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Choosing robot grippers for cylindrical parts.

Choosing robot grippers for cylindrical parts

Learning what's available, and following a recommended procedure for analyzing your application, will help ensure a productive, cost-effective installation.

Do you plan to use robots or pick-and-place arms for tending lathes, turning centers, or grinders? If so, it will pay to consider the end-effectors, particularly mechanical grippers, early in your planning. In some cases, choice of grippers affects the size, capacity, and design of the arm. This has a strong bearing on efficiency and productivity of the installation.

At the outset, there are two main courses of action you may pursue: Design your own custom grippers for the specific installation, or select from standard, modular grippers and other end-effector components already on the market. Unless your application is highly unusual, the latter course of action is better.

Standard, third-party end-effectors meet over 95 percent of application needs involving cylindrical or rotating parts. Being mass-produced, these end-effectors cost much less than custom units. Standard units and their components are stocked by distributors as well as manufacturers, enabling you to minimize inventories.

Further, manufacturers offer application engineering, turnkey start-up service, training, and maintenance. These services help bring an installation into production quickly, and enable it to operate efficiently and with minimum downtime.

What's available

As a first step, it's a good idea to learn the range of end-effectors and components available. There are a number of manufacturers in this field, each offering products that differ in size, capacity, diversification, and specialization. A review of manufacturers' catalogs and price lists will help you make a "first cut."

The line offered by ISI Mfg Inc is modular in design (Figure 3). This means various gripper heads, adaptors, cylinders, and rotate units can be interchanged readily. This feature is especially valuable where production batch sizes are small and part configurations vary from batch to batch.

A relatively new offering is the quick-change design (Figure 4). Here the head is mounted to the actuating cylinder by an adaptor plate, and is locked in place by a detented pin. To change heads, you simply remove the pin, slide off "Head A" laterally, slide on "Head B", and reinsert the pin. No special tools or skilled labor are needed.

Gripper heads come in three basic size ranges: Miniature, standard, and heavy duty. These classifications pertain to the sizes and weights of parts that can be handled. Generally speaking, mini heads are for parts weighing 10 lb or less. Standard heads are for parts in the 10 to 60 lb range, while heavy-duty heads handle parts 60 lb and over.

There are two gripper-head designs suitable for cylindrical parts. In one design (Figure 5), jaws are joined at the inner or back end, and provide a scissors action. Both jaws operate in unison, providing a maximum finger opening of 43 degrees. At 80 psi in the air operating cylinder, the gripper applies over 400 lb of finger pressure.

The second design suited to handling cylindrical parts is the parallel-motion gripper (Figure 6). In this design, slide arms move in a synchronous, parallel motion, allowing jaws to open and close equal distances at front and back.

Parallel-motion grippers develop less force than scissors-type grippers. Maximum force at 80 psi, for example, is 88.2 lb, versus over 400 lb in the scissors type. Nevertheless, a parallel-motion gripper is a better choice where part diameter changes significantly during processing.

Other components

In addition to gripper-head designs and sizes mentioned, there are other end-effector components that should be considered. One of these is a scissors-type head that provides jaw action but does not have fingers. This enables you to design your own custom fingers, and weld them to the head's jaws.

For simple wrist rotation at the head, you can choose from two types of accessories. One, powered by an air cylinder, provides rotation of 90 and 180 degrees (Figure 7). This unit gives an economical solution for reorienting small or medium-sized parts within a machine or between machines.

The second accessory, called a rotary actuating unit, permits turning a part to any degree of rotation (Figure 8). Turning motion is provided by a chain and sprocket or belt and pulley. A large-bore cylinder extends and retracts the gripper head up to 4".

A relatively new type of accessory is the adaptor swivel (Figure 9), which mounts between gripper head and cylinder. This enables you to keep the cylinder mounted square, but rotate and lock the gripper head for setup in any position through 360 degrees. Four socket-head screws provide for rapid adjustment and locking.

Also new are two types of proximity switches. One, designed for standard and mini gripper heads, affords two-position proximity sensing with indicator lights (Figure 10). These show whether the gripper's fingers are open or closed. Mounted on the side of the head, this type of sensor is compact and lightweight.

In the other design, a pair of cylinder proximity switches are used to sense the fingers open and closed states as the cylinder strokes.

Choosing a gripper

Assuming your application can be satisfied through use of standard, off-the-shelf gripper heads and other end-effector components, your next step is analysis of the part to be handled, along with details of the preloading fixtures and machine tools. Consider the following factors:

1. Over-all size of the part. What size of gripper head can comfortably handle this part? If it is relatively long, should you consider gripping it at two points with two small heads, rather than at one point with one large head?

As a rule, if you can hold a part comfortably and steadily with one hand, a single gripper should suffice. But if the part tends to wobble, or if the application doesn't allow gripping a part at its balance point, perhaps you should apply two grippers.

2. Weight of the part. What size of gripper head will be needed to handle the weight? Will the head grip the part at some place other than the balance point, and if so, what will be the forces on the fingers?

3. Areas available for gripping. These effect your choice of size, number, and spacing of gripper heads.

4. Change of part diameter during machining. If you're turning or grinding a lot of material off a cylindrical part, perhaps you should use parallel-motion grippers rather than scissors-type grippers. If the fingers are designed correctly, they will continue to close on even the smallest diameters.

5. Surface finish. If you intend to handle parts having a fine finish, you must select or design fingers and protective inserts accordingly.

6. Part orientation. Will the preload fixture present the part in the same orientation needed for loading the machine? Do you want the robot or pick-and-place arm to rotate the part for machining both ends, or to change part orientation between two machines? A rotary actuating unit may be called for.

7. Visual indication. Is it necessary or advantageous for your operators to be able to determine--from a distance--whether a gripper is in the open or closed position? If "yes," consider installing a proximity switch.

8. Head orientation. After an end-effector's cylinder and gripper have been installed, is it possible you'll need to rotate the head relative to the cylinder? In some cases, this is necessary for correctly adjusting head attitude when approaching a part. Consider installing an adaptor swivel.

9. Batch sizes. Are the sizes of batches you machine tending to be small? Are there considerable variations in part size, shape, and weight from one batch to the next? Do you need to minimize changeover time between batches? If the answer to any of these is "yes," look seriously at quick-change adaptors and heads.

An example

Say, for instance, you want to use a robot or pick-and-place arm to load and unload automotive camshafts to and from a horizontal grinding machine. The part to be ground is 6" diameter by 19" long, and weighs 10 lb. How do you select grippers for this part?

First, consider that part balance points are at the center of the shaft and between cam lobes. The part is to be machined to close tolerances, but not to a fine finish. Standard steel fingers thus would be suitable.

Orientation of the camshaft at a preloading station cannot be guaranteed. The best choice for gripping, therefore, would be at the concentric or bearing surfaces. Rough locators could be installed to provide front-to-back and left-to-right orientation in the fixture.

A good choice for this application would be use of two mini gripper heads located on 17" centers (Figure 2). Jaws would be fitted with specially contoured fingers spaced 7" apart.

Reviewing what's available, and analyzing your application as suggested here, will help ensure success of your application.

PHOTO : 1. Standard-size, scissors-type grippers on pick-and-place arms load and unload an

PHOTO : automatic grinder. Fingers are custom-designed for small, finely finished parts.

PHOTO : Air-operated pressure switches sense presence or absence of parts in grippers.

PHOTO : 2. Setup for gripping automotive camshaft.

PHOTO : 3. Components in a line of standard, modular, air-operated end-effectors.

PHOTO : 4. Components in a quick-change gripper system.

PHOTO : 5. Scissors-type gripper head.

PHOTO : 6. Parallel-motion gripper head for gripping part OD. A similar design for gripping part

PHOTO : ID also is available.

PHOTO : 7. Rotating unit for turning head through 90 and 180 degrees.

PHOTO : 8. Rotating unit for turning head through any degree of rotation. Drive may be chain and

PHOTO : sprocket or belt and pulley.

PHOTO : 9. Adaptor swivel.

PHOTO : 10. Proximity switch mounted on a quick-change head.

Ron Micallef Automation Div, ISI Mfg Inc. Fraser, MI
COPYRIGHT 1989 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1989 Gale, Cengage Learning. All rights reserved.

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Author:Micallef, Ron
Publication:Tooling & Production
Date:Oct 1, 1989
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