Exacting task

Today’s press-produced components must be measured and calibrated to a degree of accuracy unheard of even a decade ago. Mark Venables looks at what is on offer.

Recent years have seen a rapid improvement in the quality of manufactured parts, meaning that a much higher tolerance of inspection is needed.

This has resulted in more accurate and repeatable measuring equipment — plus the ability to calibrate them accurately to comply with ever more stringent regulations.

If anyone had told Mark Eldridge a decade ago that he would be inspecting press-produced components to micron tolerances, he would not have believed them. Yet in his current position as quality manager at Wild Manufacturing Group, he is doing just that — on an LK co-ordinate measuring machine (CMM). One example of the accuracy needed is in the ball washer that forms part of the front suspension for a major automotive manufacturer.

The West Midlands pressworker makes 33,000 per week, the bore being the most critical dimension at 17.67mm diameter, +0, -0.04mm. This level of accuracy is required after heat treatment and plating, both of which use up tolerance — so the washers must be accurate to within 30 microns when they come off the presses.

Wild has been making these high-alloy steel components since the late 1990s but even in that relatively short space of time, quality standards have risen in the automotive industry to the point where the company’s old Birmingham toolroom CMM was not up to the job of supporting its statistical process control (SPC) procedures. These are necessary to enable very tight tolerances to be maintained and to achieve high process capability figures.

The importance of maintaining such close quality control is critical because if the washer is faulty, the bore pin can fracture, leading to suspension failure and expensive vehicle recall. This is why, in addition to putting three washers every two hours on to the CMM to monitor the production process, every component is checked on an optical sort machine before delivery to the customer.

‘The problem with our previous CMM was its poor repeatability and reproducibility (R&R), which resulted in as much as 10 microns measuring uncertainty from component to component,’ said Eldridge.

‘R&R needed to be under 10 per cent so with the measuring process taking a maximum of three microns of the washer’s working tolerance off the press, we were left with seven microns to play with.’

Cycle time for inspecting the ball washer is now two minutes, including component reversal and re-clamping, compared with 7.5 minutes on the old CMM, so measuring productivity is higher too.

There is a further module, called Launchpad, that uses on-screen component photographs to guide metrology staff when fixturing parts and instigating measuring programs. As well as satisfying the ball washer contract, the ability to measure accurately and repeatedly has resulted in new contracts for Wild and is helping to expand its business into the manufacture of complex assemblies.

For example, a tier-1 automotive supplier recently asked the company to quote for producing the front and side panels for a complex radio housing. Tolerances were fairly open at +/-0.1mm but there were 400 dimensions on each side of the front panel that needed to be measured.

‘Manufacturing the components was not a problem but manual inspection would have been impossibly time-consuming and our old CMM would have been too inaccurate,’ said Eldridge.

‘The CMM allowed us to take on this work and fully inspect a front panel in 49 minutes using a program generated from the customer’s IGES file.

‘Investment in CMM technology to guarantee quality for customers is not a luxury any more — it is essential to winning and retaining business,’ he said. ‘It takes SPC to the next level, allowing suppliers to exercise much closer control over their production processes to meet ever more stringent quality standards demanded in today’s manufacturing environment.’

It must be remembered however, that even the highest precision instrument is worthless unless it is calibrated correctly and regularly. Messier-Dowty designs and manufactures landing-gear systems for the world’s best-known commercial airliners, jet fighters and military transport aircraft. In a sector well known for its commitment to quality, it has a control procedure for almost everything — even monitoring the accuracy of its CMMs. And because it cannot afford to take chances with quality it invests in the very best machines and has strict quality control procedures for checking their accuracy.

But environmental factors and excessive or heavy use can occasionally compromise the accuracy of even the highest precision device.

For this reason, Renishaw’s Machine Checking Gauge (MCG) provides CMM owners with the ability to regularly ascertain the performance of their investments, alerting them to accuracy problems before they impact manufacturing or quality-control processes.

Pete Willis, a principal engineer at Messier-Dowty’s Gloucester manufacturing engineering department, has established a regular and thorough programme of checks for the company’s seven CMMs.

‘Previously we had no way to monitor the accuracy of our machines, so were unable to understand the impact on performance related to our specific working conditions. This meant that we could not determine the appropriate interval for service and calibration, but were totally dependent on the recommendations of the CMM supplier.’

Part of the challenge at the group is that its machines are located on the factory floor, rather than in clean-room environments. In addition, they are under heavy loading; operating for three shifts a day, often seven days a week.

‘We don’t have any complaints with the machines themselves,’ said Willis. ‘On the contrary, problems are normally caused because we work them very hard and because they’re susceptible to environmental conditions.

‘Operational and environmental factors gradually degrade the performance of components like the air bearings, and ultimately machine accuracy.’

The gauge is a straightforward test that checks volumetric accuracy of a CMM, whatever its size. Based on a simple alternative to the widely-used ‘ball-bar principle’ — typically used to check the accuracy of machine tools — the gauge provides fast, automatic machine evaluation in accordance with ISO 10360-2, the international standard for the acceptance and re-verification of co-ordinate measuring machines.

A counter-balanced arm of known length is located at one end on a stationary, free-standing pivot positioned on the CMM’s table. The pivot allows the arm to rotate accurately through 360º horizontally and +/-45º vertically. The other end of the arm comprises two parallel guide rods with sufficient clearance between them to allow the CMM probe to move towards the pivot position, taking a radius measurement as the probe stylus ball contacts a ball attached to the end of the arm.

As the arm is a known length, any discrepancy between this length and the measured CMM value can be calculated. A standard test provides for eight measurements at each of 0º, 45º and -45º arm elevation, giving a total of 24 points. The process is repeated three times to allow for machine repeat- ability, producing 72 results in all.

‘As part of the ISO 10360 standard, we use the gauge to perform the interim verification requirements. Following each calibration of the machine, the MCG captures and records performance values, the results serving as the reference standard against which machine deterioration can be determined,’ said Willis.

‘By graphing the results, we can watch the machine accuracy deviate on a monthly basis. If the deviation is too much, too soon, we cease measuring and the calibration company re-qualifies the machine.’