Five steps to selecting a cutting tool for your operation
Exactalloy · Updated
Selection follows the workpiece first, the operation second and the machine third. Work through material, flute count, helix and reach, tip geometry and holder before choosing a SKU.
Step 1 — Material decides substrate and coating
The workpiece grade and its actual hardness determine the substrate and coating, not the other way around. A series labelled for a hardness range is a starting filter, not proof of suitability for every material at that hardness.
- Carbon and alloy steel: TiAlN / AlTiN coated fine-grain carbide.
- Hardened steel above HRC 55: dedicated high-hardness grades with nano-composite AlTiN.
- Aluminium, including high-silicon alloys: high rake, polished flutes; PCD or diamond coating above roughly 8% Si.
- Graphite and CFRP: diamond coating or PCD.
- Titanium and nickel alloys: unequal helix and variable pitch with high-pressure coolant.
Step 2 — Operation decides flute count
Fewer flutes give more chip room and are used for roughing; more flutes give more cutting edges per revolution and are used for finishing. The number of flutes also changes the feed per revolution for the same feed per tooth.
- Roughing: 2–3 flutes for maximum chip clearance.
- General purpose: 4 flutes.
- Finishing: 6 flutes and above for a stable surface finish.
- Micro-diameter tools: 4–6 flutes with a heavy core to protect rigidity.
Step 3 — Rigidity decides helix angle and length of cut
Higher helix angles reduce cutting force fluctuation and suit deep cavities and long overhangs, but they also increase axial load. Keep the flute length to what the feature actually needs; unnecessary length reduces stiffness.
- Deep cavities and long reach: helix of 45° and above.
- General milling: 30–45°.
- Keep stickout as short as the feature allows.
- Use a shrink-fit extension rather than a long flute where possible.
Step 4 — Feature decides tip geometry
Flat, ball nose and corner-radius tools solve different geometry problems. Choose from the drawing and toolpath rather than from appearance.
- Flat: faces, shoulders and roughing.
- Ball nose: curved surfaces and finishing.
- Corner radius: corner clean-up and mould work; adds edge strength.
- Dovetail, T-slot and chamfer forms are specified by the feature angle and width.
Step 5 — The holder is part of the tool
A correct cutter in a worn or unsuitable holder will not hold tolerance. Runout is set by the whole assembly, so it must be specified together with the tool.
- Finishing and high speed: hydraulic (runout ≤3 µm) or shrink fit.
- Heavy cutting: side-lock, strong or double-contact BBT holders.
- General work: ER collet chucks.
- Deep cavities: shrink-fit extensions.