Triangular bridge design improves CMM scanning

As the CMM programmer and operator for TMS Inc. job shop, Bruce Boucher must make certain the computer housings that his company makes for computer equipment manufacturers are accurate. "If they're not accurate, the computer card guides won't fit into the housings, and that's a big problem," says Boucher. So when CMM (coordinate measurement machine) manufacturer Brown and Sharpe came up with a new CMM design that improved the accuracy of part scanning, Boucher and TMS agreed to test it for three months.

The accuracy of a CMM is a measure of its ability to provide the correct numbers when measuring dimensions of a part. Likewise, the accuracy of finished computer housings is directly related to the CMM's ability to scan without any distortion. Brown and Sharpe's efforts to improve the accuracy of its new Global CMM include designing a new bridge beam, called Tricision, for the CMM.

The bridge has a granite pedestal base, which incorporates a Y guideway that is cut into the side of the granite. On top of the base is the bridge, which includes the triangular beam used for the X guideway, as well as two legs, one of which is the drive leg.

"Typically, the X-beam on the bridge has been square on Brown and Sharpe machines," says Wym Weekers, a senior design engineer at Brown and Sharpe. "We changed that shape to a triangle."

Weekers says that the Tricision provides an optimum stiffness-to-mass ratio, with a lower center of gravity and a wider bearing separation than competitive bridge designs. "Lowering the center of gravity of the bridge brings it closer to where the driving force is applied by the motors, which also increases the overall system stiffness," he says.

When a CMM moves, the entire structure deflects under the acceleration loads placed on the machine. This deflection causes errors.

Since machine deflections are directly proportional to acceleration rates and inversely proportional to the machine's stiffness-to-mass ratio, Brown and Sharpe had to increase the ratio in order to compensate for the larger accelerations. By carefully manipulating the overall stiffness of the entire system and minimizing the mass of the moving body, they were able to minimize the deflection errors; and thus increase accuracy while increasing acceleration rates.

"We employed several different methods to increase the overall stiffness of the machine, including the use of FEA deflection analysis to size the critical structural components," says Dave Payette, another senior design engineer at Brown and Sharpe. "We also increased the spacing of the air bearings in order to lower the reaction forces during machine accelerations," he explains. The smaller forces translate into smaller deflections, which contribute to an overall increase in the machine's stiffness.

"Essentially, we provide greater scanning and machining accuracy by changing the shape of the bridge," says Payette. Engineers at the company started with a stick model and estimated the stiffness needed for each of the bridge components. "Our preliminary design was close," says Payette. "After we did an FEA analysis, we went back and stiffened up the design by eliminating pockets and beefing up the gussets," he explains.

Brown and Sharpe use FEA software from Algor (Pittsburgh, PA). "If we hadn't done the FEA analysis, we might have wasted a lot of time building prototypes that would not have met our expectations," says Weekers.

"On large cast aluminum housings, we're checking for hole location, true position, flatness, perpendicularity, and parallelism to tolerances of 0.005 to 0.002 inch," says TMS's Boucher. After the initial three-month test period, the job shop purchased two Global CMMs.

TMS uses a TP200 touch trigger-probe on the CMMs they ordered. With the TP200, the Global has a repeatability of 0.000075 inch and a scale repeatability of 0.000003 inch.

For more information about FEA software from Algor: Enter 539