Why there is no single code
Inserts have ISO 1832, which makes a code readable the same way worldwide. Solid end mills do not: standards (for example DIN 6527 for dimensions and DIN 6535 for shanks) fix sizes and shanks, but the catalogue code is the maker's choice. To compare two end mills you read the features, not the code.
1. Diameter
The first figure, and the easiest. Rule of thumb: the largest diameter the part geometry allows. A bigger cutter is stiffer, vibrates less and removes more material. In inside corners the cutter must be smaller than the radius, or it copies the radius instead of contouring it.
2. Number of flutes
- 2 flutes: plenty of chip room. For full slots and materials with long soft chips, like aluminium.
- 3 flutes: the compromise, widely used on aluminium and general work.
- 4 flutes: the classic choice for steel, contouring and finishing.
- 5, 6 or more: finishing and low radial engagement strategies (such as trochoidal milling), hard materials.
More flutes mean a thicker core and a stiffer cutter, but less chip room: in a full slot, chips that cannot get out break the tool.
3. Helix angle
- Around 30°: the general-purpose standard.
- 35–45°: smoother cut and better finish, but more axial pull: the cutter tends to pull out of the holder.
- Variable helix or pitch: flutes are not evenly spaced, so the cut does not resonate. The most effective cure for chatter, especially with long overhangs.
4. The corner
- Sharp corner: for crisp 90° shoulders. It is the weakest point of the cutter.
- Corner radius (bull nose): even a small radius strengthens the corner a lot and extends tool life. The part will have that radius in the corner, though: check the drawing.
- Ball nose: for 3D surfaces and moulds. At the centre the cutting speed is close to zero, so you work tilted or with light passes.
If the cutter has to enter head-on (plunging or ramping) it needs centre cutting: not every end mill has it.
5. Lengths: flute, neck, overhang
An end mill's deflection under load grows with the cube of the overhang: doubling the length out of the holder means roughly eight times more bending. So:
- choose the shortest flute length that covers the depth you need;
- to reach the bottom of a pocket, a necked cutter beats a very long flute;
- in the machine, keep the cutter as short as possible.
6. The shank: HA, HB, HE
Carbide end mill shanks are normally h6 tolerance, with forms set by DIN 6535:
- HA, plain cylindrical: for collet, hydraulic and shrink-fit holders. Best concentricity.
- HB, Weldon flat: for Weldon holders with a side screw. The cutter cannot pull out, but concentricity is worse.
- HE, inclined flat (Whistle Notch): for matching holders, holds the cutter under strong axial forces.
For finishing and small cutters it is almost always HA in a precise holder. HB makes sense where the cutter could pull out, for example high helix and heavy roughing.
7. Coating and grade
Carbide comes in different grades (grain size, cobalt) and the coating changes behaviour a lot. In general: aluminium-titanium based coatings for steels and cast irons, uncoated cutters or specific coatings for aluminium and non-ferrous metals, which must not stick to the chip. The exact choice, and the cutting data, come from the maker's catalogue for your material.
A checklist to compare two end mills
- Diameter and diameter tolerance.
- Number of flutes.
- Helix angle, constant or variable.
- Corner: sharp, radius (how much), ball.
- Centre cutting yes or no.
- Flute length, neck length, overall length.
- Shank: diameter and form (HA, HB, HE).
- Coating and the materials it is meant for.
If two end mills from different makers match on these points, they are comparable. If not, the price alone tells you nothing.