A pneumatic gripper for CNC machine loading must not only hold the part, but also perform the entire cycle reliably: pick up the workpiece, move it to the working area, change its position if required, place it into the chuck or fixture, and remove the machined part. An incorrect gripper choice can lead to slipping, misalignment during loading, increased cycle time, or an unplanned equipment shutdown.
For robotic CNC loading, it is important to select not just a separate gripper, but the entire gripping unit: the pneumatic gripper, gripper fingers, position sensors, rotary unit, compensation module if required, and air preparation components. The configuration depends on the part shape, its weight, center of gravity, positioning requirements, and machine operating mode.
What tasks does a pneumatic gripper perform in CNC machine tending?
Pneumatic grippers are used in metalworking automation for the following operations:
- loading a workpiece into a lathe, milling machine, or drilling machine;
- unloading the finished part after machining;
- transferring the part between machines or process stations;
- changing the orientation of the workpiece before loading;
- feeding parts into a chuck, collet, jig, or inspection device;
- simultaneous loading of a new workpiece and unloading of a machined part.
Unlike a stationary clamp, a pneumatic gripper is mounted on a robot, gantry, or moving mechanism. It must take into account not only gripping force, but also movement dynamics, load on the robot wrist, gripper finger overhang, and part placement accuracy.
Which pneumatic gripper to choose for machine loading?
Parallel two-finger pneumatic gripper
Parallel pneumatic grippers are suitable for parts that are convenient to hold by two opposite surfaces: housings, rectangular workpieces, small stamped parts, profile components, bushings, and round parts when fingers of a suitable shape are used.
For standard automation tasks, VGTR series grippers with T-shaped guides are used. For higher-load operations, long fingers, and increased stability requirements for the gripping unit, the VGFR series with a toothed guide should be considered. This design helps reduce backlash and is intended for higher moment loads.
Parallel models are often chosen for horizontal movement of round bars, lathe loading, and part transfer between stations. If a larger opening range is required, long-stroke L versions can be used, for example VGTR30-95-L or VGTR46-114-L.
Three-finger centering gripper
Centering pneumatic grippers with three fingers are used for round parts when it is important to hold the workpiece symmetrically relative to the axis. This is relevant for bushings, flanges, shafts, cylindrical housings, and other parts that are fed into a chuck, collet, or fixture with centering requirements.
The VGTZ and VGFZ series provide radial finger movement toward the center. VGTZ uses T-shaped guides, while VGFZ belongs to the versions with a toothed guide. For a small round part in a compact unit, VGFZ34-68 can be considered.
Four-finger centering gripper
Four-finger VGTF models are rational to use for square, profile, and special workpieces when two contact points are not enough and the part needs to be held from several sides. This configuration can be useful for automatic feeding of irregularly shaped parts, housing components, and profile stock. As an example, VGTF33-56 can be considered.
Gripper for heavy workpieces
For large and heavy parts, reinforced VGTJ series solutions are used. These grippers have an increased stroke and high gripping force, but the model must not be selected based only on workpiece weight. It is necessary to check the gripping method, finger length, center of gravity, moment of inertia, and allowable load on the robot or gantry. The VGTJ series is used for heavy parts, including hubs, frames, and massive metal products.
What parameters are required for gripper selection?
Part shape and gripping area
First, it is necessary to determine which surface the robot will use to grip the workpiece. For a smooth part held by friction, a larger force reserve is required than for a part with projections, grooves, or shoulders, where form-fit gripping can be implemented.
For a round part, it is important to understand whether accurate axial centering is required. If it is, a three-finger centering gripper is usually considered. If the workpiece needs to be moved horizontally by its side surfaces, a two-finger parallel design is more often suitable.
Opening range and jaw stroke
The part size must fall within the working range of the model, and the finger stroke must be sufficient for gripping, removing the part from the machine, and safe movement. For example, if a part has a diameter of 50-70 mm, the gripper fingers must provide the required opening range with allowance for tolerances, positioning, and possible workpiece misalignment.
A long-stroke version is not always better than a standard one. Increased stroke is useful with a wide range of part sizes or limited feeding accuracy, but it may provide lower force compared to a short-stroke version.
Part weight and center of gravity
When selecting a gripper, not only the workpiece weight is considered, but also the total weight of the entire moving unit:
- pneumatic gripper;
- gripper fingers;
- adapter plate;
- sensors;
- rotary or compensation module;
- the part itself.
The position of the center of gravity is especially important. The farther it is offset from the robot wrist or rotation axis, the higher the load on the mechanical unit. Even a lightweight part can create a critical moment if it is long or held with a large overhang.
Length and weight of gripper fingers
Gripper fingers are designed for the specific geometry of the part. They should be as short and lightweight as possible within the process requirements. As finger length increases, the force at the contact point with the workpiece decreases, while the moment load on the base jaws, guides, and gripper body increases.
If the finger length exceeds the allowable range, it is recommended to choose a larger gripper size. This reduces the risk of body deformation and helps preserve the service life of the equipment.
Gripping force and air pressure
The gripping force of a pneumatic gripper depends on the operating pressure. In KIPVALVE technical data, it is specified as a value depending on P — the air pressure. However, maximum pressure must not be used as a universal way to increase payload capacity: as pressure increases, loads on the fingers, body, and guides also increase.
The operating pressure range, allowable moments, force at the gripping point, and maximum finger length must be checked according to the datasheet of the specific model. For friction gripping, it is especially important to include a reserve against slipping.
How to calculate the CNC loading cycle time
The response time of a pneumatic gripper is only the opening and closing time of the base jaws. The full machine loading cycle includes more operations:
- PLC command;
- directional valve switching;
- filling pneumatic lines with compressed air;
- opening or closing of the gripper;
- movement of the robot or gantry;
- placing the part into the machine;
- position confirmation by sensors;
- robot exit from the working area.
For high productivity, it is important to calculate the entire cycle, not just the finger movement speed. For stable automation, it is useful to monitor the open and closed states of the gripper with position sensors, especially if the part is fed into a chuck, mold, or narrow fixture.
When a rotary unit is required
A rotary unit is required if the workpiece needs to be turned, reoriented, or if loading and unloading must be organized within a single cycle.
For one gripper and 180° rotation, VRTP units are used. For a configuration with two grippers and 180° head rotation, VRTG is used: this solution allows the machined part to be removed while the next workpiece is prepared for machine loading. For 90° rotation with two grippers, VRTE units are suitable.
When selecting a rotary unit, not only torque is evaluated, but also the weight of the grippers, fingers, workpieces, adapter plates, tooling overhang, and moment of inertia. For heavy parts, it is not enough to rely only on the built-in buffer: external damping may be required, and the rigidity of the mechanical structure must be checked.
Why a compensation module is needed
When feeding a workpiece into a machine, small position deviations, part tolerances, fixture inaccuracies, or displacement during gripping may occur. In such cases, compensation modules are used.
HT-ZG series modules are used for VRTG rotary units, while HT-HE is used for VRTE. These modules help compensate for deviations during part placement and, when used with an inductive sensor, can participate in detecting an unsuitable workpiece or a collision.
Air preparation and safety
A pneumatic gripper must not be connected directly to a compressor without air preparation. Moisture, oil, dust, and contaminants can cause corrosion, seal wear, jamming, and unstable mechanism operation. The pneumatic system requires filtration, as well as pressure control and connection leak checks.
For operations where a dropped part may create a risk of equipment damage or personnel injury, a safe scenario must be designed in advance in case of pressure loss. Depending on the task, versions with mechanical self-locking can be considered:
- NO — maintaining the open position;
- NC — maintaining the closed position.
The choice depends on the part weight, robot position, emergency stop logic, and the requirements of the specific cell.
What to provide for selecting a pneumatic gripper for CNC loading
To select a gripping unit without unnecessary iterations, prepare the following:
- drawing or 3D model of the part;
- material and surface condition;
- weight and overall dimensions of the workpiece;
- dimensions of the gripping area;
- machine type and part installation method;
- required orientation during feeding;
- cycle time;
- data on the robot, gantry, or manipulator;
- operating air pressure;
- need for sensors, rotary unit, and self-locking;
- environmental conditions: temperature, dust, coolant, chips, and vibration.
KIPVALVE specialists will help select a pneumatic gripper for CNC loading, gripper fingers, sensors, rotary unit, and compensation module for the actual equipment layout. There are no minor details in automation: a part that is held securely on a table does not always behave as reliably under robot acceleration.
Frequently asked questions
Which gripper should be selected for loading a round part into a lathe?
For end gripping of round parts with centering requirements, three-finger VGTZ or VGFZ models are usually suitable. For horizontal movement of a bar or cylindrical part by its side surfaces, two-finger parallel VGTR or VGFR grippers are more often used.
Can a gripper be selected only by workpiece weight?
No. Weight is only one parameter. It is necessary to consider the part shape and surface, gripping method, finger length, center of gravity, acceleration, pressure, tooling overhang, and load on the robot.
When is a rotary unit required for CNC loading?
A rotary unit is required if the part needs to be turned before being fed into the machine, or if loading and unloading need to be combined in one cycle. For selection, the rotation angle, configuration with one or two grippers, total weight, and moment of inertia of the unit are evaluated.
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