{"id":742,"date":"2026-06-03T01:48:54","date_gmt":"2026-06-03T01:48:54","guid":{"rendered":"https:\/\/planetary-gearboxes.com\/?p=742"},"modified":"2026-06-03T01:48:54","modified_gmt":"2026-06-03T01:48:54","slug":"planetary-gearbox-cnc-machine-tool-rotary-axis-selection","status":"publish","type":"post","link":"https:\/\/planetary-gearboxes.com\/es\/planetary-gearbox-cnc-machine-tool-rotary-axis-selection\/","title":{"rendered":"Selecci\u00f3n de reductores planetarios de precisi\u00f3n para ejes rotativos de m\u00e1quinas herramienta CNC"},"content":{"rendered":"
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Korea Ever-Power<\/span>
\nCNC Machine Tool Application Guide<\/span><\/div>\n

Precision Planetary Gearbox Selection for CNC Machine Tool Rotary Axes \u2014 B\/C\/A Axis and 4th\/5th Axis Guide<\/h1>\n

CNC rotary axes impose requirements that general servo guides do not address. The accuracy of a boring operation depends directly on gearbox backlash at the cutting radius \u2014 not at an arbitrary test point. A 5-axis B-axis under heavy interrupted cut demands torsional stiffness that a general servo application never requires. Flood coolant demands IP65. This guide resolves all three constraints axis by axis.<\/p>\n

Get CNC Axis Specification Support \u2192<\/a><\/p>\n<\/div>\n<\/div>\n<\/section>\n

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Three Requirements That Separate CNC Rotary Axes from General Servo Applications<\/h2>\n

Most precision planetary gearbox selection methodologies are written for general servo automation \u2014 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.<\/p>\n

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\u2460 Backlash at the Cutting Radius \u2014 Not at a Generic Test Point<\/div>\n

CNC accuracy specifications state tolerances at the workpiece \u2014 at the actual cutting radius, which could be 10mm (small bore) or 300mm (large facing). The same 8 arcmin backlash produces 23\u03bcm of tangential error at R=10mm but 1,163\u03bcm 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.<\/p>\n<\/div>\n

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\u2461 Torsional Stiffness Under Cutting Load \u2014 Not Just Positioning Accuracy<\/div>\n

CNC cutting loads are highly variable \u2014 the torque changes instantaneously with chip thickness, material variations, and tool entry\/exit. Under a 380 N\u00b7m peak cutting torque, an EP-ZDE-160 (Ct=38 N\u00b7m\/arcmin) deflects elastically by 10 arcmin \u2014 more than the specified backlash \u2014 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.<\/p>\n<\/div>\n

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\u2462 Coolant Environment \u2014 IP Rating Determines Service Life<\/div>\n

CNC machine tools use flood coolant at 2\u20138 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 \u2014 6.3mm nozzle at 12.5L\/min from any direction \u2014 which approximates flood coolant conditions on exposed rotary fixtures.<\/p>\n<\/div>\n<\/div>\n<\/section>\n

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\"Korea<\/p>\n

EP series precision planetary gearboxes are 100% tested for backlash at \u00b13% rated torque before shipment. Every unit carries a certified backlash value on its factory documentation \u2014 the traceability required by CNC machine tool OEM quality management systems. View EP series specifications \u2192<\/a><\/div>\n<\/div>\n

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Backlash at Your Cutting Radius \u2014 The Table CNC Engineers Need<\/h2>\n

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 \u00d7 tan(BL \/ (60 \u00d7 180\/\u03c0)). For boring and turning operations where the tool traces a circular path, this tangential error appears directly as roundness deviation on the finished surface.<\/p>\n

\n\n\n\n\n\n\n\n\n
Reacci\u00f3n<\/th>\nR=10mm
\nSmall bore<\/span><\/th>\n		R=25mm
\n\u03a650mm bore<\/span><\/th>\n		R=50mm
\n\u03a6100mm bore<\/span><\/th>\n		R=100mm
\n\u03a6200mm face<\/span><\/th>\n		R=200mm
\n\u03a6400mm table<\/span><\/th>\n		EP Series<\/th>\n<\/tr>\n<\/thead>\n
<3 arcmin<\/td>\n8.7 \u03bcm<\/td>\n21.8 \u03bcm<\/td>\n43.6 \u03bcm<\/td>\n87.3 \u03bcm<\/td>\n174.5 \u03bcm<\/td>\nCustom \/ special order<\/td>\n<\/tr>\n
<8 arcmin \u2605<\/td>\n23.3 \u03bcm<\/td>\n58.2 \u03bcm<\/td>\n116.4 \u03bcm<\/td>\n232.7 \u03bcm<\/td>\n465.4 \u03bcm<\/td>\nEP-ZDE\/ZDF (60\u2013160mm) \u00b7 EP-ZDS (all)<\/td>\n<\/tr>\n
<12 arcmin<\/td>\n34.9 \u03bcm<\/td>\n87.3 \u03bcm<\/td>\n174.5 \u03bcm<\/td>\n349.1 \u03bcm<\/td>\n698.1 \u03bcm<\/td>\nEP-ZDE-40 single stage; 2-stage units<\/td>\n<\/tr>\n
<25 arcmin<\/td>\n72.7 \u03bcm<\/td>\n181.8 \u03bcm<\/td>\n363.6 \u03bcm<\/td>\n727.3 \u03bcm<\/td>\n1,454 \u03bcm<\/td>\nEP-ZDWE\/ZDWF \u2014 right-angle input only<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n

\u2605 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 \u2014 application requires either tighter backlash, compensation, or larger bore tolerance specification.<\/p>\n

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Reading this table for a specific operation<\/div>\n

A VMC 4th axis boring a \u03a6100mm hole (R=50mm) with an EP-ZDE-120 (<8 arcmin) produces a maximum backlash-induced roundness error of 116.4\u03bcm per revolution. With Fanuc Series 0i backlash compensation, this reduces to approximately 12\u201320\u03bcm \u2014 compatible with IT8 tolerance. Without compensation, the same bore at IT7 (25\u03bcm for \u03a6100mm bore) fails. Specify backlash compensation in the CNC program, or upgrade to EP-ZDS to reduce backlash contribution to the error budget.<\/p>\n<\/div>\n<\/section>\n

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CNC Rotary Axis Specifications \u2014 B\/C\/A Axis and 4th\/5th Axis, Axis by Axis<\/h2>\n

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.<\/p>\n

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B-AXIS<\/div>\n
5-Axis Machining Centre \u2014 Tilting Rotary Table or Head<\/div>\n<\/div>\n
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Primary requirements:<\/strong>
\nTilt range \u00b190\u00b0 or \u00b1110\u00b0
\nPeak cutting torque: 150\u2013600 N\u00b7m depending on size
\nHeavy interrupted cuts in titanium\/Inconel
\nClamping hold torque: 2\u20133\u00d7 cutting torque
\nIP54 minimum (coolant mist); IP65 for external fixture<\/div>\n
Critical specification:<\/strong>
\nTorsional stiffness dominates above crossover torque
\nZDS-142 crossover: 352 N\u00b7m (BL \u00d7 Ct = 8 \u00d7 44)
\nZDE-160 crossover: 304 N\u00b7m (8 \u00d7 38)
\nFor T_cut > 304 N\u00b7m: elastic error exceeds backlash
\n\u2192 tighter backlash spec won’t help; higher Ct will<\/div>\n
Recommended:<\/strong>
\nMedium 5-axis (T_cut \u2264 250 N\u00b7m):
\n\u2192 EP-ZDS-115<\/a>, 20:1, Ct=20 N\u00b7m\/arcmin
\nHeavy 5-axis (T_cut 250\u2013600 N\u00b7m):
\n\u2192 EP-ZDS-142, 20:1, Ct=44 N\u00b7m\/arcmin
\nVery heavy (T_cut > 600 N\u00b7m):
\n\u2192 EP-ZDS-190, 20\u201325:1, Ct=130 N\u00b7m\/arcmin<\/div>\n<\/div>\n
Inertia check:<\/strong> B-axis tilts the entire rotary table or spindle head. For a 50kg tilting table at R=200mm: J_load = 50\u00d70.20\u00b2 = 2.0 kg\u00b7m\u00b2. i_optimal = \u221a(2.0\/0.006) = 18.3 \u2014 confirming 20:1 as the correct ratio. At 20:1, J_reflected = 2.0\/400 = 0.005 kg\u00b7m\u00b2 \u2014 an inertia ratio of 0.83:1 (slightly under-reflected), which is acceptable and allows full servo Kv.<\/div>\n<\/div>\n

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C-AXIS<\/div>\n
Turning-Milling Centre \u2014 Main Spindle Positioning and Mill-Turn C-Axis<\/div>\n<\/div>\n
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Two distinct operating modes:<\/strong>
\nTurning mode: C-axis is the spindle drive \u2014 high torque, continuous rotation, no position precision required
\nMilling\/positioning mode: C-axis indexes to exact angle for off-centre milling \u2014 position accuracy critical<\/div>\n
Precision mode requirements:<\/strong>
\nIndex accuracy: \u00b15\u201315″ of arc (arcseconds)
\n8 arcmin = 480 arcseconds \u2014 far too wide for \u00b15″ target
\nC-axis precision: requires gear + encoder closed-loop
\nThe gearbox sets the accuracy floor that the encoder must overcome through closed-loop correction<\/div>\n
Recommended:<\/strong>
\nC-axis positioning (mill-turn):
\n\u2192 EP-ZDE-160 or EP-ZDS-115 at 10\u201316:1
\nBL <8 arcmin; combined with Heidenhain\/Renishaw encoder for \u00b115″ accuracy
\nFlood coolant exposure: specify EP-ZDS-115\/142<\/div>\n<\/div>\n
Critical note on C-axis precision:<\/strong> The <8 arcmin backlash of EP-ZDE\/ZDS corresponds to 480 arcseconds. Turning-milling centres requiring \u00b15″ angular positioning use the gearbox as a torque multiplier only \u2014 the actual angular position is controlled by a scale encoder mounted on the C-axis table, not the motor encoder. The gearbox backlash determines how much pre-load the servo must apply to take up the dead band before position feedback control can take over.<\/div>\n<\/div>\n

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A-AXIS<\/div>\n
Gantry Mill Trunnion \u2014 A-Axis Tilt on Gantry Machining Centre<\/div>\n<\/div>\n
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Caracter\u00edsticas:<\/strong>
\nTilt range \u00b145\u00b0 to \u00b190\u00b0
\nSpindle plus head assembly: 40\u2013120kg
\nGravity load at maximum tilt: full head weight at R=200\u2013400mm
\nMust hold position against gravity during cutting
\nTypically no clamping brake \u2014 gearbox must hold statically<\/div>\n
Gravity load analysis:<\/strong>
\n60kg spindle head, R=300mm at full tilt:
\nT_gravity = 60 \u00d7 9.81 \u00d7 0.3 = 176.6 N\u00b7m
\nRequired holding (with SF=2.0): 353 N\u00b7m
\nThis is the static load \u2014 add dynamic cutting torque
\nTotal requirement often 400\u2013700 N\u00b7m<\/div>\n
Recommended:<\/strong>
\nLight gantry head (40\u201360kg):
\n\u2192 EP-ZDS-115 or ZDE-160, 16\u201320:1
\nHeavy gantry head (80\u2013150kg):
\n\u2192 EP-ZDS-142, 16:1, Ct=44 N\u00b7m\/arcmin
\nVery heavy (150kg+):
\n\u2192 EP-ZDS-190, 16\u201320:1, Ct=130 N\u00b7m\/arcmin<\/div>\n<\/div>\n<\/div>\n

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4th \/ 5th AXIS<\/div>\n
VMC Add-On Rotary Table \u2014 4th\/5th Axis Trunnion Unit<\/div>\n<\/div>\n
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Most common in Korean shops:<\/strong>
\nSeparate add-on rotary tables (4th axis) or dual-axis trunnion (4th+5th)
\nTable diameter: \u03a6200\u2013\u03a6400mm
\nWorkpiece weight: 20\u201380kg typical
\nOften used at full flood coolant \u2014 IP65 critical
\nPositioning only (no continuous turning)<\/div>\n
Accuracy priority:<\/strong>
\n4th axis indexing: typically \u00b110\u201330″ arc
\nCombined with worm or planetary gear and encoder
\nPlanetary approach: compact, efficient, low BL
\nEP-ZDE-120 (Ct=12 N\u00b7m\/arcmin, BL <8): adequate for most VMC 4th axis
\nHeavy workpiece or interrupted cut: upgrade to EP-ZDS-115<\/div>\n
Recommended:<\/strong>
\nLight table (20\u201340kg workpiece):
\n\u2192 EP-ZDE-120, 10\u201316:1 (IP54 if internal machine)
\nMedium table (40\u201380kg, coolant exposed):
\n\u2192 EP-ZDS-115, 16\u201320:1, IP65
\nHeavy table (>80kg or heavy interrupted cut):
\n\u2192 EP-ZDS-142, 16:1, IP65, Ct=44<\/div>\n<\/div>\n<\/div>\n<\/section>\n

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Why Torsional Stiffness Determines Part Tolerance in Heavy CNC Operations<\/h2>\n

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 \u2014 happening at the cutter contact frequency \u2014 is what causes surface roughness patterns, polygon marks on bored holes, and chatter on turned surfaces.<\/p>\n

\n\n\n\n\n\n\n\n\n
Cutting Scenario<\/th>\nT_peak (N\u00b7m)<\/th>\nZDE-160
\nelastic error<\/th>\n		ZDS-142
\nelastic error<\/th>\n		ZDS-190
\nelastic error<\/th>\n		At R=100mm
\nZDS-190 advantage<\/th>\n<\/tr>\n<\/thead>\n
Light aluminium facing<\/td>\n80 N\u00b7m<\/td>\n2.1 arcmin
\n0.122mm@R=100<\/span><\/td>\n		1.8 arcmin
\n0.105mm@R=100<\/span><\/td>\n		0.6 arcmin
\n0.035mm@R=100<\/span><\/td>\n		3.4\u00d7<\/td>\n<\/tr>\n
Steel rough milling<\/td>\n200 N\u00b7m<\/td>\n5.3 arcmin
\n0.308mm@R=100<\/span><\/td>\n		4.5 arcmin
\n0.262mm@R=100<\/span><\/td>\n		1.5 arcmin
\n0.087mm@R=100<\/span><\/td>\n		3.4\u00d7<\/td>\n<\/tr>\n
Heavy steel boring<\/td>\n380 N\u00b7m<\/td>\n10.0 arcmin
\n0.581mm@R=100<\/span><\/td>\n		8.6 arcmin
\n0.500mm@R=100<\/span><\/td>\n		2.9 arcmin
\n0.169mm@R=100<\/span><\/td>\n		3.4\u00d7<\/td>\n<\/tr>\n
Inconel interrupted cut<\/td>\n600 N\u00b7m<\/td>\n15.8 arcmin
\n0.919mm@R=100<\/span><\/td>\n		13.6 arcmin
\n0.791mm@R=100<\/span><\/td>\n		4.6 arcmin
\n0.267mm@R=100<\/span><\/td>\n		3.4\u00d7<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n

Elastic error = T_peak \/ Ct. ZDE-160: Ct=38; ZDS-142: Ct=44; ZDS-190: Ct=130 N\u00b7m\/arcmin. At R=100mm using E = R \u00d7 tan(\u03b8\/3438). Servo motor encoder cannot detect this elastic deflection \u2014 it accumulates directly as workpiece dimensional error.<\/p>\n

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Design implication:<\/strong> For CNC operations where the cutting radius exceeds 50mm and peak cutting torque exceeds 200 N\u00b7m, the elastic deflection error from a ZDE-160 (0.308mm at R=100mm, T=200 N\u00b7m) exceeds IT8 tolerance for most bore sizes. The ZDS-190 reduces this to 0.087mm \u2014 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.<\/p>\n<\/div>\n<\/section>\n

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\"High-precision<\/p>\n

EP series precision planetary gearboxes for CNC rotary axes are shipped with a factory certification document specifying backlash at \u00b13% rated torque, torsional stiffness Ct, and maximum radial\/axial force limits \u2014 the three specifications most critical for CNC machine tool axis performance verification. View EP series technical specifications \u2192<\/a><\/div>\n<\/div>\n

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Coolant Environment and IP Rating \u2014 Matching Protection to CNC Reality<\/h2>\n

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 \u2014 which in CNC environments typically manifests within 2,000\u20134,000 hours.<\/p>\n

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\u2705 IP54 Adequate \u2014 EP-ZDE\/ZDF\/ZDWE\/ZDWF<\/div>\n