# Intermediate Program 2 - Segment 1

### Base Align, Symmetry Line, Secondary Alignment

### MMB and Datum Shift

You may have noticed that even though the drawing specifies Datum K and L at MMB, the position characteristic does not include those modifiers.

Calypso is limited in its ability to calculate datum shift from datums with the MMB modifier. In this rectangular pocket, the MMC/MMB option does not show up when Datum K and L are entered. Even when datums K and L are created as a 3D "Symmetry Plane" feature of size, there is no option to use MMB. (Since Datum B plane is the spatial rotation (levelling), it is acceptable to use line features for datums K and L; as well as the fact that the pocket is relatively shallow.)

In this situation, the amount of datum shift available can be calculated with formulas in Result Element characteristics.

MMB of Datum K: (getActual("34").actual-1.045)/2

MMB of Datum L: (getActual("33").actual-1.860)/2

**It is important to remember that MMB datum shift is not the same thing as MMC bonus tolerance. **You can’t just add the datum shift amount to the tolerance.
Datum shift means there is a little bit of “wiggle room” in the datum references, so the alignment can be shifted in favor of a better true position result. This reflects the relationship with mating parts.

Let’s say the 3 holes have position measurements as follows:

Hole 1 X deviation: -.001 Z deviation: +.005

Hole 2 X deviation: .002 Z deviation: +.003

Hole 3 X deviation: -.001 Z deviation: +.004

Datum K measured size = 1.051 Datum shift from Datum K = .003

We can shift the entire alignment -.003 in the Z direction, so now the results are:

Hole 1 X deviation: -.001 Z deviation: +.002

Hole 2 X deviation: .002 Z deviation: .000

Hole 3 X deviation: -.001 Z deviation: +.001

The same concept can be applied to the X direction using the datum shift from the Datum L width.

Contact steven@victoryprogramming.com if you have any questions about this course.