Roundness & Roughness - Together at Last

August 1, 2007
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A new measuring instrument combines roughness and roundness checking into one efficient inspection station.



It's every QA manager's nightmare-having to take the blame when manufacturing's new million-dollar, multi-tasking machine tool sits idle waiting for first article inspection results. To make matters worse, modern machines can change over families of parts within minutes using programmed routines, and more flexibility in production means bigger headaches in the lab. Where's your bottleneck? Chances are good that surface finish is the culprit, especially on round parts where alignment of the component and operator skill are both critical.

A new measuring instrument* has been introduced that combines roughness and roundness checking into one efficient inspection station. Programmed routines give the unit the changeover flexibility of a machine tool and eliminate the need for dedicated operators. In addition, fully automatic alignment and positioning reduce human errors and improve the repeatability and reproducibility of the results.

*The Taylrond 385 from AMETEK Taylor Hobson, Leicester, England.

The system features automatic gauge calibration, which can be achieved in a matter of seconds, considerably reducing operation time.

Automation is Key

The instrument features fully automatic operation, which speeds the inspection process and eliminates operator intervention errors. New electronics and drive mechanisms provide complete computer numerical control (CNC) of all axes to a precision suited to exacting applications. To ensure both accuracy and repeatability, the vertical column and radial arm of the instrument have high-precision linear scales coupled with precision drives. Data point resolution and noise levels are similar to those more usually associated with surface finish instrumentation, ensuring stable and repeatable measurements.

At the heart of any roundness instrument is its spindle. The new unit uses a frictionless air-bearing spindle manufactured on a precision diamond turning machine. The air spindle has radial deviations of less than 0.02 µm using a standard 1-50 upr filter. When combined with a coning error of less than 0.0003 µm/mm, spindle accuracy remains constant regardless of the component height.

Full automation, however, cannot be achieved without the ability to program the gauge attitude and orientation. In the new instrument's patented approach to this problem, the gauge body can rotate to allow measurement of internal or external features in vertical mode. In horizontal mode, the gauge can rotate in steps of 1°, allowing measurement of coned surfaces as well as flat and cylindrical parts.

The volume of free space around the stylus tip is another important feature. This clearance allows access to multi-featured components and reduces the need for multiple styli. A simple dual-cresting mechanism ensures that the stylus tip is in line with the instrument's spindle axis regardless of arm orientation or attitude. This avoids incorrect measurement, particularly on small-diameter components.

Another feature is the unit's ability to carry out vectored arm and column movement for the measurement of coned surfaces. Finally, the system features automatic gauge calibration, which can be achieved in a matter of seconds, considerably reducing operation time.

A flaw has been detected in the surface of a cylinder.

Roundness Management

At first glance, inspection instruments and machine tools have little in common. Although both are necessary to produce high-quality products, they exist in separate worlds and at opposite ends of the manufacturing process. One specialized type of inspection tool, the roundness measuring instrument, is bridging that gap in unexpected ways. By becoming more like the machine, the instrument is better able to detect and correct any mistakes the machine tool might be making.

Most roundness measuring instruments are similar to the machines that produce the components in the first place. For example, a simple lathe has a rotating spindle, one slide way moving with the axis of the spindle and another slide way for moving across the axis of the spindle. Roundness instruments are similarly equipped with a high-precision spindle, a slide way (column) for movement along the spindle axis and another slide way (radial arm) for movement toward or through the spindle axis.

An obvious fundamental difference is that the lathe is cutting the part and the roundness instrument is measuring that part. The forces involved are considerably different, as are the axis speeds. Although the stylus of the measuring device is often following the same path as the cutting tool on the lathe, the roundness instrument must have greater positional control of the axes and a much higher level of accuracy in the reference data.

Simply stated, spindle-based roundness instruments are the best method of measuring rotationally symmetric components that were produced on a spindle-based machine tool. The manufacturing process is duplicated, albeit with a higher degree of accuracy, and any deviations from specification can easily be detected and corrected. Traditionally, roundness instruments have detected errors of form or geometry, but there is a strong case for doing more, for detecting and evaluating cutting tool effects such as surface finish.

Figure 1. A deep scratch has been detected in the highlighted area. Rolling out that section of the trace into a linear profile provides a good indication of the nature of the scratch.

Incorporating Surface Finish Measurement

It is not a surprise that surface finish and roundness specifications are nearly always indicated on the same engineering drawing; they are inseparable in the manufacturing and eventual functioning of the component. Since errors of roundness and surface finish occur simultaneously at the manufacturing stage, it seems logical that an inspection instrument would measure roughness and roundness at the same time.

Since manufacturers have one engineering drawing and one machine tool to produce the component, they would ideally have one instrument to check the finished part. In the past, this has been difficult if not impossible. Stylus force, measurement speed, system noise, data point resolution and other factors made the roundness measurement incompatible with the surface finish measurement. However, recent developments have had a direct effect on surface and roundness tolerances. Instrument manufacturers have responded with a new wave of higher-accuracy, lower-noise roundness systems. These advanced systems not only measure roundness, form and geometry, but surface finish as well.

Roughness characteristics are typically measured either along the axis of the component or around the axis of the component, depending on how the part was produced and how it has been engineered to function. Figure 1 illustrates how the new instrument was used to measure a component circumferentially using a diamond stylus and 72,000 radial data points (0.005° data point spacing). A deep scratch has been detected in the highlighted area. Rolling out that section of the trace into a linear profile provides a good indication of the nature of the scratch.

Ideally, the user should be able to measure in both directions. Roundness instruments with high-density data collection in both the rotary and linear axes provide that capability and more. With automatic centering and leveling of the component, modern roundness instruments assure flawless alignment regardless of measurement direction. More than any other error, poor alignment leads to inconsistent results with traditional surface checking instruments.

While 2-D profile information is often sufficient for problem solving, it may be beneficial to create a 3-D map of the surface. This is done by performing a series of straightness traces that are then combined into a 3-D dataset.

Together at Last

Obviously, not all roundness instruments are capable of combining surface finish and roundness measurements. Precision glass scales and precise motion control in all axes, high data density, and low instrument noise are mandatory design elements. Equally important, the dual inspection functions must be seamlessly integrated into a single software package. For true convenience, the gauge head should be dual-purpose and ready to use with both diamond- and ball-tipped styli.

When all of these elements are incorporated properly, the dual-purpose instrument exceeds the capability of many dedicated roughness checkers and offers improved repeatability and reliability. With one instrument and one operator, manufacturers can inspect components as they are engineered and produced.

For additional information regarding roughness and roundness inspection, contact AMETEK Taylor Hobson, 2 New Star Rd., Leicester, LE4 9JQ, England; (44) 0116-2763771; fax (44) 0116-246-0579; e-mail sales@taylor-hobson.com; or visit www.taylor-hobson.com.

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