The true position is the actual coordinate or place representing the nominal value, as specified by fundamental dimensions or other means. In other words, the GD&T Position tolerance determines how much your feature's placement can differ from its true position.

The true position is usually set in the center of the tolerated feature. The correct position of a hole, for example, is established at the axis of the hole. Based on the feature properties, 2D or 3D diameter is defined as the tolerance zone around it. True Position is an ideal, theoretical area commonly expressed by basic dimensions.

What Is Engineering Tolerance?

The allowable variance in measurements derived from the base measurement is engineering tolerance. Tolerances might apply to a wide range of units. For example, the working conditions may include temperature (° C), humidity (g/m3), and other limits. 

Tolerances for linear, angular, and different physical dimensions are commonly analyzed in mechanical engineering. Regardless of the unit, a tolerance specifies an allowed measurement range from the base point (nominal value).

True Position – Feature Location

The amount of variation a feature can have from a stated actual location is defined by its position in the axis, point, or plane. A tolerance zone is a two- or three-dimensional area that surrounds the true position of a feature. 

A datum is usually referenced using x and y coordinates, which are basic dimensions when expressing true position (they do not have tolerances). This means you'll know exactly where the position should be, and your tolerance will tell you how much off you can get. 

The location is usually positioned using two or three data points to pinpoint the reference position. The true position is commonly expressed as a diameter to describe a circular or cylindrical tolerance zone.

Material conditions (MMC/LMC) with True Position

When combined with Maximum Material Condition, the position becomes a very valuable control. True position with a size feature can control the feature's location, orientation, and size all at once. MMC's true position helps make functioning gauges that can be rapidly inserted into the part to check for compliance.

True position in MMC for a hole sets a minimum size and positional location of the hole to retain functional control. In contrast, true position on its own regulates where the reference point locations must lie. It accomplishes this by permitting the addition of an extra tolerance to the part. 

As a part approaches the MMC, the limitations tighten, and the hole must be closer to its original location. However, if the hole is somewhat larger (but still within specification), it can stray further from its true position while still functioning correctly (like a bolt passing, though).

True Position Using Location of a Feature

A two-dimensional cylindrical zone or, more typically, a three-dimensional cylinder, centered on the true position location specified by the datums.

If this is a hole, the cylindrical tolerance zone will continue across the thickness of the part. The entire hole's axis would have to be contained within this cylinder for the 3-dimensional tolerance zone to exist in a hole.

True Position (MMC/LMC) With Modifiers

The tolerance zone is the same as before; however, it is only used in 3D. A three-dimensional cylinder centered on the datum surfaces' true position location. If this were a through hole for the 3-dimensional tolerance zone, the cylindrical tolerance zone would continue through the thickness of the part, identical to the RFS version. 

While this is still the tolerance location, the call-out now refers to the full part's virtual state. This means that the hole's size and position are both regulated simultaneously. 

Everything that can be measured, from linear measurements to weight and material hardness, will always differ from one portion to the next. Components may not fit together or give too much slack if the design does not accommodate for variance, leading to early failure.

SCTools gives you the ability to verify the real position accuracy of all your industrial tools. Using a true position calculator can be used to determine the center axis based on measurements of a manufactured feature such as a hole or shaft.

If you have any questions about carbide cutting tools, end mills, drills, etc. be sure to reach out to us @ sctools.co/Home or call us at (877)737-0987. We help you machine better!