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Moveable Datum Target Question

Moveable Datum Target Question

Moveable Datum Target Question

Along with using the new moveable datum target symbol, figure 4-47 from 2009 added a 2X basic 45° and a basic 9 dimension when compared to figure 4-38 from the 1994 standard. Otherwise, these two figures are identical.

I have seen at least one GD&T author show their interpretation of a fixture that corresponds to the requirements of the datum targets from this common figure between 1994 & 2009. This fixture uses hemispherical tipped pins to simulate the “A” point targets (A1, A2, & A3). This fixture uses a knife edged V-block to simulate the “B” line targets (B1 & B2). The fixture uses a sliding block, which moves in-line with datum plane B (the apex of the v-block). The sliding block has hemispherical tipped pins to simulate the “C” point targets (C1 & C2). The “C” simulators are fixed to the sliding block so that the 2X basic 15 dimensions are maintained while the block moves in and out with respect to the apex of the v-block.

2009 section 4.24.6 states that, unless otherwise stated, moveable datum targets move normal to the true profile of the part. It also states that if the movement of the datum target simulator is not normal to the true profile then the direction of movement shall be clearly defined.

My question revolves around the basic 9 dimension from the figure in 2009. If the fixture described above is a correct interpretation of a fixture that would meet the requirements of the datum targets in figure 4-47 then what is the need for the basic 9 dimension? If the “C” simulators are angled at 45° to datum plane B, and are fixed at basic 15 from datum plane B, and the sliding block is moved toward the part until the “C” simulators make physical contact with the part, then the basic 9 will be almost impossible to achieve.

Or, is the fixture described above incorrect? Should the “C” target simulators, oriented 45° from datum plane B, move into contact with the part along a path that is normal to the true profile and passes through a point that is 15 from datum plane B and 149 (40+100+9) from the apex of the V-block?

Asked another way, is Figure 4-47 from 2009 describing movement of the “C” simulators that is normal or not normal to the true profile?

RE: Moveable Datum Target Question

Hi, AndrewTT:

Movement of the “C” simulators that is normal to the true profile.

Best regards,


RE: Moveable Datum Target Question

In figure 4-47 the direction is as indicated by the 45 degree markings as that is obviously not normal to the arc surface.

It's clarified in the 2018 version; the coordinate dimensions to the C targets should not have been applied to appear coincident with that surface. They locate the line along which the targets move which is at 45 degrees.

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