Here is a sample lesson from our GD&T Basics Fundamentals Course. We explain why it is much better to use a Position tolerance and Basic Dimensions over locating your feature with a coordinate dimension system.
Position is one of the most useful and most complex of all the symbols in GD&T. 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). 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.
Relative to Datum:Yes
MMC or LMC applicable: Yes (common)
GD&T Drawing Callout:
True center position of a hole (RFS w/ 2 Datums)
Position of a hole under MMC (3 Datums)
True Position is actually just referred to as Position in the ASME Standard. Many people refer to the symbol as “True” Position, although this would be slightly incorrect. The Position tolerance is the GD&T symbol and tolerance of location. The True Position is the exact coordinate, or location defined by basic dimensions or other means that represents the nominal value. In other words the GD&T “Position” Tolerance is how far your features location can vary from its “True Position”.
Although incorrect, we title this page and sometimes refer to the symbol as True Position since this is typically the term people are referencing when they are looking for the specific tolerance. However, if you want to be correct to the ASME standard, just use the term “Position”.
Position is defined as the total permissible variation that a feature can have from its “true” position. Depending on how it is called out, true position can mean several different things. It can be used with Max Material Condition(MMC), Least Material Condition (LMC), projected tolerances, and tangent planes. It may apply to everything from points to axes to planes to entire features. In these examples we will use holes, since these are the most common types of features controlled by true position. Keep in mind though that true position can be used on any feature.
Position is probably the most widely used symbol in GD&T. 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!
True Position –Location of a Feature
Position in terms of the axis, point or plane defines how much variation a feature can have from a specified exact true location. The tolerance is a 2 or 3-Dimensional tolerance zone that surrounds the true location where a feature must lie. Usually when specifying true position, a datum is referenced with x and y coordinates that are basic dimensions (do not have tolerances). 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. The location is most often positioned with two or three datums to exactly locate the reference position. The true position is usually called out as a diameter to represent a circular or cylindrical tolerance zone. (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)
Position used with Maximum Material Condition becomes a very useful control. True position with a features of size can control the location, orientation and the size of the feature all at once. MMC true position is helpful for creating functional gauges that can be used to quickly insert into the part see if everything is within spec. While true position on its own controls where the reference point locations need to lie, true position in MMC for a hole sets a minimum size and positional location of the hole to maintain functional control. It does this by allowing a bonus tolerance to be added to the part. As a part gets closer to the MMC, the constraints become tighter and the hole must be closer to its position. But, if the hole is a bit larger (but still in spec), it can stray from its true position further and still allow proper function (like a bolt passing though).
GD&T Tolerance Zone:
True Position –Location of a feature
A 2 dimensional cylindrical zone or, more commonly a 3-Dimensional cylinder, centered at the true position location referenced by the datums.
The cylindrical tolerance zone would extend though the thickness of the part if this is a hole. For the 3-dimensional tolerance zone existing in a hole, the entire hole’s axis would need to be located within this cylinder.
True Position using modifiers (MMC/LMC)
The tolerance zone is the same as above except only applied in a 3D condition. A 3-Dimensional cylinder, centered at the true position location referenced by the datum surfaces. The cylindrical tolerance zone would extend though the thickness of the part if this is a through hole for the 3-dimensional tolerance zone similar to the RFS version. While this is the tolerance zone, the call-out now references the virtual condition of the entire part. This means that the hole’s position and size are controlled together as one. (see gauging section)
Gauging / Measurement:
True Position –Location of a Feature
True position of a feature is made by first determining the current referenced point and then comparing that to any datum surfaces to determine how far off this true center the feature is. It is simplified like a dimensional tolerance but can be applied to a diameter tolerance zone instead of simple X-Y coordinates. This is done on a CMM or other measurement devices.
True Position Using material modifiers (MMC only)
When a part is checked for true position under a feature of size specification, usually a functional gauge is used to ensure that the entire feature envelope is within specification. If you have a specification for Maximum Material Condition, the desired state is that a hole will not be too small, or a pin not too large. The following formulas are used to create a gauge for true position under MMC.*
Gauging of an Internal Feature
For the true position under MMC of a hole:
Gauge Ø (pin gauge)=Min Ø of hole (MMC)-True Position Tolerance
Gauging of an External Feature
For true position under MMC of a pin:
Gauge Ø (hole gauge) = Max Ø of pin (MMC) + True Position Tolerance
Locations of the gauge pins or holes are given on the drawing as basic dimensions. All gauge features should be located in the datum true positions, but sized according to the formulas above.
Note on Bonus Tolerance:
When a functional gauge is used for True Position, any difference the actual feature size is from the maximum material condition would be a bonus tolerance. The bonus tolerance for position then increases as the part gets closer to LMC. The goal of a maximum material condition callout is to ensure that when the part is in its worst tolerances, the True Position and size of the hole/pin will always assemble together. For instance, if you had a large hole size but was still in tolerance (closer to LMC), you make more bonus tolerance for yourself making the true position tolerance larger. You can now have the hole center more out of position due to the bonus tolerance.
Bonus Tolerance = Difference between MMC & Actual condition.
Confused? No worries! For more detail on how bonus tolerances play into these callouts, see our sections on Maximum Material Condition. Or check out our GD&T Course, where we go into deep detail on the position symbol!
Relation to Other GD&T Symbols:
True Position –Location of a feature
True position is closely related to symmetry and concentricity as they both require the location of features to be controlled. However, True position is more versatile since it can be called on a feature of size or combined with other geometric tolerances to specify an entire part envelope.
True Position using features of size (MMC/LMC)
True position with used of MMC or LMC is related to axis perpendicularity when used on a hole or pin. The tolerance of both perpendicularity and true position now refers to the uniformity and cylindrical envelope of a central axis. However with true position you can make the tolerance referenced to several datum’s as opposed to just one with axis perpendicularity. When you callout true position using datums on the face, and sides of the part – perpendicularity is controlled as well. See example 2 for more details.
True Position –Location of a feature
In example 1 you can see how a hole can be called out using true position. However this can also be applied to anything in need of a location tolerance, such as a pin, a boss or even an edge of a part. When you have a hole in a part such as a bolted surface, true position is usually called out. It can be used almost anywhere to represent any feature of size.
True Position using material condition (MMC/LMC)
True position of a feature of size under MMC is used when a functional gauge is ideal for checking the part. True position is also useful for describing and controlling a bolt pattern for a pipe fitting or a bolted fixture. If you specify the control using MMC, it allows you to have a pin gauge that you can insert into the part to see if the bolt pattern is functionally accurate. You will see true position called with MMC very commonly in bolt patterns where relative location of all the bolts and necessary clearance is critical. LMC with true position is a little less common, but often used when minimum wall thickness is desired.
True Position –Location of Hole Example 1:
Four holes are to be located on a block to ensure contact is always maintained and located within a specific position. The holes need to line up with the threaded connections in the mating part.
The True position callout on a block
With true position called out the holes do not need to be in exact positions as shown below, but their centers can vary by the amount specified by the tolerance. The basic dimensions (dimensions in the squares) are un-toleranced and describe the true location the hole would be in if it was perfect. In a 2D check of the upper right hole, the true location would be 40 mm from datum A and 40 mm from datum B. The holes center is calculated, usually by a CMM and compared to the true location. As long as the holes center is in the blue tolerance zone of 0.2 mm specified by the feature control frame, the part is in tolerance.
Note: in this case, the surface of the part is called out (Datum C). This means the entire hole must have its axis align with the datum. The tolerance zone would actually ensure that the location and the perpendicularity is within the specified tolerance. Since all the central points at any cross section are controlled by true position, the parts axis (line between all central points) would be controlled for orientation.
The biggest thing to note about this design is that no matter what size hole you have, your true position would always have to be the same. This is ideal for when proper exact alignment is required for function of the part. It does, however, remove the possibility of using a functional gauge.
True Position – Hole size and location using MMC Example 2:
Taking the same example, the true position can also be specified with a maximum material condition callout. This means you are now controlling the envelope of the entire hole feature, including the size of the hole throughout its entire depth.
Adding the little “M” makes a big difference.
With an MMC callout you now can use a functional gauge to measure this part, to determine that the size and geometric tolerancing are within spec at the same time.
Formula for a the functional gauge to measure the true position of all holes:
Individual Pin Diameters = Min hole Ø -True position tolerance (bonus)
This example Pin Ø = 9.9 – 0.2 = Ø 9.7
Location of pins: Same specifications
This would be the go gauge that would measure for hole size, orientation and position. The part would be pressed down onto the gauge and if it fits the part is in specification. Notice that datum A, B and C are all included in the gauge to check the location of the hole. The desired function of the part is met by ensuring that the part touches all the datums, and that the gauge pins are able to fully go through the holes.
Top view of the part once inserted into pin gauge
As long as the gauge can go into the part, it is in spec. This makes it very easy to accurately gauge the part right on a production line. The function of the part is confirmed because as long as the surface that the part is bolted to has the same tolerances, it will always fit.
Remember the further you are from MMC when it is referenced in the feature control frame, the more bonus tolerance you are allowed. For a hole, the larger the diameter, (closer to the LMC) the more bonus tolerance you have for your true position.
Bonus tolerance = true position tolerance (measured hole size – MMC hole size)
Note: Keep in mind the opposite is true for a positive feature like a pin, where the smaller the pin means you have more bonus tolerance.
Called with or without the Ø symbol
There are two ways true position can be called out – either as a distance, in X and Y or most commonly as a diameter. When true position is called out as a distance, you are permitted to move from the tolerance in X or Y direction by the allowed tolerance. However when done this way, the tolerance zone actually forms a square. This is usually undesirable since in the corners of the square are further from the center than the sides. This also removed over 57% of your tolerance zone! Most commonly, true position with reference to location is called with the diameter (Ø) symbol to be called as a cylindrical or circular tolerance zone.
Another common way true position can be called out is with slotted features. If you have a slot in your part that must always be located correctly, you can use true position to ensure that each of the planes that make up the slot are always located in the correct position. Symmetry can also be used in this case – but only if the slots have a referenced datum plane that they are symmetrical about (and measuring symmetry is very difficult!).