Thread mills are rotating, multi-point tools that are used to produce internal or external threads. They resemble end mills or drill bits, and may be capable of producing multiple threads with a single cut. Unlike thread rolling dies then, thread mills perform metal cutting rather than metal removal operations. Some have a built-in thread pitch. Thread mills area an alternative to thread cutting or thread forming.

Thread Milling vs. Thread Forming

Thread milling is recommended for the production of taper pipe threads and use with difficult-to-machine materials, such as high-temperature alloys and some heat-treated metals. Although thread mills are generally more flexible than thread roll dies, thread mills with a single-point insert may not perform optimally with some machines. Customers with CNC milling machines may use one tool for two threads by positioning the tool holder differently.

Cutter Specifications

In addition to the number of points, buyers should consider cutter specifications such as configuration, diameter, and size. There are two choices for cutter configuration: solid construction, and holder and inserts. Thread mills with a solid construction feature a one-piece design. Devices that consist of a tool holder and insert have two pieces.

Along with the full tool-cutting diameter, buyers should consider whether the thread mill is specified according to English or metric pitch. Thread pitch refers to the number of threads to cut per axial distance (i.e., threads per inch or TPI). Coarse threads have fewer threads per axial distance than fine threads.

Thread Types

Thread mills may be used to produce various types of threads, the helical structures that are used to convert between rotational and linear movement. Symmetrical threads are generally easier to manufacture, and include designations such as Whitworth and Acme threads. Other common thread types include UNC, UNRC, UNF, UNRF, and 8UN.

Materials of Construction

The cutters on some thread mills are made of tool steel and coated with titanium nitride because of its good wear properties. If the coating becomes worn, these thread mills may be stripped and refurbished for additional use. Other cutter materials include carbide, micro-grain carbide, ceramic, cobalt, high-speed steel, and polycrystalline diamond (PCD).   

References

YouTube Video – Thread Milling up close

CTS Cutting Tool Supply – Thread Milling Basics