Datums*

What are datums?Datum: A theoretically exact point, axis, or plane derived from the true geometric counterpart of a specified datum feature. A datum is the origin from which the location or geometric characteristics of features of a part are established and measured from.Datums are abstractions (that is, they are not real objects).*

What are datum features?Datum feature: An actual feature of a part that is used to establish a datum.Unlike datums, datum features are real objectsThe true geometric counterpart of a datum feature is a datum*

What are not datum features?Things that are not real objects (that you can see and touch) are not datum featuresCalling imaginary objects (e.g. a centerline between two holes) is a common way in which GD&T is misused*

What are not datum features (cont.)?*Not a datum feature

What are not datum features (cont.)?*Not datum features

Datum feature simulatorsDatum feature simulator: A surface of adequately precise form contacting the datum feature(s) and used to establish the simulated datum(s).What are some examples of datum feature simulators?*

Datum feature simulators (cont.)Not perfect, but considered accurate enough such that deviations from perfect are insignificantly small.Datum feature simulators are often tooling, for example:Granite surface platesManufacturing machine tables and locatorsAngle blocksGagesIncreasingly, computer-aided tools are used as datum simulatorsCoordinate measuring machinesLaser trackers and laser scanners

*

Datum feature simulators - examples*Extremely flat surface simulates a datum planeBig thick heavy slab of granite

Datum feature simulators examples (cont.)*Expanding/contracting jaws simulate a cylindrical datum feature of sizeDatum feature on part

Datum feature simulators examples (cont.)*Custom fixtureSimulates three datum planes and their relationships to each other

Datum feature simulators examples (cont.)*Coordinate Measuring Machine (CMM)

Datum feature simulators examples (cont.)*Table of milling machine

Considerations when choosing datums & datum featuresFunction of the part or featureAssembly considerations (e.g. how does the part or feature relate to mating parts?)Manufacturing processes usedInspection methods available (ability to simulate datum)Ease of access to featuresAppropriate size and location for desired control

*

Datum feature symbols*Suction cup

Relationship between datum, datum feature, and simulated datum*Datum simulatorContact between datum simulator and datum features establishes the simulated datum

Placement of datum feature symbols for datum surfacePlaced on edge view or extension line in view where surface appears as a line*On extension line from surfaceOn edge view of surface

Geometric control of a datum surface*

Datum reference frameNormally three mutually perpendicular planes established by datum features used to position the partLike a datum, exists only in theory and not on the part*

Datum reference frame (cont.)Datum features used to establish the datum reference frame should be selected based on their importance to the design of the part.*

Datum order of precedenceIn feature control frame, datums are listed in an order of precedence.Indicated the order in which the datum features should contact the datum simulators.*

Datum order of precedence (cont.)Primary datum: 3 points of contact minimumSecondary datum: 2 points of contact minimumTertiary datum: 1 point of contact minimum*PrimarySecondaryTertiary

Step-by-step: Positioning the part in the datum reference frameStep one: Place the part on the primary datum plane simulatorStep two: Push the part against the secondary datum plane simulatorStep three: Push the part against the tertiary datum plane simulator*

Step one: Place part on primary datum plane simulator*Part contacts this surface at 3 points minimum

Step two: Push part against secondary datum plane simulator*Part contacts this surface at 2 points minimum

Step three: Push part against tertiary datum plane simulator*Part contacts this surface at 1 point minimumNow the part is fully constrained and it can be manufactured or measured relative to the datum reference frame

Multiple datum reference frames*4x .25LMTwo different datum reference frames

Datum targetsDatum targets: Points, lines, or areas on the part used to establish datumsOften used when its not possible or not desirable to use entire surfaces as datumsCommonly used on parts that have significantly irregular surfaces such as castings and sheet metal partsThe use of datum targets can make measurements more repeatable and consistent*

Datum target symbols*

Datum target types*Example 3-13

Datum target point*Example 3-13

Datum target line*Example 3-13

Datum target area*Diameter of areaNormally indicated with phantom line circle with section lines inside

Datum reference frame established by datum target points*

Components of datum identification*

Locating datum target points*Basic dimensions, going back to datums, are usedIf desired, chain dimensioning could be used and would not result in tolerance accumulation

Simulating datum target points*Point contactPins have spherical tops

Simulating datum target areas*Pins have flat topsOne point contact minimum for each pin Target areas shown graphically

Showing small datum target areas as pointsTypically used when areas are too small to show*Upper part of frame specifies diameter of target area

Simulating a datum target line*Minimum one point contact somewhere along line

Partial datum features (using only part of a surface for datum)*Chain line indicates area to be used for datum

Referencing coplanar feature as a single datumCoplanar: Two or more features share the same theoretical plane.*

Cylindrical object as a datum feature*Center axis is datum axis

Placement of datum feature symbol for datum axes*On dimension shoulder or stubOn extension line, aligned with dimension arrowOn edge of surfaceOn feature control frame

Simulated datum axis external featureSimulator needs to be dynamicFor mechanical simulator, needs to squeeze down to contact datum feature (assuming datum is applied RFS)

*

Simulated datum axis external feature: Example*Datum feature simulator (jaws of chuck)Datum feature (surface of part)

Simulated datum axis internal featureSimulator needs to be dynamicFor mechanical simulator, needs to expand out to contact datum feature (assuming datum is applied RFS)

*

Simulated datum axis internal featureSimulator needs to be dynamicFor mechanical simulator, needs to expand out to contact datum feature (assuming datum is applied RFS)

*

Coaxial datum featuresCoaxial: Two or more features share the same theoretical axisA single datum axis is established by two coaxial datum featuresOrder of datum letters doesnt matterNormally used as a primary datum*

Datum axis of screw threads, gears, splinesFor screw threads, axis is established from pitch cylinder unless otherwise notedIf it is desired to use another feature of the thread, the note MAJOR DIA or MINOR DIA should be used.For gears and splines, the note MAJOR DIA, MINOR DIA, or PITCH DIA should be used.The use of threads, gears, and splines as datums should be avoided when possible.

*

Datum axis of screw threads, gears, splines (cont.)*MAJOR DIASpecifying the screw feature for datum

Datum axis established with datum target points one diameterTwo sets of three points establish an axis*

Datum axis established with datum target points two diametersAgain, two sets of three points establish an axis*L-M

Cylindrical datum target areas and lines*Target lineTarget area

Secondary datum axis from target points*End surface of cylinder is primary constraintPoints on cylindrical surface complete the constraint

Datum center planes rectangular, symmetric featureThe datum is the center plane between the two opposing surfaces*

Placement of datum feature symbol for datum center planes*On extension line, aligned with dimension arrowOn feature control frame

Simulated datum center plane external featureSimulator needs to be dynamicFor mechanical simulator, needs to squeeze down on datum feature (assuming datum is applied RFS)

*

Simulated datum center plane internal featureSimulator needs to be dynamicFor mechanical simulator, needs to expand out to datum feature (assuming datum is applied RFS)

*

Center of a pattern of features as the datum axisWill be covered more in Chapter 7.*