Throw enough darts at the board and eventually you’ll hit the bulls-eye. Does that mean it was an accurate throw, or did you just get lucky? Could you do it again? And again?
What we’re talking about here is accuracy and precision. People trained in statistical quality tools might prefer the word “repeatability” over “precision”. The same people will tell us it’s repeatability that matters most, and here’s why: If you can hit the same spot on the dartboard with every throw, you’re pretty precise, or repeatable. Add a little compensation and you’ll hit that bulls-eye every time, no luck required.
Here’s something to think about though: that bulls-eye isn’t a spot but an area. A pretty small one, admittedly, but it covers a region. You could say that it’s diameter is the tolerance zone. Your throws don’t need to be be perfectly repeatable, they just need to land within that zone.
So what relevance does a pub game have to metal fabrication? Quite a lot. Here’s why.
Every production process we use has some variability. You might say they’re inaccurate because they can’t produce exactly the same dimension every time. Does that mean our machines are in poor condition? Not at all! Browse Tiki- Talk a little and you’ll find posts like, “The Importance of Preventive Maintenance in a Metal Fabrication Shop” that discuss how we look after our equipment.
What it does mean is that our machines and processes are not perfectly accurate. Perfection in this sense is impossible. What we can do though is measure how close we come to a target size, time after time. The distribution of these measurements gives us a measure of the capability of a process.
Metal cutting shears for instance are capable to something like +/- 0.01”. The turret punches and lasers can do a bit better. Welding is a bit more of a challenge. There’s always some play in the initial fit up, and then variation in how the heat is applied. As a result, the amount of distortion that takes place is variable and hard to predict.
The way we handle this process variation is through tolerances.
Two Reasons for Tolerances
As fabricators we need a tolerance on every dimension of the parts we’re producing. Tolerances are also needed to help those parts go together. This is particularly important if we’re making a number of copies of a fabrication.
For production we really need the part tolerances to be greater than the capability of the process. If they’re smaller we’ll need to measure each piece and scrap the ones that are outside the limits. Or switch to a process with higher capability. (An alternative is to sort the parts into grades, but that’s a complex topic best left for another time.)
From a finished product perspective, tolerances help ensure functionality and interchangeability. (Students of manufacturing technology history might recall this as something Eli Whitney addressed back in 1798 when he won a contract to make rifles.)
If you design parts that need tight tolerances, it’s not a problem. It will however influence which production processes we use and how much time the parts take to make. We do understand though that your finished product has to work, and therefore tight tolerances are sometimes unavoidable.
Let’s Explore Some Options
If you’re looking for ways to save money, talk to us about the tolerances on your prints. We can discuss the fabrication process implications and between us we’ll probably come up with some ideas worth pursuing.
And if you’re throwing for the bulls-eye, you may be playing the game wrong. In English darts the bull is worth fifty points while a treble twenty scores, well you know. The treble twenty is about three inches above the bull, so aim high to win.