The fastest metal to machine and the most dangerous one to deep drill. At 1.74 g/cm³ magnesium is about two-thirds lighter than aluminum, drills at speeds that would destroy steel tooling — yet fine magnesium chips flash at roughly 473°C. Deep hole drilling combines high speeds with enclosed chip evacuation: the ideal fire scenario unless the process is engineered, cooled and guarded properly.
| Property | Magnesium Value | Consequence for Deep Holes |
|---|---|---|
| Density | 1.74 g/cm³ (~33% lighter than Al, ~1/5 of steel) | Thin-wall lightweight bores; low workpiece rigidity |
| Machinability | Fastest of any metal; 4–5× tool life | Very high Vc, low cutting forces, high MRR |
| Melting point | ≈650°C | Heat-sensitive; low thermal headroom before chip ignition |
| Chip ignition | ≈473°C for chips/fines | Spark, friction or hot tool edge can ignite fine swarf |
| Reactivity | Reacts with water, releasing H₂ | Water-based coolant is forbidden; hydrogen/oxyhydrogen risk |
| Damping | Low damping coefficient | Chatter and vibration in long, slender drills |
| Alloy | Nominal Composition | Tensile Strength | Machinability | Typical Applications |
|---|---|---|---|---|
| AZ91 | 9% Al, 1% Zn | ≈230 MPa | Good | Die-cast housings, transmission and electronic enclosures |
| AM60 | 6% Al, 0.2% Mn | ≈240 MPa | Good | Crash structures, seat frames, steering wheels; best general corrosion resistance |
| ZK60 | 6% Zn, 0.5% Zr | ≈360 MPa | Moderate | Aerospace structural parts, motorsport, high-strength <150°C service |
| AZ31 | 3% Al, 1% Zn | Wrought, moderate | Good | Sheets, plates, biomedical prototypes |
| AZ61 | 6% Al, 1% Zn | Wrought | Good | Plates, forgings, structural parts |
| Operation | Alloy | Vc (m/min) | Feed (mm/rev) | Notes |
|---|---|---|---|---|
| Gundrill, HSS, φ6–12 mm | AZ91 / AZ31 | 45–75 | 0.03–0.06 | Point angle 70–118°; internal coolant oil; resharpen before edge rounds |
| Gundrill, carbide, φ6–20 mm | AZ91 / AZ31 | 100–180 | 0.04–0.09 | High-pressure oil; thick, well-broken chips mandatory — watch for ignition |
| BTA solid boring | ZK60 / wrought | 80–150 | 0.10–0.25 | Larger bores; internal chip removal contains the swarf stream |
| Twist drill (macro) | AZ91 | 70–120 | 0.10–0.40 | Thick chips carry heat away; HSS or carbide; avoid thin finishing cuts |
| Micro gundrill, φ0.138 mm | Pure Mg | 14–18 | 0.002–0.0045 mm/tooth | Peck strategy, low Vc for burr control; AR up to 36:1 (Micromachines, 2024) |
| Chip Shape | What It Means | Fix |
|---|---|---|
| Long ribbon / continuous | Runs extremely hot, easily ignited | Raise feed, add a chip breaker, lower speed |
| Fine powder / dust | Flashes like dust; hardest to collect safely | Increase feed for thick chips; capture fines under oil or dry-vacuum separately |
| Entangled / intertwined clusters | Most likely to give continuous sparks | Break the chip with feed change; correct tool geometry |
| Small, well-broken, discontinuous | Ideal — carries heat away from the cut face | Maintain with sharp edges, adequate feed, proper geometry |
When coolant is required for a deep hole, use a light mineral oil or a low-viscosity synthetic oil — high flash point, and keep free fatty acids under ~0.2% (acids attack the alloy and raise fire risk). Oil is inert to magnesium, suppresses chip ignition, extends tool life and permits the highest cutting speeds. A 2024 AZ31 study found a low-viscosity synthetic lubricant delivered the best hole perpendicularity at 0.1 mm/rev feed. For deep twist-drill holes, a 40–45° helix evacuates chips best.
Magnesium fires are unique: burning magnesium sustains combustion in nitrogen, carbon dioxide and water. The wrong extinguisher feeds the fire or explodes the scene. Only Class D agents and approved dry media are acceptable.
| Agent | Use on Mg Fire? | Notes |
|---|---|---|
| Met-L-X (sodium chloride powder) | Yes | Melts and crusts over the metal, excluding air and moisture; Class D |
| G-1 dry powder / other Class D agents | Yes | Metal fire powder rated for combustible metals |
| Dry sand | Yes (small fires) | Smothers; keep a bucket at the machine |
| Dry cast iron chips | Yes | Classic shop remedy; smothers without reacting |
| Argon inert gas | Yes | Floods the enclosure; used in automated suppression systems |
| Water | Never | Decomposes to H₂ and O₂; violent explosion and splatter |
| CO₂ / nitrogen | Never | Magnesium burns in both; fire continues |
| Foam / standard ABC dry chemical | Never | Class A/B/C agents are contraindicated on combustible metal fires |
E-stop the machine, stop coolant flow, kill power, and evacuate anyone not fighting the fire. Isolate from other combustibles and ferrous tools.
Apply dry sand or Class D powder gently at the edge of the burning metal and work inward. High-velocity discharge scatters burning swarf and makes it worse.
Layer the agent 50–100 mm deep over large fires. The melted crust excludes air and draws heat away. Do not rake, stir, or disturb the pile.
All four react or fail on burning magnesium. Keep Class D units (e.g., 30 lb Met-L-X) mounted near the machine and checked per NFPA 10.
Keep the crust in place until the mass is below ignition temperature — hours for large piles. Only then move the residue into a sealed steel drum for disposal.
Magnesium’s low hardness and ductility cause heat-softened material to adhere to the flutes, forming built-up edge (BUE), poor finish and rapid wear. For high-aspect-ratio holes, published deep-hole microdrilling work (Politecnico di Milano, in Micromachines) used an intermittent peck drilling strategy with tool retraction to promote heat dissipation, break the chip, and evacuate the confined flute. A light-flow lubricant was used specifically to avoid deflecting the tiny tools, and a pilot drill established the centerline.
Heat softens Mg and welds it to the lip. Raise feed for a cleaner cut, sharpen the tool, check clearance and point angle (70–118°).
Thin ribbon chips or speed too high. Switch to thick, broken chips, reduce Vc, and machine dry or with light oil, never water-based fluid.
Plastic burr from high speed and thermal softening. Lower cutting speed, use a peck strategy, and deburr the exit with a chamfer tool.
Magnesium damps poorly and long drills whip. Rigid setup, pilot hole, guide bushing, and a steady rest for slender workpieces.
High-speed runout enlarges the mouth. Reduce spindle speed, improve toolholding concentricity, and check the guide bushing fit.
Dull edge or under-feed causing ploughing. Sharpen or index the tool, increase feed to cut (not rub), and verify coolant oil quality.