Process control and verification

“Confidence in machine tool build quality has risen to the point that there is now a strong drive towards fully traceable ‘on-machine’ measurement for process control and verification. This in turn has opened the door for 24-hour, seven-day-a-week unmanned machining strategies. Unlike full verification, which generally requires a complete co-ordinate-measuring machine (CMM) inspection of the component, process verification focuses on critical areas that would govern the ‘health’ of subsequent machining processes or determine whether a billet/casting is positioned so that it can be machined within the required constraints of the program.

If these are within the process limits the rest of the machining processes can be relied on to be correct, as can the process control used to oversee the manufacturing as a whole. Naturally, the best results require that the machines are well maintained and mapped so that an understanding is gained of temperature-related drift and positioning anomalies. On the other hand, process verification has its sceptics, and is often subject to considerable criticism, chiefly on account of established arguments. These include ‘doing anything but cutting on a machine tool wastes precious time’ and ‘you should never measure a part using the same machine that made it.’

While once relevant, most would agree that these arguments do not stand up firmly against today’s high-end machining centres that are temperature compensated and capable of holding micron accuracies. In fact, many of today’s machines are more accurate than shop floor measuring equipment — further supporting the case for this approach to process control. If set up correctly, on-machine verification is fast, often eliminating interim checks before undertaking further processes, and promotes confidence in the manufacturing process as a whole, as it allows detailed Statistical Process Control (SPC) analysis of the process to be undertaken automatically.

Of course, there is no suggestion that final inspection in a temperature-controlled environment would not be necessary if the limits of a component required it. Furthermore, if the components are to undergo secondary processes such as plating, shot peening and stress relieving, then subsequent inspection will be necessary to determine what changes have occurred. On-machine verification and process control requires automated probing sequences to be undertaken as part of the numerical control (NC) program. The results are then used to set alarms so that manual intervention can be taken, or fed back to the control system so that automated offsets can be made. Either mechanical touch trigger/scanning probes or laser probes can be used for this task depending on working conditions. As mentioned previously, under a process verification strategy, the amount of probing is restricted to critical ‘control features’ only. Thus if a series of diameters were being produced with one tool on a turning centre, there would be no need to check every turned feature in order to verify the process.

If a tool offset is required, it will show up on any of the diameters cut using that tool. As there is only one offset to adjust, just one piece of representative data is needed to control it. The measurements taken for process verification can also be used to adjust the wear offsets of finishing tools, so on-machine verification can be combined with process control. The trick is to identify the measurements most likely to highlight problems.

Data captured during machine tool programmed probing cycles can now be automatically processed to provide remote up-to-the minute Statistical Process Control (SPC) information in addition to generating warning alerts when a measured feature is moving out of its tolerance band.

Communication for this ‘real time’ information has been developed by Seiki Systems, in conjunction with GS Productivity Solutions resulting in a plug-in module that will benefit users of Seiki’s Direct Numerical Control (DNC) and production monitoring Networked Manufacturing System (NMS) and GS Productivity Solutions’ Computer Numerical Control (CNC) Reporter software.

Using Seiki Systems’ probe data collection software, GS Productivity Solutions’ CNC Reporter creates real-time CMM inspection reports from the machine tool to provide automated sequential reporting of measured results across all components being produced. This includes a production history with live updating to create a fully traceable record while generating the statistical environment to predict production trends.

To explain the process in simple terms, think of a scenario where more than one part is being produced. A machine tool is set-up, and a first off component generated. As normal, the first off component is inspected independently of the machine tool using a calibrated device such as a CMM before being passed off so that the remainder of the batch can be manufactured. The quality of each part from this point on is unknown unless some level of inspection is implemented to assure part acceptance.

Using the same Renishaw touch trigger probe that is now locating piece parts on the machine tool, GS Productivity Solutions would implement its CNC Reporter to monitor the probed results of each sequential piece part. To minimise the effect on production time, critical piece part features (which are indicative of the process stability) can be inspected on a frequency basis tailored to your process requirements. By tracking and comparing these critical features across multiple components and aligning the results back to independently ‘offline inspected’ piece parts, a picture of the machine’s output in quality terms can quickly be created and stored providing complete confidence in the process.

The offline inspection methods assure component accuracy, while the machine tool inspection results simply track the process using critical features only to maximise machining time. This procedure is known as ‘foot-printing,’ and is easy to implement using the CNC Reporter & Productivity+ together with your existing probe hardware. CNC Reporter collects, sorts and analyses probing results in real-time from either the machine tools RS232 interface or via file import.

The key features of CNC Reporter include ‘CMM’-style formatted component inspection reports, feature based control charts generated in real-time, and archiving of protected results as a traceable and auditable record of component quality. Because this is MS Excel based, it’s fully customisable and the data can be easily imported into any further Overall Equipment Effectiveness (OEE) or SPC package.

The Productivity+ software is available as a standalone product that can be used in conjunction with G-Code created manually or via a CAM system. It is also available as a plug-in for GibbsCAM, which allows a probing program to be generated through the selection of features on a component’s solids design model. As the software is ‘feature based’ the format of the results is perfectly geared to match the output required for CNC Reporter.

Finally, apart from reducing costs and manual intervention, this strategic milestone simplifies the process of manufacturing components and is claimed to improve all-round consistency.”