Ultimate Guide to Machining Accuracy Checks

Touch probes offer more comprehensive support for efficient, high-quality and cost-effective production than just part setup. Verifying the geometric accuracy of machined parts directly on the machine is equally important in ensuring quality.

Through this direct accuracy check, programming errors, tooling defects, and adverse consequences of the machining process can be quickly identified and addressed. However, these inspection methods are often not utilized due to the complexity of programming or lack of confidence in the machine’s accuracy.

The basic accuracy of the machine is crucial for the machining of precision parts, and it is recommended that the machine be periodically checked for its geometrical parameters using a laser interferometer system such as the ML10 GoldStandard by Renishaw. Additionally, the use of QC10 circular gauges for periodic inspection is recommended.

This subsequent measurement on the machine also helps to establish conditions for process control. The measurement on a coordinate measuring machine only confirms that the workpiece has been machined according to requirements and cannot distinguish the effects produced by the machine, tool, or process.

In case deviations in machining are detected for a part, it can be challenging to determine the root cause of the deviation, making it difficult to determine appropriate corrective measures. The results of such measurements alone may not provide reliable information for corrective actions.

Measuring tasks on machine tools

There are four common user requirements for measuring tasks in machine tools:

  • Simple workpiece setting and tooling measurement
  • Incidental measurement tasks based on drawings, where the measurement cycle is integrated into the program flow, and the process is controlled and measured results are outputted.
  • Complete integration of all measurement tasks with the machining program, controlled by measurement results, and programmed with CAD data, with collision inspection through graphical simulation and measurement results outputted in tables.
  • Inspection of the geometric accuracy of the workpiece after processing, programmed with CAD data, collision checked through graphical simulation, with measured values processed statistically and analytically, and results expressed and analyzed in tables and graphs.
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Renishaw’s measurement experts offer various packages to meet these different requirements. If the user only requires setting the workpiece and measuring the tool, the widely used cycle measurement package for control is recommended. This package allows for easy and convenient programming of workpiece setup and tool measurement.

Control accuracy instead of just checking the accuracy

At Renishaw, the software family for productivity can be used regardless of the machine control system employed. With the support of graphical interaction, programming all the measurement programs for the milling machine is made simple.

The ActiveEditor and ActiveEditorPro are utilized to integrate all workpiece and tool measurement programs into the machining program, incorporating additional CAD data when programming. These programs can be adjusted to the process through direct feedback from measurement results.

This not only enables correction of the machine’s coordinate system and tool data but also allows for the implementation of logical decisions based on these data, to some extent automatically, without the need to understand the control instructions. The simple and hierarchical structure of the rules also allows the user to employ special NC commands without in-depth knowledge of the program structure.

The operator interface provides suitable predetermined values and checks that all necessary inputs have been implemented. The program and instructions for the selected control are generated through the operation of the post-processor.

As a result, individual parameters can be precisely adjusted, even in the event of a sudden change in the machine’s machining schedule. With the CAD input ActiveEditorPro, programming is further simplified and allows for a complete representation of the measured motion and a measurement simulation, including collision check, taking into consideration the clamping tool.

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Measuring and recording accuracy

Renishaw has developed the OMV software in collaboration with Delcam UK, experts in CAD/CAM, to inspect complex or large parts directly on the machine after machining. By clicking on the features and surfaces of the entered CAD part model, the OMV software generates the required measurement program.

The ActiveEditor feature in the software provides predetermined values for actual use and performs credibility testing during input. The program then displays a graphical simulation of the measuring process, which boosts the user’s confidence in the accuracy of the measurements.

The OMV software utilizes a highly accurate resistive strain-gauge based spindle-trigger probe that measures in all directions. With the compact OMP400 or the proven MP700 touch-trigger probe, points on the surface are measured, and the data is transmitted to the PC for analysis and processing.

The measurement calculation method used by the software is similar to that of a CMM, allowing it to measure complex free-form surfaces. This also improves cycle times as each part requires a small number of measuring points. The touch-trigger probes MP700 and OMP400 are calibrated in all vector directions and can measure in all directions immediately.

Based on the CAD model, Renishaw’s OMV software not only displays the deviation in measurement, but also represents the deviation value in colored dots, providing a visual representation of the accuracy of the part. The colored graph produced by these dots enables quick analysis and statistical representation of the measurement results.

The measurement points can be adapted to the CAD model using an optimal fit function to prevent errors during part set-up and datum selection. This allows for errors to be identified and corrected before the part is removed from the machine, saving time and increasing the reliability of the machining process.

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