Nickel-iron and cobalt-based magnetic alloys are among the most difficult materials to drill in the deep hole drilling spectrum. They are soft enough to gall and smear — but tough enough to work-harden instantly if the cutting edge rubs instead of cuts. They produce stringy, ductile chips that are difficult to evacuate from a deep hole. And they are expensive — a single scrapped part can represent thousands of dollars in material and machining time. Drilling these materials requires specific tool geometry, carefully selected parameters, and rigorous chip control.
Material Characteristics
Common Magnetic Alloys
| Alloy | Nominal Composition | Typical Hardness (Annealed) | Magnetic Property | Typical Applications |
|---|
| Mu-metal | 80% Ni, 15% Fe, 5% Mo | 80–100 HB | Very high permeability — low coercivity | Magnetic shielding — transformers — sensors |
| Permalloy 45 | 45% Ni, 55% Fe | 100–130 HB | Medium permeability | Transformer cores — relays — magnetic amplifiers |
| Permalloy 80 (HyMu 80) | 80% Ni, 20% Fe | 90–120 HB | Very high initial permeability | Magnetic recording heads — thin-film memory |
| Supermalloy | 79% Ni, 16% Fe, 5% Mo | 80–110 HB | Extremely high permeability — lowest coercivity | Precision magnetic instruments — sensors |
| Hiperco 50 (Permendur) | 49% Co, 49% Fe, 2% V | 200–280 HB | High saturation flux density | Aerospace actuators — generators — motors — pole pieces |
| Vacoflux (Co-Fe) | 49% Co, 49% Fe, 2% V | 220–300 HB | High saturation — high permeability | High-performance magnetic circuits |
| Vacofer / electrical iron | 99.8% Fe | 80–120 HB | High saturation — low coercivity | DC magnets — pole shoes — yokes |
| Amumetal / amorphous metal | Fe-Si-B amorphous | 700–900 HV (hard) | Very low coercivity | High-frequency transformers — sensors |
| Invar | 36% Ni, 64% Fe | 140–180 HB | Low thermal expansion — not magnetic | Precision instruments — thermal stability components |
Material Challenges for Drilling
| Challenge | Cause | Effect on Drilling | Severity |
|---|
| High ductility | Alloys are designed to be soft and ductile for magnetic property optimization — especially Ni-Fe alloys | Stringy chips — difficult to break — challenging to evacuate from deep holes — chip packing risk | High |
| Work hardening | Alloys work-harden rapidly when deformed — cutting edge rubs instead of cuts | Hardened layer forms at hole surface — reduces tool life on subsequent passes — difficult to ream | High |
| Galling and smearing | Soft alloys have high coefficient of friction against tool materials | Material adheres to cutting edge — built-up edge forms — poor surface finish — tool geometry altered | High |
| Low thermal conductivity | Ni-Fe and Co-Fe alloys have lower thermal conductivity than steel | Heat concentrates at cutting edge — higher tool tip temperature — reduced tool life | Moderate |
| Chip evacuation difficulty | Stringy chips are long and difficult to break | Chips pack in chip flute — coolant pressure drops — chip evacuation failure leads to tool breakage | High |
| Burr formation | High ductility causes material to deform rather than shear cleanly at exit | Large burrs at hole exit — secondary deburring operation required — burrs can affect assembly | Moderate |
| Magnetic property sensitivity | Drilling-induced stress and heat can alter magnetic properties | Reduced permeability — increased coercivity — potential part rejection | Moderate (application-dependent) |
| Feature | Recommendation | Why |
|---|
| Cutting edge condition | Sharp — honed edge radius 5–15 µm | Sharp edge cuts cleanly — reduces work hardening — reduces built-up edge formation |
| Rake angle | High positive rake — 10–15° | Positive rake reduces cutting forces — reduces smearing — improves chip flow |
| Relief angle | Standard to high — 8–12° | Adequate relief prevents rubbing on work-hardened surface |
| Point angle | 130–140° (standard) — or 120° with notch | More robust point reduces chipping in sticky material — point notch improves chip breaking |
| Flute finish | Polished — mirror finish | Reduces chip adhesion — improves chip flow — reduces friction |
| Flute design | Wide open flutes — large chip space | Accommodates stringy chips — reduces chip packing risk |
| Coating | AlTiN or AlCrN (smooth finish) | Reduces built-up edge formation — lower coefficient of friction — thermal barrier |
| Coolant hole size | Larger than standard for deep hole | Ensures adequate coolant flow for chip evacuation — critical in stringy materials |
| Tool Material | Hardness | Wear Resistance | Toughness | Suitability for Magnetic Alloys | Recommended For |
|---|
| Micro-grain carbide | 1600–2000 HV | Good | Good | Good — preferred substrate | Most magnetic alloys — general drilling |
| Ultrafine-grain carbide | 1800–2200 HV | Very good | Good | Very good — best substrate | High-production — best wear resistance |
| Standard carbide (C-2) | 1500–1700 HV | Moderate | Moderate | Marginal — edge may chip | Low-production — acceptable for low-ductility alloys |
| HSS (M42, T15) | 900–1000 HV | Low | Very good | Poor — rapid edge wear — built-up edge | Not recommended — emergency use only |
| CBN (cubic boron nitride) | 4000–5000 HV | Excellent | Low | Good for Co-Fe alloys — overkill for Ni-Fe | Harder Co-Fe alloys (Hiperco, Permendur) — high production |
| PCD (polycrystalline diamond) | 6000–8000 HV | Excellent | Low | Good for very soft Ni-Fe — excellent surface finish | Finishing — reaming — not for interrupted cuts |
Drilling Parameters
Recommended Parameters by Alloy
| Alloy | Cutting Speed (m/min) | Feed Rate (mm/rev) | Coolant Pressure (bar) | Coolant Type | Expected Tool Life (m) |
|---|
| Mu-metal (80% Ni-Fe) | 20–35 | 0.02–0.05 | 80–120 | Semi-synthetic 8–10% | 5–15 |
| Permalloy 45 | 30–45 | 0.03–0.06 | 60–100 | Semi-synthetic 8–10% | 8–20 |
| HyMu 80 | 20–35 | 0.02–0.05 | 80–120 | Semi-synthetic 8–10% | 5–15 |
| Supermalloy | 20–30 | 0.02–0.04 | 80–120 | Semi-synthetic 8–10% | 5–12 |
| Hiperco 50 (Permendur) | 15–25 | 0.02–0.04 | 80–150 | Synthetic or semi-synthetic 10–12% | 3–10 |
| Vacoflux (Co-Fe) | 15–25 | 0.02–0.04 | 80–150 | Synthetic 10–12% | 3–10 |
| Vacofer (electrical iron) | 35–50 | 0.03–0.07 | 50–80 | Semi-synthetic 8–10% | 15–30 |
| Invar | 25–40 | 0.03–0.06 | 60–100 | Semi-synthetic 8–10% | 10–25 |
Parameter Adjustment Guidelines
| Condition | Adjustment | Reason |
|---|
| Built-up edge observed | Increase cutting speed 10–15% — increase coolant concentration | Higher speed reduces BUE tendency — higher concentration improves lubricity |
| Stringy chips — not breaking | Reduce feed rate — add chip breaker geometry — increase coolant pressure | Lower feed produces thinner chip — chip breaker encourages breaking — high pressure evacuates before tangling |
| Tool wear rapid | Reduce cutting speed 15–20% — check coolant concentration — verify coating selection | Lower speed reduces thermal load — verify coating suitable for adhesive wear |
| Poor surface finish | Increase coolant concentration — check coolant pressure — inspect cutting edge sharpness | Better lubricity — adequate chip evacuation — sharp edge required |
| Hole oversize | Check guide bushing clearance — reduce feed rate — verify spindle alignment | Magnetic alloys tend to deflect drill — lower feed reduces deflection force |
| Exit burr large | Reduce feed at exit (to 30–50% of drilling feed for last 2–3 mm) — use back-up support | Lower feed reduces material deformation at breakout |
Chip Control Strategies
| Strategy | Method | Effectiveness | Implementation |
|---|
| Chip breaker geometry | Grind notch or step on drill point — creates chip curl and break | High — most effective method | Requires specialized grinding — may affect hole straightness — test before production |
| High coolant pressure | 100–150 bar at the drill tip — ensures chip breaks and evacuates | High — essential for deep holes | Verify system capability — may require pressure upgrade |
| Controlled pecking | Retract drill periodically (every 5–10× diameter) to clear chips | Moderate — reduces production rate | Program peck cycle — ensure rapid retract — reduces chip packing risk |
| Coolant nozzle optimization | Direct coolant flow to break chips at the cutting edge | Moderate — supplementary | Adjust nozzle position — verify chip breaking at source |
| Reduced depth of cut per pass | Multiple passes — each removes less material | Low — time-consuming | Not practical for production — acceptable for small batches |
| Chip flute modification | Increase flute cross-section — polish flutes | Moderate — tool modification | Custom tool design — longer lead time |
Quality Considerations
Burr Control
| Method | Technique | Effectiveness | Cost Impact |
|---|
| Reduced feed at exit | Reduce feed to 30–50% of drilling feed for last 2–3 mm | High — significant burr reduction | No additional cost — programming change only |
| Back-up support | Support workpiece at exit face — rigid clamping | Very high — best method | Requires fixturing modification |
| Drill geometry optimization | Sharper cutting edge — higher rake angle | Moderate | Tool geometry change — no recurring cost |
| Secondary deburring | Mechanical or manual deburring after drilling | High — but adds operation | Additional operation cost — cycle time |
| Exit chamfer pre-drill | Pre-drill chamfer at hole exit before drilling through | High — eliminates exit burr | Additional machining step |
Magnetic Property Preservation
| Concern | Effect on Magnetic Properties | Mitigation |
|---|
| Machining-induced stress | Increases coercivity — reduces permeability — affects magnetic domain alignment | Use stress-relief anneal after machining (typically 800–1000°C in hydrogen atmosphere for Ni-Fe alloys) |
| Heat-affected zone | Local changes in magnetic properties at hole surface | Minimize cutting speed — use adequate coolant — avoid tool rubbing |
| Work-hardened layer | Reduces magnetic permeability at surface | Use sharp tools — adequate feed rate — final stress-relief anneal |
| Contamination — ferrous particles | Embedded particles alter local magnetic properties | Use dedicated coolant filtration — magnetic separation — clean parts thoroughly after machining |
Coolant Requirements
| Parameter | Recommendation | Why |
|---|
| Coolant type | Semi-synthetic (Ni-Fe alloys) — synthetic (Co-Fe alloys) | Semi-synthetic provides good lubricity for Ni-Fe — synthetic provides better cooling for Co-Fe |
| Concentration | 8–12% (higher than standard 5–8%) | Higher concentration improves lubricity — reduces built-up edge — improves surface finish |
| Extreme pressure (EP) additives | Recommended — sulfur or chlorine containing | Improves lubricity at cutting edge — reduces adhesive wear — reduces built-up edge |
| Coolant pressure | 80–150 bar (higher for deep holes) | High pressure essential for chip evacuation in stringy materials |
| Filtration | < 20 µm (10 µm preferred) | Fine particles recirculating cause surface scratches — magnetic particles can embed in bore surface |
| Biocide control | Essential — monitor weekly | Stringy chips carry coolant — increased drag-out — higher biocide depletion rate |
FAQ
What makes nickel-iron and cobalt-based magnetic alloys difficult to deep hole drill?
These alloys present multiple simultaneous challenges: high ductility (they are designed to be soft and ductile for optimal magnetic properties — but ductile materials produce stringy, continuous chips that are difficult to break and evacuate from a deep hole — chip packing is the most common cause of tool breakage in these materials). Work hardening (the alloys work-harden rapidly when the cutting edge rubs instead of cutting — a single dwell mark or worn tool creates a hard spot that destroys subsequent cutting edges — sharp tools and consistent feed are essential). Adhesive wear (the alloys have a strong tendency to gall and smear against the tool material — built-up edge forms quickly, altering the effective tool geometry — surface finish degrades — coolant pressure requirements increase). Low thermal conductivity (heat concentrates at the cutting edge rather than being carried away by the workpiece — tool tip temperature is higher than in steel at the same cutting speed — speed must be reduced). These challenges compound in deep hole drilling — where chip evacuation is already difficult, and tool condition cannot be visually monitored during the cut.
AlTiN (aluminum titanium nitride) and AlCrN (aluminum chromium nitride) are the best coatings for drilling magnetic alloys. Both provide: low coefficient of friction (reduces the tendency for material to adhere to the cutting edge — the primary failure mode in these alloys), thermal barrier (reduces heat transfer to the tool substrate — important since these alloys have low thermal conductivity and concentrate heat at the cutting edge), chemical inertness (resists reaction with the workpiece material — reduces built-up edge formation), and smooth surface finish (polished coating surface — reduces chip adhesion in the flute). AlCrN has slightly better oxidation resistance and is preferred for cobalt-based alloys (Hiperco, Permendur) where cutting edge temperature is higher. For very soft, high-nickel alloys (Mu-metal, Supermalloy), a smooth AlTiN coating with a sharp cutting edge provides the best combination of wear resistance and anti-sticking properties. Avoid TiN coating — it has higher friction and is more prone to built-up edge formation with these materials.
What cutting parameters should I use for deep hole drilling magnetic alloys?
Starting parameters by alloy type: Ni-Fe alloys (Mu-metal, Permalloy, HyMu 80, Supermalloy) — cutting speed 20–35 m/min, feed rate 0.02–0.05 mm/rev, coolant pressure 80–120 bar. Co-Fe alloys (Hiperco, Permendur, Vacoflux) — cutting speed 15–25 m/min, feed rate 0.02–0.04 mm/rev, coolant pressure 80–150 bar. Invar — cutting speed 25–40 m/min, feed rate 0.03–0.06 mm/rev, coolant pressure 60–100 bar. Key adjustments: if built-up edge is observed, increase cutting speed 10–15% and increase coolant concentration to 10–12%. If tool wear is rapid, reduce cutting speed 15–20% — do not increase feed to compensate (higher feed increases cutting forces and the tendency for the drill to deflect). If chips are stringy and not breaking, increase coolant pressure (100–150 bar minimum for deep holes), add chip breaker geometry to the drill point, or use a peck cycle (retract every 5–10× diameter). The most important parameter is coolant pressure — these materials produce stringy chips that require high pressure to evacuate from deep holes.
How do I control burrs when drilling magnetic alloys?
Burr control strategies for magnetic alloys: reduce feed rate at hole exit (the most effective single method — program feed reduction to 30–50% of drilling feed for the last 2–3 mm of hole depth — this significantly reduces the exit burr size by lowering the deformation force at breakout). Use back-up support (support the workpiece material at the hole exit face — a solid backing plate or close-clearance fixture prevents the material from deforming outward as the drill breaks through — this is the most effective method for eliminating exit burrs). Use sharp cutting edges (a sharp drill produces a cleaner shear at exit — a worn or dull drill pushes the material out rather than cutting it — replace tools before they become dull). Consider secondary deburring (if burrs are unavoidable, plan for a secondary deburring operation — mechanical deburring with a carbide burr or manual deburring with a chamfer tool — add cycle time for this operation). For holes that require subsequent heat treatment (stress-relief anneal), deburr before heat treatment — the burrs can become more difficult to remove after annealing.
Can deep hole drilling affect the magnetic properties of these alloys?
Yes — deep hole drilling can affect magnetic properties through: machining-induced stress (the mechanical deformation of drilling introduces residual stresses in the material surrounding the hole — these stresses affect magnetic domain alignment — increasing coercivity and reducing permeability — the effect extends approximately 0.1–0.5 mm from the hole surface). Heat-affected zone (heat generated at the cutting edge can cause localized changes in the magnetic domain structure — the effect is smaller than the stress effect but can be significant in high-permeability alloys like Mu-metal and Supermalloy). Work-hardened layer (the surface layer of the hole becomes work-hardened — this layer has different magnetic properties than the bulk material — it can affect the performance of magnetic components that rely on consistent material properties). The standard mitigation is stress-relief annealing after machining — typically 800–1000°C in a hydrogen atmosphere for Ni-Fe alloys (the specific temperature and atmosphere depend on the alloy and the required magnetic properties — consult the material supplier). For applications where post-machining annealing is not possible (assembled components, size-sensitive parts), use the sharpest tools, lowest cutting forces, and adequate coolant to minimize the affected layer.
Nickel-iron and cobalt-based magnetic alloys are among the most challenging materials for deep hole drilling — requiring specific tool geometry, carefully optimized parameters, and rigorous chip control. Use sharp, coated carbide tools (AlTiN or AlCrN) with high positive rake angles and polished flutes. Maintain cutting speeds at 15–35 m/min (lower for Co-Fe, higher for Ni-Fe) and coolant pressure at 80–150 bar to ensure chip evacuation. Control burrs with feed reduction at exit and back-up support. Plan for stress-relief annealing after machining if magnetic properties are critical to the application. These materials are expensive — invest in process development and tooling to ensure first-pass success. This article reflects industry practice as of 2026.
