{"id":11144,"date":"2026-06-05T10:21:13","date_gmt":"2026-06-05T10:21:13","guid":{"rendered":"https:\/\/weldomachining.com\/?p=11144"},"modified":"2026-06-05T10:22:08","modified_gmt":"2026-06-05T10:22:08","slug":"what-is-a-datum","status":"publish","type":"post","link":"https:\/\/weldomachining.com\/ko\/what-is-a-datum\/","title":{"rendered":"\uc5d4\uc9c0\ub2c8\uc5b4\ub9c1 \uc81c\uc870\uc5d0\uc11c \ub370\uc774\ud140\uc774\ub780 \ubb34\uc5c7\uc778\uac00\uc694?"},"content":{"rendered":"\n

As is well known, the elevation of a mountain is referenced to sea level, and an economic growth index is calculated using a previous point in time as the reference datum. Similarly, in the field of machining, there is also a datum, which serves as a basic reference for design, production, and assembly. It can be a point, line, or plane. The following will discuss datums in the field of machining.<\/p>\n\n\n\n

\"What
What Is a Datum<\/figcaption><\/figure>\n\n\n\n

What Does Datum Mean?<\/strong><\/strong><\/h2>\n\n\n\n

A datum is an ideal point, line, axis, or plane fitted from the theoretical geometric boundary corresponding to a datum feature. A datum is the starting reference point for measuring other dimensions and tolerances.<\/p>\n\n\n\n

Why Are Datums Important in Manufacturing?<\/strong><\/h3>\n\n\n\n

Role in Drawings<\/strong><\/p>\n\n\n\n

In drawings, datums are mainly used to determine the starting points for dimensioning and tolerance annotations, clarify the positional relationships between the features of a part, and clearly express design intent. At the same time, using unified datums ensures that different people interpret the drawings consistently and reduces design deviations caused by differences in annotation or understanding.<\/p>\n\n\n\n

Role in Machining<\/strong><\/p>\n\n\n\n

In the machining process, datums are used for workpiece positioning and clamping. They help the workpiece maintain the correct position in the machine tool or fixture, thereby improving machining accuracy <\/a>and stability. Machining usually follows the principle of “datum first”: the datum surface or datum feature is machined first, and then other areas are machined based on it to reduce error accumulation.<\/p>\n\n\n\n

Role in Quality Inspection<\/strong><\/p>\n\n\n\n

In inspection, datums serve as the reference basis for measurement and judgment, ensuring consistency and accuracy of measurement results. With datums, the geometric tolerances of a part can be accurately checked against requirements, thereby determining whether the part is qualified.<\/p>\n\n\n\n

Common Types of Datums and How to Determine Them<\/strong><\/strong><\/h2>\n\n\n\n

Datum Point<\/strong><\/strong><\/h3>\n\n\n\n

A datum point is a reference point used in drawings to determine the position, orientation, and dimensions of other geometric features of a part. It is the unified basis for dimensioning, geometric tolerance annotation, machining, measurement, and assembly. It can be used to determine tolerance relationships such as the position of holes and the coaxiality of shafts, ensuring that parts are manufactured and inspected accurately according to design intent. In drawings, it is usually clearly marked with a lettered datum symbol and associated with dimension lines or tolerance frames.<\/p>\n\n\n\n

Datum Line<\/strong><\/strong><\/h3>\n\n\n\n

A line datum is a specific straight line used to determine geometric relationships, dimensioning, and positional references of a part. Common forms include rotational axes, symmetry centerlines, contour lines, or boundary lines. For example, shaft-type parts often use the axis as the datum for radial dimensions and coaxiality, while symmetrical parts often use the centerline as the reference for symmetric dimensions and positional tolerances.<\/p>\n\n\n\n

The main functions of a line datum are to unify dimensioning, guide machining positioning and inspection measurement, and control geometric tolerances such as position and coaxiality. In drawings, it is usually marked with a lettered datum symbol and establishes a clear relationship with the relevant dimensions, tolerance frames, or geometric features.<\/p>\n\n\n\n

Datum Plane<\/strong><\/strong><\/h3>\n\n\n\n

In mechanical drawings, a plane datum is a reference plane used to determine geometric relationships and dimensioning of a part. Stable, flat, easy-to-machine, or functionally important planes are usually selected as datums, such as bottom surfaces, end faces, and symmetry planes. In drawings, it is generally marked with a lettered datum symbol (A, B, C); the surface indicated by the symbol or its extension line represents the datum plane.<\/p>\n\n\n\n

The main function of a plane datum is to serve as a unified reference for dimensioning, geometric tolerances, machining positioning, and inspection measurement. Attention should be paid to the constraint relationship of plane datums, and the priority should not be reversed (generally, A constrains B, and A and B constrain C).<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Datum Axis<\/strong><\/strong><\/h3>\n\n\n\n

An axis datum is a reference datum used to determine the direction or positional relationship of a part axis. It is usually formed as an ideal axis fitted from actual features such as holes, shafts, or cylindrical surfaces. It is often used in geometric tolerance annotations to control requirements such as position, coaxiality, and symmetry of other axes, holes, or surfaces.<\/p>\n\n\n\n

An axis datum is generally marked with datum symbols such as “A”, “B”, and “C”, pointing to the corresponding axis feature. Its main function is to provide a unified reference for machining positioning, assembly orientation, tolerance control, and inspection measurement, thereby ensuring part interchangeability and assembly accuracy.<\/p>\n\n\n\n

What Does a Datum Symbol Look Like?<\/strong><\/strong><\/h2>\n\n\n\n

Common datum symbols mainly include the following types. Different standards (such as ISO and ASME) differ slightly in details, but their core forms are similar:<\/p>\n\n\n\n

Combination of Datum Letter and Triangle<\/strong><\/strong><\/p>\n\n\n\n

Form: It consists of an uppercase English letter (excluding I, O, and Q) and a triangle. The triangle may be filled or unfilled.<\/p>\n\n\n\n

Meaning: The letter represents the datum identifier, and the triangle points to the datum feature (such as a surface, axis, center plane, etc.).<\/p>\n\n\n\n

Example: Letters such as A, B, and C are combined with filled or unfilled triangles to annotate datums such as planes, end faces, and axes.<\/p>\n\n\n\n

Datum Target Symbol<\/strong><\/strong><\/p>\n\n\n\n

Form: A thin double-dot chain line is used to indicate the datum target line or area, and a letter or number identifier may be added.<\/p>\n\n\n\n

Meaning: It is used to specify a particular point, line, or area as the datum, and is commonly used for complex shapes or local datum annotation.<\/p>\n\n\n\n

Example: X……X indicates a datum target line, and (X) indicates a datum target point.<\/p>\n\n\n\n

Composite Datum Symbol<\/strong><\/strong><\/p>\n\n\n\n

Form: Multiple datum letters are connected with hyphens, such as A-B and A-B-C.<\/p>\n\n\n\n

Meaning: It indicates a composite datum formed jointly by multiple datum features, commonly used to define a common axis or a complex datum system.<\/p>\n\n\n\n

Example: A-B indicates a common datum defined jointly by datum features A and B.<\/p>\n\n\n\n

Datum Modifier Symbol<\/strong><\/strong><\/p>\n\n\n\n

Form: A modifier is added after the datum letter, such as [MD] (major diameter), [LD] (minor diameter), [PD] (pitch diameter), [PL] (plane), and [SL] (line).<\/p>\n\n\n\n

Meaning: It is used to clarify the specific feature or reference position of the datum, such as the crest or root of a thread, or a specific plane or axis.<\/p>\n\n\n\n

Example: A[MD] indicates that the major diameter of datum A is used as the reference.<\/p>\n\n\n\n

Common Datum Letters: A, B, and C<\/strong><\/strong><\/h3>\n\n\n\n

Commonly used datums generally use the letters A, B, and C. Additional reference plane datums may use E and F. If more reference datum planes are needed, other letters except I, O, Q, and X are used for datum annotation. In very rare cases, datum plane symbols composed of two letters may be used for annotation.<\/p>\n\n\n\n

How to Annotate Datums in Mechanical Drawings<\/strong><\/strong><\/h2>\n\n\n\n

Datum annotation in mechanical drawings must follow local machining standards (such as ASME, GB\/T 1182, or ISO 1101<\/a>). Common datum annotation methods are as follows:<\/p>\n\n\n\n

Single Datum Feature Annotation<\/strong><\/strong><\/p>\n\n\n\n

Profile features (such as surfaces and edges): The datum symbol consists of an uppercase letter, a frame, and a triangle. The triangle is placed on the contour line or contour surface of the datum feature, or near the extension line of the contour, but it must be offset from the dimension line. For example, when annotating a plane datum, the triangle points to the plane, and the letter is shown inside the frame.<\/p>\n\n\n\n

Center features (such as axes and center planes): The triangle of the datum symbol is aligned with the extension line of the dimension line and can replace one arrow of the dimension line. For example, when annotating the datum of a cylindrical axis, the triangle is aligned with the dimension line of the axis.<\/p>\n\n\n\n

Common Datum Annotation<\/strong><\/strong><\/p>\n\n\n\n

When two or more features jointly serve as a common datum, a datum symbol must be marked on each feature, and the corresponding letters are connected with a hyphen in the tolerance frame. For example, two adjacent planes used as a common datum are annotated as “A-B”.<\/p>\n\n\n\n

Three-Plane Datum System Annotation<\/strong><\/strong><\/p>\n\n\n\n

If three mutually perpendicular features are required to form a three-plane datum system, a datum symbol should be marked on each datum feature, and the letters should be entered in the tolerance frame from left to right in datum order (such as A, B, C).<\/p>\n\n\n\n

Local Datum Annotation<\/strong><\/strong><\/p>\n\n\n\n

When the datum feature refers only to a local area, the local range is drawn with a thick chain line, the necessary dimensions are added, and then the datum symbol is marked.<\/p>\n\n\n\n

Datum Target Annotation<\/strong><\/strong><\/p>\n\n\n\n

If specific points, lines, or local surfaces on a datum feature need to be specified as datums, datum targets must be annotated:<\/p>\n\n\n\n

Point datum: Indicated by “x”.<\/p>\n\n\n\n

Line datum: Indicated by a thin line, with an “x” added on the edge.<\/p>\n\n\n\n

Local surface datum: The local surface shape is drawn with a double-dot chain line, and thin solid lines at 45\u00b0 to the horizontal line are drawn on it.<\/p>\n\n\n\n

How Datum A, B, and C Work on Drawings<\/strong><\/p>\n\n\n\n

In mechanical drawings, datums A, B, and C are usually used to establish a three-plane datum system. Their functions are as follows:<\/p>\n\n\n\n

Datum A (Primary Datum)<\/p>\n\n\n\n

Function: It constrains three degrees of freedom of the part and usually corresponds to the main functional surface or largest contact surface of the part, such as the bottom surface or mounting surface. It determines the basic position of the part in space and serves as the basis for subsequent datums and tolerance measurement.<\/p>\n\n\n\n

Annotation method: It is indicated by a framed uppercase letter “A” and associated with the actual datum feature on the part (such as a plane or axis) through a datum triangle.<\/p>\n\n\n\n

Datum B (Secondary Datum)<\/p>\n\n\n\n

Function: It is perpendicular to datum A and constrains two degrees of freedom. It is used to determine the position and direction of the part in the direction perpendicular to datum A, such as a vertical side surface or axis.<\/p>\n\n\n\n

Annotation method: In the tolerance frame, it is placed after datum A and indicated by “B”, and it must be ensured to maintain a perpendicular relationship with datum A.<\/p>\n\n\n\n

Datum C (Tertiary Datum)<\/p>\n\n\n\n

Function: It is perpendicular to both datum A and datum B and constrains the final degree of freedom. It is used to fully constrain the spatial attitude of the part, ensuring that the part has a uniquely determined position during assembly and measurement.<\/p>\n\n\n\n

Annotation method: In the tolerance frame, it is placed after datum B and indicated by “C”, and together with datums A and B, it forms a three-plane datum system.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

How to Dimension Drawings to a Datum?<\/strong><\/strong><\/h2>\n\n\n\n

To dimension drawings using a datum as the reference, the following principles and methods should be followed:<\/p>\n\n\n\n

Select Datums<\/strong><\/p>\n\n\n\n

Design datum: Select it according to part function and assembly relationships, such as important end faces, symmetry planes, rotary body axes, and mounting surfaces, to determine the position and geometric relationship of the part in the machine.<\/p>\n\n\n\n

Process datum: Consider machining and measurement convenience, such as machining positioning surfaces and measurement datum surfaces. Make the design datum and process datum coincide as much as possible to reduce errors.<\/p>\n\n\n\n

Annotate Important Dimensions<\/strong><\/p>\n\n\n\n

Important dimensions (such as fitting dimensions, mounting dimensions, and positioning dimensions) must be directly indicated from the design datum and should not be obtained through conversion, ensuring machining and assembly accuracy.<\/p>\n\n\n\n

For example, the diameter of a shaft-type part is dimensioned based on its axis, and the positions of mounting holes in a box-type part are dimensioned based on the mounting surface.<\/p>\n\n\n\n

Handle Auxiliary Datums<\/strong><\/p>\n\n\n\n

If there are multiple datums in one direction, select one primary datum and use the others as auxiliary datums. Auxiliary datums and the primary datum should be connected by direct dimensions to facilitate machining and measurement.<\/p>\n\n\n\n

For example, in the height direction of a support, the bottom surface is used as the primary datum, the top boss surface is used as the auxiliary datum, and the threaded hole depth is dimensioned with reference to the boss surface.<\/p>\n\n\n\n

Avoid Closed Dimension Chains<\/strong><\/p>\n\n\n\n

Dimensions in the same direction should not be connected end to end to form a closed chain; otherwise, machining errors will accumulate and accuracy will be difficult to ensure. Usually, the dimension with the lowest accuracy requirement is not indicated, or it is marked in parentheses as a reference dimension.<\/p>\n\n\n\n

Consider Machining and Measurement Convenience<\/strong><\/p>\n\n\n\n

When dimensioning, give priority to datums that are convenient for machining and measurement, such as indicating hole depth from an end face and indicating symmetrical structure dimensions from a symmetry plane.<\/p>\n\n\n\n

For complex structures, dimensioning can be performed in stages: first annotate the main datum dimensions, then annotate auxiliary dimensions.<\/p>\n\n\n\n

\"Finished
Finished parts datum inspection<\/figcaption><\/figure>\n\n\n\n

Datum in GD&T and Part Inspection<\/strong><\/h2>\n\n\n\n

Datums play a vital role in geometric tolerance annotation and part inspection, as follows:<\/p>\n\n\n\n

Role in Geometric Tolerance (GD&T) Annotation<\/strong><\/strong><\/p>\n\n\n\n

Determining geometric relationships: Datums are the reference basis for geometric tolerance annotation and are used to define the orientation, position, or runout relationship of the measured feature. For example, when annotating tolerances such as parallelism, perpendicularity, and position, it is necessary to clarify which datum plane, datum line, or datum point is used as the reference so that the geometric relationship between the measured feature and the datum can be accurately expressed.<\/p>\n\n\n\n

Establishing the tolerance frame: In the tolerance frame, datum letters are used to identify datum features. Multiple datums can form a datum system (such as a three-plane datum system) to fully constrain the degrees of freedom of the part in space and ensure the completeness and accuracy of tolerance annotation.<\/p>\n\n\n\n

Guiding design intent: By reasonably selecting datums, designers can clearly communicate the functional requirements and assembly relationships of a part, enabling machining and inspection personnel to accurately understand the design intent and avoid misunderstandings or errors caused by unclear datums.<\/p>\n\n\n\n

Role in Part Inspection<\/strong><\/strong><\/p>\n\n\n\n

Providing a measurement datum: During inspection, the datum is the reference for placing and positioning gauges or inspection equipment, ensuring the consistency and comparability of measurement results. For example, when using a coordinate measuring machine to inspect a part, it is necessary to establish a measurement coordinate system starting from a datum plane or datum axis in order to accurately measure the geometric errors of the measured feature.<\/p>\n\n\n\n

Verifying tolerance conformity: By comparing the actual deviation between the measured feature and the datum with the tolerance requirements, it can be determined whether the part is qualified. The accuracy of the datum directly affects the reliability of the inspection result. If the datum itself has errors, misjudgment may occur.<\/p>\n\n\n\n

Supporting inspection plan design: Based on the selection of datums, suitable inspection fixtures or tooling can be designed to ensure stable positioning of the part during inspection, reduce measurement errors, and improve inspection efficiency and accuracy.<\/p>\n\n\n\n

\"haas<\/figure>\n\n\n\n

Common Mistakes When Reading Datum Symbols<\/strong><\/strong><\/h2>\n\n\n\n

Common mistakes when reading datum symbols mainly include the following categories:<\/p>\n\n\n\n

Missing or Omitted Datums<\/strong><\/p>\n\n\n\n

In orientation tolerances (such as parallelism and perpendicularity), position tolerances (such as position and coaxiality), or runout tolerances, necessary datum symbols are not marked, resulting in a tolerance without a reference system that cannot be effectively inspected or machined.<\/p>\n\n\n\n

Incorrect Datum Order<\/strong><\/p>\n\n\n\n

The datum order (such as datums A, B, and C) is not correctly arranged according to the “3-2-1” locating principle or functional requirements. For example, a large plane that should serve as the primary datum is mistakenly set as the secondary or tertiary datum, affecting positioning stability.<\/p>\n\n\n\n

Improper Selection of Datum Surfaces<\/strong><\/p>\n\n\n\n

Surfaces with small area, poor stability, or susceptibility to deformation are selected as datums, such as using the edge of a small hole or a thin-wall surface as a datum, resulting in poor measurement repeatability or deviations during assembly.<\/p>\n\n\n\n

Confusing Datum Types<\/strong><\/p>\n\n\n\n

A plane datum is mistaken for an axis datum, or vice versa. For example, when annotating an axis datum of a cylindrical surface or inner hole, the datum triangle points to the outer surface of the cylinder instead of the axis, causing coaxiality and runout inspection to fail.<\/p>\n\n\n\n

Misreading a Common Datum<\/strong><\/p>\n\n\n\n

The “A-B” form of a common datum is mistaken for two independent datums, A and B, rather than representing a common axis or common plane, resulting in an incorrect measurement logic.<\/p>\n\n\n\n

Nonstandard Datum Symbol Annotation<\/strong><\/p>\n\n\n\n

The datum triangle style is incorrect (such as using an unfilled triangle instead of a filled triangle), prohibited characters are used for datum letters (such as I, O, and Q, which are easily confused with numbers), or the datum leader line is tilted or bent, affecting the recognition and interpretation of datum symbols.<\/p>\n\n\n\n

Incorrectly Adding a Datum to a Form Tolerance<\/strong><\/p>\n\n\n\n

A datum is incorrectly added to form tolerances (such as roundness, cylindricity, and straightness), while form tolerances themselves do not require a datum, resulting in incorrect drawing annotation.<\/p>\n\n\n\n

Ignoring the Meaning of Maximum Material Requirement (\u24c2)<\/strong><\/p>\n\n\n\n

The \u24c2 symbol is understood only as a change in the tolerance value, while its function of indicating additional tolerance compensation at the maximum material condition is ignored, which may cause interference or excessive clearance during part assembly.<\/p>\n\n\n\n

Conclusion<\/h2>\n\n\n\n

The above covers the relevant knowledge about datums. I believe you now have a basic understanding of the concept of datums. If you would like to obtain more in-depth information, you can consult our professional machining engineers<\/a>. We will provide you with more professional help and answers.<\/p>\n\n\n\n

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As is well known, the elevation of a mountain is referenced to sea level, and an economic growth index is calculated using a previous point in time as the reference datum. Similarly, in the field of machining, there is also a datum, which serves as a basic reference for design, production, and assembly. It can […]<\/p>\n","protected":false},"author":2,"featured_media":11146,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[5],"tags":[],"class_list":["post-11144","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-blog"],"_links":{"self":[{"href":"https:\/\/weldomachining.com\/ko\/wp-json\/wp\/v2\/posts\/11144","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/weldomachining.com\/ko\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/weldomachining.com\/ko\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/weldomachining.com\/ko\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/weldomachining.com\/ko\/wp-json\/wp\/v2\/comments?post=11144"}],"version-history":[{"count":2,"href":"https:\/\/weldomachining.com\/ko\/wp-json\/wp\/v2\/posts\/11144\/revisions"}],"predecessor-version":[{"id":11152,"href":"https:\/\/weldomachining.com\/ko\/wp-json\/wp\/v2\/posts\/11144\/revisions\/11152"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/weldomachining.com\/ko\/wp-json\/wp\/v2\/media\/11146"}],"wp:attachment":[{"href":"https:\/\/weldomachining.com\/ko\/wp-json\/wp\/v2\/media?parent=11144"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/weldomachining.com\/ko\/wp-json\/wp\/v2\/categories?post=11144"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/weldomachining.com\/ko\/wp-json\/wp\/v2\/tags?post=11144"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}