Trunnion fixtures

In the worked trunnion example, a 40 kg assembly with its center of mass 0.10 m from a horizontal axis, using g = 9.81 m/s², gives up to 39.24 N·m gravity torque and 0.40 kg·m² offset inertia. At 0.20 m, those terms become 78.48 N·m and 1.60 kg·m². Total inertia also includes inertia about the center of mass. Check braking, acceleration and swept clearance for the actual assembly.

Use Choosing workholding to compare location, support and restraint across fixture types.

Two related meanings

A bridge between a driven rotary and an opposite-end support carries the fixtures. Kurt’s VMC example uses five 3600 VersatileLock vises, a Koma Precision indexer and a hydraulic rotary union to reach three sides of five parts. Kurt, custom VMC trunnion application, undated

A dual-axis trunnion instead uses a tilting cradle carrying a rotary platter. Haas describes TR160 for 3+2 positioning and full five-axis machining. Haas TR160 product specification, undated

Indexing is different from simultaneous cutting

In indexed machining, rotation presents a new orientation and the cut takes place with that orientation held. With 3+2 machining, two rotary axes orient the part for a three-linear-axis cut. Simultaneous five-axis machining coordinates rotary and linear motion during the cut. These descriptions concern motion, not merely the number of axes installed.

For the TR160, Haas specifically requires a compatible Haas mill with fourth- and fifth-axis drives and NGC software 100.21.000.1130 or later for full operation. Its table separates the A-axis continuous torque (203 N·m) from brake torque (271 N·m at 6.9 bar). Brake holding capacity is consequently a different specification from continuous drive torque. Haas TR160 specification

Structure, location and support

The structural load path runs from the part through its clamps and locators into the rotating fixture and its supports, then into the machine table. Opposite-end support reduces cantilever loading but adds a mounting and alignment interface. Engineering synthesis: support height, coaxial alignment, span and fixture stiffness influence bending and twist; a tail support does not itself eliminate torsional compliance.

Powerhold's trunnion brochure illustrates mechanical, hydraulic and pneumatic clamping, locating pins, quick-change subplates, tailstocks and custom unions. Its window-fixture example allows a 180° index for opposite-side access, provided the headstock and tailstock are high enough for the rotating envelope. These features are custom fixture choices, not inherent properties of all trunnions. Powerhold, trunnion-fixture brochure, document code CN040108; date not stated

Rotating envelope and tool access

Platter diameter describes an interface; it does not describe every permissible part shape. Haas's TR160-2 layout drawing, Revision 1 dated 29 October 2019, separately identifies a 160 mm platter, 266.7 mm B-axis part swing and 241.3 mm A-axis centerline height. This is TR160-2 geometry, not TR160 geometry. The drawing warns that sheet-metal stackups can vary by 13 mm and that dimensions are subject to change. Haas TR160-2 drawing, pp. 1–2

Geometric synthesis: the swept envelope includes the fixture, jaws, fasteners, loaded parts and service lines at every intended angle. Available machine Z travel and toolholder clearance can constrain a setup even if the rotating assembly clears the table. A nominal swing diameter also does not establish clearance for an eccentric rectangular part at all combinations of tilt and rotation.

Clamping services and braking

Martin Trunnion Tables' 5 November 2025 RoboDrill installation uses a custom union with the DDR rotary to prevent hydraulic-line tangling. It incorporates a SCHUNK KSH3 hydraulic vise, internal manifolds, protected hard lines and a TS1200 hydraulic brake at the opposite end synchronized with the rotary's brake. A 1 inch riser provides full-rotation clearance in that particular build. The union transfers services; the brake restrains rotation; the vise holds the part. Their roles are separate. Neither that riser height nor that brake configuration establishes compatibility with another machine. Martin, RoboDrill application

Mass, gravity moment and inertia

Original illustrative calculation: assume the complete moving fixture-and-part assembly has mass m = 40 kg, its center of mass lies r = 0.10 m from a horizontal rotation axis, and g = 9.81 m/s². Maximum gravity torque is

Tg,max = m g r = 40 × 9.81 × 0.10 = 39.24 N·m.

At the same mass with a 0.20 m offset, maximum gravity torque doubles to 78.48 N·m. The offset contribution to rotational inertia, m r², increases from 0.40 to 1.60 kg·m². The assembly's inertia about its own center must also be included; these values are only the parallel-axis contribution.

Thus unchanged payload mass can mean twice the gravity torque and four times the offset inertia. Acceleration requires additional torque I α; cutting forces contribute moments according to their lever arms. The example excludes friction, drive efficiency, dynamic vibration and structural limits. It is not a capacity check against any named rotary, and catalog brake torque is not a complete allowable cutting-load envelope.

Scope and related reference

The cited examples do not establish universal table loading, permissible overhang, rotary inertia, indexing accuracy or machining force. Fixture mass, workpiece mass, center of mass, orientation, services and machine/control compatibility all affect a real assembly. Reduced handling can preserve one clamping relationship between faces, but does not remove locator error, rotary positioning error or elastic deflection.

See Workholding and fixture references for locating, support and restraint principles.