Clamping technology vital to productivity.
Do you buy state-of-the-art machine tools and equip them with antiquated workholding devices? You wouldn't be alone. Workholding is often the weakest link in the automation chain of a flexible machining cell. This article gives the facts about modern hydraulic clamping.
A flaw in machine-tool acquisition is the fact that many engineers don't explore workholding for machine tools until after they buy the machine. Had they investigated first, they might have chosen a different machine tool and material-handling solution--probably at less cost.
Two main categories of workholding--mechanical clamping and hydraulic clamping--have to be considered, plus power sources, sensors, controllers, and robots.
Mechanical clamping typically serves where pallets are loaded and unloaded at a remote station, and where time and consistent clamping force are not critical. Clamp types and uses are numerous.
For limited-run palletized systems, for example, mechanical clamping offers a low-cost solution, which can include modular fixturing. However, after you complete a cost-benefit analysis, you may select a hydraulic clamping solution, the subject of this article.
Pressurized on pallet
Hydraulic clamping that remains pressurized during the machining cycle allows use of the widest range of hydraulic workholding components. Most traditional hydraulic fixturing components used on tables and pallets operate using this methodology.
Pressurized hydraulics have advantages over manual clamping. They offer increased speed of clamping and unclamping, and the stroke of the hydraulic clamp allows a wide variation in part size. Consistent clamping forces allow predictable workpiece holding and deflection.
Pressurized-on-pallet hydraulic clamping has the additional advantages of makeup, space, and force. Makeup is the ability of the system to adjust if the part deflects slightly during machining, requiring a change in clamping height. Constant hydraulic pressure allows the clamp to reclamp at the new height. Space on a palletized fixture is at a premium, and hydraulics offer the highest repeatable force in the smallest area.
Use of multiple pallet systems may require that fixtures be clamped for a long time between initial clamping and final unloading. Accumulator pressure changes, hydraulic seepage, and temperature changes will affect pressurized hydraulic systems over these extended periods.
When hydraulic pressure decreases over time, clamping force drops, and the workpiece can slip. A method to prevent this movement uses hydraulically activated, mechanically locked clamps. The three types of clamps employed in this method use conical-disk springs, wedge locks, or collect locks.
Conical-disk-spring clamps have a very low profile and typically hold large, flange-type workpieces such as oil pans for transmission cases. Hydraulic pressure compresses the springs for plunger retraction, and instantaneous clamping occurs with release of the hydraulic pressure. Clamping force is proportional to plunger deflection.
One conical type has a fully threaded cylindrical body with hollow plungers and plunger threads. It exerts axial force, and clamp capacity ranges from 1000 to 10,000 lb.
The swing clamp is a second type of conical-disk-spring design. When hydraulics are applied, the swing clamp retracts from the workpiece. When clamping is required, the arm is either mechanically or hydraulically rotated into position. The hydraulics are released, and the disk springs form a positive clamp.
The wedge-locking clamp uses internal and external wedges. Internal-wedging designs use a piston driven by applied hydraulic force. A locking angle, machined into the piston, engages a corresponding angle machined on a linchpin to provide mechanical locking between the two surfaces.
The wedge-locking principle can be used for rocker, toe, swing, block, and die clamps. Capacities range from 400 to 10,000 lb clamping force.
External wedging clamps use a plunger that is hydraulically extended at a 5-deg angle to the base plane. The plunger has a 5-deg taper on its nose. When the taper contacts the workpiece parallel to the surface, the plunger wedges into place, and hydraulic pressure is no longer required. To release the clamp, hydraulics retract the plunger. These clamps have limited clamping height and are used for fixturing flat plates and quick-change fixture plates.
Collet-locking clamps are suggested for today's flexible machining cells and systems. These clamps use a collet that locks into a collar, providing zero deflection after the hydraulics are released. Hydraulic pressure moves the plunger to the clamp position, applying pressure to a collet ring for locking. This allows the clamp to be locked at any plunger position. After machining, hydraulic pressure unlocks the collet and retracts the plunger.
Collet-locking cylinders are available as swing, push, and work-support cylinders. They have full-body threads or flange-mounting styles, with capacities that range from 1000 to 8800 lb clamping force.
The primary advantage of collet-locking clamps over other hydraulically activated, mechanically locked clamps is the large stroke available. Increased stroke lets the same clamp hold workpieces with greater size variations. Mechanical locking power is the same at any point in the cylinder stroke.
Collet-locking clamps are compact, because the locking mechanism is axial to the clamping action. The footprint may not be as small as with live hydraulic clamps, but it's not significantly larger.
Multiple porting options add size to the units, but they allow part clamping at variable pressure levels and higher locking force. This ensures distortion-free clamping of the workpiece--or maximum holding force when required to withstand high cutting-tool forces.
The next step is to select a power source. The two distinct methods of power supply are pallet mounted and non-pallet mounted. Pallet-mounted solutions are most frequently specified for clamping technologies requiring constant hydraulic pressure.
Simple manual screw pumps can mount on pallets to supply hydraulic power in low-cost systems, but they don't allow system automation, and they have limited oil volume. Also, screw pumps don't have any makeup capability without accumulators.
An electric-hydraulic method uses a battery that runs a pallet pump to supply pressure to the workholding clamps. Control of the unit is accomplished by an infrared beam from a remote controller. Advantages include automated control and continuous monitoring. In addition, the entire power supply is internally located in the pallet or underneath the fixture, reducing required pallet space. However, internal mounting is expensive.
A third pallet-mounted power source uses a pump driven by the machine-tool spindle. This requires an accumulator to maintain pressure when the spindle stops.
Power sources off the pallet are most cost effective when handling multiple pallets, and where pallets don't have enough space for fluid-power pumps. Usually these systems require only one hydraulic power source.
Non-pallet-mounted-power systems must transfer either mechanical energy or fluid pressure. In one type, a master cylinder energizes a slave cylinder on the pallet by mechanical transfer of power. The cylinders are positioned plunger-end to plunger-end, and the master cylinder is energized and extended, causing oil in the slave to be compressed and thus pressurized.
Alignment brackets guide the master cylinder, and a manual valve lets oil out of the slave cylinder. The valve is closed when pressure is obtained. Because no oil transfer occurs, no chips or debris are infused into the hydraulic oil.
This solution suits nonautomated load and unload stations, where initial cost is a factor. It works well with the conical-disk-spring clamps. These require a small amount of oil to energize clamping.
The second non-pallet-mounted power option is similar to the machine-spindle-driven pump. In this solution, an external motor with a mechanical coupler on its shaft end is positioned to mate with a corresponding coupler on the pallet pump. The motor is energized and hydraulic pressure is generated on the pallet.
Currently, the most common method of non-pallet-mounted power supply is the manual hydraulic coupler, which uses hydraulic-fluid transfer through flexible hoses. The operator connects a hose-and-coupler assembly from the power supply to a mating coupler-half on the pallet to build hydraulic pressure.
Manual couplers provide versatility, and they are inexpensive, but the load and unload cycle cannot be automated, because the operator must activate the hydraulic connections.
Manual couplers serve both pressurized-on-pallet and depressurized-on-pallet systems. The pressurized solution requires a shutoff or pilot-operated check valve to lock pressure on the pallet. It's best to use pallet accumulators to ensure fluid makeup. Several manufacturers offer a combination shutoff, check, and accumulator package with a coupler-half in the block for ease of plumbing.
The fourth non-pallet-mounted solution uses an automatic coupler system or autocoupler, which is useful for flexible machining systems. It consists of a base section and a receiver mounted on the pallet. Each section has a coupler-half.
The receiver, with appropriate piping to the hydraulic clamps, has three options: full flow, accumulator addition, and sequence-valve addition. The sequence-valve option allows the receiver-half to control clamping and locking force for collet-locking clamps. This eliminates the cost of additional valves and pipes, and you can set a specific pressure on each pallet.
The base unit automatically moves a plunger into the receiver-half, where proximity or pressure switches verify its position. When the fixture is pressurized or hydraulically activated, thereby locking the clamps, the base station retracts the plunger and uncouples.
The base unit can be manually controlled by electrical push-buttons, or automatically operated by a programmable controller (PLC) or CNC unit. Automatic controls ensure proper sequence of coupling, uncoupling, and pressurization.
The autocoupler non-pallet power solution is currently the most effective method for total palletized automated clamping. Pallet shuttling into the load/unload station is sensed, and the controller depressurizes or deenergizes the clamps to release the workpiece. The autocoupler repressurizes the pallet on command to return it to the machine cell.
Sensors, controls, robots
Electronic feedback is necessary for a totally automated solution. Proximity sensors are key feedback tools and can verify workpiece orientation and mounting. Their use in hydraulic clamping devices to acknowledge advance and retraction is especially important where high-speed cutting tools require that clamps move out of the way for cutting passes. Also, fault systems can stop cutting tools if clamps are not positioned properly.
One of the drawbacks to traditional controllers has been communications capability. Lower-cost standalone PLCs could not communicate with other PLCs. Those that could were costly. But this is no longer the case.
New controllers communicate with other PLCs as well as larger control systems. This permits a centralized computer to program and control the machine-tool and pallet system from a remote location. A pallet pool can cycle pallets into any machine tool, with the central computer uploading the programming for each workpiece as needed.
New controls also handle robots for total automation, including material handling. Combined with sensors, robots can load and unload workpieces without human intervention. For robotics, engineers must examine additional factors. For example, there must be maximum clearance between the fixture and workpiece to allow for gripper inaccuracies. Clamp cylinders must be of the hydraulic-clamp, hydraulic-return type.
Much of the technology described is available off the shelf. The key is to properly execute from conception to final solution. Robotics, controllers, sensors, palletized machine tools, and advanced workholding can now be incorporated into one control system.
PHOTO : Collet-locking clamps at work. Models of this type offer 1000 to 8800 lb clamping force.
PHOTO : Layout of a non-pallet-powered hydraulic-clamping system using an automatic coupler or autocoupler.
Jim Boldt Manager of New Product Development Enerpac, a unit of Applied Power Butler, WI
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|Publication:||Tooling & Production|
|Date:||Nov 1, 1991|
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