Introduction to Hydraulic Workholding Tools
What is it?
Hydraulic workholding is the use of hydraulic systems to position, support, and clamp parts for machining. The hydraulic systems are made up of cylinders, pumps, valves, and other components to control and automate the process.
What is it?
Properly implemented, hydraulic workholding provides these benefits:
- Vastly reduced setup time
- Less scrapped parts
- More consistent results
- Increased safety
Who should use it?
From the simplest fixture to robotic assisted machining centers, Enerpac workholding products can perform the positioning, support, and clamping functions in a wide array of manufacturing processes. With a setup time per part of only 5-10% of manual methods, hydraulic workholding offers a huge productivity increase in processes such as machining, punching, pressing, and many others.
Not only are fabrication processes accelerated, but less parts are rejected and results are more consistent because each part is positioned, supported, and clamped in exactly the same manner. And, the automated nature of hydraulic workholding systems allows them to be readily integrated with operator safety systems.
The benefits of hydraulic workholding don’t apply just to high-volume manufacturing; it’s often attractive for short and medium run production. A system can be set up quickly and inexpensively for short runs. Center-hole cylinders utilize standard stud thread sizes and are easily interchanged with manual clamps. Many manual elements can be used directly with hydraulic cylinders. Add a simple air-operated pump and controls, and your fixture is converted to a hydraulic setup. For medium runs, a little more automation pays off. For example, swing cylinders allow easy loading and removal of parts.
Hydraulic cylinders produce the forces required to do three jobs: positioning, supporting, and clamping a part in place during machining. They are available in a huge variety of styles and sizes to perform these three functions. (Future issues of EU will provide more details on cylinders.)
Used to position the part properly on the fixture and also to hold it in place during machining. The part is simply placed on some basic locators, and the positioning cylinders push or pull it into place each time.
Used in the same manner as traditional stud and strap setups to hold the part in place during machining. The most popular and versatile is the swing cylinder. The attached clamp arm on these cylinders begins its cycle at a position 90 degrees offset from the clamping location to allow unobstructed part loading and unloading. When hydraulic pressure is applied, the arm swings over the part and comes down to apply the clamping force. Clamping cylinders can be designed with a linkage to reach into tight spaces on the fixture and retract out of the way when not in use.
Automatically conform to the size and shape of the part. They help to maintain dimensional control and to prevent deformation. They also absorb a lot of the vibration generated by cutting operations. These cylinders can be set up so that they are normally retracted out of the way and engage the part only when needed.
The force produced by a cylinder and the time the cylinder takes to operate are primarily dictated by the size and type of pump supplying the hydraulic oil. Many machine tools have built-in hydraulic systems that can be used to operate hydraulic workholding setups. If not, a separate pump can be used. These free-standing pumps are powered by either compressed air or electricity. (Future issues of EU will provide more details on pumps.)
Air operated pumps
Simple and cost-efficient. They are powered by the existing shop air supply and can be configured and adjusted easily. Air operated hydraulic pumps are best suited for low to medium duty cycles.
Electric operated pumps
The most versatile and are best suited for situations where automation is required. Electric pumps and corresponding controls can be completely integrated into machine tool controls, accepting and sending signals during the machining cycle. Electric pumps are recommended for high duty cycle applications. They provide the highest level of performance, durability, and automation compatibility.
If the machine tool hydraulic system does not have sufficient pressure and an external pump is not practical, a hydraulic intensifier can be used to boost the machine tool hydraulic pressure to a level suitable for the workholding cylinders. Intensifiers are simple, compact units that can be integrated directly into the fixture to provide the necessary pressure and flow.
The workholding system’s control devices determine the flow path, flow rate, and pressure of the hydraulic oil. Successful machining of a part also depends upon controls to provide proper operating sequence of the positioning, supporting, and clamping cylinders. Controls save time and money by automating the operational sequence. (Future issues of EU will provide more details on control devices.) Valves, are the fundamental control devices. There are many kinds of valves, but any given valve serves to determine the flow path, flow rate, or pressure of the hydraulic oil. As you can see from the example illustrations below, Enerpac valves are of modular design, which simplifies system assembly.
Directional control valves
Just what the name implies. They are multiport valves used to direct hydraulic oil. Single-acting cylinders are usually operated by 3-way valves, while double-acting cylinders are operated by 4-way valves.
Directional control valves that control the order of operation of various branches of the hydraulic circuit. These valves are activated by a sensed pressure. When one part of the hydraulic circuit reaches a preset pressure, the sequence valve opens to permit oil to flow to another part of the circuit. An example is activation of a support cylinder first, then the corresponding clamping cylinder.
The sequence valve contains a check valve in parallel with the directional valve mechanism. The result is sequenced/directed flow in one direction (toward the loads, during clamping) and unrestricted flow in the other direction (unclamping).
Used to allow the flow of hydraulic oil in only one direction. A special version, the pilot operated check valve, is used to hold pressure in one part of a circuit and release the pressure upon command. A pilot operated check valve works the same as a regular check valve, but also has an additional port. The application of a low pressure (about 15% of working pressure) to this extra port causes the check valve to permit oil flow in both directions, releasing the contained pressure.
Flow control valves
Control the operating speed of hydraulic components by means of an adjustable orifice. They also contain a check valve in parallel with the metering orifice. The result is restricted flow in one direction (toward the load) and unrestricted flow in the other direction (return). The most common use of flow control valves is to regulate the operating speed of cylinders. When the holding pressure on a cylinder is released, a spring in the cylinder produces a small pressure, causing the check valve to open and thereby providing rapid cylinder release.
Pressure reducing valves
Do just that; they are adjustable pressure regulators, used to provide reduced oil pressure to a secondary part of the circuit. A common use is to control the clamping force of a cylinder. Note: Pressure limiting valves perform a more specialized function. Like pressure reducing valves, they deliver a settable lower pressure to a secondary part of the circuit. Unlike pressure reducing valves, when the set pressure is reached they do not supply any makeup oil to maintain that set pressure.
Pressure relief valves
Limit the maximum pressure in the hydraulic circuit. For safety, any hydraulic system should include a relief valve. An Enerpac relief valve does not act instantly. As pressure approaches the set point, the valve permits a very small amount of oil to pass before further pressure rise results in full opening of the valve. Therefore, a relief valve should not be adjusted under conditions different from use. For example, don’t set the valve with a hand pump and then use it with a power pump.
Sense the hydraulic oil pressure, operating their contacts when the pressure reaches a set value. Enerpac pressure switches are adjustable and are available with NO, NC, SPDT, or DPDT contact configurations. Both mechanically operated and electronic pressure switches are used for monitoring and control of system pressure. Electronic switches are well-suited for high cycle, automated applications.
The use of hydraulic workholding for machining yields large paybacks, even for modest size production runs, in particular due to reduction of up to 20:1 in setup time.
Several important concepts occur in three’s:
Three Elements of a workholding system
- control devices
Three Functions of cylinders
Three Types of pumps
- Air operated
- Electric operated
- Intensifier (hydraulic operated)
Three functions of valves
- Control fluid flow path
- Control fluid flow rate
- Control fluid pressure