Quality Innovations: Roundness Tester Picks Up Pace

September 1, 2003
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A new roundness-measuring system has achieved superior accuracy while reducing measurement time.

The RA-H5000 roundness measuring system is 50% more accurate than its predecessor. Photo: Mitutoyo America Corp.


A roundness-measuring system has been developed that its distributor says is 50% more accurate than its predecessor and more accurate than other systems in its class. The level of precision has been achieved without sacrificing speed through a manufacturing process involving constant redesign, prototyping and experimentation.

The Roundtest RA-H5000 series by Mitutoyo America Corp. (Aurora, IL) has a rotational accuracy of (0.02+6H/10,000) micron. The “H” value in the equation is the probing height of the customer’s part. For instance, if the probing height was 50 millimeters, then: 0.02 + 6(50)/10000 = 0.05 micron. This value will change depending on the customer’s part.

It has a maximum rotating speed of 20 rpm, and actual measurement speeds of 2, 4, 6 or 10 rpm.

The body base is the foundation that supports the turntable, vertical column, horizontal arm and workpiece under measurement. The base needs to be rigid to combine these components into an instrument that can achieve the required accuracy. Mitutoyo created its new base by applying the results of the Finite-Element Method (FEM) structural analysis, in which the company simulated the distribution of stress throughout the base.

The RA-H5000’s highly rigid base exhibits little flexure as a result of load fluctuation during measurement and movement. The base also limits, to a small degree, the adverse effects on parallelism between the table’s rotating axis and the vertical column, thus ensuring the greatest possible accuracy of measurement.

“By adjusting the table to a fixed column, high accuracy is achieved,” says Robert Wasilesky, form product manager at Mitutoyo. “Most systems adjust the column to the table. By limiting the degrees of freedom in the column and making adjustments in the table, Mitutoyo has created a robust fine-tunable system.”

The vertical slider is driven up and down by a precision ball screw along the column-guide surface, which is

precision-finished. This slider has been designed with consideration for the center of gravity during movement, virtually eliminating the effect that the moment of inertia might otherwise have on accuracy. A proprietary run-out eliminator removes minute runouts during movement, and a counterbalance system with air cylinders is included. As a result, these models have a high-accuracy column with a straightness of 0.05 micron/100 millimeters within a narrow range, according to Wasilesky.

The straightness of the R-axis is affected by the rigidity of the horizontal arm and the frame supporting the arm. The arm was designed to ensure the highest possible rigidity through FEM structural analysis and the simulation of locus at the stylus tip of a driven arm. The use of ceramic material and other new technologies have achieved a horizontal arm straightness of 0.5 micron/200 millimeters in the RA-H5000 Series.

“The rigid ceramic arm has twice the cross-section and a straightness value that rivals surface roughness equipment,” Wasilesky says.

The series incorporates a turntable suspended by a high-precision air bearing. Mitutoyo employs a complex restrictor method for the air bearing. The complex restrictor on the RA-5000 improves the rigidity, and thus the accuracy, of the turntable by adding a grooved air-bearing channel along the edge of the table in combination with the pinhole air bearing in the center of the table.

The turntable’s rotational accuracy, which the company says is currently the highest in the world, has been achieved by improving the accuracy of the rotor, stator and other components for a greater air-bearing effect, and also by optimizing the orifice and grooves. Moreover, the capacity for load mass has been increased by as much as 30% over previous models, thereby permitting the measurement of heavier objects.

All adjustment axes of the turntable incorporate Mitutoyo’s high-accuracy linear scales, which minimize positioning error during table adjustment and contribute to high-speed automatic centering and leveling of the workpiece. This facilitates substantial reductions in overall measurement time, starting from the setting of the workpiece and ending with the display of results. This automatic system even operates when measuring a notched workpiece.



Sidebar: Quality Specs

1. The turntable has a rotational accuracy of (0.02+6H/10,000) micron.

2. Vertical column’s (Z-axis) straightness is 0.05 micron/100 millimeters, and parallelism with rotating axis is 0.2 micron/350 millimeters.

3. Horizontal arm (R-axis) has a straightness of 0.5 micron/200 millimeters, with squareness against rotating axis of 0.5 micron/200 millimeters.

4. Turntable has a maximum rotating speed of 20 rpm (2, 4, 6 or 10 rpm during measurement).

5. Vertical column’s (Z-axis) maximum speed of movement is 30 millimeters/second (0.5, 1, 2 or 5 millimeters/second during measurement).

6. Horizontal arm’s (R-axis) maximum speed of movement is 20 millimeters/second (1 or 5 millimeters/second during measurement).

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