Established in 1952

Page: 4740

Simplified C.M.M Evaluation

     In the modern Coordinate Measuring Machine environment, a quick and dirty once a year calibration of your C.M.M. just won’t cut it any more. You need to do a frequent, multi-step, in depth evaluation of your machine’s geometric envelope. This is much simpler and far less expensive than you can imagine.
     The secret weapon in this battle is the ultra accurate but very inexpensive Ball Bar (Dumbbell) System.
     Are the elements of your C.M.M.’s metrology frame straight and square enough to accurately guide the measuring probe on its assigned task?
     As modern measuring probes have become more complex, they are presenting even greater probabilities that serious machine errors will manifest themselves as erroneous measurements.
     Compound, Star and Tree test probes place stringent new disciplines on “Z” axis, rotation or roll. When using complex probes on older machines, a serious evaluation of the “Z” axis roll is even more critical as these machine designs have only crude provisions to control this rotational problem.
     Articulating measuring probe heads add position-indexing accuracy to the already complex quality equation.
     When you add automatic measuring probe changers to the equation, the potential error budget of the overall system can be doubled again.
     Many very substantial error sources such as loose probe tips, perturbations in air pressure and temperature variations have simple solutions once the error sources have been identified, using the simple, inexpensive Ball Bar (Dumbbell) system.
     One company found large disparities between the hole pattern they had programmed and the resulting hole pattern that was measured, by a brand new Coordinate Measuring machine. When they checked this brand new, Coordinate Measuring Machine with a Ball Bar, (Dumbbell) they found that this, certified in place machine, had a serious “X” to “Z” axis squareness problem. This error had somehow been overlooked by the calibration technician, when the machine was originally set up and “certified”.
      A Ball Bar (Dumbbell) system consists of a Ball Bar, (Dumbbell), a very rugged supporting stand and some arrangement to rigidly attach the Ball Bar (dumbbell) to the stand (Clamp).
     A Ball Bar or Dumbbell is made up of two very spherical balls of exactly the same diameter securely attached to a rather long rigid bar. By measuring the Ball Bar (Dumbbell) in a number of different positions a full evaluation of the Coordinate Measuring Machine’s 18 geometry related rigid body error sources can be made. All of the potential elastic or bending errors are variations of the 21 parametric error sources that can be evaluated in the same way.
     By using multiple Ball Bars (dumbbells) that have an accurately calibrated length, the three potential scale errors can be added to this equation, thus completing a full evaluation of the 21 potential parametric machine errors.