In the 1960s, a group of auto enthusiasts got together to find innovative ways to improve their hot rods. They did not have a lot of money so they had to work with stock products that came from the big three auto manufacturers. At that time, mass production at companies like General Motors and Ford were very erratic. Close enough was good enough. Sometimes you got a good engine and a good car. More often, you had a real lemon that had to be rebuilt after a few thousand miles.
The hot rod and muscle car enthusiasts did not have the skill to redesign the engines, but to their surprise, found that the engines had very effective designs. The only problem was the engines were never built to those exacting specifications. This is where the idea of “templating” or balancing developed. They wondered how the engine would perform if every feature of every engine part was manufactured as close to the nominal specification as possible. In other words, make it the way it was designed, not as it was built. There was no way of achieving with the mass production and limited manufacturing precision of the big three auto manufacturers. But when the enthusiasts rebuilt engines, they were rebuilt exactly to the specification and magic happened. Each engine component was measured precisely and remanufactured to within tenths (10,000th of an inch) of the original specifications. Engine components now fit together and engines for the first time were manufactured and assembled exactly as they were designed – a novel concept! Vibration and uneven wear was eliminated and performance went through the roof. This was a stunning revelation for these hot rod enthusiasts. The design of the engine was golden but traditional manufacturing methods at the big three auto makers contained so many variations from the nominal specifications that they were never able to achieve the performance capabilities that the designers had intended.
With modern CNC equipment, it is possible to make machine components that are a few tenths from the nominal every time they are manufactured. Knowing that’s possible, why don’t all manufacturers produce parts that are exactly to spec or very close to the nominal? One of the difficulties is finding a process that can monitor any variation from the nominal so you can detect any process drift. The other difficulty is the tolerances that the customer allows creates the environment for a go or no-go within the specification. The customer also has the fear that if they ask for tighter tolerances or closer to the nominal then the price will go up. What is very rarely considered is that a minor cost increase at this point of the manufacturing process can dramatically reduce the costs later on and can improve the capabilities of the finished product.
This principle of trying to manufacture to a target nominal is called the Taguchi principle. Almost any variation from that nominal will cause problems in the assembly process. This is dramatically illustrated in very large assemblies like aircraft. The individual variance may not be a significant problem but when you accumulate all the variances you can end up with serious fit issues that require shims and other work-arounds.
One of the origins of tolerance variance comes from a simple deviation in the manufacturing process. When a part is manufactured on a traditional machine tool, the operator's main motivation is to avoid scrapping the part. Most parts are made up of two distinct features: 1) Surfaces 2) Holes.
The operator knows if they adjust the target nominal to the maximum tolerance with offsets, then they can still manufacture parts within specifications with a machine that does not have the precision for holding the required tolerances. If they target the nominal on the surface they then have an increased chance of being undersized and have to scrap the part. The identical process in reverse applies to holes. Whereas surfaces are adjusted to the maximum tolerance, holes are made to the minimum tolerance to avoid being oversized. If a process variation occurs, you can always make the hole bigger. It is real challenge to make a hole that is too large smaller.
Imagine the impact when you combine 200,000 parts of an airplane that are all within specifications but all surfaces are at the maximum and all the holes are at the minimum, you are creating an airplane that is very difficult to fit together and will exceed the design weight of the aircraft by as much as 1-2 %. This is what happens when the suppliers of the components deliver parts with the maximum material condition but still within tolerance.
With the improved capability of machinery and accurate measuring devices, the aerospace industry can start to take a more dramatic shift towards precision manufacturing, just as the automotive industry did years ago. Contact us to learn how our software can help you or your suppliers implement the Taguchi principle within your own manufacturing facilities and help lead the charge.