0055// Calculate MMB for Primary, Secondary and Tertiary Datum

Recently, I conducted various knowledge sharing to my intern student and some engineer regarding how to interpret datum from drawing. However, I realize that most of them don't understand the differences between MMB for primary, secondary and tertiary datum.

Therefore, this week, my post will share one of the example how to calculate the correct size of the MMB.

Case 1: Primary Datum for Datum D

Note: General Tolerance = +/- 0.2

The drawing above shown a through hole in a shaft with 11mm diameter and positional control with cylindrical tolerance zone and constrain by datum D with MMB.

In this drawing, datum D is primary datum. Therefore, the MMB for the feature D is

MMC diameter 15.2.

Tips: Assume datum A, B and C transparent...

Diagram:

Case 2: Secondary Datum for Datum D

Note: General Tolerance = +/- 0.2

The drawing above shown a through hole in a shaft with 11mm diameter and positional control with cylindrical tolerance zone and constrain by primary datum A and secondary datum D with MMB.

In this drawing, datum D is secondary datum. Calculation need to consider collective effect from datum A because of datum precedence. Therefore, MMB for datum is

MMC (diameter 15.2) + perpendicularity tolerance (diameter 0.05) = diameter 15.25

Tips: Assume datum B and C transparent...

Diagram:

Case 3: Tertiary Datum for Datum D

Note: General Tolerance = +/- 0.2

The drawing above shown a through hole in a shaft with 11mm diameter and positional control with cylindrical tolerance zone and constrain by primary datum A and secondary datum B with MMB and tertiary datum D with MMB.

In this drawing, datum D is tertiary datum. Calculation need to consider collective effect from datum A and datum B with MMB because of datum precedence. Therefore, MMB for datum is

MMC (diameter 15.2) + positional tolerance (diameter 0.1) = diameter 15.3.

Since perpendicularity here is a refinement, no need to add in the MMB calculation.

Tips: Consider collective effect from datum A and B and treat datum C transparent...

Diagram:

Hope this post can help you understand the concept.

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