What machinability means
Machinability is a loose measure of how easily a material can be cut: tool life, the cutting speed allowed, the force needed, the surface finish achieved, and how the chips behave. It depends on the material’s hardness and strength, but also on how it conducts heat, whether it work-hardens, whether it welds to the tool, and whether it breaks into short chips. Soft metals can be hard to machine (copper is gummy) and hard metals can be easy.
- Hardness and strength raise cutting force and heat.
- Thermal conductivity. Aluminum and copper carry heat away, so the tool stays cool. Titanium and stainless carry little heat away, so the cutting edge gets hot.
- Work hardening. Austenitic stainless steel and some nickel alloys harden as they are cut, so a rubbing tool makes the next cut harder.
- Adhesion. Soft, ductile metals (aluminum, copper, low-carbon steel at low speed) weld to the tool as a built-up edge.
- Chip form. Long, stringy chips are dangerous and tangle; free-machining additives (lead, sulfur, bismuth) make them break short.
Cutting speeds by material
The same cutting speed ranges used on Speeds & Feeds are shown here: typical starting ranges for HSS and for carbide tools, in meters per minute and surface feet per minute (SFM = m/min × 3.281). Use the lower end on a small or light machine, without coolant, or in a long, thin tool.
| Material | HSS (m/min) | Carbide (m/min) | Carbide (SFM) |
|---|---|---|---|
| Aluminum alloys | 75–100 | 180–600 | 591–1 969 |
| Brass (free-cutting) | 45–90 | 120–230 | 394–755 |
| Plastics (acetal, nylon, acrylic) | 60–150 | 150–300 | 492–984 |
| Mild steel (1018, A36) | 25–35 | 75–150 | 246–492 |
| Gray cast iron | 20–30 | 75–150 | 246–492 |
| Alloy steel, annealed (4140) | 15–25 | 60–120 | 197–394 |
| Stainless steel (304) | 15–25 | 60–120 | 197–394 |
| Titanium (Ti-6Al-4V) | 6–12 | 30–60 | 98–197 |
Aluminum
| Alloy | Character | Notes |
|---|---|---|
| 6061-T6 | The general-purpose alloy: strong, easy to machine, weldable, anodizes well | First choice for most parts; typical yield strength about 275 MPa (see Material Properties) |
| 7075-T6 | Very strong, harder; aircraft grade | Machines well, a little more abrasive; poor for welding; anodizes |
| 2024-T3 | Strong, fatigue resistant | Machines well; corrodes unless protected |
| 5052, 3003 | Soft, formable sheet | Gummy: sharp tools and lubricant |
| Cast (A356, 380) | Contains hard silicon | Abrasive: carbide or coated tools |
- Fast and light. Use a high cutting speed (200–400 m/min in carbide), a good chip load, and shallow engagement on small machines. Aluminum cuts best fast.
- Tools: 2- or 3-flute, high-helix (40–45°), polished or ZrN/DLC coated carbide; uncoated is fine. Avoid TiAlN, which sticks.
- Built-up edge and chip welding. The cure is more speed, a sharper polished tool, lubricant (WD-40, mist, or cutting fluid) and chip evacuation. Re-cutting a chip welds it to the tool.
- Chip clearance. Aluminum makes large, bulky chips: use 2 or 3 flutes, an air blast or mist, and do not slot too deep.
- Thin walls move as they are cut and flex away from the tool; finish them last with a light pass.
- Burrs. Soft aluminum burrs easily; use a sharp tool, climb cuts, and a chamfer.
- Stress and flatness. Use stress-relieved plate (T651, MIC-6 cast tooling plate) for flat parts.
Mild, free-machining and alloy steels
| Steel | Hardness | Machinability | Notes |
|---|---|---|---|
| 12L14 (free-machining, leaded) | Soft | Excellent: short chips, the standard for turned parts | Contains lead: do not weld; see hazards |
| 1018 / A36 (mild steel) | Soft | Good: tends to form long chips and a built-up edge at low speed | Weldable, cheap |
| 1045 | Medium | Fair | Stronger; harder on tools |
| 4140 / 4340 (alloy) | Annealed to pre-hardened (28–32 HRC) | Fair; pre-hard is slower | Machined before heat treatment where possible |
| Tool steels (A2, D2, O1) | Annealed: soft; hardened: 58–62 HRC | Annealed: fair; hardened: very hard | Machine annealed then harden; grind or use CBN after |
- Moderate speed and heavier chip load. Steel prefers 100–200 m/min in coated carbide and a chip thick enough to carry away heat. A light, rubbing cut wears the tool.
- Tools: 4-flute (or more), 30–40° helix, TiAlN or AlTiN coated carbide. HSS-Co works at low speed on a flexible machine.
- Coolant: flood or mist improves life and finish; for carbide at speed, an air blast is sometimes better than a thermal shock from intermittent coolant.
- Chips. Long, hot strings: break them with chip-breaking cycles, feed rate and pecks.
- Pre-hardened and hardened steel need a rigid machine, a smaller depth, and often a different tool grade; above 45 HRC use hard-milling tools or CBN.
- Scale. Hot-rolled bar has a hard surface scale; take the first cut deeper than the scale, or use conventional milling.
Stainless steel
| Grade | Character | Notes |
|---|---|---|
| 303 | Free-machining austenitic | The easiest stainless to machine; less corrosion resistant |
| 304 / 316 | Austenitic: tough and work-hardening | Sharp tools, positive geometry, steady feed; 316 is tougher still |
| 17-4 PH | Precipitation-hardening martensitic | Machine in the solution-treated condition (H900 hardens it) |
| 410 / 416 | Martensitic | 416 is free-machining; harden after machining |
| Duplex (2205) | Strong, work-hardening | Slower than 304; rigid set-up |
- Never rub or dwell. The surface hardens, so the next pass cuts harder material. Keep the feed up, take a firm cut below the hardened layer, and do not stop the tool in the cut.
- Low speed, high feed. 40–100 m/min in carbide with a feed per tooth of at least 0.03–0.05 mm on a 10 mm tool.
- Sharp, positive-rake tools with a strong coating (AlTiN) and a rigid holder. Use fewer flutes (4) to leave chip space, and a small, constant engagement.
- Coolant in quantity: stainless conducts heat poorly, so the edge runs hot.
- Light finishing pass with a sharp tool; do not leave too little stock, or the tool rubs.
- Chips are tough and stringy; use a chip breaker or peck.
Brass, bronze and copper
| Material | Character | Notes |
|---|---|---|
| 360 brass (free-cutting) | The easiest metal to machine | High speed, short chips, excellent finish; contains lead |
| Naval brass, yellow brass | Softer; can be gummy | A zero or slightly negative rake tool prevents digging in |
| Bronze (phosphor, aluminum) | Tough; abrasive aluminum bronze | Moderate speed; sharp tools |
| Copper (C110) | Soft, gummy, high conductivity | Sharp polished tools, lubricant, high speed; chips stick |
| Beryllium copper | Strong, springy | Dust and fumes from machining are toxic: see hazards |
- Brass can run at 90–230 m/min with carbide, little force and a good finish. Use a tool with zero or negative rake to stop the tool from pulling into the work.
- Copper is gummy: the chips weld to the tool. Use sharp, polished tools, a high feed, and lubricant.
- Dust. Brass and bronze dust is fine and heavy; clean it off the machine, since it can stain and work into slides.
Titanium
- Heat stays at the edge. Titanium conducts heat poorly, so the cutting edge and chip get very hot. Use a low cutting speed (30–60 m/min in carbide), plenty of coolant aimed at the cut, and sharp tools.
- Strong and springy. It deflects away from the tool, so use a rigid set-up, a short tool and constant engagement, and reduce the speed rather than the feed.
- Never dwell. A stopped tool in titanium burns the edge and smears the surface.
- Adaptive paths with a small radial depth and high axial depth work well, because the heat spreads along the flute.
- Chips and fines can burn. Fine titanium chips and dust can ignite; keep the machine clean and never use water on a titanium fire (use a Class D extinguisher or dry sand).
- Tools: carbide with an AlTiN or a titanium-specific coating, 4–6 flutes, variable pitch for chatter.
Cast iron and other materials
- Gray cast iron breaks into short chips and is easy to machine, but it is abrasive and the graphite makes fine dust. Machine it dry with an air blast and extraction; use coated carbide and avoid cutting through the hard skin with a light cut.
- Ductile (nodular) iron is tougher and cuts like a hard steel.
- Magnesium machines easily, but the fine chips and dust are a fire hazard: use sharp tools, large feeds (no rubbing), clean the machine, and use a Class D extinguisher; never use water. Do not machine it unless you know the precautions.
- Nickel and cobalt alloys (Inconel, Hastelloy) are tougher than titanium and need low speeds and strong carbide or ceramic tools; leave them to specialist shops.
- Graphite is dusty and abrasive: use diamond-coated tools, full extraction and a respirator.
- Tool steel (hardened): hard milling with CBN or coated carbide, light cuts at high speed, rigid set-up; or EDM and grinding.
Coolant and lubrication
| Method | Use | Notes |
|---|---|---|
| Dry | Cast iron, plastics, wood; carbide at speed in some steels | Chips carry away the heat; use an air blast to clear them |
| Air blast | Most hobby milling | Cheap; clears chips and cools a little |
| Mist / MQL (minimum quantity lubrication) | Aluminum, steel, drilling and tapping | A fine oil mist: lubricates, little mess; needs ventilation |
| Flood coolant | Production milling and turning of steel, stainless, titanium | Best cooling and chip flushing; needs an enclosure, a pump and fluid care |
| Through-tool coolant | Deep holes, drills, titanium | High-pressure coolant to the cutting edge |
| Cutting oil / paste | Tapping, threading, hand work | Apply by brush or spray |
- Water-soluble coolant is mixed to 5–10 % in water; check the concentration (with a refractometer) and replace it when it smells.
- Thermal shock. On carbide inserts at speed, an intermittent coolant supply can crack the edge. Use a steady, full flood or none.
- Skin and lungs. Coolant mist and fluid can cause skin and breathing problems; use guards and extraction, and wash your hands.
- Keep coolant away from the electronics, and from fires in magnesium or titanium.
Starting values
The table gives a first cut for a 10 mm solid carbide end mill. These are starting points for a rigid machine; reduce them for a hobby machine or a long tool, and take the tool maker’s recommendation as the authority. For other sizes, use CNC Feeds & Speeds Calculator.
| Material | Vc | Flutes | fz (mm) | Spindle speed | Feed rate |
|---|---|---|---|---|---|
| 6061 aluminum | 250 m/min | 3 | 0.07 | 7 958 rpm | 1 671 mm/min |
| 1018 mild steel | 100 m/min | 4 | 0.05 | 3 183 rpm | 637 mm/min |
| 304 stainless | 80 m/min | 4 | 0.04 | 2 546 rpm | 407 mm/min |
| Ti-6Al-4V | 45 m/min | 4 | 0.04 | 1 432 rpm | 201 mm/min |
Worked example: the power for a 10 mm deep, 3 mm wide cut
- 6061 aluminum: Vf = 1 671 mm/min; P = 10 × 3 × 1 671 × 700 ÷ 60 000 000 = 0.58 kW.
- 304 stainless: Vf = 407 mm/min; P = 0.49 kW at less than a tenth of the feed rate: steel uses less power per minute, but much more per cubic centimeter.
- The specific cutting force kc is typically 700 N/mm² for aluminum, 1 800 for mild steel, 2 400 for stainless and 1 400 for titanium alloys (it varies with chip thickness); see Sandvik’s material tables.
- The cutting speed in titanium is low (about 45 m/min), so the spindle speed is also low and the heat is high; this is the reason for flood coolant.
Hazards by material
| Material | Hazard | Precaution |
|---|---|---|
| Leaded steel, brass, bronze | Lead in chips and dust | Wash hands before eating; do not weld or burn it; collect swarf; avoid breathing the dust |
| Beryllium copper | Beryllium dust and fume cause chronic lung disease | Machine only with extraction and wet; use a respirator; many shops refuse it |
| Magnesium, titanium (fines) | Chips and dust burn fiercely | Keep the machine clean; Class D extinguisher or dry sand; no water |
| Stainless, nickel, chromium alloys | Fume and dust (chromium, nickel) irritate and sensitize | Extraction; avoid dry-grinding; wash hands |
| Cast iron, graphite | Fine silica and carbon dust | Extraction and a respirator |
| Coolant mist | Skin and lung irritation, bacteria | Guarding, extraction, clean coolant |
| Hot chips | Burns and cuts | Eye protection; use a brush or hook, not hands |
See Troubleshooting, Accuracy and Safety and Workshop Safety.
Related
- CNC Machining: the overview of this guide.
- Milling: Operations and Tooling and Turning: Lathes and Operations: tools, speeds and feeds in detail.
- Machining Plastics, Wood and Composites: plastics, wood and composites.
- Material Properties and Speeds & Feeds.
FAQ
Which aluminum is best to machine?
6061-T6 is the standard choice: easy to machine, strong enough for most parts, and it anodizes well. 7075 is stronger and machines almost as well. Cast tooling plate is the best for flatness.
Why does stainless steel dull my tools so fast?
It conducts heat poorly, so the edge runs hot, and it work-hardens when rubbed. Keep the feed up so the tool cuts instead of rubbing, use a sharp coated tool, and add coolant.
Can I machine steel on a hobby CNC?
Only with a rigid machine, a small cutter, very light cuts, and patience. Aluminum, brass and plastics are more realistic on a router or light mill.
Do I need coolant for aluminum?
Not always, but an air blast or mist helps clear chips and keeps the tool from welding. In deep cuts or long runs, a lubricant improves finish and tool life.
Is machining brass or bronze dangerous?
Normal precautions are enough for most: avoid breathing the dust, wash your hands and keep food out of the shop, since many brasses contain lead. Beryllium copper is the exception and needs special controls.
References
- Oberg, E. et al., Machinery’s Handbook, 31st ed., Industrial Press: cutting speeds, machinability, materials.
- Sandvik Coromant, Material groups and specific cutting force (kc1) tables.
- ASM International, ASM Handbook, Volume 16: Machining.
- Tool-maker speed and feed data: Harvey Tool, Helical Solutions, Kennametal, Seco (material-specific starting values).
- OSHA and HSE guidance on metalworking fluids and on beryllium, lead and magnesium hazards.
Last reviewed: October 2026. Cutting data are starting points: machines, tools and materials differ, so test on scrap and follow the tool maker’s data.