“I cannot see the point of investing in accurate and repeatable CNC machine tools and then checking the components they produce by hand,” says Nick Groom, managing director of Qualiturn products, a precision turned parts manufacturer in Hertford.
“You simply take the risk of human error out of the machining process only to reintroduce it in the quality control department.”
The company, which holds ISO 9001:2000 registration for the quality management of its contract machining service, has largely moved away from manual metrology practices using plug and ring gauges, micrometers, verniers and height gauges. In their place are to be found CNC inspection machines, including two key pieces of equipment installed in 2007 and another in 2006.
They represent a capital investment of £120,000 plus the ongoing cost of regular calibration. In the same period, the subcontractor has also purchased two MecWash aqueous cleaning machines with ultrasonic capability, an Autocrib tool store and a Couzens tower storage system for raw material. All are linked into a PSL Datatrack production management system, and so too are the quotation, sales order processing, purchasing, stock control, workshop scheduling and finance functions to create a virtually paperless environment.
Qualiturn manufactures components with mill-turned features tied up to very tight tolerances, sometimes down to five microns total, so customers appreciate the security of results that CNC inspection gives. The subcontractor, whose policy is to check the first-off component in the quality room before running a batch, finds that programming its optical scanning system for round parts or its motorised vision / touch-probe measuring machine is just as quick as inspecting the parts manually.
So there is no time penalty when checking the first-off, which means that the batch run can start just as quickly without any loss of production. Where Qualiturn really benefits, however, is when the customer reorders the same job, as the measuring program as well as the machining program are stored against the product code for immediate reuse. The job is therefore much faster into production on subsequent occasions, resulting in significantly higher spindle uptime and profit.
With around one-third of orders at Qualiturn being repeat work, the financial benefit is clear. It is central to Qualiturn’s ability to juggle three conflicting pressures – amortising the capital expenditure on the CNC inspection equipment, increasing margins and responding to cost-down demands from its customers.
The savings continue throughout the batch run, as before the last-off is inspected, mid-batch samples are checked regularly in the inspection department, the frequency dictated by the customer’s requirements, the length of the production run, the material being machined and the drawing tolerances. Each of these interim checks is faster than before, reducing the labour cost content and potentially minimising scrap by indicating more promptly if the part has drifted out of tolerance.
An additional advantage of CNC metrology is that measured results are logged in the works order file, together with a digital copy of the drawing and all other job-related data, within the computerised production management system. The information can be interrogated at any time by the subcontractor’s quality staff and/or the customer, either during production or even months later if any future quality issues arise.
Furthermore, as the data is already in electronic form, results are immediately available for statistical process analysis, control and reporting, if required.
A Tesa Scan 50 Plus non-contact system, installed in 2006, is used for inspecting the outside profile of round parts. Its high resolution CCD linear sensors allow fast and accurate analysis of the geometry of any number of features when the part is illuminated. Straightness and circularity is checked by combining rotation with axial movement. A typical inspection cycle takes 15 to 20 seconds, including saving the documentation against the works order number.
It is an easy matter to set the drawing tolerance for each dimension on the screen of a linked PC. The measured result for each parameter is displayed on an LSL/USL bar chart, from which it is easy to see at a glance if it falls within the central green section or in the out-of-tolerance red areas at either end. Such systems allow Qualiturn to achieve production process averages consistently better than the acceptable quality limit, or AQL.
Two- and three-dimensional inspection is carried out on a Tesa Visio 300 DCC vision measuring system with additional touch-probe capability running under PC-DMIS, the standard operating system in the metrology environment. Co-ordinate stages along the vertical axis are displaced by means of DC servomotors and are controlled by a joystick. It allows geometric features to be located and provides for automatic, motor-driven execution of application programs.
Modern lathes produce exceptional turned finishes, to such an extent that it is no longer necessary to grind many components, as would have been required a decade or so ago. However, the finish achieved on the OD often has to be measured and recorded, so in the last 12 months, Qualiturn has taken delivery of surface tester from Tesa, which is also PC-linked for reporting.
The latest piece of metrology equipment to be installed at the Hertford works is a Tesa Scope 300V profile projector, which joins a similar piece of equipment from Mitutoyo. Both of these are included in the regular calibration service contract that covers all of the new CNC measuring machines.
Of the 24 staff at Qualiturn, three are dedicated inspectors responsible for checking up to 35 first-off components per day plus mid-batch and final inspections. Nick Groom asserts that this structure helps to place Qualiturn at the forefront of turned parts manufacture in the UK and indeed continental Europe, from where an increasing number of orders are being won owing to the current weakness of the pound against the Euro.
He concluded, “Firms tend to be reticent when buying metrology equipment because it does not produce anything and people see it as just an overhead. What they forget is that the speed of CNC inspection can boost the output of all the other plant on the shop floor by getting the spindles running quicker.
“We are working more and more for OEMs, many of which are closing their in-house inspection facilities, relying on the metrology capabilities of their suppliers to inspect and certify the parts they deliver; yet they are demanding ever more stringent quality control.”