Additional Help for True Position Dimensions

Introduction

This document has been written as an additional resource for answering common questions that arise with true position dimensions. It should be used after understanding the True Position section of the Dimension Options chapter in the PC-DMIS help file, because questions of basic use of these dimensions are explained there. However, PC-DMIS V3.2 included some new enhancements to true position dimensions and several questions have been presented by users. This document will address some of these questions.

Datums

PC-DMIS V3.2 introduced the ability to select multiple datums. One advantage of this is that the user can set the datum(s) directly with the dimension, instead of having to create an alignment with a circular feature to be used in the DD axis. Another advantage is that by arranging the order of the datums, the user can control the directions that are used for the X, Y, and Z axes. A third advantage of the ability to select multiple datums, and the main reason for the new enhancement to PC-DMIS, is that more than one datum can be a circular feature and have MMC or LMC defined according to the ASME Y14.5M 1994 Dimensioning and Tolerancing standard. However, with these advantages comes the responsibility to correctly choose the order of the datums. In some cases, changing the order of some of the datums can result in an unexpected measured, deviation, or bonus tolerance value.

Types of Features Used as Datums

One common scenario for designs that utilize True Position dimensions is to use a circle or cylinder as the sole datum feature. Another accepted practice is to select a set of datum features that follow 3-2-1 alignment principles. (Remember, the minimum definition for a datum is 3 datum points to describe the first datum, 2 datum points to describe the second datum and 1 datum point to describe the third datum.) This means that the selected features would be a plane, a line, and then a single point. However, any of the datums could have more points than the minimum. This means that 3-2-1 alignment principles can also be used with plane/line/line, plane/line/circle, plane/plane/plane, plane/cylinder/cylinder, and many other combinations.

Order of Features Used as Datums

A combination of datums that is important to discuss here is the plane/circle/ circle because the order of the circles is important in order establish the correct datum reference frame, which is essentially an alignment for the particular dimension.

Figure 1 Example part.

Figure 1 shows an example part from which the following dimensions LOC1 and LOC2 were created:

DIM LOC1= TRUE POSITION OF CIRCLE CIR3 UNITS=IN ,$

GRAPH=OFF TEXT=OFF MULT=1.00 OUTPUT=BOTH DEV PERPEN CENTERLINE=OFF DISPLAY=DIAMETER

AX NOMINAL +TOL -TOL BONUS MEAS DEV DEVANG

X 8.0080 0.0000

Y 1.0000 0.0000

DF 1.0000 0.0100 1.0000 0.0000 ----#----

D1  PLANE PLN1 AT RFS

D2 1.0000 0.0100 1.0000 CIRCLE CIR1 AT MMC

D3 1.0000 0.0100 1.0000 CIRCLE CIR2 AT MMC

TP  MMC 0.0200 0.0000 -156.3706 #--------

END OF DIMENSION LOC1

DIM LOC2= TRUE POSITION OF CIRCLE CIR3 UNITS=IN ,$

GRAPH=OFF TEXT=OFF MULT=1.00 OUTPUT=BOTH DEV PERPEN CENTERLINE=OFF DISPLAY=DIAMETER

AX NOMINAL +TOL -TOL BONUS MEAS DEV DEVANG

X 8.0080 0.0000

Y 1.0000 0.0000

DF 1.0000 0.0100 1.0000 0.0000 ----#----

D1  PLANE PLN1 AT RFS

D2 1.0000 0.0100 1.0000 CIRCLE CIR2 AT MMC

D3 1.0000 0.0100 1.0000 CIRCLE CIR1 AT MMC

TP  MMC 0.0200 0.0100 0.0000 #--------

END OF DIMENSION LOC2

Notice that the X, Y, and Z axes are reported with respect to the current alignment. However, it is important to note that the deviations of the position of the feature are calculated first with respect to the datum reference frame (DRF), but then are converted to the current alignment, in order to read them easier. It has been suggested that most customers prefer interpreting their results when the X, Y, and Z measured and nominal values report in the current alignment, and not in the internal alignment of the DRF, which can be difficult to visualize. However, it is important to visualize how the internal DRF is oriented.

The effect of the ordering of the datums is crucial, as the X, Y, and Z axes give different results. However, if the order of the datums is switched to PLN1, CIR2, and CIR1, the Z axis is still the same as the PLN1 normal vector, but the X axis now is in the direction of the line between the centers CIR2 and CIR1, with the origin at CIR2. This is shown in Figure 3.

Notice that the order of the datums for D2 and D3 have been reversed for the two dimensions, as displayed in Table 1.

Table 1 Datum reference frames for LOC1 and LOC2.

Figure 2 LOC1 Datum Reference Frame

Figure 2 shows that by selecting the datums in the order of PLN1, CIR1, and CIR2, the Z axis is the same as the PLN1 normal vector, and the X axis used in the dimension is in the direction of the line between the centers of CIR1 and CIR2. The origin of the reference frame is positioned at CIR1. Then, the true position of circle CIR3 is measured from this reference frame.

Figure 3 LOC2 Datum Reference Frame

In summary, there are some datum sets where the X axis of the datum reference frame must be calculated between two features, as opposed to directly taking the vector from a line or a slot. Care must be taken in order to select the appropriate sequence of datums.

Bonus Tolerance

Whenever a circular feature is used in a true position dimension, whether as the main feature or a datum, the Maximum Material Condition (MMC) or Least Material Condition (LMC), or Regardless of Feature Size (RFS) can be selected. If the circular feature is a hole, MMC is the smallest hole allowed by the diameter tolerance, while LMC is the largest hole allowed. If the circular feature is a pin, MMC is the largest pin allowed by the diameter tolerance, and LMC is the smallest pin allowed. For most cases, the MMC is the appropriate material condition to use. See the Dimensioning and Tolerancing standard ASME Y14.5M 1994 for special cases where LMC is the preferred method.

Figure 4 shows the increase in positional tolerance (bonus tolerance) due to deviation from the MMC for a hole, as interpreted from the ASME standard.

Figure 4 Increase in Positional Tolerance where Hole is not at MMC.

One Circular Datum

Versions of PC-DMIS prior to V3.2 only had the ability to use one circular datum. As Figure 5 shows, in the case where there is only one circular datum the total bonus tolerance for the features TP axis is a sum of the increase in individual bonus tolerance of the feature (DF axis) and the datum (D1 axis). In other words, the feature is allowed to move its position even more than the initial TP plus tolerance by the total amount that the feature and datum deviate from their MMC conditions, as long as their diameters are within their own diameter tolerances.

Figure 5 Increase in Positional Tolerance for Both the Feature and the Datum.

Multiple Circular Datums

However, a confusion arises from the case that is illustrated in Figure 6, where there are two circular datums (actually there are three datums the top plane, and then the two circles, but for our purposes here the plane isnt illustrated). In this case, it is NOT correct to assume that the total bonus tolerance of the feature is the sum of the individual bonus tolerances of the feature and the two datums. The reason is that while the bonus tolerance from datum 1 allows the feature to move in one direction, the bonus tolerance from datum 2 allows the feature to move in another direction, and additionally the bonus tolerance from the feature itself allows for movement of the position in any direction. The restrictions from these datums depend upon the distances from the feature to the datums, as well as the directions to those datums. If the bonus tolerance from datum 1 is smaller than the bonus tolerance from datum 2, the features position may not be able to move much in the Y direction, even though there is enough bonus tolerance there.

Figure 6 Increase in Positional Tolerance for the Feature and Two Circular Datums

In order to calculate the correct bonus tolerance for the entire dimension (TP axis), PC-DMIS creates an initial guess of the coordinate system or datum reference frame. The initial guess is from the 3-2-1 alignment system created from the three datums. Then the reference frame is moved using 2D or 3D best fit methods as it is adjusted within the degrees of freedom and within the allowable positional tolerance. Because of this, when you change the plus tolerance of the TP axis, the axes will be recalculated and the measured and deviation values may differ slightly than those of a true position dimension that only differs by the plus tolerance of the TP axis. This best-fit method calculates the position of the datum reference frame that provides the largest tolerance allowed for the dimension (both from the total bonus tolerance and the TP plus tolerance).

Composite Positional Tolerance

Figure 7 shows a composite positional tolerance of a pattern of holes. This is called composite positional tolerance because there are two separate tolerance values that are interrelated. This type of tolerance is used to locate the entire feature pattern as well as define the position and orientation of each of the features in the pattern set. For specific interpretations of the tolerance zones please refer to Section 5.4 of the Dimensions and Tolerances standard, ASME Y14.5M 1994. However, one important note here is that the top part of the composite position dimension governs the individual holes, while the bottom part of the dimension is applied to the entire pattern.

In PC-DMIS V3.2, true position dimensions can be defined to allow for these types of dimensions. To do this, first create a true position dimension for each of the four small circles, making sure you select the appropriate datums, either the planes A, B, and C, or features created from these planes that represent a 3-2-1 alignment. The TP plus tolerance for this example would be the 0.8, as defined on the top part of the composite position dimension. Then, construct a feature set with the four small circles as inputs. Next, construct another true position dimension using the feature set. The TP plus tolerance for this dimension would be 0.25.

This method results in five true position dimensions for this case: one for the position of each of the four circles, and one that relates the individual circles to each other and defines their position. If any one of these five dimensions is not acceptable the entire pattern is unacceptable.