Thread rolling is a metal forging process that forms threads into the mirror image of a roller die. The process is different from metal cutting, grinding, and chasing because it does not remove any metal from the workpiece. Instead, thread rolling uses hardened steel dies to displace and mold ductile metals. The high-pressure process physically changes the properties of rolled metal parts to make the base part and the threads harder and stronger. 

The tooth form of the hardened steel dies protrudes into the outside diameter of plain cylindrical blanks to reform the surface. Each tooth exudes extreme pressure against the workpiece’s surface, pushing the blank material outward toward the crests in the thread roll profiles, thus producing a part with a thread mirroring that of the die. That displaced metal in the crests forms the threads.

Thread rolling has several advantages that traditional thread cutting processes lack. One key advantage is the strengthening force of the pressure—instead of cutting through the grain of the part’s microstructure, and intrinsically weakening the workpiece, thread rolling realigns the grain lines while leaving them largely intact. It also hardens the entire affected material. Manufacturers that use thread rolling processes produce parts that are smoother, stronger, long-lasting, and more resistant to handling damage.

Tools and Design Considerations

Designing parts with rolled threads is different from designing parts with cut threads. With cut thread parts, designers need to make the major diameter of the shaft equal to the major diameter of the threads. This allows machinery to cut away excess material without making the thread diameter too small.

With rolled thread parts, designers must instead account for how the metal flows across the rolled surface rather than being removed. Here are some factors that must be accounted for:

• Diameter of the part blank diameter should be about .002 inches smaller than the maximum part pitch diameter. Blank diameter is the diameter prior to rolling thread.

• When rolling close to the major diameter, every 0.001 inches of blank diameter adjustment changes the thread diameter by 0.003 inches, a 3:1 ratio after rolling thread. Harder metal pieces, however, may require a smaller chamfer angle of 25-28° if part print allows. This will produce a shorter angle of 40-42°, primarily used to maintain thread roll life in harder materials.

• The workface of the thread roll needs to be longer than the thread itself to allow for the material’s displacement over the length of the part, without chipping the thread rolls over the ends of the rolled part. Thread roller dies should overhang the workpiece on each side by 1.25 threads, or 2.5 threads over the entire part length.

• Thread rolling can produce straight or tapered threads on parts.

CJWinter has the fastest delivery of standard and custom thread rolls in the industry. We ship 80% of our rolls in just 24 hours! No matter what specifications govern your product, we can design, manufacture, and deliver long-lasting, high-quality thread rolls that meet your exact specifications. 

Materials

Both thread rolling and thread cutting have a valuable role in creating threaded parts. Some metals respond well to rolling; other materials to cutting. To utilize thread rolling, the material needs to have an elongation factor of 12% or more. This property determines if the metal can safely handle the displacement forces of thread rolling. Metals that meet this requirement will result in a finished product with better form geometry, finish, fatigue resistance, strength, and wear resistance.

Some materials, like cast iron, are too brittle to be viable for thread rolling. These materials must be cut instead. Manufacturers also use thread cutting for components that don’t need blank accuracy, or need thread to run up against the shoulder of the part. 

One rule to keep in mind when choosing an appropriate material for thread rolling is the Formability Index. Softer metals can better handle thread rolling. The harder the metal, the faster it wears down the thread rolls. Stainless steel workpieces, for example, produce a beautiful thread but can quickly wear down the thread rolls if blank diameter is not prepared correctly with a sharp cutting insert. A dull insert can cold work the blank diameter, creating a hardened outer surface and reducing thread roll life.

Benefits of Thread Rolling vs. Thread Cutting

Other than choosing the proper thread fabrication process based on material properties, it’s also important to consider the needs of the product and of your company.

Benefits of Thread Rolling

Thread rolling offers many benefits, especially when working with softer metals. Those benefits include:

• Better production quality. Thread form geometry is better, harder, stronger, and more resistant to wear. This extends the life of the part and gives it better functionality. Thread rolling is also much more accurate, and the finishes are 32 micro-inches or less (which is generally half that of parts with cut thread).

• Lower costs. Since thread rolling doesn’t remove material, there’s very little process waste. The process itself is also much faster – usually about 10% of thread cutting cycle – and has lower tooling costs, which reduces labor and materials costs as a whole.

• Faster production/shorter lead times. Thread rolling’s faster pace facilitates the production of larger orders than cutting in the same amount of time. Thread rolling requires one pass compared to thread cutting’s 10 required passes. Companies can get completed orders with more accurate pieces in a much shorter period of time.

Benefits of Thread Cutting

Thread cutting, however, does have some applications where it can perform better. The benefits of thread cutting include:

• Handles materials with high tensile strength. If the metal is too hard or brittle, it won’t be compatible with thread rolling. Thread cutting can accommodate materials with an elongation factor below 12% and metals with a strength greater than 150 kilo pounds per square inch.

• Works with leaden materials. Pressed lead forms flakes, which creates a poor-quality finish and can contaminate the coolant used in thread rolling processes. This makes thread cutting a better choice.

• Compatible with larger material stock. It’s not always practical to roll diameters greater than 16 inches, so thread cutting is the default process for larger items.

Thread Rolling Applications

At CJWinter, we specialize in producing thread-rolled parts for myriad industrial applications. Our components can be used in factory applications, equipment, and commercial or residential settings. Some of the most popular applications for thread-rolled parts include the following:

• Aerospace parts

• Automotive parts

• Oil & gas parts

• Medical parts

• Plumbing parts

• Fasteners

• Fittings

• Fluid connectors

• HVAC components