How to predict machinability to reduce scrap,
change and wastage

2nd March 2011

As a design engineer, one doesn't always have all the necessary experience of manufacturing materials, processes and treatments to produce a design that will result in a product that requires minimal engineering change and will keep scrap and wastage to a minimum. One of the ways this can be achieved is to provide the designer with the expertise of a manufacturing or machining professional. Here, Martin Raines, of tolerance software provider Tolcap, suggests five ways that good communication and accurate tolerancing can be applied to accurately predict, and thus improve, machinability.

Encourage feedback: The first point is to encourage feedback from the machine shop and associated manufacturing specialists and absorb as much of it as you can. Don't be the designer that takes offense when someone suggests a change; an improved design can only reflect well on you.

Spend time on the shop floor: It's a good idea to spend a few days helping a machinist make one of your more complex parts. As part of this process, tell your machinist which features are absolutely necessary and which can be tweaked; he or she may well have some good suggestions to improve the machinability. If a machinist tells you that a particular aspect of a component is challenging, consider simplifying it.

Invest in the right tolerancing software: Parts that don't fit together are not necessarily the fault of the manufacturing department. Double check your tolerance stack, and then check it again, visualize it and write it down. And don't just use your calculator, invest in good tolerancing software.

Bear in mind that, in tolerancing stack design, it is absolutely crucial that any software employs capable tolerances. If it doesn't, it really isn't worth the code it is written in. If it does not work with capable tolerances, then it needs to be used alongside Tolcap, which will allocate process capable tolerances to any design characteristic. Tolcap manages tolerance allocation and predicts process capability at the early stages of design before production starts. This reduces production costs, improves product quality and increases product profit. The software is Web based, readily usable and fully supported for business and the education and training of engineering students.

Consider material and process: You should consider the material and process you are using as a priority. If the material isn't crucial to the design, simply choose the easiest material to machine, because time is usually the more expensive of the two quantities. This will reduce capital expenditure as well as reducing scrap and wastage attributable to machining difficulties.

Equally, if the process is producing a tolerance that is greater than required, choose a different process to reduce the cost. It may well be that the component doesn't really require a fine process, such as grinding, for instance. Good tolerancing software will help you select the right process and material; by predicting tolerance you can choose exactly the right level of accuracy, without expensive over-specification.

Spend time with the QA department: While your new part is being qualified and tested, spend time with the quality assurance team and whoever handles industrial compliance in your business. It isn't worth designing an easily machinable part if it doesn't have the physical attribute required to pass muster and reach the end user.

However, I'm very conscious that if you work in a multinational manufacturing organisation many of these suggestions are nothing short of utopia. Your design department may be on a different site from the production plant. In fact, it's frequently in a different country! So spending time with your machinist is impossible.

If this is the case, then point three is the only way you can get the benefits that closer integration will bring. By investing in tolerancing software, such as Tolcap, potential problems will never make it to the shop floor. The potential waste is never produced and the design is more elegant, more efficient and less costly. No matter what the product costs, and irrespective of whether you are using Tolcap in Rolls Royce or Dyson or on behalf of a micro manufacturer, the enormous costs of engineering change, rework and scrap will be reduced.

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.