09 March 2012
Making perfect parts
How can you be sure, when you press the cycle start button, that you will machine a perfect part? Metrology Software Products (MSP) has developed a suite of software that can make that possible. Andrew Allcock reports (extended article)
Perfect parts – guaranteed. That's the claim from MSP for its recently introduced NC- PerfectPart offering, whose creation derives from knowledge gleaned over a number of years of chasing the sources of errors. Its journey may offer some surprises for those in the metalcutting business.
MSP, located in Ainwick, Northumberland (01665 608193), was started by Tony Brown and Peter Hammond in 2002. Peter Hammond hailed from the metrology arena, CMM maker Eley Metrology, as director of metrology. He was involved in applications – deciding how to measure, measuring software development and the accreditation of CMMs, regarding their accuracy (ISO 10360).
Tony Brown was involved with 2D CAD and the pair initially got together with the purpose of interfacing such a CAD system with a CMM, so as to automate the measurement of components. Mr Brown then moved to Delcam and became involved with that company's PowerMILL CNC programming product. The pair maintained contact and discovered that there was a problem to be solved.
Says Mr Hammond: "I was finding that people were producing parts that were wrong, after they had machined them, while Tony was involved with complex machining and, when parts were wrong, it was always the machining program that was the culprit, never the machine. So, we thought, if we could perform measurement on the machine, that would stop them making bad parts."
Adds Mr Brown: "Quite often, it was the part set-up that was at issue. People would blame metrologists for the way they measured it, or would blame the software for the way it had driven the machine, but it was often part set-up – specifically part location. We talked about this and decided there was a solution and, in 2002, concluding that there was a product opportunity, established MSP. We didn't have any software, so we 'specced' up what would become NC-PartLocator, but were not in a position to invest in developing it immediately."
NC-PartLocator would support the execution of probing sequences for a part, with the results of those sequences used to identify any mismatch between the nominal position used in the CAM system to generate the toolpaths and the actual position of the workpiece on the machine. The software would then send a computed realignment to the machine tool control to compensate for datum shift or rotation. It would also highlight condition of supply – that is, whether the part had enough material on to be machined successfully.
Image: Eurofighter foreplanes are manufactured with the help of MSP technology - see later and box item
In the meantime, the company's first customer was CMM maker OGP, for which MSP wrote a software 'wrapper', ezLink, for Renishaw's Universal CMM Controller (UCC) for use with Renishaw's 3-axis SP25 scanning probe. This saw OGP CMM users able to employ high level language that allows them to think and work in quality and metrology data manner, rather than the command level, pure motion commands of the UCC.
"The key point here is that they were getting metrology quality data off the CMM controller, which is the same issue when we come to probing on machine tools," explains Mr Brown.
To underline this point about the importance of metrology quality data, Mr Hammond explains further. "When a touch-trigger probe makes contact with a part, the registered position will need to be modified to take account of many things. For example, a correction for the probe, corrections for the attitude of the probe, corrections for the geometry of the machine; if it's a scanning probe, there may be some filtering required. Whether it's a CMM or machine tool, those factors can affect how good that single point of measurement actually is, as regards its use in a measurement calculation. What we do is get the very best quality of data we can for any particular set of circumstances.
"Our principle is, irrespective of the device [CMM or machine tool], to get measurement results back in a standard format and then apply standard algorithms [these including compensations for the various elements described above] that will give metrology quality data that we know are valid.
"In that way, we separate the measurement from the device on which it is taken. So, if we know the algorithm is good, it is good for all devices it is connected to [only the device-specific compensation values vary]."
The OSG ezLink development, which Renishaw also adopted for UCC/SP25 packages, helped fund the company's own software development, with NC-PartLocator launched in 2004 – the first of the three software packages that make up NC-PerfectPart.
This product was focused on part set-up – correct location and orientation of the part to be machined –with a particular emphasis on 5-axis machining centre technology. If parts are not correctly located, then, while the machining program will be applied correctly, the result may not be correct.
Explains Mr Hammond: "If it is machined out of solid and there is plenty of material on it, that's not an issue, unless it doesn't clean up, of course. But as we move to near net shape, or free-form parts, or we want to get lighter parts, the material isn't there, so part set-up is even more important."
NC-PartLocator was sold to a number of companies, including BAE Systems. It was, however, most visibly offered through Delcam, while similar MSP CNC probe path technology was developed for Delcam's PowerINSPECT OMV (on-machine inspection technology), launched in 2005 (see YouTube video clip). Incidentally, NC-PartLocator can also be employed after machining to measure machined components.
MSP's resources were given a further fillip in 2005 when Renishaw struck a deal with MSP to put the ezLink software on every one of its UCC Servers, provided that Renishaw could buy 50% of the company. The deal was agreed, with MSP becoming a Renishaw associate company. Incidentally, Renishaw also has a shareholding in Delcam, 20% (see here).
The following year, 2006, NC-PartLocator technology installed at BAE Systems saw it win the BAE Systems' chairman's award for Eurofighter foreplane cost reduction (see box item, below).
Following that, some yet-to-be key knowledge was gained with another major project. This saw the company develop the 5-axis probing interface for Renishaw's award-winning REVO technology, unveiled in 2007. Again this was about obtaining metrology quality data from the 5-axis scanning probe system, with MSP gaining detailed knowledge about Rotate Tool Centre Point (RTCP) technology – tool centre point programming. "What we learned here informed our own software developments later," explains Mr Brown.
In parallel with its own software developments, MSP software has gone on to play a critical part in other Renishaw developments, most recently the Equator, parallel kinematic machine-based comparator (see article here). "Measurements from the Equator are as good as the system's repeatability, which is excellent," stresses Mr Hammond.
But what of NC-PartLocator? All was well? No. NC-PartLocator was not, admits Mr Brown, "the total solution we had predicted – it wasn't solving all the problems of part location and we weren't achieving zero concessions".
The problem was one of position variation related to machine kinematics – essentially, different combinations of axis position used to command the same tool/probe position gave varying results. So, for example, if you have a 5-axis machine tool with a compound head, it is possible to position the head in different positions to measure the same point – the problem was that the results were not the same at the various locations. "People were seeing this and thinking they had a part set-up problem, but they didn't," says Mr Brown.
What was particularly interesting was that even seemingly identical machines employing NC PartLocator were giving results that varied, with these often related to differences in set-up during original machine commissioning, says MSP. The company discovered this issue in 2005 and set about developing the next part of NC-PerfectPart – NC-Checker.
NC-Checker comprises a set of probe-based tests that can be configured for the specific machine and controller, which are used in conjunction with a sphere located in the machining envelope. Now, NC-Checker does not correct errors, but merely highlights them, flagging the need for attention by maintenance, for example, and preventing the machining of scrap parts. It also helped explain to customers why there was variation between parts, even if there were no scrap components.
Key is that NC-Checker can be applied without breaking down a machine set-up to employ, for example, a Renishaw ball-bar; although, if problems are found, it may be that maintenance does then employ such devices.
Image: Via its NC Checker software, MSP developed a set of metrics to highlight to machine tool setters whether a machine is capable of machining a part within tolerance, with this illustrated graphically for ease of understanding. A full check might take two hours, but a partial check could take just a few minutes. A partial check might incorporate the 5-axis head, the most elaborate check, with a positive result here basically saying that the machine is good to go, because that will be the largest error source. The probe performance check is undertaken before every part, however. NC Checker is also an aid to machine maintenance scheduling – either shortening or extending the planned event, as this will be affected by how hard a machine is worked.
So, it's now 2008, and NC-Checker is fully developed (with Delcam also offering it), save for new, machine-specific checks that are added as additional machines are tackled. MSP now believes that it has the complete answer. Again, no.
Mr Hammond: "So, it was the same machine with the same controller, producing the same parts. We had used NC-PartLocator and NC-Checker, but the results were different – why?
"We discovered that a touch-probe on a machine used in 5-axis mode requires proper 5-axis calibration for the 5-axis measurement cycles used in NC-Checker. All of the software supplied on machines supports 3-axis calibration and, while that delivers consistent results, these are nowhere near as good as they could be. So we developed an off-controller 5-axis calibration technique, with the necessary corrections then fed back into the controller. The result was an improvement in apparent probing accuracy almost of the order of 10 times better. Probe calibration became a part of NC-Checker, with this the first thing you do when applying it to check a machine." Certain NC-Checker operations are always before NC-PartLocator and, therefore, prior to every component.
YET MORE ERRORS?
But, yet again, this was not enough to trap all sources of error. Mr Hammond continues: "What we found in our first few iterations was that either the on-machine probing macros themselves, or the calibration data that those macros was relying on, gave errors greater than the stated geometry errors of the machine. That was clearly impossible. We didn't want to write bespoke software for every single machine and every single controller, so we took the probing macros outside of the controller and used a suite of our own cycles that understand how to apply the corrections gained via our 5-axis calibration procedure. That really did close the loop."
And so was born the third and final element of NC-PerfectPart, NC-Macro. With this completed in 2010, the whole package, NC-PerfectPart, was launched early last year, with a technical sales partnership struck with Delcam this year.
So, basically, in order to make NC-PartLocator work perfectly, NC-Checker is required to make sure a machine will position a tool/probe where it is commanded. But to make sure NC-Checker delivers accurate probe measurements required proper, external-to-the-CNC 5-axis probe calibration (because CNCs have in-built 3-axis routines only), while NC-Macro is required to avoid use of in-built CNC probe macros, eliminating misinterpretation or variation of interpretation of the 5-axis probe calibration. MSP started at the end of the error chain, rather than the beginning, but it took a few years to get right to the beginning.
Image: A schematic showing the various component parts and data flows
MSP has worked with just a handful of major OEMs using multiple multi-axis machine tools that are machining high value parts and for whom a scrapped part has multiple issues, not least of which is schedule disruption (Delcam has more customers and, incidentally, with its OMV system writes 'option files' for each machine, so doesn't see the variation that NC Macro sets out to counter).
Working with these customers, MSP has been developing and refining its complete solution. Up till now, the company has been aware of the shortcomings of its initial offerings and so there has not been a harder push. Now, with all sources of error identified and successfully tackled, its OEM customers are trickling this down their supply chains, where appropriate, and MSP is taking its message to market, most recently at the Aero Engineering exhibition in September last year. This, together with the recently inked technical sales agreement with Delcam, will likely see the technology gain a more visible presence.
But even for those not requiring such sophistication, MSP's journey does point up some interesting issues that might inform their on-machine measurement variation dilemmas.
BAE Systems' installation
Box item 1
MSP and Renishaw have a joint patent relating to NC-PerfectPart. This patent relates to the communication of design data between the PC and machine controller that negates the need for CAD on the shopfloor. Mr Hammond: "An absolute key factor for our technology is that for generating a probing path for part location via a CADCAM system, NC PerfectPart does not require a CAD model to do all the analysis. All the design intent is embodied within the probing program. The reason this is vital is because in the aerospace industry, it is not allowed to have CAD on the shopfloor."
Box item 2
Substantial cost and technology benefits were achieved at BAE Systems Samlesbury to support Typhoon assembly manufacturing. The project team of MSP and BAE Systems, for the fixture, and Delcam, for the systems integration, has now achieved maximum benefits. Analysing production data of over 1,000 foreplanes, machining time has been reduced in excess of 60%, and part quality and production scheduling has been improved.
This small front wing for the Typhoon is an essential component of the strategic and tactical performance of the aircraft. It is designed with inherently unstable flight and only remains airborne due to flying attitude corrections being applied to the component 100s of times a second. Consequently, it is a critical flying component, which makes its manufacture of crucial importance to BAE Systems.
This complex, high-value part is diffusion bonded out of a number of titanium sheets then super-plastic formed. The process involves extremely high temperatures and, after completion, only a loose relationship exists between the part's new state and any previously defined datums. Due to the nature of the SPF process, these deviations can be as much as 4 mm.
After forming, excess material required for the SPF process has to be milled away so that the machined surface blends closely with the adjoining formed aerofoil surfaces. The shallow, tapered shape adds to the problems of recovering from the datum shift to proceed with the milling operations.
The original process started with the foreplane being located manually on a fixture to machine one side of the part. It was then turned over and re-located by the operators prior to running another part program. This was repeated two further times, each time requiring a different fixture. The quality of the finished part was totally reliant on the ability of the operator to manually adjust the foreplane position and machining supports relative to the fixture datums.
Although this four-stage setup compensated for the SPF-created datum shifts, it inevitably took many hours. Furthermore, it suffered from a high degree of process variability due to the large amount of manual setting. This not only created quality issues that required manual reworking to achieve specification, but also had substantial adverse effects on job scheduling.
BAE Systems recognised that something needed to be done to improve the process to reduce variable part quality and stabilise the erratic production times.
After a detailed study of the process, BAE Systems' engineers and machine tool and metrology experts Metrology Software Products (MSP) designed an innovative new process to solve the problems. Drawing on the experience of the team, the original fixtures have been replaced by a single vacuum fixture, which allows the part to overhang. This removes the need to turn the part over and all key areas can be machined in one operation. To complete the solution, manual alignment has been entirely replaced by a computerised fixturing process.
The heart of the new system is NC-PartLocator adaptive machining software developed jointly by MSP and Delcam. The foreplane now only needs roughly locating and the workspace alignment is automatically calculated, removing most of the manual set up and adjustments necessary in the old scheme. Even the alignment upload from the host PC to the machine-tool controller is automated using a high-speed data link.
The details of the process are as follows:-
• The part is located roughly (to within 4 mm), then secured on the vacuum fixture.
• The operator enters part details (serial no. etc.) into NC-PartLocator. These are used later to archive all the data and results to ensure the process is fully traceable.
• A 5-axis probing program measures a set of carefully selected points on the part surface.
• NC-PartLocator uses the results to calculate a new workspace alignment adjustment. Axis constraints are used, due to the gentle curvature of the part surfaces. They were defined by BAE Systems and MSP after a detailed set of trials. There are checks to allow the operator to verify that the movement of the datums does not exceed pre-defined tolerances.
• NC-PartLocator automatically uploads the new machine workspace alignment to the controller before the machining programs are run to cut the part.
• The part programs are run and machine another perfect foreplane.
Over 1,000 production Foreplanes have now been produced and the benefits of the new process are:
• Manufacturing time has been reduced by in excess of 60%; each foreplane used to require 20-30 hours.
• Improvement in quality and reduction in process variability has virtually eliminated the need for manual rework, in contrast to the previous process where almost every item needed rework.
• Manual set-up has been dramatically reduced; the part now fixtures in one hit.
• Consistent set-up times have been achieved, due to the single fixture, rough initial location and automated alignment, significantly reducing scheduling disruption. Coupled with the reduction in machining hours, this allowed BAE Systems to bring subcontract work back in-house.
• Redundant fixtures have released storage floor space, handling equipment and, if the process had been active from day one, would have saved the cost of their manufacture.
• Operator variability has been further reduced with in-process tolerance checks.
• The project has been recognised internally and won a coveted BAE Systems chairman's award for the improvements made.
First published in Machinery, March 2012
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