An end mill is a rotating cutting tool for milling machines with cutting teeth on its end as well as around its sides, so it can cut sideways through material, not just plunge like a drill. It is the main tool for machining slots, pockets, profiles and 3D contours, and is usually made of high-speed steel or carbide.
In short: the end mill is the workhorse cutter of CNC milling. Most milled features are shaped by one.
How Does an End Mill Work?
An end mill is held by its shank in a collet or holder on the machine spindle. As it spins, the helical cutting edges, called flutes, shear off chips from the workpiece. Because teeth run along the periphery as well as across the end, the tool can cut with its side while the machine moves it across the part, and with its end when it moves down.
Flutes can be straight or spiral, and spiral flutes are made right-hand or left-hand. The spaces between the flutes carry chips out of the cut. Fewer flutes leave more room for chips; more flutes give more cutting edges per revolution.
How fast an end mill can run is set by its surface speed, in surface feet or meters per minute, which depends on the tool material and the workpiece material. Carbide tools run considerably faster than high-speed steel ones. The spindle speed and feed rate are then calculated from the tool's diameter and number of flutes.
Types of End Mills
| Type | Shape and feature | Typical use |
|---|---|---|
| Flat (square) end mill | Flat end with sharp corners | Flat-bottomed pockets, slots, square shoulders, side profiles |
| Ball-nose end mill | Hemispherical tip | 3D contoured surfaces and molds, finished with closely spaced passes |
| Corner-radius (bull-nose) end mill | Flat end with rounded corners | Pockets with filleted floors; a stronger edge than a sharp corner |
| Two-flute end mill | Two cutting edges, deep flutes | Slots and keyways; can plunge to start its own hole |
| Four-flute end mill | Four cutting edges | The most common general-purpose cutter |
| Shell end mill | Large diameter (normally over 2 in / about 50 mm), mounted on an arbor | Face milling large flat surfaces |
End Mill vs Drill Bit
A drill bit cuts only at its point and is designed to move straight down its own axis to make a round hole. An end mill cuts on its sides as well as its end, so it can move sideways to widen a hole, cut a slot or trace an outline. A center-cutting end mill, such as a two-flute cutter, can also plunge, but a drill remains the usual tool for plain holes.
How to Design Parts for End Milling
The tool shapes the part, so a few design choices make milling faster and cheaper:
- Round internal corners. A spinning tool cannot cut a sharp inside corner. Give vertical inside corners a radius slightly larger than the cutter radius, so a standard tool can turn the corner smoothly.
- Keep pockets shallow relative to their width. Deep, narrow pockets need long, thin tools that deflect and must run slower. Shorter, stiffer tooling cuts more accurately.
- Match floors to the tool. A flat floor with a small corner fillet suits a corner-radius end mill; a sharp floor corner needs a flat end mill and more passes.
- Expect scallops on 3D surfaces. A ball-nose end mill leaves small ridges between passes. Closer passes or a larger ball make them smaller, at the cost of machining time.
NICE Rapid mills parts in-house on 3-, 4- and 5-axis CNC machines, with an engineering review for machinability before toolpaths are programmed.
Frequently Asked Questions
What is an end mill used for?
An end mill is used on a milling machine to cut slots, pockets, steps, outside profiles and 3D contours. Because it cuts with its sides as well as its tip, it can move sideways through material instead of only drilling straight down.
What is the difference between an end mill and a drill bit?
A drill bit cuts at its point and only moves along its own axis to make round holes. An end mill has teeth on its sides and end, so it can cut sideways as well. A center-cutting end mill can also plunge, but plain holes are usually drilled.
How many flutes should an end mill have?
Two-flute end mills leave more room for chips and suit slots and plunging. Four-flute end mills are the most common general-purpose cutters, with more cutting edges for a smoother finish at a given feed. The right count depends on the material and the cut.
Related Services at NICE Rapid
- CNC Machining — 3-, 4- and 5-axis milling with tolerances to ±0.01 mm and as-machined finishes of Ra 0.8–3.2 μm.
- Rapid Prototyping — machined prototypes in metal and plastic, typically in 3–5 days.
- Request a Quote — send a STEP file and drawing; an engineering review is included with every prototype quote.
Sources
- US Army TC 9-524, Fundamentals of Machine Tools, Chapter 8: Milling Operations (hosted by Carnegie Mellon University) — end mill "has teeth on the end as well as the periphery"; straight or spiral flutes, right- or left-hand; four-flute spiral cutter most common; two-flute cutters mill slots and keyways and drill their own starting holes; shell end mills normally over 2 in diameter; cutters of high-speed steel, stellite and cemented carbide; RPM from cutting speed and diameter — uhv.cheme.cmu.edu
- University of Florida, EML2322L Design and Manufacturing Laboratory, "Drilling and Milling Speeds and Feeds" — cutter speed measured as surface feet per minute; recommended speeds by material; carbide speeds 2.5× HSS; stiffness of tooling ("shorter is better") affects the speed you can run — web.mae.ufl.edu
- Harvard GSD Fabrication Lab, "Selecting a Stepover" — ball mills used for 3D toolpaths, alongside flat end mills and bull mills; scallops between passes grow with stepover and shrink with a larger tool — wiki.harvard.edu
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