Is your process not working, or not capable?

23rd May 2011

In the current economic climate no manufacturer can afford substantial waste product and the costs associated with it. However, many companies have to deal with scrap and re-work as a direct result of out of tolerance manufacture. In an ideal world, manufacturing engineers would have sufficient input in the early stages of the design process to minimise such quality losses. Here Martin Raines, a director of Tolcap, explains how manufacturing engineers can help mitigate the problem by applying the correct software tools.

The root cause of out of tolerance manufacture normally comes down to one of two issues. Either, the tolerances are too tight in the first instance or the manufacturing process is not performing to its full potential. Fixing the problem comes down to deciding which of these two issues to address.

Making the wrong decision at the incipient stage of your production process can be costly in both time and money. As well as the scrap and re-work, one should consider all the time and effort put in by design, quality and manufacturing engineers, analysing and solving production problems, when they could be working on the next generation of the product!

Up to 50% of manufacturing errors are directly attributable to incorrect allocation of tolerances before the manufacturing process even started. This is a major component of the quality costs for a business that can absorb as much as 20% of an organisation's turnover.

At Tolcap, we have often seen companies spending large amounts of capital chasing process improvement, when the manufacturing process is already performing to standards of good practice. In this case there is very little scope for improved capability as this is ultimately limited by the laws of physics.

By applying process improvement techniques where tolerances are fundamentally inappropriate for the process, you can only ever make the manufacture of scrap more efficient. There's very little value in having an extremely lean, 5S compliant, line if that line is producing 30% scrap. In a utopia of course, one would be manufacturing to specified tolerances, thus reducing losses, and applying process improvement techniques, making manufacturing more efficient.

On the other hand, there are cases where the manufacturing processes are not operating under conditions of good practice, thus leading to out of tolerance components. Even though the tolerances may be correct, the desired benchmark may not be achieved due to such things as poor maintenance of tools and machinery or incorrect set-up and operator errors.

How you go about solving these problems depends on their root cause. If the manufacturing process is not performing as it should be, then the manufacturing engineer can work on improving the performance of the process using the traditional approach.

Alternatively, if the manufacturing process used is not capable of producing the required tolerances, then the only solution is to examine the design to see if the tolerances can be relaxed, the geometry of the design changed, or alternative material selected. Tolcap will show you whether the tolerances you put on a drawing will be process capable before the production process begins. This right-first-time tool prevents costly manufacturing and assembly problems. Equally, it can be used to analyse an existing process in order to correct the faults you've identified.

Tolcap is an easy to use web application, incorporating a knowledge base containing more than eighty commonly used manufacturing processes including casting, machining, moulding and forming. It deals with the affects of different materials, such as steels, non-ferrous alloys and plastics, on manufacturing tolerances. An appraisal of the geometry of a part, based on half a dozen straightforward questions, provides the user with a capability prediction (Cpk) or a suggested capable tolerance. In carrying out an analysis, the software assumes good practice in manufacturing operations.

The target Cpk is set by each company, in view of their customer requirements. Hence, if the predicted Cpk is below the specified target of your organisation, then there is a good chance that there will an unacceptable number of components out of tolerance which need to be scrapped. If the software predicts a Cpk below the target then there is little chance of achieving a satisfactory result with process improvement.

Alternatively, if Tolcap predicts a Cpk significantly above the target value, which isn't being achieved in reality, then this is a good indication that the process is not meeting standards of good practice. The reasons why this situation occurs should be investigated by manufacturing and engineering with the aim of achieving the required capability. For example, in some cases there may be a problem with the way in which machines are operated or an issue with tool maintenance or machine set-up.

Using Tolcap can deliver excellent results for both SMEs and large companies. For instance, one high value products manufacturer in the Midlands placed a component for construction with a supplier boasting an excellent international reputation. On manufacture, the supplier found that they could not achieve the tolerances required capably. This raised the issue of whether the tolerance was achievable using the processes and technology available. Without the use of Tolcap this is a commonplace issue that is very difficult to resolve. By carrying out a Tolcap analysis it was found that the tolerances specified were inappropriate for the manufacturing process. Also, after examining the design it was possible to relax the tolerances without any reduction in functional performance, consequently resolving the problem.

It's proven that the costs of re-work, scrap and returns can exceed 20% of sales revenue and that up to half of these expenses are caused by incorrect allocation of tolerances. Closer attention to tolerances alleviates these problems at an embryonic stage, increasing profitability and sending massive savings straight to the bottom line. The only question you have to ask yourself as a manufacturing engineer is; are the tolerances too tight or is the manufacturing process not performing as it should? Tolcap allows you to answer this critical question.

Written by:

Dr Martin Raines CEng, MIMechE

Dr Martin Raines CEng, MIMechE

Martin is a founding member of the Capra Technology team and specialises in software solutions for capable design and manufacture. Martin's expertise in quality and reliability engineering is called upon by an international client base including BAe and Rolls Royce.