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Thread mills are used for cutting internal and external threads. They are ideal for threading exotic materials and soft, nonferrous alloys where tap breakage is often a problem. Thread mills also provide smaller cutting forces, allowing for the threading of thin-walled parts. They can be used in materials up to 60 HRC and can cut any large size internal thread if the pitch stays the same. For example: an 11/16-16 internal thread can be cut by a thread mill that is rated for the 9/16-16 thread.

Diagram of a Thread Mill

Thread Milling Diagram
Thread Milling Flute Diagram

Advantages of Thread Milling:

  • Complete operation in one setup
  • No upper limits to bore size
  • One tool will cut many different diameters
  • Short, manageable chips produced
  • Threads can be machined within 1/2 pitch of the bottom of blind holes
  • One tool cuts right- and left-hand threads
  • Easily machine threaded parts with interrupted cuts
  • Shorter cycle times due to higher cutting speeds and rapid feed rates
  • Better finish produced over tapping
  • Lower horsepower required than for tapping
  • Hard materials are easier to thread mill compared to tapping
  • Better thread quality in soft, gummy materials where taps normally tear the material


Types of Thread Mills:

Helical flute thread mills Image

Helical flute thread mills are used for cutting internal and external threads in high-speed applications. The helical design allows for reduced cutting pressure, providing a smoother finish on the workpiece. Ideal for tough and thin-walled materials.

Straight flute thread mills Image

Straight flute thread mills are used for cutting internal and external threads in general-purpose applications. There is one tooth per thread on each face. Ideal for strong, fast cutting; great in rigid setups.

Single profile thread mills Image

Single profile thread mills are used for cutting internal and external threads. The multiple flute design allows for cutting threads in a variety of pitch ranges, lengths and diameters. Ideal for unfavorable length-to-diameter ratios and applications where very low side cutting pressure is required.


Bright provides a smooth, polished finish on the tool. It increases chip flow in softer materials such as aluminum, wood and plastic.

Titanium Nitride (TiN) is a multi-purpose coating which increases chip flow in softer materials. The heat and hardness resistance allows the tool to run at higher speeds than uncoated tools.

Titanium Carbonitride (TiCN) is harder and more wear resistant than TiN. It is used on stainless steels, cast iron and aluminum alloys.

Oxide, also known as black oxide or steam oxide, is a surface treatment that prevents chip building, galling and welding on the workpiece. It is commonly used on low carbons, stainless steel and ferrous metals.

Chrome Plate is a bright electroplated coating that offers excellent anti-friction properties. It is commonly used on steel, aluminum, brass, copper or other nonchromium materials.

Nitride is a thin, hard-shell coating that supports surface hardness of the tool. It is used where abrasive or wearing conditions exist.

Aluminum Chromium Nitride (AlCrN) has higher-heat resistance than AlTiN. It is commonly used for machining aircraft and aerospace materials, nickel alloys, stainless steel, titanium, cast iron and carbon steel.

Thread Milling Image

Aluminum Chromium Titanium Nitride (AlCrTiN) is a high heat and wear resistant multilayered PVD coating. It is designed for enhanced tool life and superior thread finish.

TiCN PLUS Titanium Carbon-Nitride (TiCN) plus Titanium Nitride (TiN) is an all-purpose finish designed to increase tool life by two to four times more than TiN coated tools. The heat and hardness resistance allows the tool to run at higher speeds than uncoated tools.

Uncoated tools do not feature supportive treatments on the cutting edge. They are used at reduced speeds in general applications on nonferrous metals.

Cobalt is harder than high speed steel and provides better wear resistance. It is commonly used on high tensile alloys.

High Speed Steel (HSS) provides good wear resistance and can be used in general purpose applications for both ferrous and nonferrous materials.

Solid Carbide provides better rigidity than high speed steel. It is extremely heat resistant and used for high speed applications on cast iron, nonferrous materials, plastics and other tough-to-machine materials.

Vanadium High Speed Steel (HSSE) is made of high speed steel, carbon, vanadium carbide and other alloys to increase abrasive wear resistance and toughness. It is commonly used in general applications on stainless steels and high silicon aluminums.

Powdered Metal (PM) is tougher and more cost effective than solid carbide. It is commonly used on highly abrasive materials including high silicon aluminums.

Thread Milling Image


Terminology:

Pitch: The distance between a point on a screw thread and a corresponding point on the next thread. The pitch is equivalent to one divided by the number of threads per inch.

Threads per Inch (TPI): refers to the total number of threads within a one-inch space on the mill. Higher numbers of threads per inch provide smoother cuts, while fewer numbers of threads provide coarser cuts

Cutting diameter: refers to the cutting portion of the thread mill. The cutting diameter is used for programming the speeds . The internal threads must be smaller than the nominal diameter of the drilled hole.

Number of flutes: refers to the number of grooves or valleys that are cut into the body of the thread mill. Higher numbers of flutes increase the strength of the thread mill and reduce space or chip flow. It can also decrease cycle time and increase tool life.

Thread Millling Image