Why Magnesium Is Different from Aluminum
If you have ever drilled aluminum at production scale, you know the common headaches: built-up edge, stringy chips that tangle around the tool, and the need for high-pressure coolant to break those chips. Magnesium is in a completely different league.
Magnesium has one-third the density of aluminum and roughly half the Young’s modulus. Cutting forces are dramatically lower. The coefficient of thermal expansion is higher, which means the hole closes in on the tool faster as it cools. But the biggest difference is how it burns.
Aluminum chips can ignite under extreme conditions — I have seen it happen with sub-micron dust in a fire-suppressed enclosure. Magnesium chips, however, will ignite at approximately 450 °C in ambient air, and the fire is intense enough to burn through steel tooling. The chip form, the coolant selection, and the feed rate all become safety parameters, not just quality parameters.
I have spent years drilling both materials, and I treat magnesium as a completely separate discipline. The techniques that work beautifully on 6061 aluminum will get you hurt on AZ91.
Chip Formation: Fine Chips vs. Stringy Chips
The chip morphology in magnesium is unique among common structural metals. Magnesium’s hexagonal close-packed crystal structure limits the slip planes available during shear deformation. The result: chips that fracture cleanly and consistently into small, segmented particles rather than the continuous ribbons you get from aluminum or steel.
This is good and bad.
Good: Small, well-broken chips evacuate through the gun-drill chip flute with minimal friction. There is virtually no risk of chip packing or clogging inside the bore — a problem I fight constantly with aluminum deep-hole drilling. The low chip load keeps coolant return velocities manageable even at moderate pressures.
Bad: Fine magnesium chips present a massive surface-area-to-volume ratio. A handful of coarse turnings might not ignite, but the same mass of fine, powdery chips can flash in an instant. The chip breaker geometry you choose directly controls the fire risk. I run chip breakers that produce thumbnail-sized chips — roughly 2–5 mm — rather than dust. This means balancing feed rate against surface finish requirements, which I will detail in the parameters section.
The stringy chip problem that plagues aluminum deep-hole drilling simply does not exist with magnesium. That saves you one headache, but do not relax — the fire risk more than makes up for it.
Fire Risk and Coolant Strategy
This is the section that matters most. Read it twice.
Never, under any circumstances, use water-based coolant with magnesium.
Water and magnesium are chemically incompatible at cutting temperatures. When the drill tip contacts the workpiece, the fresh magnesium surface reacts with water to produce hydrogen gas:
Mg + 2H₂O → Mg(OH)₂ + H₂↑
The hydrogen accumulates in the chip flute and around the cutting zone. A single spark from a worn tool edge or a hot chip impacts the gas pocket, and you have an explosion inside the bore. I have seen the aftermath of a water-coolant magnesium fire at a job shop — the gun-drill snapped, the workpiece was scrap, and the fire suppression system dumped CO₂ across the entire cell.
The reaction accelerates with temperature. At 100 °C it is slow. At 300 °C — easily reached at the tool-workpiece interface — it is vigorous. And the hydrogen does not need an ignition source if the magnesium chips themselves ignite, because magnesium burns at over 2,000 °C and provides plenty of spark.
Coolant Types That Work (and What Is Dangerous)
The table below summarizes what I use and what I avoid.
| Coolant Type | Magnesium Compatibility | Fire Risk | Notes |
|---|---|---|---|
| Neat oil (mineral oil) | Excellent | Low | My default for all magnesium gun-drilling. High flash point (≥200 °C) preferred. |
| Straight mineral oil, high viscosity | Excellent | Low | Best chip evacuation. Keeps chips coated and cool. |
| Synthetic ester oil | Very Good | Low | Better lubricity than mineral oil for surface finish. More expensive. |
| High-oil emulsion (≥60 % oil) | Marginal | Elevated | Only acceptable with oil-to-water ratio ≥60:40 and constant monitoring. Not recommended. |
| Standard soluble oil emulsion (5–10 %) | Dangerous | High | Produces hydrogen. Do not use. |
| Semi-synthetic / synthetic (water-based) | Dangerous | High | Hydrogen evolution is guaranteed. Do not use. |
| Straight water / flood water | Extremely dangerous | Extreme | Violent hydrogen evolution. Catastrophic fire risk. Never use. |
My recommendation: use a neat mineral oil with a viscosity between 10–22 cSt at 40 °C and a flash point above 200 °C. I have had excellent results with gun-drilling-specific oils that include EP additives — they reduce the friction at the guide pads and keep the burnishing zone cool enough that chip ignition is virtually impossible at normal parameters.
For deep-hole drilling, the coolant must serve double duty: lubricate the guide pads and evacuate chips. Oil does both for magnesium. The one downside is the oily mess on the machine and parts — budget for a wash station after the drilling operation.
Cutting Parameters for Common Magnesium Alloys
I run different parameters depending on the alloy because the machinability varies significantly. Here is my reference table after years of tuning.
| Alloy | Condition | Drill Ø (mm) | Spindle Speed (RPM) | Feed Rate (mm/rev) | Coolant Pressure (bar) | Coolant Flow (L/min) | Typical MRR (cm³/min) |
|---|---|---|---|---|---|---|---|
| AZ31 | Wrought, annealed | 5–15 | 4,000–8,000 | 0.020–0.040 | 40–80 | 30–60 | 8–25 |
| AZ31 | Wrought, as-rolled | 5–15 | 3,500–6,000 | 0.025–0.050 | 40–80 | 30–60 | 10–30 |
| AZ91 | Die-cast | 5–20 | 3,000–5,000 | 0.030–0.060 | 50–100 | 40–80 | 12–40 |
| AZ91 | Sand-cast, T6 | 5–20 | 2,500–4,500 | 0.020–0.040 | 60–100 | 40–80 | 8–25 |
| ZK60 | Wrought, aged | 5–15 | 3,000–5,000 | 0.015–0.030 | 60–120 | 40–80 | 6–18 |
| ZE41 | Sand-cast, T5 | 5–20 | 2,500–4,500 | 0.020–0.035 | 50–100 | 40–80 | 7–20 |
| Elektron 21 | Cast, T6 | 5–15 | 2,000–4,000 | 0.015–0.030 | 60–120 | 50–100 | 5–15 |
Key observations:
- AZ91 is the most forgiving — the high aluminum content stabilizes chip fracture and the casting microstructure produces consistent, short chips. I start every new magnesium job on AZ91 if possible.
- ZK60 is harder and tougher. It requires lower feed rates and higher coolant pressure. The chips are thinner and more prone to producing fine dust if the feed drops below 0.015 mm/rev.
- AZ31 sits in the middle. Wrought material can work-harden locally if the feed is too low, so I keep the feed rate aggressive enough to maintain a minimum chip thickness above the edge hone radius.
I always use climb-drill peck cycles in conventional drilling, but with gun-drilling the continuous cut means the parameters above must keep the chip form consistent over the entire bore length.
Surface Finish Achievable
Magnesium rewards you with excellent surface finish when the parameters are right. Here is what I consistently achieve.
| Condition | Typical Ra (µm) | Typical Rz (µm) | Notes |
|---|---|---|---|
| Gun-drilling, oil coolant, AZ91 die-cast | 0.4–0.8 | 3.0–6.0 | Best results. Die-cast surface feeds the burnishing zone consistently. |
| Gun-drilling, oil coolant, AZ31 wrought | 0.6–1.2 | 4.0–8.0 | Slightly higher due to microstructural variation in wrought material. |
| Gun-drilling, oil coolant, ZK60 | 0.5–1.0 | 3.5–7.0 | Good finish but requires stable feed to avoid chatter. |
| Gun-drilling, oil coolant, all alloys (suboptimal feed) | 1.2–2.5 | 8.0–15.0 | Happens when feed is too low or guide pads are worn. |
The low cutting forces mean the tool deflects less, and the burnishing action of the gun-drill guide pads produces a surface that often requires no secondary operation for sealing applications like hydraulic valve bodies or compressor housings. I have held 0.6 µm Ra over 300 mm bore lengths in AZ91 with a single-pass gun-drill at 4,500 RPM and 0.040 mm/rev.
The one surface defect I watch for is the “smear layer” — a thin, re-deposited magnesium layer that masks porosity in cast alloys. If you are drilling for a leak-tight application, account for this in your NDT process.
Safety Protocols
I follow these protocols on every magnesium drilling job. They are not optional.
Coolant verification. Before the first part, I verify the coolant type and concentration with a refractometer for emulsions or a flash-point check for oils. I label the coolant tank clearly: “MAGNESIUM — OIL ONLY — NO WATER.”
Chip handling. Magnesium chips are collected separately from steel, aluminum, and brass chips. I store them in a covered steel container with a tight lid. No open containers, no plastic bins. The chip container is placed in a designated fire-safe area at least 3 m from the machine.
Machine preparation. I remove all combustible residues from the machine interior — accumulated oil mist, paper filters, plastic covers. The machine must have a Class D fire extinguisher (metal fire) within arm’s reach of the operator station, not a Class ABC extinguisher that will not control a magnesium fire.
Tool monitoring. I track tool wear more aggressively with magnesium than with any other material. A dull tool generates higher temperatures, and higher temperatures mean ignition risk. I set a maximum tool life of 80 % of the expected flank-wear limit and replace the tool at that threshold.
Coolant flow interlock. The machine coolant pump is interlocked with the spindle. If coolant pressure drops below the minimum threshold, the spindle stops. No exceptions.
Fire drill. Every operator on the cell runs a magnesium fire drill quarterly. They practice using the Class D extinguisher and the dry sand bucket (backup method). They know not to use water — never water.
Post-operation cleaning. At the end of each shift, the machine interior is vacuumed with an explosion-proof vacuum to remove any accumulated chip fines. Chip bins are emptied into the central fire-safe collection.
These protocols came from experience — my own and others’. I have seen what happens when you skip step 2 or step 4. A magnesium fire in a deep-hole drilling machine is not a small fire. It is a machine-destroying event.
Related Reading
- Gun Drilling Aluminum: Chip Breaking and Surface Finish — I cover the aluminum-specific challenges that magnesium drilling avoids, including built-up edge and stringy chip management.
- High-Pressure Coolant Safety Systems — Coolant pressure interlocks and filtration safety are even more critical when drilling with flammable chip materials.
Key Takeaways
- Magnesium machines with very low cutting forces and produces clean, broken chips — but the fine chip form creates a significant fire hazard that demands a different approach to coolant and safety.
- Never use water-based coolant with magnesium. The hydrogen evolution reaction makes it explosive. Use neat oil or mineral oil with a flash point above 200 °C.
- AZ91 is the most forgiving magnesium alloy for deep-hole drilling. ZK60 requires lower feeds and higher coolant pressure.
- Surface finish of 0.4–0.8 µm Ra is achievable in cast alloys with proper parameters. Watch for the smear layer in porosity-sensitive applications.
- Follow strict safety protocols: dedicated chip collection, Class D fire extinguishers, coolant-flow spindle interlocks, and regular fire drills. A magnesium fire will destroy your machine if you are not prepared.
- The low tool wear and consistent chip evacuation make magnesium one of the most productive materials to gun-drill — when you respect the fire risk and set up your process correctly.