On-machine verification (OMV) is a recent innovation that combines existing technologies to solve more complex measurement problems on machine tools. Many machine tools are equipped with probing systems, and using the probe for simple part setting is an established process. Simple macro-based probing cycles allow the user to measure basic features such as faces, corners, and bosses, and these can be combined to create rudimentary inspection reports. These basic solutions are restricted to simple 2-D measurement because 3-D measurement is just not practical. Although skilled operators can sometimes adapt probing macros to measure along compound angles, this becomes too difficult and too time-consuming for complex, curved surfaces.
OMV solves these problems using graphical 3-D software methods to program the measuring sequences. The programming and reporting tools from inspection software are combined with machine tool post-processor expertise to create a measuring solution for machine tools.
Increasingly there is a trend toward multi-axis CNC machines that allow the machining of complex shapes—including those with undercuts—in a single setup. Four-, five-, and six-axis machines allow for more complex machining operations and, in turn, this accentuates the need for more sophisticated programming techniques, both for machining and measurement.
Multi-axis probing capability adds the ability to check these more complex components without any need for repositioning the work- piece. It becomes easy to check typical five-axis features, such as undercuts, in a single setup. Similarly, it is possible to check inside features that are not accessible from the Z direction—for example, to inspect a series of holes at different angles in a single operation. In addition, shorter styli can be used to check deeper pockets or walls by lowering the head of the machine tool toward the workpiece and inclining the stylus toward the surface to be measured.
Successful inspection requires suitable hardware as well as software. For OMV the probe on the machine tool is an important consideration. Not all probes are equal, and using a strain gauge probe, such as Renishaw’s MP700 or OMP400, is recommended. Strain gauge probes provide the most consistent response in all directions, giving users the most accurate system.
A strain gauge probe coupled with software, such as Delcam’s PowerINSPECT OMV, can capture 3-D measurement data and compare them directly against the computer-aided design (CAD) model, leaving no doubt as to whether the component is correct. The software generates a program that will inspect the part using the probe on the CNC machine. This uses a post-processor option file in the same way that a computer-aided manufacturing ( CAM) system does for cutting operations.
The final stage of OMV is simulation of the inspection program before it is sent to the machine. Although not essential, this is advisable to guard against programming errors that might result in collisions and damage. This step requires that the OMV software have internal machine tool models for the most popular machine tools so that it can help with collision detection.
The main benefit of OMV is for measuring during the manufacturing process. Because modern machine tools either come with or can be retrofitted with probing capabilities to assist in setup of the job, it’s a logical extension to use this for measurement as well. The same equipment used for machining can easily be fitted for initial quality checks at a minimal cost and minimal interruption of a company’s machining operations.
Traditionally, measurement of a component after machining involves breaking down the machine, moving the part to a coordinate measuring machine (CMM) in a temperature-controlled room, and setting up the part on the CMM. For a larger component, this process requires the use of lifting equipment and may take several hours. The inspection sequence may take only a few minutes, but if the operator finds a problem, the part has to be moved back to the machine tool and the setup operation repeated.
Measuring directly on the machine tool allows errors to be detected earlier, and to be corrected more quickly and at lower cost. The ability to run the same software on the inspection equipment and the machine tool is a big benefit because it saves on programming time and allows direct comparisons to be made to the CAD model.
The ability to measure on a machine tool is also beneficial for companies that have no inspection equipment, or for those that occasionally manufacture components larger than their inspection devices can measure. This is especially prevalent in the aerospace and energy industries.
This type of inspection has helped Birmingham-based Pro-Mil move into large-scale 3-D machining for aerospace, automotive, marine, and railway products. “We have never had a CMM,” says Ian Hilton, Pro-Mil’s general manager. “Due to the size of the parts that we produce, it would be impossible to justify the cost. OMV has been an ideal fit with the work that we do.”
Before it had OMV software, Pro-Mil created probing programs manually. “With PowerINSPECT OMV, the process is much easier and much faster,” says programmer Steve Davies. “This means I can take more measurements in less time. In addition, I have customized the report template to our company standard, so our customers can confirm quickly that the job is within the specified tolerances. I have also started using the system to assist in setups, especially to check the amount of material left on castings that are sent to us for finish machining.”
With the specialist nature of its business, Pro-Mil is less vulnerable than many other subcontractors to competition from overseas. “The equipment and expertise we have built up here would be very difficult to duplicate anywhere else,” says Hinton. “In addition, the weight of the components would make shipping very expensive. However, unlike many companies that are always looking to cut costs, even when it means risking a drop in quality, we are prepared to spend what we need to provide the best possible service to our customers.”
Similarly, the extent of any damage caused by tool breakage, for example, can be assessed accurately and a decision made immediately to determine whether the part can still be completed within tolerance or whether it will have to be scrapped. Due to advancements in software and systems, newer OMV programs have been designed to be easily used by machine-tool operators rather than specialists. The instant reporting makes it easy to spot potential errors, and decisions can be made on the shop floor as to whether extra measurements need to be taken before the part leaves the machine.
By carrying out an initial verification of the part on the machine, errors can be detected and corrected that might otherwise not be found until after the component had been shipped to the inspector. This helps to increase confidence that everything possible has been done to make sure that the part is correct, even if the next step is to submit it for final inspection.
Companies that already have suitable inspection equipment may question the accuracy of a machine tool against their current methods and see this as an unnecessary operation that can lose precious machining time. However, in this instance the point is not usually to replace final inspection but to detect errors at the earliest opportunity.
Machine tools are often equipped with scales and probes that are comparable to those used on CMMs. However, CMMs are not subjected to the forces and thermal loads experienced by machine tools. Machines used for OMV should be regularly checked for accuracy, and checking the consistency of the measurement results for a known component is one way to monitor the performance.
Programs such as Delcam’s NC-Checker should be used to monitor the performance of the machine. Such programs provide an easy way to confirm the accuracy of the machine with standard probing equipment. It can be used before machining starts to confirm that the equipment has been set up correctly, and then applied during the production run to detect any movements out of tolerance that might have been caused by wear or temperature changes.
The operator must first use the software to calibrate the probe and the machine tool setup against a known artifact, normally a sphere or a set of three spheres. The software then generates a series of probing and performance tests for regular use to ensure that the machine is operating as it should. This process takes many more measurements than are typically used for checks on the machine-tool control and so offers much greater confidence in the results. Test routines can be applied at various stages during a production run. The frequency of such tests will vary depending on the value of the parts, the complexity of the shapes, and the accuracy that is required. Clearly, more regular checks will be needed when producing more expensive or more complex components with stricter tolerances. All the results can be archived to provide a traceable history of the performance of each machine. Any deterioration in performance over time can be detected, possibly signaling the need to schedule maintenance work, even before any out-of-tolerance parts have been produced. The software can also be used after any problems in machining, such as might result from tool breakage or from a collision with a part that has been loaded incorrectly. It can again provide a quick confirmation on whether the production run can continue with high confidence in the quality and accuracy of the finished parts.
Even with regular machine accuracy checking, it should be remembered that OMV is not comparable to a final independent inspection on a CMM in a temperature-controlled room. What it offers is the earliest possible detection and rectification of any problems. When the whole process is considered, there is potential for extreme reduction in delivery times. Consider the typical part flow without OMV. A machined part is transferred to a dedicated CMM for inspection. The CMM inspection reveals errors. The component must now be returned to the machine tool and reclamped into position before being machined again. This is time-consuming for any component but can take many hours for a heavy item, such as a large aircraft structure or a press tool for an automotive body panel. In addition, any mistakes during the setup back onto the machine tool could result in a new series of errors in the component, and so lead to a further cycle of inspection and remachining.
With OMV, the part can be checked at each stage. A final inspection using specialist measuring equipment only needs to be undertaken once at the end of the manufacturing process. The more regular verification on the machine tool ensures that there will be greater confidence that the component will be produced within specifications.
The advantage of the latest OMV software is that it offers all the advantages associated with the software used on dedicated inspection equipment. It gives both quick and easy comparison of tooling and sample components against CAD data and produces clear, comprehensive reports that can be understood by everyone involved in the product-development process, not just inspection specialists. Newer systems also allow off-line programming of the inspection sequence, with fully integrated simulation.