threads in a single rotation because it creates long chips which quickly jam the tap (an effect known as "crowding"), possibly breaking it. Therefore, in manual thread cutting, normal wrench usage is to cut the threads 1/2 to 2/3 of a turn (180 to 240 degree rotation), then reverse the tap for about 1/6 of a turn (60 degrees) until the chips are broken by the back edges of the cutters. It may be necessary to periodically remove the tap from the hole to clear the chips, especially when a blind hole is threaded.
For continuous tapping operations (i.e., power tapping) specialized spiral point or "gun" taps are used to eject the chips and prevent crowding.
Single-point threading
Single-point threading, also colloquially called single-pointing (or just thread cutting when the context is implicit), is an operation that uses a single-point toolTool bit
The term tool bit generally refers to a non-rotary cutting tool used in metal lathes, shapers, and planers. Such cutters are also often referred to by the set-phrase name of single-point cutting tool. The cutting edge is ground to suit a particular machining operation and may be resharpened or...
to produce a thread form on a cylinder or cone. The tool moves linearly while the precise rotation of the workpiece determines the lead of the thread. The process can be done to create external or internal threads (male or female). In external thread cutting, the piece can either be held in a chuck
Chuck (engineering)
A chuck is a specialized type of clamp used to hold an object, usually an object with radial symmetry, especially a cylindrical object. It is most commonly used to hold a rotating tool or a rotating workpiece...
or mounted between two centers
Lathe center
A lathe center, often shortened to center, is a tool that has been ground to a point as to accurately position a workpiece about an axis...
. With internal thread cutting, the piece is held in a chuck. The tool moves across the piece linearly, taking chips off the workpiece with each pass. Usually 5 to 7 light cuts create the correct depth of the thread.
The coordination of various machine elements including leadscrew
Leadscrew
A leadscrew , also known as a power screw or translation screw, is a screw designed to translate turning motion into linear motion...
, slide rest, and change gears was the technological advance that allowed the invention of the screw-cutting lathe
Screw-cutting lathe
A screw-cutting lathe is a machine capable of cutting very accurate screw threads via single-point screw-cutting, which is the process of guiding the linear motion of the tool bit in a precisely known ratio to the rotating motion of the workpiece...
, which was the origin of single-point threading as we know it today.
Today engine lathes and CNC
Numerical control
Numerical control refers to the automation of machine tools that are operated by abstractly programmed commands encoded on a storage medium, as opposed to controlled manually via handwheels or levers, or mechanically automated via cams alone...
lathes are the commonly used machines for single-point threading. On CNC machines, the process is quick and easy (relative to manual control) due to the machine's ability to constantly track the relationship of the tool position and spindle position (called "spindle synchronization"). CNC software includes "canned cycles", that is, preprogrammed subroutines, that obviate the manual programming of a single-point threading cycle. Parameters are entered (e.g., thread size, tool offset, length of thread), and the machine does the rest.
All threading could feasibly be done using a single-point tool, but because of the high speed and thus low unit cost of other methods (e.g., tapping, die threading, and thread rolling and forming), single-point threading is usually only used when other factors of the manufacturing process happen to favor it (e.g., if only a few threads need to be made, if an unusual or unique thread is required, or if there is a need for very high concentric
Concentric
Concentric objects share the same center, axis or origin with one inside the other. Circles, tubes, cylindrical shafts, disks, and spheres may be concentric to one another...
ity with other part features machined during the same setup).
Thread milling
Threads may be milledMilling machine
A milling machine is a machine tool used to machine solid materials. Milling machines are often classed in two basic forms, horizontal and vertical, which refers to the orientation of the main spindle. Both types range in size from small, bench-mounted devices to room-sized machines...
with a rotating milling cutter
Milling cutter
Milling cutters are cutting tools typically used in milling machines or machining centres . They remove material by their movement within the machine or directly from the cutter's shape .-Features of a milling cutter:Milling cutters come in several shapes and many sizes...
if the correct helical
Helix
A helix is a type of smooth space curve, i.e. a curve in three-dimensional space. It has the property that the tangent line at any point makes a constant angle with a fixed line called the axis. Examples of helixes are coil springs and the handrails of spiral staircases. A "filled-in" helix – for...
toolpath can be arranged. This has been possible mechanically since the early nineteenth century, but it was never a commonplace method of threading until the widespread dissemination of affordable, fast, precise CNC. Since that development, internal and external threads are often milled. Some advantages of thread milling, as compared to single-point cutting, are a better surface finish
Surface finish
Surface finish, also known as surface texture, is the characteristics of a surface. It has three components: lay, surface roughness, and waviness.-Lay:...
; improved concentricity in some cases; and that a left- or right-hand thread can be created with the same tool. Additionally, for large, awkward workpieces (such as a fire hydrant
Fire hydrant
A fire hydrant , is an active fire protection measure, and a source of water provided in most urban, suburban and rural areas with municipal water service to enable firefighters to tap into the municipal water...
casting), it is simply easier to let the workpiece sit stationary on a table while all needed machining operations are performed on it with rotating tools, as opposed to rigging it up for rotation around the axis of each set of threads (that is, for the "arms" and "mouth" of the hydrant).
There are various types of thread milling, including several variants of form-milling and a combination of drilling and threading with one cutter, called thrilling.
Form-milling uses either a single- or multiple-form cutter. In one variant of form-milling, the single-form cutter is tilted to the helix angle
Helix angle
In mechanical engineering, a helix angle is the angle between any helix and an axial line on its right, circular cylinder or cone. Common applications are screws, helical gears, and worm gears....
of the thread and then fed radially into the blank. The blank is then slowly rotated as the cutter is precisely moved along the axis of the blank, which cuts the thread into the blank. This can be done in one pass, if the cutter is fed to the full thread depth, or in two passes, with the first not being to the full thread depth. This process is mainly used on threads larger than 1.5 in (3.8 cm). It is commonly used to cut large-lead
Lead (engineering)
Lead is the axial advance of a helix or screw during one complete turn The lead for a screw thread is the axial travel for a single revolution....
or multiple-lead threads. A similar variant using a multiple-form cutter exists, in which the process completes the thread in one revolution around the blank. The cutter must be longer than the desired thread length. Using a multiple-form cutter is faster than using a single-form cutter but it is limited to threads with a helix angle less than 3°. It is also limited to blanks of a substantial diameter and no longer than 2 in (5.1 cm).
Another variant of form-milling involves holding the cutter's axis orthogonally (no canting to the thread's helix angle) and feeding the cutter in a toolpath that will generate the thread. The part is usually a stationary workpiece, such as a boss
Boss (engineering)
In engineering, a boss is a protruding feature on a workpiece.A common use for a boss is to locate one object within a pocket or hole of another object...
on a valve body (in external thread milling) or a hole in a plate or block (in internal thread milling). This type of thread milling uses essentially the same concept as contouring with an endmill or ball-nose mill, but the cutter and toolpath are arranged specifically to define the "contour" of a thread. The toolpath is achieved either using helical interpolation (which is circular interpolation in one plane [typically XY] with simultaneous linear interpolation along a third axis [typically Z]; the CNC control model must be one that supports using the third axis) or a simulation of it using extremely small increments of 3-axes linear interpolation (which is not practical to program manually but can be programmed easily with CAD/CAM software). The cutter geometry reflects the thread pitch but not its lead; the lead (thread helix angle) is determined by the toolpath. Tapered threads can be cut either with a tapered multiple-form cutter that completes the thread in one revolution using helical interpolation, or with a straight or tapered cutter (of single- or multiple-form) whose toolpath is one or more revolutions but cannot use helical interpolation and must use CAD/CAM software to generate a contour-like simulation of helical interpolation.
The tooling used for thread milling can be solid or indexable. For internal threads, solid cutters are generally limited to holes larger than 6 mm (0.236220472440945 in), and indexable internal thread cutting tools are limited to holes larger than 12 mm (0.47244094488189 in). The advantage is that when the insert wears out it is easily and more cost effectively replaced. The disadvantage is the cycle time is generally longer than solid tools. Note that solid multiple-form thread cutting tools look similar to taps, but they differ in that the cutting tool does not have a backtaper and there is not a lead-in chamfer. This lack of a lead-in chamfer allows the threads to be formed within one pitch length of the bottom of a blind hole.
Thrilling
Thrilling is the process of drilling and threading internal threads using a specialized cutting tool on a CNC mill. The cutting tool tip is shaped like a drill, while the shank has a thread shaped form. The cutter first plunges to drill the hole. Then the thread is circularly interpolated just like the multiple-form cutter described above. The advantage is this process eliminates a tool, tool holder, and tool change. The disadvantage is that the process is limited to hole depth no greater than three times the diameter of the tool.
Thread grinding
Thread grinding is done on a grinding machineGrinding machine
A grinding machine, often shortened to grinder, is a machine tool used for grinding, which is a type of machining using an abrasive wheel as the cutting tool...
using specially dressed grinding wheel
Grinding wheel
A grinding wheel is an expendable wheel that is composed of an abrasive compound used for various grinding and abrasive machining operations...
s matching the shape of the threads. The process is usually used to produce accurate threads or threads in hard materials; a common application is ball screw mechanisms. There are three types: center-type grinding with axial feed, center-type infeed thread grinding and centerless thread grinding. Center-type grinding with an axial feed is the most common of the three. It is similar to cutting a thread on a lathe with a single-point cutting tool, except the cutting tool is replaced with a grinding wheel. Usually a single ribbed wheel is used, although multiple ribbed wheels are also available. To complete the thread multiple passes are commonly required. Center-type infeed thread grinding use a grinding wheel with multiple ribs that is longer than the length of the desired thread. First, the grinding wheel is fed into the blank to the full thread depth. Then the blank is slowly rotated through approximately 1.5 turns while axially advancing through one pitch per revolution. Finally, the centerless thread grinding process is used to make head-less set screw
Set screw
A set screw is a type of screw generally used to secure an object within or against another object. The most common examples are securing a pulley or gear to a shaft. Set screws are most often headless , meaning that the screw is fully threaded and has no head projecting past the major diameter of...
s in a similar method as centerless grinding
Centerless grinding
Centerless grinding is a method of material removal through grinding, similar to centered grinding except for the absence of the spindle. It has high throughput, i.e. a large number of parts can be manufactured in a short time....
. The blanks are hopper-fed to the grinding wheels, where the thread is fully formed. Common centerless thread grinding production rates are 60 to 70 pieces per minute for a 0.5 in (1.3 cm) long set screw.
Thread lapping
Rarely, thread cutting or grinding (usually the latter) will be followed by thread lappingLapping
Lapping is a machining operation, in which two surfaces are rubbed together with an abrasive between them, by hand movement or by way of a machine.This can take two forms...
in order to achieve the highest precision and surface finish achievable. This is a toolroom
Toolroom
The term toolroom can refer to three related concepts. The concepts have evolved over the past two centuries as technology itself has evolved.- Storing tools :...
practice when the highest precision is required, rarely employed except for the leadscrews
Leadscrew
A leadscrew , also known as a power screw or translation screw, is a screw designed to translate turning motion into linear motion...
or ballscrews
Ball screw
A ball screw is a mechanical linear actuator that translates rotational motion to linear motion with little friction. A threaded shaft provides a helical raceway for ball bearings which act as a precision screw. As well as being able to apply or withstand high thrust loads, they can do so with...
of high-end machine tools.
Threading with EDM
Internal threads can be electrical discharge machinedElectrical discharge machining
Electric discharge machining , sometimes colloquially also referred to as spark machining, spark eroding, burning, die sinking or wire erosion, is a manufacturing process whereby a desired shape is obtained using electrical discharges...
(EDM) into hard materials using a sinker style machine.
Thread forming and rolling
Thread forming and thread rolling are processes for forming screw threads, with the former referring to creating internal threads and the latter external threads. In both of these processes threads are formed into a blank by pressing a shaped die against the blank, in a process similar to knurlingKnurling
Knurling is a manufacturing process, typically conducted on a lathe, whereby a visually attractive diamond-shaped pattern is cut or rolled into metal.- Uses :...
. These processes are used for large production runs because typical production rates are around one piece per second. Forming and rolling produce no swarf
Swarf
Swarf, also known as turnings, chips, or filings, are shavings and chippings of metal — the debris or waste resulting from metalworking operations including milling and grinding. It can usually be recycled, and this is the preferred method of disposal due to the environmental concerns regarding...
and less material is required because the blank size starts smaller than a blank required for cutting threads; there is typically a 15 to 20% material savings in the blank, by weight. A rolled thread can often be easily recognized because the thread has a larger diameter than the blank rod from which it has been made; however, necks and undercuts
Undercut (manufacturing)
In manufacturing, an undercut is a special type of recessed surface. In turning it refers to a recess in a diameter. In machining it refers to a recess in a corner. In molding it refers to a feature that cannot be molded using only a single pull mold...
can be cut or rolled onto blanks with threads that are not rolled. Also, the end of the screw usually looks a bit different from the end of a cut-thread screw.
Materials are limited to ductile materials because the threads are cold formed. However, this increases the thread's yield strength, surface finish, hardness, and wear resistance. Also, materials with good deformation characteristics are necessary for rolling; these materials include softer (more ductile) metals and exclude brittle
Brittle
A material is brittle if, when subjected to stress, it breaks without significant deformation . Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Breaking is often accompanied by a snapping sound. Brittle materials include most ceramics and glasses ...
materials, such as cast iron
Cast iron
Cast iron is derived from pig iron, and while it usually refers to gray iron, it also identifies a large group of ferrous alloys which solidify with a eutectic. The color of a fractured surface can be used to identify an alloy. White cast iron is named after its white surface when fractured, due...
. Tolerances are typically ±0.001 in. (±0.025 mm), but tolerances as tight as ±0.0006 in (±0.015 mm) are achievable. Surface finishes range from 6 to 32 micro-inches.
There are four main types of thread rolling, named after the configuration of the dies
Die (manufacturing)
A die is a specialized tool used in manufacturing industries to cut or shape material using a press. Like molds, dies are generally customized to the item they are used to create...
: flat dies, two-die cylindrical, three-die cylindrical, and planetary dies. The flat die system has two flat dies, the bottom one is held stationary and the other slides. The blank is placed on one end of the stationary die and then the moving die slides over the blank, which causes the blank to roll between the two dies forming the threads. Before the moving die reaches the end of its stroke the blank rolls off the stationary die in a finished form. The two-die cylindrical process is used to produce threads up to 6 in (15.2 cm) in diameter and 20 in (50.8 cm) in length. There are two types of three-die processes; the first has the three dies move radially out from the center to let the blank enter the dies and then closes and rotates to roll the threads. This type of process is commonly employed on turret lathe
Turret lathe
The turret lathe is a form of metalworking lathe that is used for repetitive production of duplicate parts, which by the nature of their cutting process are usually interchangeable...
s and screw machine
Screw machine
A screw machine may refer to a:* Screw machine , a small- to medium-sized automatic lathe that is mechanically automated via cams...
s. The second type takes the form of a self-opening die head
Die head
A die head is a threading die that is used in the high volume production of threaded components.Die heads are commonly used on lathes, turret lathes, screw machines and CNC lathes. They may be used for either cutting a thread or rolling a thread...
. This type is more common than the former, but is limited by not being able form the last 1.5 to 2 threads against shoulders. Planetary dies are used to mass produce threads up to 1 in (2.5 cm) in diameter.
Thread forming is performed using a , or , which closely resembles a cutting tap without the flutes. There are lobes periodically spaces around the tap that actually do the thread forming as the tap is advanced into a properly sized hole. Since the tap does not produce chips, there is no need to periodically back out the tap to clear away chips, which, in a cutting tap, can jam and break the tap. Thus thread forming is particularly suited to tapping blind holes, which are tougher to tap with a cutting tap due to the chip build-up in the hole. Note that the tap drill size differs from that used for a cutting tap and that an accurate hole size is required because a slightly undersized hole can break the tap. Proper lubrication is essential because of the frictional forces involved, therefore a lubricating oil
Oil
An oil is any substance that is liquid at ambient temperatures and does not mix with water but may mix with other oils and organic solvents. This general definition includes vegetable oils, volatile essential oils, petrochemical oils, and synthetic oils....
is used instead of cutting oil
Cutting fluid
Cutting fluid is a type of coolant and lubricant designed specifically for metalworking and machining processes. There are various kinds of cutting fluids, which include oils, oil-water emulsions, pastes, gels, aerosols , and air or other gases. They may be made from petroleum distillates, animal...
.
When considering the blank diameter tolerance, a change in blank diameter will affect the major diameter by an approximate ratio of 3 to 1. Production rates are usually three to five times faster than thread cutting.
| Description | Application |
|---|---|
| Flat dies | Machine, tapping and wood screws |
| Cylindrical in-feed 2 dies | Large or balanced screws |
| Cylindrical in-feed 3 dies | Tube fitting, spark plugs |
| Planetary dies | High volumes screws, sheet metal screws, and drive screws |
| Thread diameter [in.] | Flat dies [pieces/min] | Cylindrical [pieces/min] | Planetary [pieces/min] |
|---|---|---|---|
| 1/8 | 40 to 500 | 75 to 300 | 450 to 2000 |
| 1/4 | 40 to 400 | 60 to 150 | 250 to 1200 |
| 1/2 | 25 to 90 | 50 to 100 | 100 to 400 |
| 3/4 | 20 to 60 | 5 to 10 | - |
| 1 | 15 to 50 | 1 to 50 | - |
Thread casting and molding
In castingCasting
In metalworking, casting involves pouring liquid metal into a mold, which contains a hollow cavity of the desired shape, and then allowing it to cool and solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process...
and molding
Molding
Molding or moulding may refer to:*Molding , feature formed from marble, plaster, wood, etc. and used in interior design*Molding , process used in manufacturing to shape materials*Car body molding...
the threads are directly formed by the geometry of the mold cavity in the mold
Molding (process)
Molding or moulding is the process of manufacturing by shaping pliable raw material using a rigid frame or model called a pattern....
or die
Die (manufacturing)
A die is a specialized tool used in manufacturing industries to cut or shape material using a press. Like molds, dies are generally customized to the item they are used to create...
. When the material freezes in the mold, it retains the shape after the mold is removed. The material is heated to a liquid, or mixed with a liquid that will either dry or cure (such as plaster or cement). Alternately, the material may be forced into a mold as a powder and compressed into a solid, as with graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
.
Although the first thoughts that come to mind for most machinists regarding threading are of thread cutting processes (such as tapping, single-pointing, or helical milling), Smid points out that, when plastic bottles for food, beverages, personal care products, and other consumer products are considered, it is actually plastic molding that is the principal method (by sheer volume) of thread generation in manufacturing today. Of course, this fact highlights the importance of the moldmaker
Moldmaker
Moldmaker is the designation for a profession in the metalworking and ceramics industry. It is a variety of the profession of the toolmaker. The moldmaker produces molds for the injection molding , die casting and ceramics industries....
s getting the mold just right (in preparation for millions of cycles, usually at high speed).
Cast threads in metal parts may be finished by machining, or may be left in the as-cast state. (The same can be said of cast gear
Gear
A gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine....
teeth.) Whether or not to bother with the additional expense of a machining operation depends on the application. For parts where the extra precision and surface finish is not strictly necessary (although it might be nice), the machining is forgone in order to achieve a lower cost. With sand casting parts this means a rather rough finish; but with molded plastic or die-cast metal, the threads can be very nice indeed straight from the mold or die. A common example of molded plastic threads is on soda (pop) bottles. A common example of die-cast threads is on cable gland
Cable gland
A cable gland is a device designed to attach and secure the end of a cable to the equipment...
s (connectors/fittings).
Additive methods
Many, perhaps most, threaded parts have potential to be generated via additive manufacturingAdditive manufacturing
Additive manufacturing is defined by ASTM as the "process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies, such as traditional machining...
, of which there are many variants, including fused deposition modeling
Fused deposition modeling
Fused deposition modeling is an additive manufacturing technology commonly used for modeling, prototyping, and production applications. The technology was developed by S...
, direct metal laser sintering
Direct metal laser sintering
Direct metal laser sintering is an additive metal fabrication technology developed by EOS out of Munich, Germany, sometimes also referred to by the terms selective laser sintering or selective laser melting . The process involves use of a 3D CAD model whereby a .stl file is created and sent to...
, 3D printing
3D printing
3D printing is a form of additive manufacturing technology where a three dimensional object is created by laying down successive layers of material. 3D printers are generally faster, more affordable, and easier to use than other additive manufacturing technologies. However, the term 3D printing is...
, solid freeform fabrication, layered object manufacturing, and stereolithography
Stereolithography
Stereolithography is an additive manufacturing technology for producing models, prototypes, patterns, and in some cases, production parts.-Technology description:...
. Most additive technologies are still on the laboratory end of their historical development, but further commercialization
Commercialization
Commercialization is the process or cycle of introducing a new product or production method into the market. The actual launch of a new product is the final stage of new product development, and the one where the most money will have to be spent for advertising, sales promotion, and other marketing...
is picking up speed. Additive methods today generally produce a rough surface finish
Surface finish
Surface finish, also known as surface texture, is the characteristics of a surface. It has three components: lay, surface roughness, and waviness.-Lay:...
, which suggests that their earliest commercial wins will be in parts that don't require secondary finishing by subtractive methods.
Combinations of subtractive, additive, deformative, or transformative methods
Often subtractive, additive, deformative, or transformative methods are combined in whatever ways are advantageous. Such multidisciplinary manufacturing falls under classifications including rapid prototypingRapid prototyping
Rapid prototyping is the automatic construction of physical objects using additive manufacturing technology. The first techniques for rapid prototyping became available in the late 1980s and were used to produce models and prototype parts. Today, they are used for a much wider range of applications...
, desktop manufacturing, direct manufacturing
Direct Manufacturing
Direct Manufacturing, or DM for short, is one of many available technologies used to produce a three-dimensional near net shape part or component using an additive layer over layer process, which is commonly referred to as Additive Manufacturing. DM, however, is the first commercially available,...
, direct digital manufacturing, digital fabrication, instant manufacturing, or on-demand manufacturing.
Inspection
InspectionInspection
An inspection is, most generally, an organized examination or formal evaluation exercise. In engineering activities inspection involves the measurements, tests, and gauges applied to certain characteristics in regard to an object or activity...
of the finished screw threads can be achieved in various ways, with the expense of the method tailored to the requirements of the product application. Shop-floor inspection of a thread is often as simple as running a nut
Nut (hardware)
A nut is a type of hardware fastener with a threaded hole. Nuts are almost always used opposite a mating bolt to fasten a stack of parts together. The two partners are kept together by a combination of their threads' friction, a slight stretch of the bolt, and compression of the parts...
onto it (for male threads) or a bolt into it (for female threads). This is plenty good enough for many applications (e.g., MRO
Maintenance, repair, and operations
Maintenance, repair, and operations or maintenance, repair, and overhaul involves fixing any sort of mechanical or electrical device should it become out of order or broken...
or hobbyist work), although it is not good enough for most commercial manufacturing. Higher-precision methods are discussed below.
Commercial-grade inspection of screw threads can involve most of the same inspection methods and tools used to inspect other manufactured products, such as micrometer
Micrometer
A micrometer , sometimes known as a micrometer screw gauge, is a device incorporating a calibrated screw used widely for precise measurement of small distances in mechanical engineering and machining as well as most mechanical trades, along with other metrological instruments such as dial, vernier,...
s; vernier or dial calipers
Caliper
A caliper is a device used to measure the distance between two opposing sides of an object. A caliper can be as simple as a compass with inward or outward-facing points...
; surface plate
Surface plate
A surface plate is a solid, flat plate used as the main horizontal reference plane for precision inspection, marking out , and tooling setup. The surface plate is often used as the baseline for all measurements to the workpiece, therefore one primary surface is finished extremely flat with...
s and height gauge
Height gauge
A height gauge is a measuring device used either for determining the height of something, or for repetitious marking of items to be worked on...
s; gauge blocks; optical comparator
Optical comparator
An optical comparator is a device that applies the principles of optics to the inspection of manufactured parts...
s; white light scanner
White light scanner
A White Light Scanner is a device for measuring the physical geometrical characteristics of an object using White light interferometry. Frequency domain analysis is an alternate approach that uses all of the information available in the interferogram...
s; and coordinate-measuring machine
Coordinate-measuring machine
A coordinate measuring machine is a device for measuring the physical geometrical characteristics of an object. This machine may be manually controlled by an operator or it may be computer controlled. Measurements are defined by a probe attached to the third moving axis of this machine...
s (CMMs). Even industrial radiography
Industrial radiography
Industrial Radiography is the use of ionizing radiation to view objects in a way that cannot be seen otherwise. It is not to be confused with the use of ionizing radiation to change or modify objects; radiography's purpose is strictly viewing. Industrial radiography has grown out of engineering,...
(including industrial CT scanning
Industrial CT Scanning
Industrial CT scanning is a process which uses X-ray equipment to produce three-dimensional representations of components both externally and internally. Industrial CT scanning has been used in many areas of industry for internal inspection of components...
) can be used, for example, to inspect internal thread geometry in the way that an optical comparator can inspect external thread geometry.
Conical micrometer anvils, specifically suited to resting on the sides of the thread, are made for various thread angle
Thread angle
The thread angle of a screw is the angle between the threads. This is a defining factor for the shape of a screw thread. Standard values include:...
s, with 60° being the most common. Mics with such anvils are usually called "thread mics". Users who lack thread mics rely instead on the "3-wire method", which involves placing 3 short pieces of wire (or gauge pins) of known diameter into the valleys of the thread and then measuring from wire to wire with standard (flat) anvils. A conversion factor
Conversion factor
A conversion factor changes something to a different version or form. A factor is something that brings results or a cause, while conversion is an action of changing the "version" of a thing....
(produced by a straightforward trigonometric calculation) is then multiplied with the measured value to infer a measurement of the thread's pitch diameter. Tables of these conversion factors were established many decades ago for all standard thread sizes, so today a user need only take the measurement and then perform the table lookup (as opposed to recalculating each time). The 3-wire method is also used when high precision is needed to inspect a specific diameter, commonly the pitch diameter, or on specialty threads such as multi-start or when the thread angle is not 60°. Ball-shaped micrometer anvils can be used in similar fashion (same trigonometric relationship, less cumbersome to use). Digital calipers and micrometers can send each measurement datum as it occurs through an interface (commonly RS-232
RS-232
In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports...
) to storage and as input to software, in which case the table lookup is done in an automated
Automation
Automation is the use of control systems and information technologies to reduce the need for human work in the production of goods and services. In the scope of industrialization, automation is a step beyond mechanization...
way, and quality assurance
Quality Assurance
Quality assurance, or QA for short, is the systematic monitoring and evaluation of the various aspects of a project, service or facility to maximize the probability that minimum standards of quality are being attained by the production process...
and quality control
Quality control
Quality control, or QC for short, is a process by which entities review the quality of all factors involved in production. This approach places an emphasis on three aspects:...
can be achieved using statistical process control
Statistical process control
Statistical process control is the application of statistical methods to the monitoring and control of a process to ensure that it operates at its full potential to produce conforming product. Under SPC, a process behaves predictably to produce as much conforming product as possible with the least...
.
History
Each method of thread generation has its own detailed history. Therefore a comprehensive discussion is beyond the scope of this article; but much historical information is available in related articles, including:- Screw > History
- Screw-cutting lathe > History
- Automatic lathe > History
- Screw thread > History of standardization
- Turret latheTurret latheThe turret lathe is a form of metalworking lathe that is used for repetitive production of duplicate parts, which by the nature of their cutting process are usually interchangeable...
[various sections] - Casting (metalworking) and its family of articles (e.g., Sand casting > History)
- Grinding (abrasive cutting)Grinding (abrasive cutting)Grinding is an abrasive machining process that uses a grinding wheel as the cutting tool.A wide variety of machines are used for grinding:* Hand-cranked knife-sharpening stones * Handheld power tools such as angle grinders and die grinders...
and its family of articles - Additive manufacturing > Historical development and broadening applications
- Various specific additive manufacturingAdditive manufacturingAdditive manufacturing is defined by ASTM as the "process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies, such as traditional machining...
articles (e.g., digital fabricator, direct digital manufacturing, 3D printing3D printing3D printing is a form of additive manufacturing technology where a three dimensional object is created by laying down successive layers of material. 3D printers are generally faster, more affordable, and easier to use than other additive manufacturing technologies. However, the term 3D printing is...
, rapid prototypingRapid prototypingRapid prototyping is the automatic construction of physical objects using additive manufacturing technology. The first techniques for rapid prototyping became available in the late 1980s and were used to produce models and prototype parts. Today, they are used for a much wider range of applications...
, solid freeform fabrication) - List of emerging technologies