Inspection software to verify potential errors on machined parts are critical to ensuring wastage and eventually improving cost savings. By Peter Dickin, Marketing Manager, Delcam.
On-machine verification with Delcam’s PowerInspect inspection software allows initial checking of machined parts to be carried out on site on any CNC machine tool rather than having to transfer them to coordinate measuring machines for inspection. The main advantage is that any mistakes are discovered where they can be corrected – on the machine tool. Repeated cycles of machining and inspection, interspersed with long set-up times on the respective pieces of equipment, are avoided, meaning that overall manufacturing times can be reduced.
The most obvious benefit of on-machine verification is for those companies that do not have existing inspection capabilities. Most modern machine tools either come with, or can be retrofitted with, probing capabilities to assist in the set-up of the job. With PowerInspect, this same equipment can now be used for verification at little extra cost.
On-machine verification can give huge time savings by enabling the quality of the component being machined to be monitored at each stage in the manufacturing process. This allows any errors to be detected earlier, and so corrected more quickly and at lower cost. For example, it is possible to check that the correct amount of stock has been left on the component after a roughing operation, rather than having to wait until all machining operations have been completed before discovering that an error has been made.
Similarly, the extent of any damage caused, for example, by a tool breakage, 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.
Of course, there are already a variety of manual methods for undertaking such checks between machining operations. However, like all manual operations, these are time-consuming and prone to human errors and inconsistencies. Furthermore, they are based on inspection against drawings, when most design data is now issued as CAD files. On-machine verification is a more automated and more consistent process than manual measurement, and also allows checking against CAD data.
Companies that already have suitable inspection equipment might think that on-machine verification is an unnecessary operation that can lose machining time. However, if the whole process is considered, there is considerable potential to reduce delivery times. After all, a machine is only earning money if it is producing good parts.
When a part has to be transferred to a dedicated CMM and the inspection shows any errors, the component must be returned to the machine tool and re-clamped in position before being machined again. This is time-consuming for any component but can take many hours for any large, heavy item, such as a press tool for an automotive body panel. In addition, any mistakes during the set-up 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 re-machining.
With on-machine verification, the part can be checked at each stage without being moved. The inspection on specialist measuring equipment only needs to be undertaken once at the end of the manufacturing process. This more regular in-process verification ensures that there will be greater confidence that the component will be produced within specification.
Furthermore, the verification sequence can be simulated on the computer before it is run in exactly the same way as a machining program. The user can confirm that the measurements will be taken in the required areas and that no collisions will take place, so minimising the set-up time on the machine.
One user that regards on-machine verification as “an invaluable tool” is process application engineer, Stewart Seedhouse, from the Makino-NCMT Grinding Division. The technology saves the company and its customers considerable amounts of time and money.
NCMT has represented the complete range of the company’s machining centres in the UK for many years. In 2006, in recognition of the company’s leadership in the application of Viper grinding, the grinding division was set up in Coventry to market the technology throughout Europe.
Viper grinding employs small, vitrified aluminium oxide wheels in creep-feed grinding mode on the machining centres, in place of conventional, plated cubic boron nitride wheels. It is mainly used in the production of components from nickel-based alloys, principally in the aerospace and power generation industries but also in the motorsport and medical sectors.
The technology is not only up to eight times faster than the conventional process but also reduces the cost of consumable significantly. Furthermore, the Makino range is touted to be the first to combine creep-feed grinding with milling, drilling and turning in a multi-processing environment, reducing the number of separate operations and, therefore, the set-up times.
Even so, the process remains extremely challenging due to the use of difficult-to-machine materials, such as inconel, the complexity of the shapes that need to be produced and the high levels of accuracy that are required. In addition, many of the components are produced from forgings or castings, which can be both expensive and in very limited supply.
“In a typical project, our customer might be required to produce at least 30 conforming parts from 32 high-value castings,” explained Mr. Seedhouse. “Anything that can make the process more reliable and more accurate is hugely valuable.”
The first stage of most projects uses the PowerInspect software to check the machine kinematics by taking measurements from a standard sphere in a known position on the machine. This takes one to two hours but is essential to ensure that the accuracy of future measurements can be trusted.
The next stage is to inspect the datum locations of all the fixtures to be use in the process and, in some cases, their clearances. This is necessary, said Mr. Seedhouse because “although we expect to receive the fixtures in the correct state, being able to ensure they are correct to what we expect at this stage can save a whole lot of time and effort trying to find a problem later on.”
Following these checks, the first part is loaded and on-machine verification is used to measure the amount of stock to be removed. If necessary, the position of the part can be adjusted or a modification made to the datum being used in the machine-tool control to give a more even distribution of stock around the required final shape. The complex shapes found in many of the components would be difficult to check with physical measurement but are easy to measure with PowerInspect OMV.
“With new parts and possibly multiple casting suppliers, we often check the amount of stock on the component prior to machining,” explained Mr. Seedhouse. “Too much stock on the component can lead to metallurgical problems such as cracking and burning, or wheel breakdown leading to geometrical problems, and, in extreme cases, even damage to the machine. Once you know that there is more stock than expected, it is easy to add in an extra cut to remove this excess material safely.”
Another potential problem is that residual stress within a forging or casting can be released during machining and change the shape of the part. “If we think this might happen, we carry out an initial cut with extra material left on to release the stress,” said Mr. Seedhouse. “We can then check if any movement within the part has occurred with OMV and so allow the finish machining to be performed accurately.”
OMV is also used to check fixture and part positions after any heavy cuts that might have moved the part or if any problems have occurred during machining.
“PowerInspect OMV is an invaluable tool when making any new component or installing a new machine,” claimed Mr. Seedhouse. “It gives you a high degree of confidence almost immediately because it is very, very quick to provide basic measurements on the key points of the part.”
“It can save weeks of time that might be spent waiting for a CMM to be available and for a metrologist to analyse the results,” he added. “OMV even gives us the confidence to ask the customer to check the CMM if the measurements show any errors. In my experience, if the part gives the correct results with OMV, there is not going to be much wrong with it.” This faster commissioning benefits customers because initial test batches can be produced more quickly with high confidence that the parts will be to the required standard.
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