3.5 Dimensions, Tolerance, and Allowance
D.M. Donner
As a machinist, you will spend the majority of your day measuring, calculating, and verifying dimensions from a print. Using the proper equipment to obtain measurements and then comparing them to the engineer’s drawing is an important part of what we do. Here we will focus on obtaining dimensions from a print and determining what the tolerance is for the measured feature.
A tolerance is the accepted amount of variation between the measured dimensions of a part and the nominal dimensions from the print. The center area of a dart board is called a bull’s eye. Any dart that lands within the bull’s eye will score points. Think of tolerance as a bull’s eye that is allowed to change size. If a part tolerance can vary by a greater amount, the bull’s eye can be larger, and any measurement that falls within that tolerance is acceptable. If the engineer requires a tighter tolerance for a dimension, the bull’s eye will be smaller, allowing less variance; however, any dimension that falls inside the tolerance is acceptable.
You may hear tolerance and allowance used interchangeably, but in fact, they are different. An allowance is the engineered difference from nominal, which allows parts to function as necessary. If a slip fit needs to exist between a shaft and a hole, the engineer will use an allowance of .01″ between the two part dimensions. In order to ensure the allowance is met, the engineer may allow a .005″ tolerance to the nominal dimensions.
A nominal dimension refers to the standard or intended size, value, or dimension of a component or part without considering variations or deviations that may occur due to manufacturing processes or other factors. It represents the theoretical or target value used as a reference point for design, analysis, and communication in technical specifications.
In simpler terms, the nominal value is the ideal or planned measurement, size, or characteristic that a part or feature is expected to have according to the design or specification. However, due to inherent variability in manufacturing processes and other factors, the actual measured values may deviate slightly from this nominal value. These deviations are taken into account using tolerances and allowances to ensure that the manufactured part is still within an acceptable range of functionality and performance.
All manufactured parts have inherent variations caused by many factors, such as work holding, tool holding, tool wear, human interaction with the system, temperature, and stock variations, just to name a few. There is no such thing as the perfect part. It is vital that you understand this principle because many hours can be wasted chasing this impossible standard. The machinist does not operate in the perfect world, but we do exist in the realm of allowed variation. The engineers who design the parts we make also understand this principle, and they design this variation into the part.
Overall Dimensions (O