| Established in 1952 | |
Page: 1510 |
Technical Data Sheet: CMM-18
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A Ball Bar (Dumbbell) consists of two very precise spheres of exactly the same diameter securely attached to the opposite ends of a rather long rigid bar. Invar® Ball Bar (Dumbbell)s with their ultra low rate of thermal expansion have long been used in Coordinate Measuring Machine research, but until now they have been far too expensive for general use. The 36% Nickel steel alloy known as Invar has a coefficient of thermal expansion of 0.68 microinches per inch per degree Fahrenheit. This is about one tenth that of steel and one twentieth that of aluminum. It is the lowest rate of thermal change of any commercially available metal. By purchasing the material in entire mill run volume and dedicating our company to serial production of Invar Ball Bar (Dumbbell)s, we have brought the Invar Ball Bar (Dumbbell) price down to a level for everyday use. |
The actual change in length of a Ball Bar (Dumbbell) with a given variation in temperature is affected by three factors:
The first factor is the Coefficient of Thermal Expansion of the material that the Ball Bar (Dumbbell) is made of.
The second factor is the length of the BallBar being measured. The longer the Ball Bar (Dumbbell), the greater will be the change in length.
The third factor is very subtle but it is just as important as the other two. This is the speed at which a material exchanges heat with its environment. Aluminum exchanges heat very efficiently, so it changes dimension quickly. Invar, on the other hand, does not exchange heat very efficiently at all, so it is very slow to move. This phase lag gives the calibration technician more time to perform the multitude of measurements required before even that very small changes in dimension actually take place. In addition, any short lived ups and downs, referred to as perturbations, in room temperature will be factored out entirely.
Using a Ball Bar (Dumbbell) with a low rate of thermal expansion is important, because such long dimensions are involved. Also, the long time period of well over an hour that is required to measure the Ball Bar (Dumbbell) in all of the 20 to 35 positions required to meet the ANSI-B89.4.1-1997 specifications for "Methods for Performance Evaluation of Coordinate Measuring Machines".
The Invar ball bar price is $100.00 more than the standard steel Ball Bar.
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Length (sphere center - center) |
Invar Part # | Invar Price | |
| 100 mm | 3.9370" | PBB-INV-100 | $330.00 |
| 200 mm | 7.874" | PBB-INV-200 | $370.00 |
| 300 mm | 11.8" | PBB-INV-300 | $375.00 |
| 350 mm | 13.779" | PBB-INV-350 | $375.00 |
| 400 mm | 15.748" | PBB-INV-400 | $386.95 |
| 450 mm | 17.7" | PBB-INV-450 | $386.95 |
| 500 mm | 19.685" | PBB-INV-500 | $389.87 |
| 550 mm | 21.653" | PBB-INV-550 | $389.87 |
| 600 mm | 23.622" | PBB-INV-600 | $397.75 |
| 650 mm | 25.59" | PBB-INV-650 | $397.75 |
| 700 mm | 27.559" | PBB-INV-700 | $401.85 |
| 750 mm | 29.528" | PBB-INV-750 | $401.85 |
| 800 mm | 31.496" | PBB-INV-800 | $421.13 |
| 850 mm | 33.464" | PBB-INV-850 | $427.70 |
| 900 mm | 35.433" | PBB-INV-900 | $427.00 |
| 950 mm | 37.041" | PBB-INV-950 | $440.63 |
| 1000 mm | 39.37" | PBB-INV-1000 | $440.63 |
| 1100 mm | 43.307" | PBB-INV-1100 | $540.63 |
| 1600 mm | 62.992" | PBB-INV-1600 | $578.23 |
