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Variability Reduction in Manufacturing

by Ron Kurtus (updated 21 November 2022)

Variability reduction is a manufacturing method where a target part size is used as opposed to specifying a size with tolerances. It is based on a manufacturing approach devised by Genichi Taguchi in Japan.

The problem with specifying tolerances is the possibility of tolerance buildup when parts are combined to make a product. It also allows for carelessness in making parts and products. The Taguchi method focuses on the target size, with a probability curve drop-off. It greatly reduces tolerance buildup and results in a more reliable product.

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This lesson will answer those questions.

Problem of tolerance buildup

When you specify a part to be a certain size plus or minus a tolerance, there is no incentive to make that part to the exact size. This may be fine for an individual part. But when a number of parts are assembled, the tolerances may build up in one direction or the other. This can result in items fitting too loose or too tight, resulting in possible failures or lack of reliability.

Fro example, a part may be specified to be 10 cm long with a tolerance of +/- 0.1 cm. If one part was 0.1 cm too big and another was 0.1 cm too small, the combination would even out to an acceptable size.

Part with a specified tolerance

Part with a specified tolerance

But it is also possible that two or more parts could be oversized or two or more could be undersized. That could result in an assembly that was unacceptable and didn't function properly.

Minus or plus tolerance buildup unacceptable

Minus or plus tolerance buildup unacceptable

Variability reduction

By focusing on the target size and not accepting parts on the fringes of the tolerance, you can reduce the variations and practically eliminate tolerance buildup.

Most parts are correct size with variability reduction

Most parts are correct size with variability reduction

The bell-shaped curve shows that the number of items at the edge of the tolerance is now only 1%. Some companies have improved their manufacturing processes such that they get only 1 millionth of the parts in the outside ranges. This is often called six-sigma.

More reliable

When parts fit together better the assembly or machine works better and does not break down as often. If some parts are slight too large and fit together tightly, there is more wear or strain, and they do not last as long. If the parts are too small and fit together loosely, there is the chance of them rattling and breaking down after a short while.

The person credited with popularizing variability reduction, Taguchi, showed that any variation from the target costs a company money. Thus, the further a part is from the target size, the more it can cost a company down the road in required repairs due to failures.

Story of Ford and Mazda

A story of what happened between Ford and Mazda illustrates the value of the variability reduction methods:

A number of years ago Ford Motor Company bought an interest in the Japanese automobile company, Mazda. Some of the cars and trucks Ford sold were then made by Mazda but sold under the Ford name.

Shortly after the partnership started, quality officials at Ford Motor Company observed some usual statistics concerning warrantee repairs on the transmissions of their Ford pickup trucks. The found that transmissions made by their Ford plant had 7 times the breakdowns as those made by their Mazda transmission plant. Because of their part ownership in Mazda, they had some of the excess transmission manufacturing performed by the Japanese company.

The first thing the Ford officials thought was that Mazda had changed the design, but after comparing the transmissions from both facilities, they were the same.

Then, while measuring the parts on several Mazda-made transmissions, to make sure they were according to specification, the inspector noticed that every Mazda-made gear was exactly the same size. He thought something was wrong with his micrometer, so he tried another and still found the gears were the same size.

The blueprints allowed a tolerance of plus or minus a few millimeters, and while the American-made gears varied within the allowable tolerance, the Japanese-made gears were all right on the desired value with a negligible amount of variation.

It was then that the Ford engineers found out about the Japanese method of variability reduction. Ford now uses that method in all their manufacturing.


Variability reduction is a manufacturing method where a target part size is used as opposed to specifying a size with tolerances. It is also called the Taguchi method or approach. Specifying tolerances can result in tolerance buildup when parts are assembled.The Taguchi method focuses on the target size, with a probability curve drop-off. This method greatly reduces tolerance buildup and results in a more reliable product.

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