Three Requirements That Separate CNC Rotary Axes from General Servo Applications
Most precision planetary gearbox selection methodologies are written for general servo automation — conveyors, robots, indexers. CNC machine tool rotary axes introduce three additional requirements that these guides do not address, and failing to account for any one of them leads to a gearbox specification that is technically correct for torque and speed but wrong for the application.
CNC accuracy specifications state tolerances at the workpiece — at the actual cutting radius, which could be 10mm (small bore) or 300mm (large facing). The same 8 arcmin backlash produces 23μm of tangential error at R=10mm but 1,163μm at R=500mm. CNC specifications must always be evaluated at the actual cutting radius, not at a representative intermediate value. Backlash that is acceptable for one operation may fail completely for another on the same machine.
CNC cutting loads are highly variable — the torque changes instantaneously with chip thickness, material variations, and tool entry/exit. Under a 380 N·m peak cutting torque, an EP-ZDE-160 (Ct=38 N·m/arcmin) deflects elastically by 10 arcmin — more than the specified backlash — producing a tool-position error that servo feedback cannot detect or correct because the motor encoder is on the input side of the gearbox. This load-dependent error is invisible to the servo, directly accumulates in the workpiece, and worsens with cutting force amplitude.
CNC machine tools use flood coolant at 2–8 bar pressure, cutting oil mist, and periodic internal machine washdown. A gearbox installed on an external rotary table positioned under the spindle can receive direct coolant impingement at full pump pressure. IP54 (standard for EP-ZDE/ZDF/ZDWE/ZDWF) protects against directional splashing but not sustained direct jets. Only IP65 (EP-ZDS) withstands the IPX5 test — 6.3mm nozzle at 12.5L/min from any direction — which approximates flood coolant conditions on exposed rotary fixtures.
Backlash at Your Cutting Radius — The Table CNC Engineers Need
The following table converts standard planetary gearbox backlash specifications into tangential positioning error at representative CNC cutting radii. All values use the exact formula: E_tangential = R × tan(BL / (60 × 180/π)). For boring and turning operations where the tool traces a circular path, this tangential error appears directly as roundness deviation on the finished surface.
| Reaksi | R=10mm Small bore |
R=25mm Φ50mm bore |
R=50mm Φ100mm bore |
R=100mm Φ200mm face |
R=200mm Φ400mm table |
Seri EP |
|---|---|---|---|---|---|---|
| <3 arcmin | 8.7 μm | 21.8 μm | 43.6 μm | 87.3 μm | 174.5 μm | Custom / special order |
| <8 arcmin ★ | 23.3 μm | 58.2 μm | 116.4 μm | 232.7 μm | 465.4 μm | EP-ZDE/ZDF (60–160mm) · EP-ZDS (all) |
| <12 arcmin | 34.9 μm | 87.3 μm | 174.5 μm | 349.1 μm | 698.1 μm | EP-ZDE-40 single stage; 2-stage units |
| <25 arcmin | 72.7 μm | 181.8 μm | 363.6 μm | 727.3 μm | 1,454 μm | EP-ZDWE/ZDWF — right-angle input only |
★ Standard EP-ZDE/ZDF/ZDS series. Values are maximum backlash error before CNC backlash compensation. With Fanuc/Siemens backlash compensation active, residual error at slow feed rates is typically <10% of uncompensated value. Values in bold red exceed IT7 tolerance for that bore diameter — application requires either tighter backlash, compensation, or larger bore tolerance specification.
A VMC 4th axis boring a Φ100mm hole (R=50mm) with an EP-ZDE-120 (<8 arcmin) produces a maximum backlash-induced roundness error of 116.4μm per revolution. With Fanuc Series 0i backlash compensation, this reduces to approximately 12–20μm — compatible with IT8 tolerance. Without compensation, the same bore at IT7 (25μm for Φ100mm bore) fails. Specify backlash compensation in the CNC program, or upgrade to EP-ZDS to reduce backlash contribution to the error budget.
CNC Rotary Axis Specifications — B/C/A Axis and 4th/5th Axis, Axis by Axis
The five rotary axis types used in Korean CNC machine tools have different primary design drivers. The same EP series unit that is correct for a 4th-axis trunnion on a VMC will be inadequate for a B-axis on a 5-axis machining centre, and wrong for a C-axis on a turning-milling centre that experiences full turning torques. The axis-by-axis analysis below resolves the correct specification for each.
Tilt range ±90° or ±110°
Peak cutting torque: 150–600 N·m depending on size
Heavy interrupted cuts in titanium/Inconel
Clamping hold torque: 2–3× cutting torque
IP54 minimum (coolant mist); IP65 for external fixture
Torsional stiffness dominates above crossover torque
ZDS-142 crossover: 352 N·m (BL × Ct = 8 × 44)
ZDE-160 crossover: 304 N·m (8 × 38)
For T_cut > 304 N·m: elastic error exceeds backlash
→ tighter backlash spec won’t help; higher Ct will
Medium 5-axis (T_cut ≤ 250 N·m):
→ EP-ZDS-115, 20:1, Ct=20 N·m/arcmin
Heavy 5-axis (T_cut 250–600 N·m):
→ EP-ZDS-142, 20:1, Ct=44 N·m/arcmin
Very heavy (T_cut > 600 N·m):
→ EP-ZDS-190, 20–25:1, Ct=130 N·m/arcmin
Turning mode: C-axis is the spindle drive — high torque, continuous rotation, no position precision required
Milling/positioning mode: C-axis indexes to exact angle for off-centre milling — position accuracy critical
Index accuracy: ±5–15″ of arc (arcseconds)
8 arcmin = 480 arcseconds — far too wide for ±5″ target
C-axis precision: requires gear + encoder closed-loop
The gearbox sets the accuracy floor that the encoder must overcome through closed-loop correction
C-axis positioning (mill-turn):
→ EP-ZDE-160 or EP-ZDS-115 at 10–16:1
BL <8 arcmin; combined with Heidenhain/Renishaw encoder for ±15″ accuracy
Flood coolant exposure: specify EP-ZDS-115/142
Tilt range ±45° to ±90°
Spindle plus head assembly: 40–120kg
Gravity load at maximum tilt: full head weight at R=200–400mm
Must hold position against gravity during cutting
Typically no clamping brake — gearbox must hold statically
60kg spindle head, R=300mm at full tilt:
T_gravity = 60 × 9.81 × 0.3 = 176.6 N·m
Required holding (with SF=2.0): 353 N·m
This is the static load — add dynamic cutting torque
Total requirement often 400–700 N·m
Light gantry head (40–60kg):
→ EP-ZDS-115 or ZDE-160, 16–20:1
Heavy gantry head (80–150kg):
→ EP-ZDS-142, 16:1, Ct=44 N·m/arcmin
Very heavy (150kg+):
→ EP-ZDS-190, 16–20:1, Ct=130 N·m/arcmin
Separate add-on rotary tables (4th axis) or dual-axis trunnion (4th+5th)
Table diameter: Φ200–Φ400mm
Workpiece weight: 20–80kg typical
Often used at full flood coolant — IP65 critical
Positioning only (no continuous turning)
4th axis indexing: typically ±10–30″ arc
Combined with worm or planetary gear and encoder
Planetary approach: compact, efficient, low BL
EP-ZDE-120 (Ct=12 N·m/arcmin, BL <8): adequate for most VMC 4th axis
Heavy workpiece or interrupted cut: upgrade to EP-ZDS-115
Light table (20–40kg workpiece):
→ EP-ZDE-120, 10–16:1 (IP54 if internal machine)
Medium table (40–80kg, coolant exposed):
→ EP-ZDS-115, 16–20:1, IP65
Heavy table (>80kg or heavy interrupted cut):
→ EP-ZDS-142, 16:1, IP65, Ct=44
Why Torsional Stiffness Determines Part Tolerance in Heavy CNC Operations
In interrupted milling cuts, face milling with large cutters, and turning operations on difficult materials, the cutting torque varies rapidly between near-zero (at air cut) and full cutting torque (at full engagement). Each engagement/disengagement cycle applies an impulse to the gearbox that causes elastic wind-up and spring-back of the output shaft. This elastic oscillation — happening at the cutter contact frequency — is what causes surface roughness patterns, polygon marks on bored holes, and chatter on turned surfaces.
| Cutting Scenario | T_peak (N·m) | ZDE-160 elastic error |
ZDS-142 elastic error |
ZDS-190 elastic error |
At R=100mm ZDS-190 advantage |
|---|---|---|---|---|---|
| Light aluminium facing | 80 N·m | 2.1 arcmin 0.122mm@R=100 |
1.8 arcmin 0.105mm@R=100 |
0.6 arcmin 0.035mm@R=100 |
3.4× |
| Steel rough milling | 200 N·m | 5.3 arcmin 0.308mm@R=100 |
4.5 arcmin 0.262mm@R=100 |
1.5 arcmin 0.087mm@R=100 |
3.4× |
| Heavy steel boring | 380 N·m | 10.0 arcmin 0.581mm@R=100 |
8.6 arcmin 0.500mm@R=100 |
2.9 arcmin 0.169mm@R=100 |
3.4× |
| Inconel interrupted cut | 600 N·m | 15.8 arcmin 0.919mm@R=100 |
13.6 arcmin 0.791mm@R=100 |
4.6 arcmin 0.267mm@R=100 |
3.4× |
Elastic error = T_peak / Ct. ZDE-160: Ct=38; ZDS-142: Ct=44; ZDS-190: Ct=130 N·m/arcmin. At R=100mm using E = R × tan(θ/3438). Servo motor encoder cannot detect this elastic deflection — it accumulates directly as workpiece dimensional error.
Design implication: For CNC operations where the cutting radius exceeds 50mm and peak cutting torque exceeds 200 N·m, the elastic deflection error from a ZDE-160 (0.308mm at R=100mm, T=200 N·m) exceeds IT8 tolerance for most bore sizes. The ZDS-190 reduces this to 0.087mm — within IT7 tolerance range. The same backlash specification (<8 arcmin) applies to both series; the stiffness difference alone produces the accuracy improvement, which tighter backlash specification cannot replicate.
Coolant Environment and IP Rating — Matching Protection to CNC Reality
The IP rating decision for a CNC rotary axis gearbox is not a generic “inside machine = IP54” choice. The actual coolant exposure depends on gearbox position relative to the coolant flow path, the machine enclosure design, and whether the rotary axis is integrated into the machine or added externally. Making the wrong IP choice results in the contamination failure described in the failure causes guide — which in CNC environments typically manifests within 2,000–4,000 hours.
- Integrated 5-axis head (B-axis) inside a fully enclosed machine — coolant is directed away from the B-axis drive by machine design
- VMC 4th axis trunnion mounted away from direct spindle coolant discharge
- C-axis positioning on turning-milling centres with an enclosed coolant chamber below the gearbox
- EDM rotary C-axis (dielectric fluid, not water-based coolant)
- Any axis where the gearbox is above the coolant line and receives only mist, not direct jets
- External rotary tables positioned on the VMC table — directly in the machine’s coolant discharge zone
- Horizontal machining centre (HMC) pallet rotary — pallet rotation brings the drive unit through the coolant zone
- Transfer line rotary indexers with HACCP-compliant (food/medical) washdown
- Any CNC rotary axis below the spindle centerline in a machine without a splash guard specifically protecting the drive
- Outdoor CNC cutting operations (waterjet, plasma, laser with water table)
- Integrated B-axis in a machine without a full enclosure — depends on whether the coolant management system protects the drive unit
- Retrofit 4th axis added to a machine not originally designed for it — coolant routing may not account for the new unit’s position
- High-pressure through-spindle coolant (TSC) machines where coolant spray patterns are unpredictable
- When in doubt: specify IP65 (EP-ZDS). The cost premium for IP65 is far less than the cost of a contamination-induced failure and unscheduled line stoppage
Complete CNC Rotary Axis EP Series Selection Matrix
| CNC Application | T_peak (N·m) |
Perbandingan | AKU P | Min Ct (N·m/arcmin) |
Recommended EP Series | Primary Spec Driver |
|---|---|---|---|---|---|---|
| 5-axis B-axis, medium (50kg table) | 150–300 | 20:1 | IP54 | 20 | EP-ZDS-115 | Stiffness + inertia at 20:1 |
| 5-axis B-axis, heavy (100kg table) | 300–600 | 20:1 | IP54/65 | 44 | EP-ZDS-142 | Crossover at 352N·m; heavy Inconel/Ti |
| HMC rotary table, flood coolant | 400–900 | 16–25:1 | IP65 | 44–130 | EP-ZDS-142/190 | IP65 + highest Ct + high torque |
| C-axis mill-turn, precision index | 50–200 | 10–16:1 | IP54 | 12–38 | EP-ZDE-160 | BL <8; encoder provides ±15″ accuracy |
| Gantry mill A-axis, 60kg head | 250–500 | 16–20:1 | IP54 | 20–44 | EP-ZDS-115/142 | Gravity hold + cutting torque |
| VMC 4th axis, 30kg workpiece | 80–200 | 10–16:1 | IP54 | 12 | EP-ZDE-120 | Cost-effective; light interrupted cut |
| VMC 4th axis, flood coolant exposed | 100–250 | 16:1 | IP65 | 20 | EP-ZDS-115 | IP65 primary; Ct adequate for load |
| EDM rotary C-axis (precision) | 10–50 | 5–10:1 | IP54 | 38 | EP-ZDE-160 | BL <8; dielectric fluid, not water coolant |
| Laser rotary attachment (light) | 5–30 | 3–8:1 | IP54 | 4.5 | EP-ZDE-80 | Speed + low mass priority; light load |
| Transfer line indexing, IP65 washdown | 500–1,800 | 16–25:1 | IP65 | 44–130 | EP-ZDS-142/190 | Highest torque + IP65 + Ct |
CNC Backlash Compensation — What It Can and Cannot Fix
Modern CNC controls (Fanuc Series 30i/31i/32i, Siemens SINUMERIK 840D, Heidenhain TNC 640) include backlash compensation that programs a counter-motion at each direction reversal to take up the dead band before resuming the commanded trajectory. This capability does not eliminate the need for low backlash — it extends the accuracy of a given backlash specification to higher feed rates and more complex trajectories.
- Static positioning error at low feed rates (<500 mm/min) where the servo loop has time to execute the compensation pulse before resuming trajectory
- Systematic angular error during slow circular interpolation — the control knows exactly when direction changes and applies compensation at that point
- Index-to-index repeatability at low speed — each index completes from the same approach direction with compensation active
- Elastic torsional deflection under cutting load — this is a stiffness issue, not a backlash issue, and the compensation algorithm has no knowledge of applied torque
- High-speed circular interpolation errors — at rapid contour speeds (>2,000 mm/min), the compensation pulse creates a velocity discontinuity that shows as a surface mark
- Vibration from drivetrain resonance excited by the compensation pulse itself
- Any dynamic accuracy degradation — compensation only acts at quasi-static direction reversal points
Practical guidance for Korean CNC machine tool builders: Specify backlash compensation as standard in the CNC program for all rotary axis operations. Set the compensation value from the gearbox factory certificate (for example: 7.5 arcmin measured at ±3% rated torque). Re-measure at the 5,000h maintenance interval and update the compensation value if backlash growth has occurred — an outdated compensation value is worse than no compensation at all, as it over-compensates and introduces a systematic positioning error in the opposite direction.
CNC Rotary Axis Gearbox Specification Checklist — 8 Parameters to Verify Before Ordering
Calculate maximum cutting torque at the worst-case operation (largest cutter, hardest material, largest depth of cut). Apply SF = 1.5 for smooth cuts, 2.0 for interrupted cuts, 2.5 for heavy interrupted cuts in difficult materials.
From your part tolerance and cutting radius: θ_max = arctan(tolerance/R). Then Ct_required = T_peak / θ_max. If Ct_required exceeds 38 N·m/arcmin (ZDE-160 maximum), specify EP-ZDS series.
Use the table in Module 2 to determine whether <8 arcmin (standard EP-ZDE/ZDS) is adequate for your cutting radius and tolerance. If not, verify that backlash compensation will be used and that the resulting compensated error is within tolerance.
Determine whether the gearbox position receives direct coolant impingement, indirect splash, or only mist. IP54 for mist/splash only. IP65 for any direct jet or flood coolant situation.
Calculate J_load for all rotating elements plus reflected linear mass. Find i_optimal = √(J_load / J_motor). Select nearest EP standard ratio that satisfies both inertia and torque.
For rotary tables and trunnions: verify that the gravity component at maximum tilt angle, combined with cutting force radial component, does not exceed the gearbox output bearing limits. EP-ZDS axial: 12,000–28,000N. EP-ZDE-160 axial: 3,000N.
Verify n_motor = n_output × i ≤ 3,000 rpm (recommended) and ≤ 4,500 rpm (maximum) at maximum required indexing speed. CNC rotary axes rarely operate above 100 rpm output — speed is usually not the binding constraint.
Specify motor model at time of order for EP series to supply matched input flange. For right-angle configurations (ZDWE/ZDWF), specify motor exit direction (L/R/U/D). Verify concentricity ≤0.02mm TIR at installation to prevent input bearing failure.
Korea Ever-Power application engineering provides complete CNC rotary axis specification including Ct requirement from your part tolerance, backlash accuracy table for your cutting radius, IP rating assessment, and gear ratio optimisation — in Korean and English. Provide your axis type, cutting torque, tolerance requirement, and coolant environment for a complete specification recommendation.
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Editor: Cxm
