CMM design incorporates user analysis and CAD techniques

Modern Machine Shop, April, 1994

Machine shops are doing more measurement and more shop people are involved in doing measurement. (It's all part of that quality-SPC-TQM thing going on). So making people more efficient is as important as making the measurement machine more efficient.

As a case in point, when Brown & Sharpe (North Kingston, Rhode Island) introduced the MicroXcel manual coordinate measuring machine (CMM) last year, its features signaled an approach to machine development that reflected a close analysis of manual measurement needs. (Computer analysis of the design also played a big part in the shaping of this machine.)

One of the features that makes this CMM different is the "Zmouse," a track-ball computer interface that controls the probe and software operation of the machine directly from the Z axis. In analyzing manual measurement techniques, the company found that operators spent a great amount of time moving between the machine and the computer. The Zmouse reduces this wasted operator movement. Engineers there figure that this feature alone speeds manual measurement by 30 to 50 percent, program editing by 25 to 35 percent, and execution of a pre-written program by 15 to 20 percent.

Along with a lot of walking back and forth, manual operation of a CMM may also involve a lot of physical pushing and pulling for the operator. This partially explains why all of the structural components in the MicroXcel, including the bridge, are made from thermally diffusive aluminum.

Most CMM designs use heavy materials such as granite, cast iron, or steel in the bridge to reduce the affect of deflection on measurement. These bridges can weigh between 350 and 500 pounds. In a manual machine, however, heavier bridge materials can often have the opposite effect on accuracy. Because of their weight, they can be difficult to position accurately. The added weight can also tire an operator who has to physically push it around all day.

The aluminum bridge and other structural components were designed using finite element analysis techniques. By using this design approach, engineers could predict deflection of various designs before construction and use the design that provided the most rigidity with the least weight.

As a result of the study, the total weight of this CMM was reduced from more than 600 lbs in its initial design to 190 lbs. A fortunate side-effect of this weight reduction was considerably lower manufacturing costs. Yet, according to the company, the MicroXcel has a volumetric accuracy of 10 ||micro~meter~, which they ascribe to the rigid structure of its aluminum components. The result is a manual measuring machine with a price/performance ratio that seems right for its intended market.

With a measuring envelope of 29" x 27" x 20", this CMM can accommodate 80-plus percent of the workpieces produced by most shops (again, by Brown & Sharpe's analysis). The X axis (side to side) is the longest axis (unlike many other machines where the Y axis--front to back--is longest) to make it easier for the operator to position large workpieces.

It is interesting to note that CAD technology was applied to modal analysis in the design phase to reduce machine and environmental vibrations--a technique that is generally used to analyze machine performance after it is built rather than before. That design technique is paying off this year with the introduction at Quality Expo International of the MicroXcel PFx "personal flexible gage"--a direct computer control (DCC) version of the MicroXcel manual machine.

The repeatability of CMMs can be degraded by machine-induced and environmental vibrations. In a DCC machine, the drive system itself induces vibrations from belt runout, motor pole ripple, servo loop band width, and other conditions. By predicting the degree of vibration through modal analysis, engineers claim they were able to reduce its affect on the measurement process even before the drive system was installed on the manual machine.

The MicroXcel PFx is equipped with a disengageable drive that lets users toggle from DCC to either free floating manual or manual joystick operation and back again. This extends the capability of the machine for the small to medium shop. In its manual mode, this CMM can be used as a walk-up measurement station for first piece or one-of-a-kind measurements. In the DCC mode, it can be used for automated measurement of multiple workpieces, or part as a flexible gage that can be configured to measure a variety of workpieces or families.

The MicroXcel PFx uses the company-developed Micromeasure IV software for intuitive manual operation, and easy-to-use DCC part programming and operation. This software provides 3D measurement capability with an icon-based editor and screen prompts. An optional DataPage statistics package collects data, performs statistical analyses, and prints a concise statistical report.

With more people from various levels in a shop likely to use a CMM these days, accommodating users at all skill levels is an important consideration. In keeping with this trend, a standard operator interface (SOI) for this CMM allows all basic operator functions to be selected and executed via standard icons. This is the poke-the-picture approach. For experienced users, a more advanced graphics language permits user-definable real-time graphic outputs to screen, printer, or plotter.