Tolerance Basics
This term refers to the total amount a specific dimension is allowed to vary.
What is Tolerance?
This common format permits variation in both directions from the specified dimension, usually shown with a plus/minus symbol.
What is Bilateral Tolerance?
This term describes the minimum clearance or maximum interference between two mating parts.
What is Allowance?
This is the common name for the bell-shaped curve that represents a normal distribution of data.
What is the Bell Curve?
This value is a measure of the spread or variation of a set of data from its mean
What is Standard Deviation?
To calculate the tolerance of a part, you find the difference between these two values.
What are the Upper and Lower Limits?
This format provides an allowable variation in only one direction (either larger or smaller).
What is Unilateral Tolerance?
In this type of fit, there is always a space between the two mating parts.
What is a Clearance Fit?
In a normal distribution, most of the data points cluster around this central value.
What is the Mean (or Average)?
If a set of manufactured parts are very consistent and close to the target size, the standard deviation will be this (high or low).
What is Low?
This is the condition of a part when it contains the largest amount of material (e.g., the largest pin or the smallest hole).
What is Maximum Material Condition (MMC)?
This method provides the maximum and minimum allowed values directly on the drawing, rather than using a plus/minus symbol.
What are Limit Dimensions?
In this type of fit, the internal part is actually larger than the hole it is meant to fit into.
What is an Interference Fit?
According to the empirical rule, this percentage of data falls within exactly one standard deviation of the mean.
What is 68%?
This spreadsheet software is commonly used by engineers to automate the calculation of standard deviation for large sets of data.
What is Microsoft Excel or Google Sheets?
This is the condition of a part when it contains the smallest amount of material (e.g., the smallest pin or the largest hole).
What is Least Material Condition (LMC)?
Given a dimension of 2.000 +/- .005, these are the specific Upper and Lower limits.
What are 2.005 and 1.995?
To calculate allowance, you subtract the MMC of the external feature (the shaft) from the MMC of this feature
What is the Internal Feature (or the Hole)?
This percentage of data falls within two standard deviations of the mean.
What is 95%?
When engineers only have a small subset of data rather than the entire population, they use this specific version of the standard deviation formula.
What is Sample Standard Deviation?
Engineers must balance the need for high precision with this specific negative factor, which increases as tolerances get tighter.
What is High Cost?
If a hole is dimensioned as 0.500 with a unilateral tolerance of +.004 / -.000, this is the total tolerance (the range) of the part.
What is 0.004?
If the MMC of a hole is 9.85 mm and the MMC of a shaft is 10.00mm, what is the allowance, and what type of fit does it represent?
What is -0.15mm and an Interference Fit?
Known as the "Three-Sigma Rule," this specific percentage of all data falls within three standard deviations of the mean.
What is 99.7%?
In the formula for Sample Standard Deviation, the "sum of squared differences" is divided by this value (represented by n-1).
What is the Sample Size minus one?