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Support Cylinders

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Supporting the Part

The previous two chapters discussed cylinders for positioning and clamping-two of the three steps in workholding. In this chapter we look at cylinders used to implement the third step: supporting the part. Supports add rigidity to fixtured parts to minimize part deflection and machining vibration. Proper support is necessary for high quality, repeatable machining results..

It’s important not to confuse work supports with the support points and planes inherent in the workpiece positioning step. As explained in Chapter 2, rigid support points or surfaces additional to those used in the 3-2-1 locating process must not be introduced.

Hydraulic work supports are just what the name implies: hydraulic devices used to support a workpiece during a machining process. These cylinders can be set up so that they are normally retracted out of the way and engage the part only when needed. After a part has been positioned, the work support advances to contact the part with slight pressure, then locks in place.

Production runs can be limited by the labor-intensive Care must be taken to ensure that points of application of clamping pressure are aligned with support points. An offset of the centerlines will cause bending of the workpiece.

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Additionally, the direction of the clamping force must be axial to the centerline of the work support plunger. Side loading of the work support must be avoided in order to obtain safe, reliable, repeatable results.


Selecting a Hydraulic Work Support

The basic selection of a work support for a fixture depends on two factors: determination of the support force required, and then from that, the size of support cylinder required.

Determine support force needed

A work support is required if a portion of a part would be moved or deformed by forces due to clamping or machining. See Chapter 6 for a discussion of how clamping force is calculated and the next paragraph regarding machining forces.

Determine work support size

As a rule of thumb, the capacity of a work support cylinder at the hydraulic pressure to be supplied to it should be at least twice the clamping force to be applied. (By "capacity" we mean the amount of opposing force the support cylinder is designed to withstand when in its locked state.) For many applications a 2:1 margin of support cylinder capacity over clamping force is sufficient to also absorb machining forces. In cases of extreme vibration or interrupted cuts, the margin of support cylinder capacity should be increased to 4:1.

The support cylinder selection process involves both the clamping and support cylinders, and four criteria that need to be satisfied:

 

  1. The necessary clamping force must be calculated.
  2. The machining forces acting on the support cylinder should be determined, if possible.
  3. The support cylinder capacity selected should be equal to the applied clamping force plus machining force plus a reasonable margin, or else at least twice the clamping force value.
  4. A common feed pressure should be determined for both clamp and support cylinders, otherwise an appropriate pressure control valve should be used.

 

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The basic tool for cylinder selection is the pressure-force diagram. When a work support is to be used opposite a clamping force, the clamping and support cylinders are selected at the same time, using pressure-force diagrams for both types of cylinders to accomplish the tradeoffs between force, pressure, and cylinder rating (size). In some situations, it may be necessary to operate the clamping and supporting cylinders at different pressures in order to obtain the desired results.


Cylinder Configurations

The two considerations in selecting support cylinder configuration are the actuation method and the mounting method.

Actuation method

Three methods of support actuation are available:

Spring advance ¾ The cylinder plunger is extended by an internal spring, and workpiece weight compresses the spring. Spring force can be adjusted for the desired relation to the workpiece weight. When hydraulic pressure is applied, an internal sleeve grips and holds the plunger in a fixed position. These cylinders may also be capable of air advance.

Hydraulic advance ¾ Here, the plunger is normally retracted, allowing unobstructed workpiece loading. The hydraulic advance cylinder combines an external spring advance cylinder with an internal hydraulic plunger to move it into place. After the spring advance cylinder is compressed, an internal sleeve grips the cylinder to lock it in place.

Positive locking ¾ Enerpac’s unique Collet-Lok® cylinders allow work supports to remain locked in place after hydraulic pressure has been removed. As with the other two types of supports, the Collet-Lok® plunger adjusts to the contours of the workpiece during positioning. Application of hydraulic pressure to a specific port then locks the plunger in place by means of a hydraulically driven collet. After removal of hydraulic pressure, the work support remains locked in place. Unlocking is accomplished by application of hydraulic pressure to another port. Refer to Chapter 4 for more information about Collet-Lok® cylinders.

Mounting method

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Single and double-acting

Four methods of cylinder mounting are used. In each case, an internal plunger thread provides for optional end effectors.

Manifold mount ¾ (spring advance and hydraulic advance cylinders) This method requires no external plumbing. Its compact configuration is attractive when space is at a premium.

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Threaded body ¾ (spring advance, hydraulic advance, and positive locking cylinders) This method offers the ability to adjust the cylinder height. Threaded body cylinders are externally plumbed from the side or bottom.

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Lower flange ¾ (spring advance, hydraulic advance, and positive locking cylinders) Cylinders mounted by means of a lower flange can be plumbed externally or via manifold. Advantages of these cylinders are the surface mounting (vs. a hole through the fixture) and ease of assembly/disassembly.

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Cartridge style ¾ This particularly compact mounting style allows close clustering of work supports and is entirely manifold-fed.


Hydraulic System Considerations

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Swing cylinders and hydraulic work supports are very sensitive to oil flow rate. To ensure safe and reliable function of these elements, the maximum oil flow rate specified in the catalog pages and product instructions must not be exceeded. If there is a risk of high flow rates, use flow control valves to limit the flow rate.

The clamping sequence must be set up so that work supports are operated only after the workpiece has been firmly positioned against the locators. An exception is a work support directly opposite a clamping cylinder. In this case, the work support should be brought to full pressure before the clamping cylinders actuate. This sequence is accomplished via either modular (solenoid) valves or a sequence valve, as depicted in the diagram.