The short answer
An end mill is a rotating cutting tool that machines material with cutting edges on two surfaces: the periphery (the sides of the cylinder) and the face (the bottom). Because the side edges cut, an end mill can mill slots, pockets and profiles sideways across the workpiece, not just bore straight down. A drill bit, by contrast, cuts almost entirely at its tip and is made to move along its own axis. That is the whole distinction, and it is worth insisting on: end mill vs drill bit is not a matter of preference but of different tools for different directions of cut.
The word itself: "end" refers to the cutting face at the end of the tool; "mill" because these tools run in milling machines and, today, CNC machining centers. Older catalogs sometimes split hairs between end mills and slot drills - the slot drill naming note covers that history.
The numbers on the label
End mills are specified by a short list of numbers, and every one of them appears on the spec sheet in some form.
| Line | What it tells you |
|---|---|
| Diameter (D) | The cutting diameter - the width of the slot it cuts |
| Length of cut (LOC) | How deep the flutes reach axially |
| Overall length (OAL) | The whole tool, shank end to tip |
| Shank | The plain cylinder the holder grips; often equal to D on metric tools |
| Flutes | How many cutting edges around the periphery - 2, 3, 4 or more |
| Helix | The twist angle of the flutes |
| Corner | Sharp, radiused, or chamfered at the corner between face and periphery |
Each of these lines has a page or a note behind it on this site - sizes, flute count, and the geometry guides are all written for buyers rather than for sales copy.
The main geometries, in one pass
Within that basic shape, a handful of geometries cover most milling work. A square end mill is the general-purpose rectangle: sharp corners, slots and profiles. A ball nose ends in a hemisphere and is the tool for 3D contours and molds. A bull nose puts a radius at the corner so the edge survives harder alloys longer. A roughing end mill is serrated on the periphery to break chips into short pieces and move metal fast, leaving a scalloped wall for a finishing pass. A tapered end mill brings a cone to the party for draft angles and deep cavities. A chamfer mill cuts 45-degree edges; a corner rounding end mill cuts full-radius fillets. Special jobs get special shapes: T-slot cutters, dovetail cutters, thread mills. The geometry index on the products page links each family's spec sheet.
What they are made of
Two substrates dominate. Solid tungsten carbide - hard, stiff, heat-tolerant - is the default for CNC milling, usually with a thin PVD coating from the TiAlN or TiN families, except in aluminum where polished uncoated flutes often win. High-speed steel (HSS), often with cobalt added, is tougher and cheaper, and stays sensible on older machines and hand-held work. The trade-offs are on the carbide vs HSS note; the substrate line on a spec sheet should name which one you are buying.
How to spec your first set
- Start from the part. What material, what features, what sizes? The workpiece decides the geometry more than any catalog does.
- Pick the geometry family. Slots and profiles start with square; contours start with ball nose; hard alloys argue for a corner radius.
- Pick the size lines. Diameter, LOC, OAL and shank from the sizes reference.
- Pick flutes and coating. Material again: see the flute guide and the coating notes.
- Write the sheet. Every line above, per item, and send it for quotation - the quote form takes a sheet faster than a paragraph.
Center cutting, or not
One more line on the sheet trips buyers: whether the tool is center cutting. A center-cutting end mill has edges that meet at the center of the face, so it can plunge straight into solid material. A non-center-cutting tool has a hole or a recess at the center - some roughing geometries are built this way - and needs a pre-drilled start hole or a lead-in move. Neither is better; they are built for different motions. The practical takeaway is to say which you need when you order, because a non-center-cutting tool in a plunging job is a broken tool and an argument about whose fault it was.
Sizes, and the inch-metric split
End mills come in metric and inch series, and the same physical tool can be described either way. Metric lines run in clean steps with the shank usually equal to the diameter; inch lines include fractions and wire-gauge descendants, and the shank may differ from the cutting diameter. If your market sells both, spec both series explicitly rather than converting in your head - the sizes reference keeps the two apart. Mixed shipments with one series mislabeled are a classic sourcing failure that a careful spec sheet prevents.
Reading a geometry drawing
Suppliers describe tools with simple line drawings, and learning to read one pays for itself immediately. The side view shows the cylinder, the flutes spiraling along the length of cut, the plain shank behind, and dimension arrows for diameter, length of cut and overall length. The end view shows the cross-section: the circle of the tool and the flute profiles around it, which is where flute count and core thickness are visible at a glance. Ten seconds with the end view tells you more about how the tool will carry chips than a paragraph of copy - a deep-fluted two-flute and a stubby four-flute look nothing alike even at the same diameter. The spec pages on this site keep that drawing habit: geometry first, words second.
Why buyers should care about the vocabulary
Most sourcing friction in cutting tools is vocabulary friction. A request like "send your best 6 mm cutter" cannot be quoted; a sheet that reads "D6, 4 flutes, 18 LOC, 60 OAL, 6 shank, fine-grain carbide, TiAlN family" can be quoted to the digit, ground to the same digits, and inspected against them. That is the standard the desk holds programs to: the spec sheet is the agreement, and everything else - samples, inspection, packing - checks against it.