![]() ![]() Notice the theory and physical are both shown. Notice the datum feature symbol H applied to the surface it will establish a plane. Afterwards, we will cover datum features with size (These features may have datum modifiers applied). (point, line or plane) The next few slides will first deal with datum features without size (Basically plane surface datums, also can not have datum modifiers applied). The following slides show how to get from the datum feature to the datum. ![]() Remember datum features are the actual part features and datums are the theoretical perfect. T y Since datums are mandatory lets look at how datums are called out and how datums are established from datum features. 2 M - 1 x 5 H 7 ( / ) 9 3 6 X L D a t u m f e r s H n d L w i h o c l p g. 1.9ġ0 Establishing Datums from Primary Plane Surfaces 6.15 D a t u m f e r s Any feature can be a datum feature, so datum features can be of size or without size. If you remember earlier in unit 1 we talked about features of size and features without size. ![]() datums 1 point minimum contactĬircular parts also require a three-plane concept for repeatable orientation the primary datum is usually one flat surface of the part, defines a datum axis along the center of cylinder should be a construction line (or omitted) should have a center line We will need to label the datums on our drawing so that the part inspector will know from where to measure.Ĥ Datum Feature Symbol Datum Feature Symbolĥ Three-Plane Concept define primary, secondary, and tertiary datum planes the primary datum is the one that functionally is usually the most critical feature or surface of the part the primary datum feature of the part will make contact with the theoretical datum plane in a minimum of three pointsĦ Three-Plane Concept the secondary datum plane is to the primary datum secondary datum feature is the one that functionally is the 2nd most critical feature or surface of the part 2 point minimum contactħ Three-Plane Concept the tertiary datum plane is to the primary & sec. Datums are usually a plane, a line, or a point. Datums define a coordinate system from which to measure everything. These features are considered theoretically exact. ![]() Tell me how far off from ‘perfect’ is allowed.ģ Introduction Datums are nothing more and nothing less than the physical features of parts from which to make repeatable measurements. True b.Presentation on theme: "Geometric Dimensioning and Tolerancing"- Presentation transcript:Ģ GDT Establish a reference coordinate system. Runout tolerance applies to a surface that surrounds or intersects the datum axis. Tangent plane modifier cannot be used in the tolerance portion of a circular or total runout tolerance. Profile tolerance may contain MMC modifier in the tolerance portion. True profile must always be located with basic dimensions. Indicate whether concentricity tolerance is individual (requires no datum reference), related (requires datum references), or both. A position tolerance can be applied to a feature of size b. A position tolerance is related (i.e., requiring datum references) in general, but it may be individual (i.e., requiring no datum reference) when applied to coaxial features of size. A feature control frame for angularity tolerance must contain datum feature reference. A parallelism tolerance can control location and size. A perpendicularity tolerance applied to a planar surface, controls the orientation and of the surface. Where a regular feature of size is referenced as a primary datum at MMB, a fixed-size datum feature simulator is used. ![]()
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