The difference is, that in a CAD program, the constraint works on a pair of perfect elements (one plane in relation to another…) and a datum reference system links a physical, imperfect element (surface, for example) to a high precision simulator (granite plate…) of a perfect theoretical element (plane).

2020年5月19日 · The primary datum (commonly Datum A) will be the first to be constrained. It requires at least three points of contact, and controls three degrees of freedom. The secondary datum (commonly Datum B) will always be perfectly perpendicular to Datum A.

2022年4月4日 · Datums are used in Geometric Dimensioning and Tolerancing drawings to create a reference system for inspecting a manufactured part. This reference system is called a Datum Reference Frame (DRF). Datums are derived from datum features. Datum features are real, tangible features on a part and are usually important functional surfaces.

2024年3月16日 · Datums are specified in the Feature Control Frame in an Order of Precedence. This Feature Control Frame has 2 Datums specified (A and B): A Feature Control Frame... Datum A is the Primary Datum and B is the Secondary Datum. If there had been a third Datum, it would be called the "Tertiary Datum."

2023年3月28日 · The datum reference frame is the coordinate system, made up of 3 mutually orthogonal planes, created by the datums specified within a feature control frame. Our feature control frame shows Datum Feature A as the primary datum feature reference, and Datum Feature B as our secondary datum feature reference.

2021年12月11日 · Datum A is the main datum on GD&T tolerances on a part and also the main reference for the other datums (in this case datum B and datum C) on the part. In general, datum A commonly applied to the most stable feature on a part, such as the surface with the largest surface area or as the base of the part.

This page explains the indication symbols and notation used for common datums and datum systems as well as how to set datums. “Learning GD&T From Scratch,” provided by KEYENCE, walks you through the basics of geometric dimensioning and tolerancing, datums, and measurements by coordinate measuring machines.

Primary Datum (Datum A) locks down the most significant degrees of freedom. Secondary Datum (Datum B) complements the first by controlling the remaining major movements. Tertiary Datum (Datum C) typically refines the control, constraining any remaining degrees of freedom.

The datum references (the letters at the end of a feature control frame) are given in a specific order to show the relative importance of each (primary, secondary, and tertiary). They do not have to be in alphabetical order, but rather order of precedence. The modifier M …

2014年5月13日 · Each hole is specified as a datum (in this case, datums B and C, where datum A is the large flat face of the plate). I have another feature with a positional tolerance with respect to A and a common datum defined by B and C (B-C).