— × Datum target line
Datum target line · Datums y referencias · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
On the drawing, “Datum line B between two line targets” states a datum target line requirement. Line contact is embodied by a knife-edge locator or a cylinder laid on its side, which touches the part along a generatrix: that is why, in addition to translation, it locks a rotation that a single point would leave free. It is combined with the other targets following 3-2-1 logic: a line can play the role of two points in the secondary datum, or form part of the primary along with other locators. Its location and length are dimensioned with basic dimensions from the datum reference frame, and the length is part of the requirement: supporting a longer or shorter stretch changes the result..
What controls
Defines the datum by contact along a dimensioned line on the surface, not at a single point or over the entire face. On the drawing it is shown with an X at each end joined by a line, and the line is located and sized with basic dimensions. It is used where a point is not enough to seat the part but a full area would not be repeatable: casting ribs, generatrices of cylindrical parts resting in a V-block, sheet metal edges, weld beads. A line contact locks more degrees of freedom than a point, so it provides orientation as well as support. What it does NOT mean is that the line must be machined or perfect: it is the area where the part touches the fixture, not a feature to be manufactured.
How to verify
In fixturing, line contact is embodied by a cylindrical pin resting on its side or a straight edge, placed at the indicated basic location. On a CMM, a row of points is probed along the target line, a straight line is fitted through them and the datum is built from that line, without extending the probing beyond the dimensioned ends. Spread the points evenly over the whole length, because bunching them at one end biases the resulting orientation. Check beforehand that the part is not riding on a burr partway along.
Datums and notes
Line contact is embodied by a knife-edge locator or a cylinder laid on its side, which touches the part along a generatrix: that is why, in addition to translation, it locks a rotation that a single point would leave free. It is combined with the other targets following 3-2-1 logic: a line can play the role of two points in the secondary datum, or form part of the primary along with other locators. Its location and length are dimensioned with basic dimensions from the datum reference frame, and the length is part of the requirement: supporting a longer or shorter stretch changes the result.
Common mistakes when interpreting it
- Resting the part on a wide surface instead of a line, which completely changes how it seats.
- Probing more length than dimensioned or going past the ends, picking up areas that are not part of the target.
- Confusing the line target with an edge that has to be machined: it is the contact area for seating, not a feature of the part.
How to get it in the workshop / if it comes out of tolerance
Two point targets can define a datum line. Useful on castings with support ribs. In the fixture, the line is made with a ground edge, a knife-edge locator or a cylindrical pin laid on its side that touches along its generatrix; not with a wide surface. Make sure the contact area is clean and burr-free along its whole length: a single high spot turns the line into a point contact and the part pivots on it.
If the part shifts or does not repeat, first look at the condition of the contact line on the raw part: scale, sand, weld spatter or a parting line flash. If the fixture locator has worn and no longer touches along its full length, it behaves like a point and the part pivots: regrind or replace it. And when the raw part is very irregular, consider with design switching to an area target, which is more forgiving of rough surfaces, or machining a small locating pad in the first operation.
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