Tool Materials, Coatings and Geometry
HSS, cobalt, carbide, ceramic, CBN and diamond; coatings; rake and relief angles, flutes, helix and edge preparation.
Tool materials
A cutting tool must stay harder than the work at the temperature of the cut, and tough enough not to chip. The two properties pull against each other: the harder the material, the more brittle it is. The table runs from the tough and cheap to the hard and expensive.
| Material | Hardness (HV, typical) | Max working temperature (°C) | Speed against HSS | Use |
|---|---|---|---|---|
| High-speed steel (M2) | 800 to 900 | 600 | 1 | General purpose, drills, taps, low-rigidity setups; tough and cheap, sharpens easily |
| Cobalt HSS (M35, M42) | 850 to 1 000 | 650 | 1.2 | Tougher alloys and stainless; better hot hardness than M2 |
| Uncoated carbide (WC-Co) | 1 400 to 1 800 | 900 | 3 to 4 | The standard for CNC milling and turning; needs rigidity; shock-sensitive |
| Coated carbide | 1 400 to 1 800 (substrate) | 900 to 1 100 | 4 to 6 | Most inserts and end mills; the coating sets the speed and wear |
| Cermet | 1 600 to 1 800 | 1 000 | 4 to 6 | Steels at high speed for a bright finish; low toughness |
| Ceramic (alumina, silicon nitride) | 1 800 to 2 200 | 1 200 | 8 to 15 | Cast iron and nickel alloys at very high speed; brittle; usually dry |
| Cubic boron nitride (CBN) | 3 000 to 4 500 | 1 400 | 10 and more | Hardened steels (50 HRC and above), cast iron; expensive |
| Polycrystalline diamond (PCD) | 8 000 | 700 | to 20 in aluminum | Aluminum, composites, plastics; not for ferrous metals (carbon dissolves in iron) |
- High-speed steel is tough, takes shock and a sharp edge, and is easily resharpened: the first choice for drills, taps, form tools and hand and low-power machines.
- Carbide (tungsten carbide in a cobalt binder) is two to four times as fast; a higher cobalt content makes it tougher, a lower one harder.
- Ceramics, CBN and diamond are for specific, high-volume jobs; they are brittle and need a rigid, powerful machine.
Carbide grades and the ISO groups
| ISO group (color) | Material | What the grade is built for |
|---|---|---|
| P (blue) | Steels | Crater resistance; coated with TiAlN or Al₂O₃ |
| M (yellow) | Stainless steels | Toughness and a sharp, tough edge |
| K (red) | Cast iron | Abrasion resistance; hard and thick coatings |
| N (green) | Aluminum and non-ferrous | Sharp edges, polished faces, uncoated or diamond |
| S (brown) | Heat-resistant alloys and titanium | Heat and notch resistance; positive geometries |
| H (gray) | Hardened steels | Very hard grades; CBN or ceramic |
Inside each group the maker offers grades from tough (for interrupted cuts) to hard (for finishing); the catalog lists them with recommended speeds.
Coatings
A thin coating (2 to 5 µm) of a hard ceramic on the tool raises the hardness and the working temperature and lowers the friction. The coating sets how fast the tool can run, much more than the substrate does.
| Coating | Working temperature (°C, typical) | Best for |
|---|---|---|
| TiN (gold) | 600 | General purpose; the oldest coating; light wear resistance |
| TiCN | 400 | Tougher and harder than TiN; steels and cast iron at moderate speeds |
| TiAlN / AlTiN | 800 to 900 | High-speed machining of steels and stainless, dry or with minimum lubrication |
| AlCrN | 1 000 | Hard steels and high heat; good in dry milling |
| ZrN or TiB₂ or polished uncoated | — | Aluminum and non-ferrous: smooth and non-stick, no chip welding |
| DLC (diamond-like carbon) | 350 | Aluminum, graphite, non-ferrous; very low friction |
| CVD diamond | 600 | Graphite, composites, high-silicon aluminum; never on steel |
- Match the coating to the material: a TiAlN coating on a tool for aluminum makes chips weld; a polished uncoated tool is better there.
- Do not recoat or resharpen casually: sharpening removes the coating from the edge; recoated tools are usually worthwhile only for expensive tools.
- Hot work and coolant: aluminum-rich coatings (AlTiN) work best hot and dry; do not flood them on an interrupted cut.
Geometry: rake, relief, helix and flutes
- Rake angle γ: the angle of the face the chip slides over. A positive rake lowers the cutting force and the heat; a negative rake gives a stronger, blunter edge.
- Relief (clearance) angle α: the angle behind the edge that keeps the tool from rubbing on the surface; too small and it rubs, too large and the edge is weak.
- Wedge angle β = 90° − γ − α: the strength of the edge.
- Helix angle (end mills): a higher helix cuts more smoothly and lifts chips; a low one is stronger and suits brass.
- Number of flutes: fewer flutes give more chip room (aluminum, plastics); more flutes give a stronger, smoother cut and a higher feed rate (steels).
Worked example: rake angle and edge strength
- A tool with rake -5° and relief 7° has a wedge angle of 90 − (-5) − 7 = 88°: strong and blunt. A tool with rake +15° and relief 10° has 65°: much sharper and weaker.
- As a rule of thumb the cutting force falls about 1 % for each degree of positive rake: going from -5° to +15° (20°) cuts the force by about 20 %, which is why aluminum tools have a high positive rake and why a sharp positive tool helps on a light machine.
- The price is edge strength: at any distance behind the tip the sharper wedge is only 66 % as thick as the blunt one (tan(β ÷ 2) of 65° against 88°), so it chips if the cut is interrupted or too heavy.
| Material | Rake | Flutes | Helix | Note |
|---|---|---|---|---|
| Aluminum, plastics | 12 to 20° positive | 2 or 3 | 35 to 45° | Large flutes for chips; polished; sharp |
| Mild steel | 5 to 10° positive | 4 | 30 to 40° | Balance of strength and force |
| Stainless steel, titanium | 8 to 12° positive, sharp edge | 4 or 5 | 35 to 45° (variable) | Sharp edge avoids rubbing; variable helix reduces chatter |
| Cast iron, hard steels | 0 to −5° (strong edge) | 4 to 6 | 30° | Strong edge for shock and abrasion |
| Brass, free-cutting | 0° to slight negative | 2 to 4 | 20 to 30° (low) | Stops the tool “digging in” |
| Wood and MDF | up-cut, down-cut or compression | 1 or 2 | 20 to 40° | Chip direction sets the finish and dust |
Worked example: do the chips have room?
- A 12 mm three-flute end mill slotting at 6 mm deep and 0.08 mm per tooth cuts 6 × 12 × 3 × 0.08 = 17.3 mm³ of metal per revolution.
- The chips fluff up to about 4 times the solid volume: 69 mm³ must fit in the flute valleys. Three flutes of 5 mm² area over 6 mm of cut give 90 mm³: the flutes would be 77 % full: near the limit for aluminum.
- A four-flute tool of the same size has smaller flutes (about 3.4 mm² each) and a smaller chip load per tooth: with the same feed rate, 17.3 mm³ of metal per revolution makes 69 mm³ of chips for 82 mm³ of room: 85 % full: too tight in aluminum, which is why two- and three-flute tools are used there.
- A flute that is over about 70 % full packs and re-cuts chips: reduce the depth, use air blast or a coolant, or choose fewer flutes.
Edge preparation and corner radius
- Edge hone (a micro-radius of 0.01 to 0.03 mm) strengthens a sharp edge and slows chipping; too large a hone raises the cutting force and rubs on a light cut (see Cutting Force, Power and Torque).
- Corner radius on an end mill (0.2 to 1 mm) strengthens the corner and spreads the wear; use a radius that matches the corner you need to machine.
- Nose radius on a turning insert sets the finish and the strength of the tool (see Turning, Threading and Surface Finish).
- Chamfer or wiper edges on inserts improve the finish at a higher feed.
Choosing a tool
| If you are cutting… | Choose | Avoid |
|---|---|---|
| Aluminum on a CNC router or mill | Polished two- or three-flute carbide, positive rake, uncoated or ZrN | TiAlN coatings and four-flute steel tools |
| Mild steel on a rigid mill | Four-flute coated carbide (TiAlN or AlTiN) | Long reach, uncoated HSS at high speed |
| Stainless steel | Sharp, positive, five-flute variable-helix coated carbide | Dull tools and low chip loads |
| Titanium | Variable-helix carbide with a heat-resistant coating, high-pressure coolant | Dry cutting, long dwell |
| Plastics | Single-flute “O” tools, polished, sharp | Multi-flute steel tools that heat and melt |
| Hardwood and MDF | Compression or up-cut spiral carbide | Dull bits that burn |
| Drilling steel by hand or on a drill press | Cobalt HSS drills with a 135° split point | Cheap uncoated bits |
Troubleshooting
| Symptom | Likely cause | Try |
|---|---|---|
| Coating wears off the edge quickly | Speed too high, or wrong coating for the material | Lower the speed; use the coating made for the material |
| Edge chips in a few seconds | Too brittle a grade or too sharp for the cut | Tougher grade, hone the edge, lower the feed |
| Aluminum welds to the tool | Coating with aluminum (TiAlN), too low a speed | Polished uncoated or ZrN, higher speed, lubricant |
| Tool rubs and squeals on a light cut | Edge too blunt, rake too small | Sharper positive tool, raise the feed |
| Tool breaks in a deep slot | Chips packed in the flutes | Fewer flutes, air blast, reduce the depth |
| Chatter on a long tool | Low stiffness | Variable-helix or variable-pitch tool, shorter reach (see Dialling In: Chatter, Rigidity and Troubleshooting) |
Related
- Speeds and Feeds: the overview of this guide.
- Tool Life, Wear and Cutting Economics: how the tool material sets the Taylor constants.
- Speeds and Feeds by Material: the speeds each tool type runs at.
- Milling: Operations and Tooling and Turning: Lathes and Operations.
FAQ
HSS or carbide?
HSS for drills, taps, hand work and low-rigidity machines (tough, cheap, easy to resharpen); carbide for CNC milling and turning (two to four times the speed) when the setup is rigid.
Which coating should I buy?
Match the coating to the material: AlTiN or TiAlN for steels and stainless, a polished uncoated or ZrN or DLC tool for aluminum, diamond for graphite and composites.
How many flutes do I need?
Two or three for aluminum and plastics (chip room), four or more for steels (strength and a faster feed rate), one for plastics and wood in some cases.
What does rake angle do?
A positive rake lowers the cutting force and heat (about 1 % per degree) but weakens the edge; a negative rake is stronger and needs more power.
References
- Oberg, E., et al., Machinery’s Handbook (cutting tool materials, coatings, tool geometry).
- ISO 513, Classification and application of hard cutting materials for metal removal with defined cutting edges (P, M, K, N, S, H groups).
- Smith, G. T., Cutting Tool Technology: Industrial Handbook, Springer; tool makers’ catalogs (Sandvik Coromant, Kennametal, Seco, OSG).
Last reviewed: October 2026. Hardness, temperatures and speed factors are typical: use the maker’s data.