Thread rolling dies are used to cold-form external threads or splines by pressing a hardened die into a rotating workpiece. Thread rolling produces stronger threads than those machined with a cutting tool and is generally less expensive. Applications include the production of threaded shafts and ball screws.
Metal Removal vs. Metal Cutting
Thread rolling uses cold forming, a forging process that shapes metal blanks without removing material. As the blank is compacted and compressed, the amount of force applied determines the metal’s hardness and other material properties, such as fatigue resistance. Changes to the metal grain structure make rolled threads stronger than those produced by cutting devices such as thread mills and threading inserts.
Thread Rolling Machines and Standard Workface
Thread rolling dies are designed to fit thread rolling machines made by companies such as Winter, Landis, Detroit, Davenport, Reed and Salvo. When selecting products, buyers should specify both the manufacturer and model. Most equipment suppliers offer several different models, each of which has a standard workface.
The workface or working face (WF) of a thread rolling die should always exceed the length of the thread to roll. As a general rule, thread roll suppliers recommend allowing the thread to overhang each end of the metal blank by at least 1.25 threads. For root-to-root or crest-to-crest measurements, 1 thread may be sufficient.
Thread Type and Thread Size
In addition to thread rolling machine/model and standard workface, key specifications for thread rolling dies include thread type and thread size or pitch.
- Thread type refers to the helical structure that is used to convert between rotational and linear movement. Symmetrical threads are easier to manufacture and include Whitworth and Acme threads. UNC, UNRC, UNF, UNRF, and 8UN threads are other common thread types.
- Thread pitch refers to the number of threads to roll per axial distance (i.e., threads per inch or TPI). As a rule, coarse threads have fewer threads per axial distance than fine threads. Coarse threads also have a larger thread form relative to the diameter.
Although thread rolling dies are relatively inexpensive, buyers may wish to consider the total cost of owning and operating thread rolling equipment – especially if thread mills and threading inserts are suitable alternatives. Generally, however, thread rolling is recommends for higher volumes and external threads. Thread rolling dies can be used to produce knurling and splining, but not internal threads.