⇀ A Movable datum target
Movable datum target · Datums y referencias · ISO 1101
Example in plan
How to interpret it
On the drawing, “Note: movable datum target on tab per ISO 1101” states a movable datum target requirement. The locator is simulated like any datum target (a point with a spherical pivot, a line with an edge or a pin lying on its side, an area with a flat-faced pin of the stated diameter), but mounted on a sliding element. The movable datum target symbol is precisely that arrow indicating the direction in which the locator can move; without the arrow, the target is fixed and its location is dimensioned with basic dimensions. The direction of movement is usually perpendicular to the seating surface or toward the part axis, and it is defined by the drawing: it is not chosen on the shop floor..
What controls
A variant of the datum target in which the locating element is not fixed at a set coordinate: it can move in the direction shown by the arrow until it contacts the part, and it is that actual contact that establishes the datum. It is used when parts vary so much from one to the next (castings, forgings, weldments, flexible sheet metal) that a fixed locator would leave the part rocking or distort it when seated. What it does NOT mean is that the datum is optional or approximate, or that the part can move during measurement: the movement is limited to the indicated direction, the other degrees of freedom remain constrained and, once contact is made, the locator is locked and not touched again.
How to verify
Mount the part in the specified fixture, first seat the fixed locators of the primary datum, advance the movable locator to light contact and lock it; from there the part is measured as with any other datum. On a CMM without a fixture, the contact point is probed at the actual location where it ends up (not at the nominal coordinate) and the datum is constructed from those points: the degree of freedom that the symbol allows to translate must be left free in the alignment. If the software fixes the locator at its theoretical coordinate, it is measuring a different requirement from the one on the drawing.
Datums and notes
The locator is simulated like any datum target (a point with a spherical pivot, a line with an edge or a pin lying on its side, an area with a flat-faced pin of the stated diameter), but mounted on a sliding element. The movable datum target symbol is precisely that arrow indicating the direction in which the locator can move; without the arrow, the target is fixed and its location is dimensioned with basic dimensions. The direction of movement is usually perpendicular to the seating surface or toward the part axis, and it is defined by the drawing: it is not chosen on the shop floor.
Common mistakes when interpreting it
- Treating it like any other locator and tightening it to make the part sit nicely: forcing it distorts the part and invalidates every subsequent measurement.
- Placing the movable locator at its theoretical coordinate on the CMM instead of at the actual contact position: that measures a fixed datum, which is a different requirement.
- Assuming the locator can move in any direction: it can only move in the direction indicated by the arrow on the drawing.
How to get it in the workshop / if it comes out of tolerance
In practice it translates into a fixture with retractable locators (spring-loaded, screw-driven or hydraulic) that advance until they touch the part and are locked before measuring or machining. The operator must seat the part against the fixed locators first and only then close the movable locator, without forcing it: if it is tightened, it stops being a locator and becomes a force that distorts the part. The same fixture must be used for machining and inspection, or the results will not be comparable.
If the part rocks on the fixture, check that the movable locator is really making contact and not jammed by burrs, casting sand or scale: clean the locating area before loading. If measurements do not repeat from part to part, check that the locator is always locked in the same sequence and without overtightening. And if the part is consistently out in the area of the movable datum, the problem is usually upstream (distortion of the blank or after welding), not in machining.
Alternative symbol:↗ A
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