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The harder you work, the easier it looks


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The harder you work, the easier it looks

The ideal assembly process yields exactly what you want, when you want it, with zero waste and no inventory.

A lot of problems are caused by failing to allow for production variations. I had a reminder this week, just how easy it is to get this wrong. I've been talking to foundries about castings. One of the most important questions: What's a practical working tolerance? My design must guarantee that the castings fit directly into the plastic mouldings, without the foundry, or the plastic moulder, doing anything unusual.

CNC mill

The client wants to build one boat at a time. Murphy's Law states that if he's going to have trouble, it will happen when he has an urgent order, while, at the same time, he hasn't sold any boats for a while and his overdraft is maxed out.

It's not difficult to design the product to accommodate small variations in the length, width, or height of the castings. There are two possible materials, and three or four potential variations on the casting process. I haven't worked with these materials before. I've also never worked with the relevant foundries before. Obviously, they know the process, so I have to get their advice on the appropriate dimensional tolerances.

I've come to expect each supplier to respond differently. But I coudn't believe what I heard from one foundry. According to this company, our castings won't vary. Not ever. The "tolerance" is zero millimetres. And, when I asked about the practicality and cost of permanent moulds (which foundries call "dies") in various materials, I was told that this is a trade secret. Maybe. But if so, it's one of the worst-kept secrets in the world.

I'd already estimated the "worst-case" magnitude of production variations using metal dies. It was large enough to create a problem. However, I would not expect a real foundry to get anything like this "worst-case" variation. They would control the production parameters within narrow limits, not only for dimensional accuracy, but also to maximise the life of the dies and to ensure high-quality castings.

Other foundries were only too happy to talk about these issues. In fact, like many suppliers I've dealt with in the past, they liked the opportunity to talk about their processes. Like me, they want to prevent future problems.

Failing to allow for production variation often goes hand in hand with failing to stick to the marketing brief. Engineers assume the customer will use the finished product the way they'd use it; And they assume every part will perfectly match the drawing. "Perfect", of course, is a relative term. Does it mean "perfect" within a hundredth of a millimetre? Or perfect within half a millimetre.

I've seen too many examples of products that just would not go together. The designs were so bad that every part had to be hand-fitted. They are company-killers. Production managers can't predict how long it'll take to fill an order, nor can they predict what it will cost. Sales managers have no idea if the product is being sold at a profit because they don't know what it'll cost to build.

On this project I've found contractors who understand these issues. I'm absolutely rapt about that. My client won't suddenly find, when he's putting together boat number 19, that the parts don't fit. That means I won't get a phone call, months or years after I've forgotten about the project, saying: "Kev, these castings don't fit."

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