(ASME Y14.5-2009, ISO 1101:2012)
Our monthly tip following Benny’s suggestion – how do datums affect the control of position with regards to groups of holes?

Figure 1 shows position control for the four holes in the part.

It is easy to treat position control as a dartboard; for each controlled element (the axis of each of the four holes), the control specifies:

•     What is the size of the target area where the axis of each of the four holes needs to be?
  (a 0.3-diameter cylinder at MMC)
•     In what orientation in relation to the datums that have been called out (90° and 180° are treated as BASIC angles)?
•     In what location in relation to the datums that have been called out (requires BASIC dimensions)?

Geometric tolerances illustrate functional interfaces of the part and thus enable functional testing of the item, even when the rest of the parts are not available!
Figure 2 shows an implementation of a control gage for the position of the holes in the part.

The part (colored yellow) needs to be attached to the simulator surface of Datum A, then to B and finally to C.

The M modifier of the position tolerance makes the size of the tolerance zone adaptable. In MMC mode, the tolerance is a 0.3-diameter cylinder. Control gages in the Virtual Condition dimension of the holes (4.5-0.3=4.2) must be inserted into the four holes simultaneously. The pins are positioned according to the BASIC dimensions in the drawing, relative to each other and to the datum reference frame (perpendicular to A and at a distance of 6 mm from B and C respectively).

The following illustrations show the possible effect of changing the datums in the Feature Control Frame. In each figure, the holes’ position requirement is presented, and the three figures show parts that meet the requirement presented (in each one) for controlling the position of the holes:

Orientation and position are maintained within the group only!!!

In summary:

• Controlling the position of a group of features, controls, first and foremost, the relative position and orientation among the features in the group.
• Controlling the position of a group is the exception in which there is significance to a position tolerance without datums.
• Each added datum applies orientation and position requirements to the group in relation to it.
• The selection of datums in FCF represents a functional constraint!!!

Questions? Ideas? Suggestions?
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