Finer Tolerances Demand Better Measurement Methods

The new generation of shaft measuring systems must be fully functional on the production floor as well as the measuring lab environment.The stability of the system ensures changes in temperature, humidity, and air pressure have no effect on measurement accuracy.

Chatter marks produced during milling or turning operations are not uncommon. These chatter marks are usually visible to the naked eye, and are determined by a simple visual quality control check. Chatter marks are usually identified by inspecting ground surfaces under intense light. Manual light inspection is typically not a satisfactory method of quality control because length and depth of visible defects cannot be quantified. Produced during the grinding operation, chatter marks call for a better inspection method in order to be even detectable.

Achieving improvements to surface finish qualities is currently through utilization of modern machine tools and new production processes. However, with the improvements in surface finish quality, even smaller sources of error occur, known as chatter. This type of error is typically not detectable by just a standard roundness evaluation.

Chattermarks on precision shafts, even at low amplitudes, can cause disruptive noises and/or vibration within an assembly such as an engine. Defining chatter marks can only be through the use of a fast fourier transform (FFT) routine on a roundness profile. The FFT routine displays the individual harmonics and the amplitude of the chatter mark and determines circumferentially which errors are in excess. The basis of the quantification of chatter marks are on quality control standards from the engine manufacturers to ensure low engine noise levels.

Not every frequency in the profile will be harmful to the performance of the workpiece, in a sense there is good chatter and bad chatter. In fact, depending on the application, there is usually only a small bandwidth of frequencies considered bad chatter. With data obtained during the shaft inspection, manufacturers will be able to link inspection data to specific manufacturing methods, processing speeds, forces, feedrates, and/or tool wear issues.
 

Software for the new type system can be used for standard measuring tasks such as roundness, cylindricity, concentricity, coaxiality, radial run-out, positional tolerance, as well as the FFT calculation for measuring chatter marks.

Shaft Measuring Systems
The challenges in measuring shafts start with utilizing the appropriate measuring system. When measuring concentric diameters on a shaft, some traditional roundness measuring systems fixture the shaft so that the shaft axis centers to the measuring system axis. This method requires long set-up times as each individual measurement requires a new axis set-up.

There is a new type of measuring system available that addresses these challenges, supporting measuring efficiency and reliability.

Design of this recently developed system is to quantify even the smallest chatter marks produced on all shaft types. It requires no special axis alignment as the shaft is fixtured on centers, providing highly accurate inspection results in regards to diameter, radial position, and straightness.

The challenge with specific shafts, such as camshafts, is due to the specific shape of the lobe. Traditional roundness systems do not have the probe range or scanning capability with suitable resolution of the R-axis to trace the eccentric features of a lobe.

The measurement of complex shafts, therefore, require a specialized roundness inspection system.

Common shaft measuring systems utilize essentially the same measuring principle – the shaft is clamps between centers, allowing for fast and accurate pre-location of the part. This location method is common, since all earlier machining operations establish datums in relationship back to the center axis of the part.

When utilizing a highly accurate shaft measuring system, there are essentially three axes: Z, R, and C. The Z-axis provides travel and length measuring capability along the center axis of the workpiece. The R-axis (or X-axis) is for measuring in the radial direction, and follows the part contour recording data points. The C-axis utilizes a high resolution drive system for part rotation.
 

The shaftscan part plot screen provides a large amount of measurement data for shaft quality control.

Superior Mechanics Count
During inspection of a pin on a crankshaft or the lobe on a camshaft, the measuring follower (R-axis) contacts the outer diameter of the surface. The shaft is driven by the C-axis. During rotation, the pins or lobes introduce forces to the measuring follower with each directional change.

Compensation of the translational forces must occur in the R-axis. Traditional systems rely on mechanical bearings to balance the friction and pre-loads associated with the axial forces onto the R-axis. Laser compensation is one available solution to monitor the R-axis for compensation. Completion of this compensation method is the addition of a glass scale and reader head.

Both, the measuring head and the laser must simultaneously be tracked for this compensation to function properly. Although this method provides accurate results, it does result in longer inspection times as the alignment of both measuring signals limits the measuring heads overall speed.

When utilizing mechanical bearings, their lack of response to high frequencies can become critical. The mechanical bearing system requires pre-loads to balance the radial and axial play of the mechanical system. The limitations of the mechanical system to detect high frequency chatter are a result of slow response to high frequencies. As chatter or higher frequency needs to be detected in the future, the natural frequency of mechanical bearings may distort, interfere with, or simply filter the actual measurement data.

During recent years, another approach has helped to pave the way in mechanical accuracy of the measuring system. Unlike standard industry shaft systems, this technology does not require laser compensation due to its unique mechanical design. Utilizing a high precision linear air bearing in combination with a vibration-free cable drive system eliminated laser compensation. Its rigid design does not introduce inaccuracies associated with axial play, and therefore eliminates the need for error compensation. An extremely small air gap and air pressure of 5 bar ensure frictionless function, guaranteeing a very high response time, and ensures there are no adverse effects from transverse forces.

The most recent approach includes a refined version of the earlier proven R-axis technology. The new system also includes an all new high precision Z-axis in combination with an improved R-axis, which together aids in decreasing measuring time and increasing capability. Even the smallest roundness deviations qualify with confidence.

This new approach also offers ease of serviceability of the entire system with very little maintenance – a clear advantage compare to laser-based systems. Traditional laser-based systems frequently require cleaning and precise adjustment of their lenses, a process that is time-consuming, complex, and quite costly.

Looking forward to increasingly demanding measuring requirements, the design of the shaft measuring systems must able to handle the requirements of today and tomorrow. Users should expect measuring density of 3,600 data points per trace, a radial runout accuracy of 0,3µm, and a repetitive accuracy of up to 0,2µm. Such a system must be able to deliver reproducible harmonic and amplitude results for even the smallest features and a cycle times in the range of eight minutes or less for a complex shaft containing 44 measurement positions.
 

A diameter of a cam lobe shows variations detected during a rapid scan cycle. Cycle times, in the range of eight minutes or less for a complex shaft containing 44 measurement positions, are typical.

Software
With this new type of system, the measurement cycle is automatically optimized by the software, ensuring short measuring times. Further optimization of measurement cycles can be through selection of shorter trace widths and repeating only individual measurements within a measurement cycle. Simple operation and programming in Windows XP/Vista can provide quick and easy-to-understand software control.

Program adjustments can occur both on-line or optionally off-line for CNC programming, position adjustment, tolerances, and nominal values. Software for the new type system is usable for standard measuring tasks such as roundness, cylindricity, concentricity, coaxiality, radial runout, positional tolerance, (Picture 3), as well as the FFT calculation for measuring chatter marks. Within the software, measuring reports easily create and output.

The new generation of shaft measuring systems must be fully functional on the production floor as well as the measuring lab environment; the stability of the system ensures changes in temperature, humidity, and air pressure have no effect on measurement accuracy.

Flexible application of the system within the production workflow is also possible. The range of accessories include axial and radial drivers as well as flat measuring followers for both types of shafts, and round measuring followers for cam shafts in varying diameters. Loading of shafts is manual, or can automate with robot or gantry loading with the assist of a pneumatic tailstock.

Furthermore, there may also be in vertical systems dual measuring heads, positioned opposite to one another, to increase the measurement speed. Both measuring heads, utilized at the same level, further increase accuracy with regard to absolute precision. The range of accessories include axial and radial drivers as well as round and flat measuring followers for a variety of shafts of various diameters.

Convenient operation and ergonomic design for the user become one in the same in the new type system. With motor-driven adjustment of the workspace height, the operator can choose to either work seated or standing.

Conclusion
Chatter marks left behind on precision part surfaces by even the most precise machining operation, particularly on the eccentric surfaces of rotating components, have a big impact on part performance. Going forward, the only reliable way to measure these fine chatter marks is with a mechanically accurate, air-bearing based measuring system.
 

Jenoptik Industrial Metrology
Rochester Hills, MI
jenoptik.com