Challenges and Benefits of On-Machine Measurement

When machine tools were less accurate and ubiquitous methods for controlling the environment in and around the machine tool were rare, there was a veritable prohibition on performing probe-on-spindle, on-machine, and in-process measurement (IPM). Manufacturers are now not prohibiting On-Machine Measurement (OMM), but there is persistent suspicion of its value and/or ignorance of how to perform OMM successfully.

Arguments given for this suspicion include:

– Measuring with the same tool (axes, spindle, and frame) with which one is machining seems manifestly suspect

– Measuring on-machine is non-value-added and reduces productivity, since it takes time away from machining operations

– Writing probing routines is typically difficult and time consuming

– The machine tool environment is so unstable and unpredictable in terms of humidity, temperature, sticky cutting fluid, and unpredictable swarf, one can never be confident that an on-machine measurement will produce accurate measurements

Answers to the above objections include:

– With increasingly accurate machine tools, one can now calibrate a machine tool, measurement probes, and spindle at will, prior to measurement, if required, and reduce measurement uncertainty

– The cost of making a mistake while machining a critical feature will sometimes constitute a larger cost than the loss of machining time caused by on-machine measuring

– The machining of large size, tight tolerances, high material cost (e.g., titanium and composites), and/or highly malleable parts tends to favor performing OMM

– Accurate probing macros and software routines are available from several
software vendors

– Probing on spindle is profitably employed a) in-process intermittent to send data to the machine controller monitoring critical conditions, e.g., errors, tool breakage, and physical dislocation of the part, and b) off-machine post-process while the part is still fixtured in the machine, augmenting if not replacing a separate coordinate measuring machine (CMM)

– Certain types of errors which are hard to eliminate via machine tool maintenance/certification are easily discovered via in-process measurement, e.g., common problems during machining, like machine deflection/distortion, tool wear, and vibration, will all be absent during measurement

– The removal of swarf and cutting fluid has been successful with various cleaning systems and techniques, e.g., tombstone fixtures and part cleaning baths.

In general, OMMs are more error-prone than measurements on a stand-alone CMM in an environmentally controlled area. The machining environment suffers from widely varying and uncertain temperature, humidity, and particulate matter conditions, all of which challenge accurate OMM. Controlling this variability can be difficult, costly, and/or time-consuming. When a machine tool goes out of calibration for machining, it simultaneously goes out of calibration for measurement. In contrast, measurements on CMMs are often performed in a particle-free environment with stable temperature and humidity, all of which decrease measurement uncertainty. For critical industries, like aerospace, accurate CMMs provide documented evidence for the compliance of the manufactured product to accepted standards.

A metrologist considers carefully the details of each measurement and its environment, when deciding which features and characteristics should be measured on-machine and which should be measured on a stand-alone CMM in an environmentally controlled area. For example, if the cost of delivering a faulty part to the customer is sufficiently high, e.g., the part is very expensive or failure in its functional role would be catastrophic, then an off-machine post-process measurement may need to be performed, even though OMM may also be performed on certain part features and characteristics.

Nonetheless, there are several circumstances that favor the use of OMM. The cost of faulty parts is known to increase exponentially along the manufacturing process timeline. Therefore, OMM may improve product quality, customer satisfaction, and profitability, if the OMM is sufficiently accurate and the cost of performing OMM is less than the cost of faulty parts. Modern high-quality machine tools can also achieve and maintain very precise performance tolerances, and therefore can produce accurate measurements, when accompanied with thoughtful measurement practices. OMM adds benefit by reducing wasted machining time and scrap parts. OMM avoids the ‘second fixation problem of the workpiece,’ when moving the part to a CMM.

OMM includes increased throughput from existing assets, which defers capital expenditure, reduces sub-contract and overtime expense, and allows additional business without increasing resources. Another benefit of OMM is automation and reduced human intervention which, in general, improves tool setting and measurement accuracy, reduces direct labor costs, enables redeployment of staff into proactive engineering roles, and increases repeatability versus the generally increased variability gotten from manual methods, e.g., avoids manual ‘cut and measure’ activities and errors manually keying in height offsets, which can cause tool crashes.

OMM generally enables reduced rework, concessions, and scrap, which improves conformance and consistency, lowers unit costs, shortens lead times. Finally, OMM enhances system capability, offering customers state-of-the-art capabilities, options to take on more complex work, and satisfying customer demands for traceability.

Using machine tools for measurement and machining requires machine tool geometric accuracies sufficient to measure a part within all the tolerances specified for that part, otherwise in-process measurement will be wasted, and the parts may not be machined to tolerance. The machine tool must be sufficiently monitored over time and calibrated to ensure that it can maintain part tolerances within those specified by the customer.

The above is an extract from a white paper from NIST titled ‘On-Machine Measurement Use Cases and Information for Machining Operations’ and available for download.

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