| Established in 1952 | |
Page: 2430 |
| CMM-20 |
 Part No.: GBB Giant Ball Bar with Tripod ( priced separately ) |
Evaluating Really Large Coordinate Measuring Machines with a Giant Invar® Ball Bar (Dumbbell)The Ball Bar (Dumbbell) evaluation technique has been popular for over twenty years. The Ball Bar (Dumbbell) has always been a rather long rigid bar with two very precise spheres of exactly the same diameter firmly attached to the opposite ends, see Technical Data Sheet CMM-1, Page 1 & 2. This design works quite well on Ball Bar (Dumbbell)s up to about 900 mm (36 inches), but as they get longer, the Ball Bar (Dumbbell) is not rigid enough to resist bending due to the contact force of the measuring probe. |
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The first success in providing greater rigidity for longer Ball Bar (Dumbbell)s is the Way Out Ball Bar (Dumbbell) support system (see Technical Data Sheet CMM-16, Page 1 & 2.). It uses a robust aluminum rail and auxiliary kinematically supported spheres to increase the stiffness of the Ball Bar (Dumbbell) five fold. The Way Out Ball Bar (Dumbbell) support system works well up to about one and one half meters (5 feet), but it becomes too cumbersome and too flexible after that. Development of a really good, long Ball Bar (Dumbbell) has taken a long time. The main difficulties have been that the Ball Bar (Dumbbell) must be rigid enough to provide good data, while still being light enough to be easily moved throughout the 20 to 35 positions outlined in the ANSI B89.4.1-1997 specifications for "Performance Evaluation of Coordinate Measuring Machines".
The chief obstacle to the development of long Ball Bar (Dumbbell)s was the industry's preoccupation with the definition of a Ball Bar (Dumbbell). By abandoning the old concept that a Ball Bar (Dumbbell) is two spheres attached to the ends of a single long bar, and redesigning it as a composite structure instead, the problem of building really long Ball Bar (Dumbbell)s has been solved. The break through design uses tubular Invar® members with their extremely low rate of thermal expansion, that is 0.68 microinches per inch per degree Fahrenheit, and a series of bulkheads to form a structural assembly. The thermal expansion of Invar® is about one tenth that of steel and one twentieth that of aluminum. Using this design, we end up with a very rigid structure that has a very low rate of thermal expansion; but is still light enough to be easily handled. Large one and one half inch (38 mm) diameter spheres are used on the Giant Ball Bar (Dumbbell). This greatly facilitates the probing of the target sphere from the long distance that separates it from the operator. Two inch (51 mm) diameter hollow spheres are offered as an option that enhances the ease of probing even more.
By kinematically supporting this tubular structure at the end bulkheads, it may be moved through a wide array of horizontal, vertical and angular positions with a minimum of distortion.
The problem of physically positioning these large structural devices is accomplished by supporting the Giant Ball Bar (Dumbbell) on two tubular framed Invar® tripods, (see Technical Data Sheet CMM-24). The thermal stability of the Invar® construction of these tripods is important because they become part of the actual measurement during each evaluation.
For giant Ball Bar (Dumbbell)s up to eight feet (2.5 meters) long, we have developed an economical light weight system that only uses a single tripod. Ask for the Single Tripod Giant Ball Bar (Dumbbell) System.
