Thread Rolling: Types, Processes and Advantages

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Introduction

This guide contains everything you need to know about thread rolling and screw machine products.

You will learn about topics such as:

• What is thread rolling?
• Thread rolling processes
• Advantages and disadvantages of thread rolling
• Common defects
• Types of thread rolling machines
• And much more…

Chapter 1: What is Thread Rolling?

Thread rolling is a threading process that involves deforming metal stock by pressing it between dies to produce external threads on the surface. This technique can also generate internal threads via a procedure known as thread forming. Unlike conventional threading methods like thread cutting, thread rolling is a non-subtractive approach, meaning it doesn't involve removing metal from the stock. Rolled threaded fasteners offer benefits such as enhanced thread strength, accurate final measurements, superior surface finish, and reduced friction coefficients.

Screw machine products include threaded machine items such as bolts, nuts, and screws. These threaded components can be grouped based on their purpose. Bolts, nuts, and screws serve as structural elements or fasteners. Threaded fasteners may also be integrated into parts, creating threaded fittings.

Threaded fasteners are utilized to establish non-permanent connections, allowing for components to be mechanically loosened or disassembled. On the other hand, power screws and lead screws function as mechanical devices or drives, serving as mechanisms to regulate movement and transmit force to other machine parts, thus playing an essential role in mechanical systems.

Screw Thread Forms

Screw threads can be classified according to their shapes.

• V-Thread: These threads are triangle-shaped and typically form an angle of 60° with flanks. Crests and roots are usually sharp, though sometimes slightly flattened due to fabrication limitations.
• American National Thread: Previously referred to as the United States Standard Screw Thread, this thread type standardizes a V-thread version with specified dimensions for crest and root flatness. It replaced the basic V-thread for general purposes.
• British Whitworth Thread: This thread type was the UK equivalent of the American National Thread.
• Unified Thread: The Unified Thread form succeeded the American National Thread, merging thread standards from the US, Canada, and the UK for interchangeable parts. It retains a V-shape with rounded or flat crests and roots, offering series like Unified Fine (UNF), Unified Coarse (UNC), Unified Extra Fine (UNEF), and Unified Special (UNS).
• Metric Thread: This thread form transitioned threading standards to the metric system under ISO, phasing out the UTS thread form in favor of metric dimensions.
• Square Thread: Designed for power transmission, square threads are ideal for mechanisms due to perpendicular load-bearing faces. However, they are impractical due to manufacturing constraints.
• Acme Thread: An adaptation of the square thread, the Acme thread has a trapezoidal shape with a narrower root than crest. It is easier to machine and stronger than the square thread.
• Buttress Thread: This thread features one perpendicular flank or slightly angled with the axis, while the other flank forms a 45° angle, designed to handle high loads in one direction.
• Knuckle Thread: Characterized by highly rounded crests and roots with a flank angle of 30°, this design allows debris to move aside, ensuring smooth thread meshing.

Chapter 2: What is an overview of threading processes?

Thread generation processes are fundamental to manufacturing, machine shops, and the production of precision fasteners, screws, and bolts. Understanding threading processes is critical for industries that rely on high-strength, accurately dimensioned threaded components. Thread production is typically classified into three main categories—subtractive, deformative (also called deformation), and additive—each distinguished by how threads are created or formed on metal, plastic, or composite workpieces. These various techniques form the backbone of industrial fastener manufacturing, and each process has unique advantages based on application, thread profile, and product requirements.

Subtractive processes, often referred to as cutting processes in the context of precision machining, are among the most widely used methods. Machinists, toolmakers, and fabrication engineers carefully select these processes based on thread type (internal vs. external), required material strength, thread finish, and production volume. Below is a comprehensive summary of common thread generation methods:

• Tapping: Tapping is a thread machining process for producing internal threads, commonly found in nuts and tapped holes. This is achieved using a tap, a cylindrical or conical cutting tool typically made from high-speed steel or carbide. The tap has multiple cutting edges that mirror the external thread form. During thread tapping, the tap is rotated and progressively advanced axially into the pre-drilled hole or bore of the metal stock, cutting the internal thread as it goes. This method is frequently used for thread manufacturing in mass production environments, especially for machined parts requiring high accuracy.

  
  

• Die Threading: This process is used to produce external screw threads, such as those on bolts, studs, and threaded rods. Its method of applying force and cutting action is similar to tapping, but a die is used to cut external threads onto the metal stock, with multiple cutting points forming the helical thread pattern. Dies are available in solid, self-opening, and adjustable varieties to suit different production requirements. Thread dies provide reliable results for low- to medium-volume production and are ideal for repairs or creating custom threads outside of automated machinery.

  
  

• Single-point Cutting: Single-point cutting is performed on a lathe or CNC turning center, where the workpiece is securely held and rotated. The cutting tool, precisely mounted on the carriage and fed linearly by a lead screw, traces the helical path to form the thread. This process is suitable for both internal and external threads and is well-suited for creating custom, variable-pitch threads or unique profiles. While single-point threading is slower than tapping or die-threading, its flexibility and ability to cut large or specialized threads make it valuable for prototyping and low-volume, precision applications.

  
  

• Chasing: This process uses a tool called a thread chaser, which is an assembly of several single-point cutting tools arranged in succession. The thread chaser is typically mounted on the lathe’s carriage, gradually indexed to cut the thread accurately along the workpiece length. Thread chasing is commonly used for large-diameter threads and for restoring worn or damaged threads where precision is critical.

  
  

• Milling: In thread milling, single or multiple rotary cutting tools are used to machine threads into a workpiece. Unlike tapping or die-threading, thread milling involves the rotation of both the cutting tool and the workpiece along their respective axes, allowing for the creation of both internal and external threads, including tapered and multi-start forms. Thread milling provides exceptional flexibility and is preferred in CNC production for quick changeover, high accuracy, and the ability to run on difficult-to-machine materials such as stainless steel or high-strength alloys.

  
  

• Grinding: Instead of traditional cutting, thread grinding employs abrasive wheels or discs to remove material and achieve a fine surface finish and precise thread dimensions. This process is often used in conjunction with other threading methods and is commonly reserved for hard materials, finishing, or high-precision threaded components such as lead screws, ballscrews, and high-tolerance fasteners. Thread grinding is essential in the manufacturing of aerospace, medical, and instrumentation threads, where tight dimensional tolerances and smooth surface finishes are mandatory.

  
  

  Deformation (deformative) processes generate threads by displacing and reshaping the metal workpiece without any removal of material. Common deformation threading processes include rolling and casting:

• Rolling: Thread rolling is an external threading process that plastically deforms the workpiece by passing it through precision-ground roller dies. These dies have the inverse of the desired thread form and, through pressure and friction, reshape the surface to form threads with improved surface finish and increased strength due to work hardening. Thread rolling is widely used in automotive, construction, and high-volume fastener manufacturing for its efficiency, accuracy, and the enhanced fatigue resistance it imparts to threads. The process is considerably faster than thread cutting and produces no waste material.

  
  

• Casting: Thread casting involves pouring or injecting molten metal into a precision die or mold that contains the negative shape of the desired threaded part. While casting is efficient for producing net-shape parts in high volume, it generally requires secondary machining—such as tapping or thread chasing—to achieve accurate threads, particularly for small or fine thread profiles. Among the most common uses of casting in thread manufacturing are for large fittings or coarse threads found in pipe fittings and valves.

  
  

  Finally, additive manufacturing processes, also referred to as 3D printing, are innovative methods for thread production by precisely depositing material layer by layer. These processes are increasingly applied for prototyping and for manufacturing both plastic and metal parts, especially when complex or custom thread geometries are needed. Currently, additive threading is often combined with secondary finishing operations—such as grinding or lapping—to meet industry tolerances.

  Key additive threading techniques include stereolithography, selective laser sintering (SLS), and fused filament fabrication (FFF):

  
  
• Stereolithography (SLA): One of the most widely used 3D printing processes for producing plastic parts, stereolithography involves a vat of photo-reactive liquid resin, which is selectively cured and solidified by a focused ultraviolet light or laser. SLA-printed threads are ideal for rapid prototyping and low-volume manufacturing, especially for precision plastic threaded components.
• Selective Laser Sintering (SLS): This process utilizes a high-powered laser beam to sinter powdered materials—plastic, nylon, or various metals—layer by layer, fusing them into a solid structure. While plastics remain the most common material, growth in sintered metal threads is enabling the on-demand production of custom metal parts with complex or internal thread geometries that are not easily machined by traditional methods.
• Fused Filament Fabrication (FFF/FDM): In this economical and accessible method, a continuous filament of thermoplastic or composite material is heated, melted, and extruded through a nozzle to build the part layer by layer. FFF is commonly used for developing prototypes with functional threads or for manufacturing replacement components in low-stress applications.

How to Select a Threading Method: Choosing the optimal thread generation process depends on several factors, including material type (such as carbon steel, alloy, aluminum, brass, or plastic), required thread geometry (standard, metric, acme, or custom thread profiles), production volume, cost considerations, and the mechanical properties required of the finished thread. For example, thread rolling is preferred for mass production of high-strength bolts, while thread grinding is chosen for aerospace or medical applications where extreme accuracy and surface finish are critical. CNC threading, incorporating both single-point and thread milling techniques, brings flexibility, efficiency, and repeatability to modern manufacturing environments.

For engineers, purchasing agents, or machinists seeking to specify or source threaded components, understanding the advantages of each technique ensures optimal performance, longevity, and cost-effectiveness. Partnering with a trusted threaded fastener manufacturer or precision thread supplier is crucial for applications with tight tolerances, custom designs, or specialized requirements.

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Chapter 3: What are the Advantages and Disadvantages of Thread Rolling?

Utilizing a thread rolled screw machine product presents a unique set of advantages and disadvantages within precision manufacturing. Thread rolling, a dominant process in the fastener manufacturing industry, offers superior mechanical properties, enhanced fatigue resistance, and excellent surface integrity. The primary benefit of thread rolling is its ability to produce stronger, more durable threads with high dimensional consistency and accuracy. However, since this process relies on plastic deformation of the material, it is generally restricted to ductile, soft metals and often involves higher initial tooling and setup costs compared to thread cutting methods such as thread grinding or thread tapping.

Below are the core benefits of rolled threads, screws, and bolts in modern manufacturing:

• High thread strength and load-bearing capability: Thread rolling is usually conducted at relatively low temperatures, making it a cold forming process. Cold working significantly increases material strength and produces threads with dense grain flow—enhancing tensile strength and fatigue life. Rolled threads are 10 to 20 percent stronger than cut or ground threads, making them the preferred choice for applications requiring superior thread integrity, such as in aerospace, automotive, and critical structural assemblies.

  
  
• Excellent surface finishes and reduced friction: The inherently smooth and burnished surface of rolled threads not only enhances appearance, but also provides functional advantages. The process removes surface irregularities, yielding roughness values as low as 8 to 24 microinches Ra—much finer compared to 64 to 125 microinches Ra for cut threads. This smoothness reduces the coefficient of friction, improving assembly efficiency and extending component lifespan by minimizing wear and galling. Rolled threads are characteristically free of defects like chatter marks, torn grain, and burrs.

• Precision threading and repeatability: The use of mirror-image dies in thread rolling ensures tight tolerances and consistent thread geometry over large production runs. Since no material is removed, the thread profile remains stable and uniform. This makes thread rolling ideal for the mass production of fasteners, lead screws, threaded rods, and precision components where high accuracy is required from batch to batch.

  
  
• Lower coefficient of friction for optimized assemblies: Superior surface finish in rolled threads translates into lower frictional resistance during mating or installation. This leads to more reliable and uniform tightness for nuts and bolts and enhances power transmission efficiency in lead screw assemblies used in automation and CNC machinery.
• Faster production rates and shorter lead times: Thread rolling machines are highly efficient, capable of producing threads at rapid speeds—often outpacing alternative techniques like thread cutting. Rolling speeds are determined by material type, thread profile, diameter, and the method of metal feeding. For example, reciprocating die machines can output 30 to 40 threaded components per minute for diameters of 5/8" to 1 1/8", while cylindrical die machines handle larger diameters, producing 10 to 30 pieces per minute. This throughput makes thread rolling a strategic choice for high-volume screw, bolt, and fastener production.

• Cost efficiency via minimized material waste: Because thread rolling is a non-cutting, chipless forming process, the original metal stock is fully utilized, eliminating the need to handle removed metal chips. This results in better material yield and decreased energy and recycling costs, making thread rolling an environmentally friendly, sustainable manufacturing process.

  
  

  Despite substantial benefits, manufacturers should consider the limitations of thread rolling as these can impact product cost and process selection. Most downsides impact the manufacturing process more than the end-user, but ultimately contribute to the final pricing of rolled thread products.

• Not suitable for high-hardness metals: Thread rolling is generally applied to malleable metals including low-carbon steel, alloy steel, aluminum, copper, and certain stainless steel grades. Processing materials harder than 40 Rockwell C dramatically reduces tool life and process efficiency. For hardened or heat-treated steels, thread grinding or other specialized methods are recommended for optimal results.

  
  
• Higher tooling investment and maintenance costs: Thread rolling dies must be manufactured to exacting standards and hardened for durability—any imperfection leads to thread inaccuracies. The precision tooling required (including die sets and machine calibration) is more expensive than the simpler cutters needed for thread cutting, impacting initial project budgets and ongoing maintenance.
• Stock diameter must be carefully controlled: The initial bar or wire stock must have precisely controlled diameter tolerances, as the thread profile is formed by material displacement—not removal. For tight-thread tolerances, pre-turning or centerless grinding may be needed to ensure correct dimensions before rolling, adding complexity to process planning and setup.

Choosing between thread rolling and cutting depends on application-specific requirements such as material hardness, thread strength, production volume, and desired surface finish. Manufacturers targeting high-strength fasteners, precision threaded connectors, or custom screw machine products should evaluate both the technical and economic aspects of thread rolling in light of their specific needs.

If you are comparing thread rolling versus thread cutting methods for your application, consider engaging with an experienced contract manufacturer or fastener supplier. Request thread samples, tolerance documentation, and load-bearing data to ensure the selected threading process aligns with your quality and budget expectations. For unique or specialty projects, prototype fabrication and application testing may be recommended to confirm optimal thread performance. Learn more about the differences between thread rolling and cutting here.

Chapter 4: What factors should be considered in thread rolling?

As with any machining process, several factors must be considered to ensure optimal operating conditions and product quality. Below are some of the key variables that impact thread rolling.

• Material Requirements: A known disadvantage of thread rolling is its incompatibility with hard materials. The materials to be rolled must have a hardness not greater than HRC 40. Materials that can be rolled are low-carbon steels, mild steels, stainless steels, copper alloys, and often, aluminum. Moreover, the material must have the right degree of ductility. The recommended range is 12 to 20% elongation factor.

• Stock Diameter: The correct stock diameter is almost the same as the pitch diameter of the screw or bolts. Usually, the space or cavity between the threads and below the pitch line is the same as the volume of the thread above the pitch line. Some adjustments for tolerances may be needed to attain the desired crest formation, especially if secondary processes such as coating, or plating are needed to be done.

  
  
• Chamfer Angle: Chamfer is the tapered conical surface at the start of a thread. Before rolling, the edge at one end of the stock must be machined to have a chamfer. A correct chamfer angle must be set to properly shape the thread at the end. The recommended chamfer angle is 30° for most cases.

• Feeding: There are three basic techniques for feeding the stock into the dies: radial infeed, tangential feed, and through feed. In radial infeed, the dies move radially towards the axis of the stock. For tangential feed, the pitch of the stock approaches the rollers from its side making square, tangential contact. Lastly, through feed involves a cylindrical die that mates against the stock causing it to move axially.

  
  
• Thread Rolling Speeds: Thread rolling speeds depend on the mechanical and power limitations of the machine, the thread diameter, and the material and hardness of the metal stock. Rolling speeds can range from 30 to 100 m/min. Low rolling speeds are required for hard materials while high speeds are for soft and ductile materials.
• Coolant and Lubricant: Coolants or cutting fluids are extensively used in thread cutting, but these are also necessary for thread rolling. Deforming the metal also generates heat which can compromise both the dies and stock. Moreover, coolants can act also as lubricants to reduce the friction between the dies and stock.

Chapter 5: What are the common defects in thread rolling?

Although the thread rolling process provides higher precision compared to other methods, defects can still occur due to upsets and irregularities in the operation. Common issues include out-of-tolerance stock dimensions, worn or misaligned rollers, and improper stock feeding. The following are the most frequent defects observed in thread rolling.

• Truncated Thread Crest: This defect is described by a non-fully formed crest or an excessively truncated crest. One reason could be an undersized stock where there is insufficient material to flow and create the crests. This is fixed by gradually increasing the size of the stock. If the pitch diameter is oversized, then the more probable root cause is a loose threading head which is solved by sizing-in. If not, then the defect is possibly caused by too much hardness of the material. Thus, it is then necessary to change into a softer material.

  
  
• Flaking: Flaking or slivers causes unusual roughness on the surface of the threads. This is usually caused by the incompatibility of the material for rolling. The root causes can be excessive lead and sulfur content, inconsistent grain structure, and sometimes cold working before rolling. If the material being used already has a good rollability, then other possible causes can be mismatched rollers or dies, rough roller surface, overfilling, or slow rolling speeds.
• Drunken Threads: This defect is seen as wavy or uneven thread crests. This is a result of mismatched dies, misaligned feeding of the stock, or poor die construction. The best solution is to check the condition of the rolls and their bushings.

• Curved Pitch Line: This is viewed as the tapering of the threads towards the ends of the threaded segments of the bolt or screw. The curvature can be concave or convex. Its root causes are inconsistent stock diameter, misaligned stock relative to the roller, wearing of rollers, or too much deformation of the material causing it to flow towards the end of the stock.

  
  
• Out-of-tolerance Helix Angle: This can be a result of a variety of causes such as unsynchronized rollers, imperfect rollers, incorrect feeding of the stock, or screw jacking. This can be solved by correctly timing and aligning the rollers, proper stock feeding, and optimizing the rolling speed.
• Poor Finish: Poor finish is a result of factors such as worn-out dies, high material hardness, oversized stock diameter, or presence of contaminants in the coolant supply.

• Cupped End: A cupped end appears as a concave-end caused by forcing the metal to flow over an insufficient chamfer. This is more evident on softer metals. The defect is solved by properly chamfering the stock, usually about 30°.

  
  

Chapter 6: What are the types of thread rolling machines?

Thread rolling is a straightforward process that begins with cutting a metal bar to length and forging it to create the bolt or screw head. Next, the bar is machined to achieve the correct stock diameter and a chamfer on one end. The prepared stock is then fed into the threading machine, where it passes through dies to shape the thread. After the thread is rolled, the stock undergoes secondary processes such as plating, anodizing, and coating.

This process summary applies to all types of thread rolling. However, thread rolling machines differ based on the type of die used. They can be categorized into flat-die, planetary, or cylindrical-die types.

• Flat-die Type: This type of thread rolling machine consists of two rectangular dies where one is stationary while the other is reciprocating. The reciprocating die moves parallel to the stationary die. The surface of the dies contains ridges representing the profile of the thread to be produced. These ridges are inclined at an angle equal to the helix angle of the thread. The distance between the crests of the dies is equal to the minor diameter of the thread.

  
  

  The threads are formed typically in one passage only. The length of the die allows the stock to be rolled around six to eight times. The stock is inserted on one end, either manually or automatically. The dies roll the stock tangentially which carries it up to the opposite end by friction.

  Contact us to discuss your requirements of Bar Thread Rolling Machine. Our experienced sales team can help you identify the options that best suit your needs.

• Segment or Planetary Type: A planetary type operates by rolling the stock through one stationary and one moving surface. However, this machine uses rotating motion instead of translation. This type involves stationary curved dies and a central rotating die. One or more stationary dies can be matched with a single rotating die. A stationary die rolls one stock at a time.

  
  

  Similar to the flat-die type, the planetary machine has a finite rolling surface that forms the thread through one passage. The stock is inserted on one end of the curved die. The rotating die then rotates a full arc of the curved die revolving the stock until ejected on the opposite end.

• Cylindrical-die Type: Cylindrical dies or rollers are regarded as dies with infinite work surfaces. These machines usually operate through the combination of radial and through feeding. Unlike the flat-die and planetary types, the cylindrical-die type deforms the metal through multiple passes as it rolls. Cylindrical-die type machines can be further divided into two major categories: two-die and three-die machines.

  
  
  1. Two-die: This type of threading machine has two parallel rollers wherein one or both can move radially to accept and penetrate the stock. The stock is positioned with a slight offset from the plane of the centerline of the dies to prevent it from rising out. A smooth roller support or rest bar is located in the middle to hold the stock as it is being threaded.
  2. Three-die: This machine has three rollers positioned 120° from each other. Typically, all rollers can move radially wherein the position of the stock is maintained at the center during penetration. Compared with two-die machines, three-die types have better force balance but are more difficult and complex to adjust.

Conclusion

• Thread rolling is a type of threading process which involves deforming a metal stock by rolling it through dies to form external threads along its surface. Internal threads can be formed using the same principle, specifically termed thread forming.
• The processes of generating threads are generally classified into three methods: subtractive, deformative, and additive. These differ on how the thread is shaped or formed.
• The main advantage of thread rolling is the stronger surface and dimensional accuracy of the product. However, the process is limited to soft metals and requires more expensive tooling.
• There are different types of thread rolling machines that vary according to the type of die used. Thread rolling machines can be a flat-die, planetary, or cylindrical-die type.

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Screw threads may be formed by rolling either by using some type of thread-rolling machine or by equipping an automatic screw machine or turret lathe with a suitable threading roll. If a thread-rolling machine is used, the unthreaded screw, bolt, or other “blank” is placed (either automatically or by hand) between dies having thread-shaped ridges that sink into the blank, and by displacing the metal, form a thread of the required shape and pitch. The thread-rolling process is applied where bolts, screws, studs, threaded rods, etc., are required in large quantities. Screw threads that are within the range of the rolling process may be produced more rapidly by this method than in any other way. Because of the cold-working action of the dies, the rolled thread is 10 to 20 percent stronger than a cut or ground thread, and the increase may be much higher for fatigue resistance. Other advantages of the rolling process are that no stock is wasted in forming the thread, and the surface of a rolled thread is harder than that of a cut thread, thus increasing wear resistance.

Thread-Rolling Machine of Flat-Die Type

One type of machine that is used extensively for thread rolling is equipped with a pair of flat or straight dies. One die is stationary and the other has a reciprocating movement when the machine is in use. The ridges on these dies, which form the screw thread, incline at an angle equal to the helix angle of the thread. In making dies for precision thread rolling, the threads may be formed either by milling and grinding after heat treatment, or by grinding “from the solid” after heat treating. A vitrified wheel is used.

In a thread-rolling machine, thread is formed in one passage of the work, which is inserted at one end of the dies, either by hand or automatically, and then rolls between the die faces until it is ejected at the opposite end. The relation between the position of the dies and a screw thread being rolled is such that the top of the thread-shaped ridge of one die, at the point of contact with the screw thread, is directly opposite the bottom of the thread groove in the other die at the point of contact. Some form of mechanism ensures starting the blank at the right time and square with the dies.

Thread-Rolling Machine of Cylindrical-Die Type

With machines of this type, the blank is threaded while being rolled between two or three cylindrical dies (depending upon the type of machine) that are pressed into the blank at a rate of penetration adjusted to the hardness of the material, or wall thickness in threading operations on tubing or hollow parts. The dies have ground, or ground and lapped, threads and a pitch diameter that is a multiple of the pitch diameter of the thread to be rolled. As the dies are much larger in diameter than the work, a multiple thread is required to obtain the same lead angle as that of the work. The thread may be formed in one die revolution or even less, or several revolutions may be required (as in rolling hard materials) to obtain a gradual rate of penetration equivalent to that obtained with flat or straight dies if extended to a length of possibly 15 or 20 feet (4.6 or 6 m). Provisions for accurately adjusting or matching the thread rolls to bring them into proper alignment with each other are important features of these machines.

Two-Roll Type of Machine: With a two-roll type of machine, the work is rotated between two horizontal power-driven threading rolls and is supported by a hardened rest bar on the lower side. One roll is fed inward by hydraulic pressure to a depth that is governed automatically.

Three-Roll Type of Machine: With this machine, the blank to be threaded is held in a “floating position” while being rolled between three cylindrical dies that, through toggle arms, are moved inward at a predetermined rate of penetration until the required pitch diameter is obtained. The die movement is governed by a cam driven through change gears selected to give the required cycle of squeeze, dwell, and release.

Rate of Production

Production rates in thread rolling depend upon the type of machine, the size of both machine and work, and whether the parts to be threaded are inserted by hand or automatically. A reciprocating flat die type of machine, applied to ordinary steels, may thread 30 or 40 parts per minute in diameters ranging from about 5⁄8 to 1 1/8 inch(15.875–28.575 mm), and 150 to 175 per minute in machine screw sizes from No. 10 (.190 inch) to No. 6 (.138 inch). In the case of heat-treated alloy steels in the usual hardness range of 26 to 32 Rockwell C, the production may be 30 or 40 per minute or less. With a cylindrical die type of machine, which is designed primarily for precision work and hard metals, 10 to 30 parts per minute are common production rates, the amount depending upon the hardness of material and allowable rate of die penetration per work revolution. These production rates are intended as a general guide only. The diameters of rolled threads usually range from the smallest machine screw sizes up to 1 or 1½ inches (25.4 or 38.1 mm), depending upon the type and size of machine.

Precision Thread Rolling

Both flat and cylindrical dies are used in aeronautical and other plants for precision work. With accurate dies and blank diameters held to close limits, it is practicable to produce rolled threads for American Standard Class 3 and Class 4 fits. The blank sizing may be by centerless grinding or by means of a die in conjunction with the heading operations. The blank should be round, and, as a general rule, the diameter tolerance should not exceed ½ to 2⁄3 the pitch diameter tolerance. The blank diameter should range from the correct size (which is close to the pitch diameter, but should be determined by actual trial), down to the allowable minimum, the tolerance being minus to insure a correct pitch diameter, even though the major diameter may vary slightly. Precision thread rolling has become an important method of threading alloy steel studs and other threaded parts, especially in aeronautical work where precision and high-fatigue resistance are required. Micrometer screws are also an outstanding example of precision thread rolling. This process has also been applied in tap making, although it is the general practice to finish rolled taps by grinding when the Class 3 and Class 4 fits are required.

Steels for Thread Rolling

Steels vary from soft low-carbon types for ordinary screws and bolts, to nickel, nickel-chromium and molybdenum steels for aircraft studs, bolts, etc., or for any work requiring exceptional strength and fatigue resistance. Typical SAE alloy steels are No. , , , , , and . The hardness of these steels after heat-treatment usually ranges from 26 to 32 Rockwell C, with tensile strengths varying from 130,000 to 150,000 psi (896– MPa). While harder materials might be rolled, grinding is more practicable when the hardness exceeds 40 Rockwell C. Thread rolling is applicable not only to a wide range of steels but for non-ferrous materials, especially if there is difficulty in cutting due to “tearing” the threads.

Diameter of Blank for Thread Rolling

The diameter of the screw blank or cylindrical part upon which a thread is to be rolled should be less than the outside screw diameter by an amount that will just compensate for the metal that is displaced and raised above the original surface by the rolling process. The increase in diameter is approximately equal to the depth of one thread. While there are rules and formulas for determining blank diameters, it may be necessary to make slight changes in the calculated size in order to secure a well-formed thread. Blank diameter should be verified by trial, especially when rolling accurate screw threads. Some stock offers greater resistance to displacement than other stock, owing to greater hardness or tenacity of the metal. The following figures may prove useful in establishing trial sizes. The blank diameters for screws varying from ¼ to ½ are from 0.002 to 0. inch(50.8–63.5 μm) larger than the pitch diameter, and for screws varying from ½ to 1 inch(12.7–25.4 mm) or larger, the blank diameters are from 0. to .003 inch (63.5–76.2 μm) larger than the pitch diameter. Blanks which are slightly less than the pitch diameter are intended for bolts, screws, etc., which are to have a comparatively free fit. Blanks for this class of work may vary from 0.002 to 0.003 inch(50.08–76.2 μm) less than the pitch diameter for screw thread sizes varying from ¼ to ½ inch(6.35–12.7 μm), and from 0.003 to 0.005 inch(76.2–127 μm) less than the pitch diameter for sizes above ½ inch. If the screw threads are smaller than ¼ inch, the blanks are usually from 0.001 to 0. inch(25.4–38.1 μm) less than the pitch diameter for ordinary grades of work.

Thread Rolling in Automatic Screw Machines

Screw threads are sometimes rolled in automatic screw machines and turret lathes when the thread is behind a shoulder so that it cannot be cut with a die. In such cases, the advantage of rolling the thread is that a second operation is avoided. A circular roll is used for rolling threads in screw machines. The roll may be presented to the work either in a tangential direction or radially, either method producing a satisfactory thread. In the former case, the roll gradually comes into contact with the periphery of the work and completes the thread as it passes across the surface to be threaded. When the roll is held in a radial position, it is simply forced against one side until a complete thread is formed. The method of applying the roll may depend upon the relation between the threading operation and other machining operations. Thread rolling in automatic screw machines is generally applied only to brass and other relatively soft metals, owing to the difficulty of rolling threads in steel. Thread rolls made of chrome-nickel steel containing from 0.15 to 0.20 percent of carbon have given fairly good results, however, when applied to steel. A 3 percent nickel steel containing about 0.12 percent carbon has also proved satisfactory for threading brass.

Factors Governing the Diameter of Thread Rolling

The threading roll used in screw machines may be about the same diameter as the screw thread, but for sizes smaller than, say, 3⁄4 inch (19.05 mm), the roll diameter is some multiple of the thread diameter minus a slight amount to obtain a better rolling action. When the diameters of the thread and roll are practically the same, a single-threaded roll is used to form a single thread on the screw. If the diameter of the roll is made double that of the screw, in order to avoid using a small roll, then the roll must have a double thread. If the thread roll is three times the size of the screw thread, a triple thread is used, and so on. These multiple threads are necessary when the roll diameter is some multiple of the work, in order to obtain corresponding helix angles on the roll and work.

Diameter of Threading Roll

The pitch diameter of a threading roll having a single thread is slightly less than the pitch diameter of the screw thread to be rolled, and in the case of multiple-thread rolls, the pitch diameter is not an exact multiple of the screw thread pitch diameter but is also reduced somewhat. The amount of reduction recommended by one screw machine manufacturer is given by the formula shown at the end of this paragraph. A description of the terms used in the formula is given as follows: D = pitch diameter of threading roll, d = pitch diameter of screw thread, N = number of single threads or “starts” on the roll (this number is selected with reference to diameter of roll desired), T = single depth of thread:

Example: Find, by using above formula, the pitch diameter of a double-thread roll for rolling a ½-inch American standard screw thread. Pitch diameter d = 0. inch and thread depth T = 0. inch.

Kind of Thread on Roll and Its Shape

The thread (or threads) on the roll should be left hand for rolling a right-hand thread, and vice versa. The roll should be wide enough to overlap the part to be threaded, provided there are clearance spaces at the ends, which should be formed if possible. The thread on the roll should be sharp on top for rolling an American (National) standard form of thread, so that less pressure will be required to displace the metal when rolling the thread. The bottom of the thread groove on the roll may also be left sharp or it may have a flat. If the bottom is sharp, the roll is sunk only far enough into the blank to form a thread having a flat top, assuming that the thread is the American form. The number of threads on the roll (whether double, triple, quadruple, etc.) is selected, as a rule, so that the diameter of the thread roll will be somewhere between 1 ¼ and 2 ¼ inches (31.75–57.15 mm). In making a thread roll, the ends are beveled at an angle of 45 degrees, to prevent the threads on the ends of the roll from chipping. Precautions should be taken in hardening, because, if the sharp edges are burnt, the roll will be useless. Thread rolls are usually lapped after hardening, by holding them on an arbor in the lathe and using emery and oil on a piece of hard wood. To give good results a thread roll should fit closely in the holder. If the roll is made to fit loosely, it will mar the threads.

Application of Thread Roll

The shape of the work and the character of the operations necessary to produce it, govern, to a large extent, the method employed in applying the thread roll. Some of the points to consider are as follows:

1. Diameter of the part to be threaded.

2. Location of the part to be threaded.

3. Length of the part to be threaded.

4. Relation that the thread rolling operation bears to the other operations.

5. Shape of the part to be threaded, whether straight, tapered or otherwise.

6. Method of applying the support.

When the diameter to be rolled is much smaller than the diameter of the shoulder preceding it, a cross-slide knurl-holder should be used. If the part to be threaded is not behind a shoulder, a holder on the swing principle should be used. When the work is long (greater in length than two-and-one-half times its diameter) a swing roll-holder should be employed, carrying a support. When the work can be cut off after the thread is rolled, a cross-slide rollholder should be used. The method of applying the support to the work also governs to some extent the method of applying the thread roll. When no other tool is working at the same time as the thread roll, and when there is freedom from chips, the roll can be held more rigidly by passing it under instead of over the work. When passing the roll over the work, there is a tendency to raise the cross-slide. Where the part to be threaded is tapered, the roll can best be presented to the work by holding it in a cross-slide roll-holder.

Tolerances on Wire for Thread Rolling

The wire mills will accept a tolerance specification of plus or minus 0.002 inch (50.8 mm) on the diameter. It is particularly important that this tolerance be maintained on stock used for long screws of small diameter. On screws of short length the material will flow, and if the wire is over size little trouble will be experienced, but in the case of screws having a length greater than ten times their diameter, the material will be confined, and “burning” will take place, if the tolerance is greater than that specified. If the wire is slightly under size, the rolled threads will have a ragged appearance due to the fact that the crest is not fully formed. On screws under the No. 10-24 size, a tolerance of plus or minus 0.001 inch should be adhered to in order to ensure good results.

Speeds and Feeds for Thread Rolling

When the thread roll is made from high-carbon steel and used on brass, a surface speed as high as 200 feet per minute can be used. However, better results are obtained by using a lower speed than this. When the roll is held in a holder attached to the cross-slide, and is presented either tangentially or radially to the work, a considerably higher speed can be used than if it is held in a swing tool. This is due to the lack of rigidity in a holder of the swing type. The feeds to be used when a cross-slide roll-holder is used are given in the upper half of the table “Feeds for Thread Rolling;” the lower half of the table gives the feeds for thread rolling with swing tools. These feeds are applicable for rolling threads without a support, when the root diameter of the blank is not less than five times the double depth of the thread. When the root diameter is less than this, a support should be used. A support should also be used when the width of the roll is more than two-and-one-half times the smallest diameter of the piece to be rolled, irrespective of the pitch of the thread. When the smallest diameter of the piece to be rolled is much less than the root diameter of the thread, the smallest diameter should be taken as the deciding factor for the feed to be used.

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