Posted May 27, 2021

What's the Difference Between Unilateral and Bilateral Tolerance?

Machined parts that require careful bilateral or unilateral tolerancing

To locate features such as holes when machining a part, engineers and manufacturers depend on geometric dimensioning and tolerancing (GD&T) for precision. These features aren’t the only aspect of the project that requires tolerancing though, the surface of the part also needs to be precisely fabricated. To specify surface tolerances in GD&T, the engineer can choose between either bilateral or unilateral tolerancing depending on what is required. This article explores the difference between unilateral and bilateral tolerances and how unequal disposed tolerances fit in to assist in making the best choice between them.

Explaining the Difference Between Unilateral and Bilateral Tolerance

Geometric dimensioning and tolerancing is a system that allows designers to communicate precise dimensions and tolerances throughout the manufacturing chain, from engineering through inspection. Using a symbol-based language, GD&T will call out specific tolerances for locations, orientations, and profiles. For example, to designate a tolerance for the surface of a part, the designer will use a surface profile symbol for its geometric characteristic in the control frame for that dimension. You can see an example of the surface profile symbol on the left side of the control frame illustrated in the picture below.

A radius of 1.8 defines the surface profile for this example and represents the ideal or “true profile” of the part’s surface. For manufacturability, one should define tolerances, allowing for a slight deviation from the true profile during fabrication. The example shows that the tolerance of .04 is designated in the control frame immediately to the right of the surface profile symbol. To pass inspection, the surface profile dimensions of the manufactured part must fall within the designated tolerance zone. However, the tolerance zone’s position in its relationship to the true profile of the surface can vary in “unilateral and bilateral tolerances.” The following section looks at these, starting with bilateral tolerances.

Bilateral Tolerance

Ignoring the control frame in the picture below for a moment, imagine the part’s radius of 1.8 has a fixed tolerance of +/- .02, allowing for cutting the radius to .02 greater or less than the dimensioned true profile of the part. The surface profile symbol in the control frame would follow with the tolerance value, which is .04, to call this radius out as a bilateral tolerance instead, thus positioning the tolerance so it’s distributed equally on either side of the true profile line. However, the control frame in the picture below has more details than just the tolerance, as explained below.

Unilateral Tolerance

In contrast to bilateral, a unilateral tolerance will allow the deviation to extend from one side of the true profile line or the other. A unilateral tolerance is a type of unequally disposed tolerance and is designated with the letter “U” in a circle in the control frame. Beside that is the amount of tolerance that determines which side of the true profile line to which the zone will extend. This value will equal zero or the total amount of the tolerance:

  • Using a zero will designate a tolerance zone that starts at the true profile line and extends into the part, allowing additional material removal. This is equivalent to a + .04, - 0 fixed tolerance. 
  • Using the entire value of the tolerance instead (.04 in our example) will designate a tolerance zone that extends outwards from the true profile line, allowing additional material retention. This is equivalent to a + 0, -.04 fixed tolerance.

The following looks at the remaining tolerance (briefly mentioned) and the unequally disposed tolerance zone

A drawing illustrating the use of an unequal unilateral tolerance

An example of an unequal unilateral tolerance called out in GD&T

What is an Unequal Disposed (Unilateral) Tolerance?

As evidenced, a bilateral tolerance is where the tolerance zone equally distributes on either side of the true profile line. And a unilateral tolerance is where the zone positions on one side of the line or the other. There are times, where you will want the tolerance zone to be distributed unequally on the true profile line. Such instances—known as an unequally disposed tolerance and an extension of a unilateral tolerance zone—are designated with the same methodology. We already know that a value of zero to the right of the U symbol extends a unilateral zone past the true profile line into the part, while the full value of the tolerance extends the zone in the other direction. If, however, one uses a partial value to the right of the U symbol, the tolerance zone will position unequally around the true profile line.

The picture above shows how the unilateral tolerance zone has ended up unequal by using a partial tolerance value in the control frame. With a value of .025, the unequal unilateral tolerance equals more than half of the total tolerance value of .04, giving the surface profile a tolerance zone that extends almost 2/3s outward of the part’s exterior. This unequal distribution of the tolerance zone allows retaining more material from the true profile line than what’s allowable during machining.

Working with a Machine Shop to Use the Correct Tolerancing

Using geometric dimensioning and tolerancing to specify the shape and contours of parts is complex but essential to ensure the greatest accuracy in the final product. Fortunately, machine shops already understand those needs and work with and apply the given dimensions correctly. They are also a great source of information when refining the dimensioning for newly designed or redesigned parts.

At Plethora, we specialize in manufacturing precision parts using the industry's most advanced CNC machining equipment and software. To ensure that your project is built precisely to specifications, our staff is thoroughly trained and experienced in all types of mechanical drawings, dimensioning and tolerancing, and CAD data. We are ISO 9001 certified, supporting our primary goal of manufacturing your parts to the highest level of quality. Our online DFM and quoting systems are ready to receive your data so we can begin working with you on your next project. To get started, upload your design files to Quote My Part or call us at 415-726-2256.
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Topics: materials, processes, Manufacturing, finishing, CNC machining, Quality, Prototyping, threading