At what point along the datum cylinder do we make our measurement?įigure 3: Establishing a common axis between two different size diameters In Figure 3 both datum A and datum B have a angular orientation roughly in the same axis but we measure in a real world and if the orientation between the two datums pointed in the opposite direction then we are faced with a dilemma. When measuring features as in Figure 3 the datum axis line must be created between Datum A and Datum B to measure the runout of the journal. In the example (.), the datum is the common axis formed by the two smallest circumscribed coaxial cylinders…įigure 2: Datum axis with two coaxial cylinders.
#Datum definition iso
When the part has two different sized diameters but share the same axis both diameters must be used to create the datum axis.įor establishing a datum being the common axis of two cylinders (see figure 2) the following definition is stated in ISO 5459:1981. In addition to the single axes it is necessary to define common axes as well. These planes are the center axis of the cylinder and will be averaged during the spatial orientation process of establishing a datum structure on your CMM. Due to the geometry of a cylinder it lies on two planes of orientation. The same will be for the smallest circumscribed cylinder respectively and consequently its axis for a shaft. For that case the standard requires an equalization of the angles a in all directions of space. The orientation of the cylinder depends on the surface condition and form of the hole and might therefore not produce an unambiguous orientation. This shows the largest inscribed cylinder and its axis for a hole.
#Datum definition simulator
If you measure the physical hole or the datum simulator an issue will arise. The gage, the pin or ring gage is the simulated datum and is the datum axis.įigure 1: Datum is the axis of a cylinder The datum feature is the surface of the part. This simulator can be a pin gage (internal) or a ring gage (external). NOTE: Simulated datum features are used as the practical embodiment of the datums during manufacture and inspection.ĪNSI Y14.5: When a diameter is designated as a datum feature, the datum axis is derived from placing the part in a datum feature simulator. A real surface of adequately precise form (such as a surface plate, a bearing, or a mandrel, etc.) contacting the datum feature(s) and used to establish the datum(s).
When creating a simulated datum the specs state: It will be imperative that you take as many points when measuring a cylinder on a part that are equally distributed around each circle segment of the cylinder to create a more consistent best-fit cylinder.Ī datum simulator is the best method to get consistent measurement values. The orientation (See Figure 1 a) is the angular deviation of 3d line through the center of the cylinder. When measuring a part surface form, waviness, and roundness deviations will cause some variation in the calculation of the cylinder thus changing the orientation of the cylinder. The surface of the datum cylinder will never be ‘true’.
When measuring a part with a datum feature requirement that requires you use a cylinder as the spatial orientation datum (base plane datum) there are several considerations you must keep in mind. Datums may by based on one or more datum features of a part. A theoretically exact geometric reference (such as axes, planes, straight lines, etc.) to which toleranced features are related.
The following is from The CMM Handbook - View HereĪ datum is defined in ISO 5459:1981 as:
Today I Learned - Datum Definition When the Datum Is Cylinder
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