Giving robots a hand means selecting the right gripper and finger design.
To handle long parts, the contact surface of each gripper finger should extend to stabilize the part during motion.
For gripping the O.D. of a cylindrical part, a three-point line contact uses a V-shaped finger opposite a flat finger.
For high-speed I.D. applications, V-shaped rounded-point fingers on both sets of grippers secure parts with a four-point line contact.
Fingers with thin profiles avoid interferences when opening to release a part.
Three-point line contact fingers can change the part centerline in relation to the gripper centerline. However, a four-point contact has two V-shaped fingers that eliminate centerline shift of the part being handled.
Dual V-shaped rounded-point fingers and a single rounded-point finger provide three-point line contact for I.D. part gripping.
It's difficult to pick up an object without using one's hands and fingers. The same holds true for robots. With grippers and gripper fingers, robots can handle practically any part type. But, unlike humans, robots need their grippers and fingers matched to the particular application at hand. When selecting these devices, shops must consider repeatability requirements, type of stroke needed, and the size, shape, and weight of the part to be gripped.
There are two major types of pneumatic grip-per motions or strokes: angular and parallel. Angular-motion grippers have jaws that rotate simultaneously from the open to the closed positions, much like a pair of scissors. This style works well in applications where accuracy is not the main concern, as with reject stations.
In addition, angular grippers are usually dedicated to picking up only one part type. Their gripping fingers match the shape of the part and no other. Hence, angular-motion grippers are not normally used for positioning or placement of parts.
As the simplest of the major gripper designs, angular grippers are usually the least expensive. Their strokes range from as little as 10° up to 180°.
Unlike angular grippers, the jaws on parallel-motion grippers move simultaneously in a linear direction, always coming together at the same point when closed. This is the most precise type of gripper motion and is widely used. The reason is that parallel grippers can pick up numerous part sizes with one set of fingers. However, parts to be gripped must be located accurately in the same place each time, typically using a part nest.
When part location varies, a nonsynchronous parallel-motion gripper is called for. The jaws on this gripper move independently of each other, making it perfect for applications where parts may shift position, like on a conveyor. These grippers do not shift parts to center, as do synchronous parallel-motion models, but, instead, the fingers conform to the part diameter. Keep in mind, however, that parts picked up off-center will also be placed down off-center. This type of gripper is found in about 1% of applications.
After shops decide on an appropriate gripper style, the next step is choosing a finger design. Considerations for this include gripper motion, gripping style, finger shape, and part weight, shape, and surface finish.
There are three primary ways to grip parts. One is squeezing them, called a friction grip, the second is cradling, called a capture grip, and the third is a combination of the two.
In a friction grip, the fingers rely totally on the force of the gripper to hold the part. It's recommended to use a 25:1 finger-force-to-part-weight ratio. So, for a part weighing 5 lb, the gripper should generate 125 lb of force.
Another rule of thumb is to keep tooling fingers as short as possible. Gripping force decreases when finger length increases. This is caused by friction on the bearing surfaces resulting from deflection of the tooling fingers. Most gripper manufacturers supply a finger-length guide that provides an accurate idea of grip force.
A cradle grip captures parts between fingers with little or no squeezing applied to the parts. Fingers bear the weight of the part, and thus, grip force is less of a factor when choosing a gripper. The cradle grip adds stability and power because it encapsulates parts, and, for this style, a 10:1 finger-force-to-part-weight ratio works best.
When designing fingers for angular grippers, it is more difficult to engineer a friction grip than a cradle grip. Due to the angular motion, gripper fingers must be parallel to hold the part securely, requiring absolute precision where the two parallel fingers make contact with the part. A preferable design is a combination of the two gripping styles. This holds a part the most securely.
Part shape and surface finish
Since workpieces come in every shape and size, shops should consider some basic part surface geometries — flat, round, convex, or concave — when designing gripper fingers. Most parts are held on their exterior surfaces using standard grippers.
However, certain jobs can require special-order grippers to grasp the I.D. of a part. Many times these parts have painted or plated surface finishes that cannot be compromised in any way, thus eliminating the option of gripping the O.D. Also, I.D. gripping works for handing parts to another gripper or lathe chuck that is gripping by O.D.
When centerline repeatability is crucial in I.D. gripping of circular parts, shops should go with a three-point-contact finger configuration. It has two V-shaped, rounded-point fingers on one side and a single rounded-point finger on the opposite side. Putting V-shaped, rounded-point fingers on both sets of grippers provides a four-point line contact to hold parts even more securely for high-speed applications.
To successfully handle long parts, a gripper must have superior bearing support to accommodate larger movements and cantilevers. Basically, the contact surface of each finger is extended not only to stabilize the part in motion but also to balance the long tooling fingers.
For round parts, particularly when using a parallel-motion gripper, a three-jaw gripper is the best choice (although, a two-jaw style is less expensive).
One option a shop has when it comes to gripping cylindrical parts is using a 3-point line contact for O.D. gripping a range of diameters. This incorporates a V-shaped finger on one side and a flat finger on the other. It should be noted, however, that the part centerline changes in relation to the gripper centerline.
To avoid this centerline shift, shops can opt for a 4-point contact, which has two V-shaped fingers. Centerlines of various diameter parts remain constant in relation to the centerline of the gripper.
Other considerations in finger design
Reduction of finger weight is an important objective, particularly for high-speed applications. One way to minimize finger weight and lower costs without sacrificing strength is by using a lightweight material such as aluminum. Or shops can narrow fingers to a thinner profile, which not only reduces weight but also helps avoid interference each time the fingers open to release a part.
Another key goal is improving finger hold. In many machining applications, parts are slick with coolant, making them difficult to grip. One way to improve a gripper's holding involves roughing or knurling the finger surface. Or if parts have an irregular surface that makes them difficult to hold, applying a urethane material to gripper fingers will improve the situation.
Some applications demand that parts are free of the slightest surface marks, which may be caused when a gripper finger makes contact. Fingers constructed of polymer do not leave marks, but polymer can flex and bend as pressure is applied, jeopardizing the part's position. If a shop uses aluminum or steel fingers, it can apply a section of urethane or plastic at the point of contact to obtain a firmer grip.
For multiple-part handling and applications that require specific grip forces, fingers that adjust to a gripper's stroke provide the most flexibility. When adjusting such fingers in the closed position, one finger should have a tapped hole with a threaded rod and two nuts. For accurate grip-force adjustment, it is best to keep the adjustment as close to the gripper as possible.
Similarly, to adjust fingers in an open position, one finger should have a tapped hole with a threaded rod and two nuts. In this case, however, the other finger must have a clearance hole that allows the threaded rod to pass through, and two additional nuts on the opposite side. This setup works best to control how far the fingers open.
Finger alignment is critical. If they do not meet accurately, the part's position is greatly compromised, as is gripper holding power. To help ensure proper alignment, dowel holes and/or a surface to key on are musts.