endmill

An endmill is a rotary cutting tool with flutes (grooves) that run along its length and converge at a pointed or flat end. Unlike a drill bit, which cuts primarily at its tip, an endmill cuts on both its end face and its cylindrical sides, making it suited to hollowing cavities, facing surfaces, and cutting walls inside a workpiece. The tool is held in a spindle and rotates at speeds typically between 500 and 3000 RPM depending on material and diameter.

Endmills are classified by several features. A flat endmill (or square endmill) has a flat bottom and is the most common variant, used for general-purpose pocketing and contouring. A ball endmill has a hemispherical tip and is preferred for 3D profiling and sculptured surfaces where a smooth radius is needed. Corner radius endmills split the difference, with a small fillet at the end to reduce stress concentration. Flute count varies: two-flute endmills suit softer materials like aluminium and plastics and evacuate chips more freely; four-flute and six-flute endmills give finer finishes on steel and cast iron. Longer tools exist for deep pockets; shorter, stiffer tools resist chatter and deflection when cutting harder materials.

The material of the endmill itself determines its speed and hardness. High-speed steel (HSS) endmills are affordable and forgiving, but slow: suitable for manual machines and interrupted cuts. Carbide endmills are much harder, hold their edge longer, and run at three to ten times the speed of HSS, but are brittle and demand rigid setups and consistent feeds. Coated carbide (TiN, AlTiN, or similar) extends tool life further on steel. Ceramic endmills exist for specialized high-temperature work but are rarely seen in general shops.

Endmill breakage usually stems from chatter, deflection, or interrupted cuts. Chatter appears as vibration and poor surface finish; it happens when the tool is too long relative to its diameter, the spindle is worn, or the workpiece clamp is loose. Deflection occurs when feed pressure exceeds the tool's stiffness, especially near the end of a deep cut; this causes overheating and dull edges. Sudden fracture happens if a carbide tool strikes an obstruction or if feed is jerked sideways during a cut. Softer materials like aluminium sometimes clog the flutes if chips are not evacuated; a peck-cutting strategy (retract periodically) helps.

The term 'endmill' emphasizes that cutting happens at the end of the tool, distinguishing it from a face mill (which cuts primarily on a flat face perpendicular to the axis) or a slot drill (an older term sometimes used loosely for the same tool, though slot drill properly refers to a tool without the end-cutting capability of a true endmill). In CNC work, endmills are the primary tool for 2.5D and 3D milling operations; manual machines use them for hand-fed cavity work, spline slots, and finishing passes where accuracy and surface finish matter more than raw speed.