Hydraulic Work Supports

What Work Supports Do?

Work supports are not clamps. They do not exert clamping force on a workpiece. Instead, they function as adjustable rests that automatically conform to the workpiece surface, lock in position, and then resist deflection during machining. 

The distinction matters. A clamp actively pushes against the workpiece to hold it in place. A work support passively conforms to wherever the workpiece happens to be, then becomes a rigid support point. The workpiece sets the position; the work support simply holds it there. 

This makes work supports essential for preventing deflection in thin-walled parts, long workpieces, and any geometry where the workpiece cannot be fully supported by fixed locators alone. Without adequate support, cutting forces push the workpiece away from the tool, creating dimensional errors, poor surface finish, and chatter marks. 

How Hydraulic Work Supports Operate 

A hydraulic work support contains a plunger that extends to contact the workpiece, then locks in that position when hydraulic pressure is applied to an internal clamping mechanism. The sequence occurs in two stages: advance and lock. 

Plunger Advance: The plunger must first move outward to contact the workpiece surface. Different advance mechanisms are available.

Spring Advance: An internal spring pushes the plunger outward with light force. When the workpiece is loaded, its weight compresses the spring until the plunger contacts the underside of the part. Spring advance is the most common configuration and works well when the workpiece is heavy enough to depress the spring-loaded plungers during loading. 

Fluid Advance: Hydraulic pressure drives the plunger outward. An internal piston pushes against a spring, which advances the plunger until it contacts the workpiece. The plunger exerts only light spring force during contact, not full hydraulic force. Fluid advance is preferred when the extended plunger would interfere with part loading, since the plunger remains retracted until hydraulic pressure is applied. 

Air Advance: Pneumatic pressure advances the plunger, allowing the contact force to be adjusted via a pressure regulator. This provides variable advance force independent of the locking mechanism. With continuous air pressure, the plunger functions as an air spring. 

Plunger Locking 

Once the plunger contacts the workpiece, hydraulic pressure engages a locking mechanism that clamps the plunger in position. The most common design uses a collet or compression sleeve surrounding the plunger shaft. 

When hydraulic pressure is applied to the locking port, a conical piston drives inward against the segmented collet. The collet compresses radially against the plunger, gripping it with friction force proportional to the hydraulic pressure. The plunger is now locked and cannot move under machining loads up to the rated capacity. 

When pressure is released, the collet relaxes and the plunger is free to move again. On spring-advance supports, the spring immediately extends the plunger back to its extended position. 

Understanding Support Capacity 

Work support capacity is not a fixed value. It increases proportionally with hydraulic pressure, following a linear relationship up to the maximum rated pressure. A support rated at 2,000 lbs at 5,000 psi will provide only 1,000 lbs of support at 2,500 psi. 

When selecting work supports, the support capacity must be sufficient to resist three categories of load: 

1. Machining forces: The cutting forces that tend to push the workpiece away from the tool 

2. Workpiece weight: Gravity loading on unsupported sections 

3. Clamping forces: Any portion of clamp force not absorbed by fixed locators 

Static vs. Dynamic Loading 

Published capacity ratings are based on static loading. Real machining involves dynamic forces from interrupted cuts, tool entry and exit, cutter vibration, and pressure fluctuations in the hydraulic system. 

For general machining with continuous cuts, select work supports with static capacity at least 1.5 times the sum of expected machining force and clamping force. 

For interrupted cuts, heavy roughing, or operations with significant dynamic loading, select work supports with static capacity at least 2 times the sum of expected forces. 

Work Supports Positioned Under Clamps 

When a work support is positioned directly beneath a clamp, the support must resist the full clamping force plus any dynamic loads from clamp actuation. This is one of the most demanding applications for work supports and requires careful sizing. 

Capacity Requirements 

The work support must resist both static and dynamic clamping loads. When a swing clamp arm accelerates toward the workpiece and stops suddenly at contact, the momentum creates an impact load substantially higher than the steady-state clamping force. This repeated hammering effect must not be overlooked. 

For work supports positioned under clamps, select capacity at least twice the nominal clamping force. This provides adequate margin for dynamic impact loads. 

Sequencing Work Supports and Clamps 

When a clamp operates above a work support, the sequence of actuation matters. If the clamp begins generating force before the work support is fully locked, the clamp can push the workpiece down before adequate support exists. This can deflect the workpiece, damage thin-walled parts, or shift the workpiece off its locators. 

A sequence valve between the work support circuit and the clamp circuit solves this problem. The sequence valve blocks flow to the clamp until the work support circuit reaches a preset pressure, ensuring the support is locked before clamping force is applied. 

The proper sequence is: (1) Work support advances and contacts workpiece. (2) Hydraulic pressure locks the work support. (3) Sequence valve opens when support circuit reaches threshold pressure. (4) Clamp actuates against the now-supported workpiece. 

Common Applications 

Thin-Wall Machining 

Thin-walled castings, aluminum housings, and sheet metal components deflect easily under cutting forces. Work supports positioned beneath or beside weak sections provide backing that prevents deflection during machining. The support automatically conforms to the actual part surface, accommodating casting variations and tolerance stack-up without manual adjustment. 

Long Workpieces 

Long steel plates, aluminum extrusions, and structural members sag in the middle due to gravity. Work supports positioned along the length distribute the load and prevent center sag. This maintains consistent cutting depth and surface finish across the entire workpiece length. 

Interior Support 

When machining the exterior of hollow parts or cases, work supports positioned inside the cavity support the walls from within. The support plunger extends through an opening in the fixture and contacts the interior surface, providing backing against outward cutting forces. This application often produces dramatic improvements in part quality and cycle time. 

Use with Manual Clamps 

Work supports do not require a fully hydraulic fixture. They can be used alongside manual clamps to improve consistency and reduce setup time. In manual clamping, operators often adjust screw jacks by feel, which introduces variation between operators and between cycles. Hydraulic work supports eliminate this variation by automatically conforming to the workpiece and locking with consistent force. 

A single hydraulic circuit can lock multiple work supports simultaneously while manual clamps secure the workpiece perimeter. This hybrid approach provides many benefits of hydraulic workholding at lower cost than a fully hydraulic system. 

Work Support Type Comparison 

Deflection and Repeatability 

Work supports are not perfectly rigid. Under load, the locked plunger deflects slightly as the collet and housing absorb force. This deflection is small but measurable, typically in the range of thousandths of an inch at rated capacity. 

What matters more than absolute deflection is repeatability. High-quality work supports position the plunger within ±0.0002 inch (0.005 mm) from cycle to cycle when operating at consistent pressure. This repeatability ensures that part-to-part variation in support position does not contribute to dimensional scatter in the finished parts. 

Factors affecting repeatability include workpiece surface finish at the contact point, end effector (contact tip) geometry, system pressure consistency, and workpiece-to-workpiece variation in the supported surface. For best results, use consistent contact surfaces and maintain stable hydraulic pressure. 

Venting Requirements 

Spring-advance work supports contain an air chamber on the spring side of the mechanism. As the plunger moves, air must flow in and out of this chamber through a filtered vent. In flood coolant environments, cutting fluid can be drawn into the air chamber during plunger retraction, eventually contaminating seals and springs. 

For work supports operating in flood coolant, route vent lines to a location above the coolant level. Many work supports include a vent port that accepts standard tubing fittings. Connect tubing from this port to a clean, dry location outside the machining envelope. 

Fluid-advance work supports in ventless configurations avoid this issue entirely since both sides of the piston contain hydraulic fluid with no air exchange. 

Controlling Advance Speed 

On fluid-advance work supports, the plunger can extend rapidly when hydraulic pressure is applied. If the plunger advances faster than expected, it may bounce off the workpiece surface and lock in a retracted position before the spring can re-extend it. 

Install meter-in flow controls on the advance port to slow plunger extension. Use flow controls with free-flow returns to avoid restricting the retract stroke. Do not use meter-out flow controls or needle valves, which can cause pressure intensification in hydraulic circuits. 

Installation Considerations 

Mounting Options 

Work supports are available in threaded body, flange mount, and cartridge configurations. Threaded bodies install into tapped holes in the fixture plate. Flange mount supports bolt to the fixture surface. Cartridge supports drop into precision-bored cavities for applications requiring minimal footprint or manifold integration. 

Contact Bolts 

The plunger contact surface can be configured with various end effectors (contact tips) to match the workpiece surface. Standard options include flat pads, spherical tips, and serrated pads for rough castings. The plunger typically includes internal threads to accept interchangeable end effectors, allowing the same support body to accommodate different workpiece geometries. 

Selection Guidelines 

• Choose spring advance when workpiece weight naturally depresses the plunger during loading 
• Choose fluid advance when the extended plunger would interfere with part loading 
• Choose air advance when variable contact force is required 
• Size capacity at 1.5× expected load for continuous cuts, 2× for interrupted cuts 
• Under clamps, size capacity at 2× the clamping force minimum 
• Use sequence valves when work supports must lock before clamps actuate 
• Route vent lines above coolant level for spring-advance supports in