EV traction motor shafts are going hollow. A gundrilled axial bore cuts rotating mass, and deep oil and cooling passages keep rotor, magnets, and bearings within thermal limits at high speed. Gun drilling is the method that delivers the highest straightness, and the discipline that keeps millions of shafts a year within tolerance.
| Challenge | Why It Matters | Deep Hole Response |
|---|---|---|
| Rotating weight | Lower inertia = faster acceleration, better efficiency | Axial gundrilled bore removes up to a third of shaft mass |
| Dynamic balance | Imbalance causes vibration, NVH, and bearing wear | Gun drilling delivers the highest achievable hole straightness |
| Rotor cooling | Copper and iron losses heat the rotor core | Axial cooling gallery circulates oil through the shaft |
| Oil delivery | Bearings and splines need a constant oil feed | Inclined (oblique) holes tap the central gallery |
| Assembly alignment | Rotor stack must sit true on the shaft | Precision centerline bore guarantees concentricity |
E-motor components generally carry tight tolerance requirements, and drilling remains central to EV production for the precision holemaking used in assembly, alignment, and structural integrity (Seco Tools). Gun drilling uses a long, thin drill with coolant fed through the center, producing deep, straight holes with high precision and accuracy — exactly what rotor shafts and stator cores need (Precihole).
| Component | Hole Type | Typical Size | Depth |
|---|---|---|---|
| Traction motor rotor shaft | Axial cooling / oil bore, blind or through | Ø20–50 mm | 400–800 mm |
| Gearbox input shaft | Internal oil gallery, step-drilled | Ø15–35 mm | 250–600 mm |
| Stator / rotor core | Alignment and cooling passages | Ø3–12 mm | 150–400 mm |
| Bearing journals | Inclined (oblique) oil feed holes | Ø3–8 mm | Short, angled up to 130° |
| Cooling galleries | Radial bleed holes to rotor surface | Ø4–10 mm | Cross from bore to OD |
| Axle / output shafts | Long hollow tubes for weight and oil feed | Ø25–50 mm | Up to 1,500 mm |
| Material | Typical Grade | Condition | Machinability | Note |
|---|---|---|---|---|
| Low-carbon steel | 1018 / 1020 / 1035 | Normalized, 150–200 HB | Good but gummy | Stringy chips — the #1 chip-control problem (Tungaloy) |
| Medium-carbon alloy | 4140 / 4340 / 40Cr | Quenched & tempered, 200–300 HB | Moderate | Balance of strength and machinability for motor shafts |
| Case-hardening steel | 20CrMnTi / 8620 | Carburized surface 58+ HRC | Difficult after case | Drill before heat treat, or hard-turn after (CBN) |
| Induction-hardened steel | 4140 / 40Cr hardened | 50–60 HRC surface | Very difficult | Hard turning with CBN inserts removes grinding (Tungaloy) |
| Aluminum | 6061 / 6082 | T6 | Excellent | Used for lightweight rotor/sleeve parts; watch built-up edge |
Three systems dominate motor shaft deep holes. Gundrilling is the default for shafts because it is the method that delivers the highest straightness (Tungaloy). Ejector drilling suits mid-range diameters where a high-pressure rotary seal is undesirable. BTA pulls chips through the tube for the largest bores at the highest metal-removal rate.
| Method | Diameter | Max Depth | Chip Evacuation | Tolerance | Roughness |
|---|---|---|---|---|---|
| Gundrill | 2–50 mm | 2,000+ mm | External V-flute, coolant through center | ±0.02 mm typical | Ra 0.8–3.2 μm |
| Ejector (DTS) | 18–150 mm (18–65 typical) | 2,500 mm | Internal, ejector-nozzle suction | ±0.04 mm, IT9–IT11 | Ra 0.8–3.2 μm |
| BTA | 20–500 mm | 3,000 mm | Internal, through the drill tube | ±0.05 mm, IT7–IT10 | Ra 1.6–3.2 μm |
In the ejector system, about 2/3 of the cutting fluid flows between the inner and outer tubes to the cutting zone, pushing chips into the inner tube, while the remaining 1/3 is sprayed at high speed through a crescent-shaped nozzle, creating a low-pressure zone that sucks chips out — combining spray and suction to clear the bore quickly (deepholedrilltools.com). Because chips never touch the machined surface, internal-chip systems finish smoother than gundrilling at comparable diameters.
| Configuration | Purpose | Process Notes |
|---|---|---|
| Axial blind bore | Weight + cooling, closed at one end | Deepest hole; straightness critical; step-drilled with multiple tool diameters |
| Axial through bore | Full-length oil gallery | Simplest chip path; through-exit eases evacuation |
| Stepped bore | Larger entry for spline, reduced bearing seat | Pilot then re-cut to each step; check concentricity between steps |
| Inclined / oblique oil hole | Feed bearings from the central gallery | Up to 130° to axis; withdraw every ~1 mm of feed to clear chips (CN103028915B) |
| Radial bleed holes | Coolant to rotor surface | Perpendicular cross-holes; align to the gallery, deburr break-out |
| Taper end + oil groove | Hydraulic shaft-coupling pressing | Glycerin injection/discharge path; oil groove must sit exactly on the hole |
| Material | Vc (m/min) | Feed (mm/rev) | Coolant | Notes |
|---|---|---|---|---|
| Low-carbon steel (1018/1020) | 60–90 | 0.05–0.12 | 60–120 bar oil | Use chip-splitting inserts; never underfeed (rubbing creates work hardening) |
| Alloy steel (4140/40Cr) | 45–70 | 0.04–0.10 | High-EP oil | Higher EP additives; resharpen on tool-life count |
| Case-hardened 58 HRC | 15–30 | 0.02–0.05 | High-EP oil | Carbide gundrill; low speed, positive feed |
| Aluminum (6061-T6) | 150–300 | 0.08–0.20 | 40–70 bar | Watch built-up edge; polished-flute gundrills help |
Motor shaft volumes reach millions of parts per year, so the machine is built around multiple spindles and automation. Multispindle gundrilling machines drill several parts at once: configurations range from 1–4 spindles (Premach ZK21, depths to 3,000 mm) up to dedicated 6-spindle and 9-spindle cells.
| Reference Cell | Configuration | Part | Key Feature |
|---|---|---|---|
| DeHoff 1024C | 6 spindles | Automotive camshafts (centerline weight-reduction hole) | 1 in capacity, 24 in slide travel, hydraulic clamping, lift-and-carry material handling |
| DeHoff 518 Cell | 9 spindles total (3 + 6) | Scroll compressor drive shafts | Spindle allocation balanced to the longer cycle of the deeper hole |
| Eldorado M75-30T | Twin spindle, independent cycles | General shafts | Counter-rotation fixture, 2,000 psi coolant, 20 gpm |
| Premach ZK21 | 1–4 spindles | Multi-workpiece shafts | CNC, depths to 3,000 mm, coolant 1–18 MPa |
Confirm bore diameter, depth, exit condition, and tolerance against spindle count and cycle budget.
Allocate spindles so deeper, longer-cycle holes don’t idle the cell — split the cell between a large shallow hole and small deep hole (DeHoff 518 pattern).
Verify through-spindle pressure, flow, and filtration at every delivery point; 138–200 bar is normal for motor shafts.
Indexable-insert gundrills remove regrinding and simplify tool management; serialize inserts with life counts.
Measure concentricity, straightness, and surface on the production fixture; feed results into SPC.
Automated load/unload, real-time torque and coolant-pressure monitoring, automated retract on threshold.
Dedicated shaft machines log spindle/feed power and coolant pressure in software (Mollart), so the line runs unattended and flags drift before a scrapped bore leaves the cell.
| Parameter | Typical Tolerance | Measurement Method |
|---|---|---|
| Bore concentricity / coaxiality | ≤0.05 mm TIR on motor shafts | CMM, runout on centers |
| Straightness | 0.05 mm/m (gundrill) | Straightness mandrel, laser bore alignment |
| Surface roughness | Ra 0.8–3.2 μm as-drilled | Profilometer, surface comparator |
| Roundness | 0.01–0.03 mm | Roundness gauge, CMM |
| Wall thickness | ±0.1 mm | Ultrasonic gauging (real-time on large shafts) |
| Cost Driver | Impact | Levers |
|---|---|---|
| Machine amortization | Dedicated multispindle is expensive per hour | More spindles, cycle balancing, lights-out utilization |
| Tooling | Brazed gundrills need regrinding; inserts don’t | Indexable-insert gundrills cut tool-management cost (Tungaloy) |
| Coolant system | High-pressure oil pumps and mist extraction consume energy | Proper filtration, temperature control, high-EP oil life |
| Cycle time | Feed rate sets hole time directly | Optimize feed vs straightness; chip splitters allow faster feed |
| Scrap & rework | One drifted bore can scrap a nearly finished shaft | Monitoring, straightness control, entry preparation |
| Symptom | Cause | Fix |
|---|---|---|
| Crescent-shaped exit / bore drift | Off-square entry, worn guide bushing | Spot-face the entry, use a hardened bushing, consider counter-rotation fixture |
| Chip blockage or tool breakage | Stringy low-carbon chips packing the flute | Chip-splitting inserts, higher coolant pressure, controlled pecking |
| Poor surface finish in bore | Low speed, worn guide pad, dirty coolant | Raise speed within limits, regrind/replace pad, check filtration |
| Taper or oversize entry | Guide pad wear, bushing clearance | Regrind tool, check bushing ID against drill OD |
| Burr at radial-hole break-out | Sudden loss of support at cross-hole | Slow feed over last 2–3 mm, chamfer intersection, alternate hole order |
| Bit fracture on entry or break-through | Sudden change in feed resistance | Controlled sequence: fast feed, slow cut-in, normal feed, slow cut-out, withdrawal (CN102784937A) |