Getting it together is getting easier

One of the great achievements of IT, in our opinion, is that parts of cars and sections of aircraft can be manufactured in different countries, and can usually be assembled without problem when they are brought together. Not so many years ago, it was not uncommon even for parts made in the same factory to fail that test first time. Every engineering company employed specialist craftsmen, called fitters, to get things to fit together, and to ensure that parts supposed to move smoothly and freely performed as intended. At Blaw Knox, where I spent four and a half years in charge of research, it was always necessary to have a fitter present when one of our road paving machines was delivered and commissioned. There he was, ready to do a little filing here, a little grinding there, and then, if necessary, apply a certain amount of carefully controlled brute force. Even in more recent years, we can think of a shower valve assembly made by a certain leading UK manufacturer, whose two main parts failed to assemble in a large number of cases. Assembly was possible or not possible according to whether the manufacturing oversizes and undersizes happened to combine to produce one part which would just go inside the other. The solution adopted by management to avoid such disasters was to require that the design office move into the same building as the production department, and consult with them at all stages of the design process. And that is the first way in which IT tools have helped the process, by improving communications between design and production engineers, and ensuring that everyone is accessing the same, latest version of the design. One company that does this is Vision Alert in Leeds, which makes configurable lines of custom light bars for emergency vehicles. It's part of the Boise, Idaho-based ECCO which also maintains an office in Tanzania. The firm uses SolidWorks Professional and PDMWorks to securely manage and share engineering design data, eliminate lost files, prevent unauthorised changes and enable concurrent design that can span the three countries in three widely different time zones. But to return to the issue of the shower valve parts, the fundamental problem was variations in manufacturing tolerances. It is only possible to make parts to very precise sizes by expensive finishing operations. In the real, cost-conscious world, a certain amount of size variation is permitted, and indicated on production drawings. The CAD model can always be made to produce virtual parts that fit together to extreme degrees of accuracy, but anyone who has ever made anything should be aware that inaccuracies inevitably occur during manufacturing, and real surfaces can never be perfectly smooth. Furthermore, even parts that are initially near perfect are unlikely to stay that way. Tolerating tolerance Despite this, if you start talking to the average design engineer about manufacturing tolerances, he or she will usually be inclined to mark shafts in holes as 'interference' or 'clearance' and dismiss the calculation of exact target sizes and tolerances as a problem for production engineers. So it is, but it should still be a part of the design engineer's responsibilities to ensure that designs are as low cost as possible to make – and that means having parts which can be manufactured by low cost techniques, without requiring excessively expensive finishing operations. In a true Six Sigma environment, the goal should be that these should still fit together and work together in 99.9997% of cases. Miles Pennington, of West London-based design consultant Design Stream told MCS: "It is taken for granted that, as a design company, we will undertake our designs on a high end parametric 3D CAD system in order to create very accurately designed virtual prototypes that can be signed off." In his company, manufacturing tolerances are specified on the basis of human knowledge of the manufacturing processes. This requires knowledge of the behaviour of different materials in different processes, whether machining, sheet metal bending, injection moulding or vacuum forming. It isn't easy. "One of the hardest things we do is striking the balance between what we want to make, and what can be made at a reasonable price," he says. Pennington says his team is currently working on a product that involves two injection moulded parts that have to fit together. Initially, the tolerance is 0.5mm, in light of knowing that it is presently working with soft tooling before later going to hard tooling that will be producing parts to 0.1mm tolerance. He says that SolidWorks enabled quick production of accurately made prototype parts, but it was necessary to remember that when fitting them together, those parts would be made from a different material. "People coming out of college often have very good CAD design skills, but they then have to start learning about real world manufacturing," he laments. We asked if tolerances were specified in accordance to ISO standards, since these cover a very wide range of fits and surface finishes, but Pennington says they're insufficient. It is, he explains, not enough to know what materials and processes a supplier might be using to make a particular component: you also need to know its experience and capabilities. Design for manufacture "One supplier, for example, may be used to manufacturing small, precision made injection moulded parts, while another's experience might be in making larger parts with thick walls at minimal cost, without having the same requirement for precise dimensions," he says. His solution: the team is careful to maintain a good relationship with production engineers in the companies it works with on both the supplier and customer sides. "If we have any doubts, we ask them." There are CAD tools, of course, that come with tolerance analysis, although in light of the above, it is clear that they should be used with some caution, taking supplier capabilities into account. Neither SolidWorks nor Solid Edge include, or are able to offer, tolerance analysis facilities or add-on modules. Catia does, and so does UGS NX. Other vendors recommend third party add-ons, such as TASys, TolWizward and the Sigmund family of products from Varatech Engineering Consultants. Autodesk products are said to work well with ToleranceCalc from Geomate and the Tolerance analysis module from MechSoft, although a certain amount of tolerance handling comes built into the basic Autodesk products. Advanced tolerance analysis is said to be particularly useful with regard to designing complex products with large numbers of components where tolerances are liable to build up. PDD in Hammersmith, arguably the UK's leading independent industrial design consultancy, is currently evaluating the three alternative tolerance analysis modules that can come with UGS NX. Until now, the team has avoided using commercially available off-the-shelf software for this process, because, in the words of development director Graham Lacy, "These have until now required too much manual input." Instead, PDD uses spreadsheets that it has developed itself. "The way we work, we embed tolerances right at the beginning. When we start our design work, we have already established process capabilities and materials, and use this information to generate a tolerance regime. We then agree this with the client and subsequently embed it in all the design work we do. At the end of the job, we check that the tolerancing intent has been maintained." The spreadsheet tools used include one for process and probabilistic tolerancing. Medical devices and precision or complex mechanism projects involve the preparation of a dimensional management spreadsheet listing all clearances and fits (others are checked on-screen or manually). The latter is used to track all design changes, and their effects on tolerances. Lacy again emphasises the importance of specifying manufacturing tolerances right from the start of the design process. "All the production intent must be present in the design model well before getting to the virtual and real prototypes. If you have not considered manufacturing tolerances at the outset of the design process, you may well find that your whole build philosophy has to be changed when you get to the end." If the materials or manufacturing processes to be used are new, knowledge based on experience falls down, and computer modelling becomes essential if much time is not to be wasted on trial and error. Packaged computer products may be inadequate, especially if modelling is based on estimated data, but Dr Daniel Csete of Ceram, formerly the British Ceramic Research Association, tells us that his organisation offers an advanced modelling facility as part of its design for manufacture service. That service has been applied successfully to the design of products and components in many sectors, from those used in the building industries to medical implants and advanced aerospace components. Charges start from £2,000.