True position with a feature of size can control the location, orientation and the size of the feature all at once. Position used with Maximum Material Condition becomes a very useful control. (However, it can also be called out as a distance for X and Y coordinates as well – see final notes) True Position using material conditions (MMC/LMC) The true position is usually called out as a diameter to represent a circular or cylindrical tolerance zone. The location is most often positioned with two or three datums to exactly locate the reference position. This means that you will have an exact point where the position should be and your tolerance specifies how far from this you can be. Usually, when specifying true position, a datum is referenced with x and y coordinates that are basic dimensions (do not have tolerances). The tolerance is a 2 or 3-Dimensional tolerance zone that surrounds the true location where a feature must lie. Position in terms of the axis, point or plane defines how much variation a feature can have from a specified exact true location. If you are looking for more information about Position or any of the other symbols, you should check out our GD&T Fundamentals Course. If you like the simplified approach to GD&T on this website and in the video above, be sure to contact us to learn more about the course! Position is probably the most widely used symbol in GD&T. Position can be used on any feature of size (but not on surfaces where we would use Profile).
In these examples, we will use holes, since these are the most common types of features controlled by true position. It may apply to any feature of size (Feature with physical dimensions like a hole, slot, boss or tab) and control the central elements of these size features. It can be used with Max Material Condition(MMC), Least Material Condition (LMC), projected tolerances, and tangent planes.
Depending on how it is called out, true position can mean several different things. Position is defined as the total permissible variation that a feature can have from its “true” position. However, if you want to be correct to the ASME standard, just use the term “Position”. In other words, the GD&T “Position” Tolerance is how far your feature’s location can vary from its “True Position”.Īlthough incorrect, we title this page and may sometimes refer to the symbol as True Position since this is typically the term people are referencing when they are looking for the specified tolerance. The True Position is the exact coordinate, or location defined by basic dimensions or other means that represents the nominal value. The Position tolerance is the GD&T symbol and tolerance of location.
Many people refer to the symbol as “True” Position, although this would be slightly incorrect. True Position is actually just referred to as Position in the ASME Standard. GD&T Symbol: Relative to Datum: Yes MMC or LMC applicable: Yes (common) GD&T Drawing Callout: True center position of a hole (RFS w/ 2 Datums) However, since this is such a useful symbol, we will continue to add content and examples for other uses of this nifty little symbol in the coming months. The two methods of using Position discussed on this page will be RFS or Regardless of Feature Size and under a material condition ( Maximum Material Condition or Least Material Condition). Position is one of the most useful and most complex of all the symbols in GD&T. We explain why it is much better to use a Position tolerance and Basic Dimensions over locating your feature with a coordinate dimension system. Here is a sample lesson from our GD&T Basics Fundamentals Course.