An example of this can be seen in Figure 3. We explain why it is much better to use a Position tolerance and Basic Dimensions over locating your feature with a coordinate dimension system.
The drawing has both symmetry and parallelism tolerances controlling the key slot orientation. This is because manufacturing a slot is simply more complex than drilling a hole in relatively thick parts. The basic dimensions dimensions in the squares are un-toleranced and describe the true location the hole would be in if it was perfect.
Although 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. American Society of Mechanical Engineers.
The following formulas are used to create a gauge for true position under MMC. All opinions are welcome!
For that matter, it is a linear spec. Same specifications This would be the go gauge that would measure for hole size, orientation and position.
The holes need to line up with the threaded connections in the mating part. The position of these holes is defined using basic dimensions with respect to the datum reference frame ABC. For more detail on how bonus tolerances play into these callouts, see our sections on Maximum Material Condition.
You can now have the hole center more out of position due to the bonus tolerance. 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 tolerance zone would actually ensure that the location and the perpendicularity is within the specified tolerance. Depending on how it is called out, true position can mean several different things. The tolerance is a 2 or 3-Dimensional tolerance zone that surrounds the true location where a feature must lie.
slot call out
The symmetry requirement states that the centerplane of the key slot blue, derived from slot walls has to locate inside the given tolerance zone. 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.
Hole and Slot as Datum Features -- Part I
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. 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.