基准A. 2. Datum C- 最大内接两平 面且垂直于A和B一致
Effect of Datum Sequence and Material Condition.
Ф 0.4 M A B M
-C-
Datum Axis B
Ф 0.1 M A
-B-
-A-
Datum center plane C
Note: 1.Datum B- 实 Ф 0.2 M A B MC M 效状态且垂直于基
The datum is the axis or center plane of the feature at the MMC limit. Where the actual size of the datum feature departed from MMC. A variation is allowed between its axis or center plane and those of the datum.
准A. 2. Datum C- 实效状态且垂 直于A又和B一致
3. Geometric Characteristic Symbols
1) Geometric Characteristic Symbols
For Individual Features
Form 形状公差
For Individual or related Features
2
Slide 2
Customer 用户
Engineering Drawing
工程图纸
Product&Tool Design 产品/模具设计
Manufacturing Including. Supplier
制造包括供应商
Geometric Characteristic Geometric Tolerance
50 50.04 50.13
Ф
25
+0 - 0.05
Ф 0.04 M A M
Virtual condition boundary 0
Ф24.95 Ф 0.09
Ф 50 Ф25.04
+0 - 0.05
50
Ф
Drawing Callout
Datum @ MMC Feature @ LMC Tolerance(max)=0.09
ＡＢＣ
Point as Primary Datum
Line as Primary Datum
Plane as Primary Datum
The Effect of Material Condition on a Datum Feature • Effect of MMC/LMC 最大/小实体的影响
▪ Geometric Tolerance 形位公差：与部件 的几何特征相关的公差
形状、轮廓、定向、定位、跳动
▪ Reference Dimension 参考尺寸：没有 公差的尺寸，仅供参考，加括号或跟随 REF。
▪ Virtual Condition (实效状态）：指实际要 素具有最大实体尺寸且相应中心要素的形位 误差等于形位公差时的状态。以控制匹配部 件的间隙或建立量具的要素尺寸。
• Simulate the Manner in which a part is placed in final assembly. 模拟装配形式
•Datum must be recorded in a specific sequence called datum precedence. 按特定的基准优先次序来记录
• Effect of RFS 独立原则的影响
Referencing a datum feature of size on an RFS basis means the a centering about its axis or center plane is necessary, regardless of the actual size of the feature.
19.979 20
Ф
50
+0.13 0
Ф 0.08 M
Virtual condition boundary
A
Ф 50.13 0.21
Ф 49.92 Ф 50
0.08 Ф 49.92
A 0.021 0.08 50 50.13
Ф
50
+0.13 0
Ф 0.08 M A Ф 0.12 A
A
0.012 0.08
X, Y, Z • 3 Rotational orientation
U, V, W
Specifying Datums in an Order of Precedence 按顺序来规定基准
• Based on the functional and three dimentsional requirements. 根据功能和三维要求
• The surface of the feature shall not extend beyond a boundary of MMC and LMC. (被测要素实体不得超 越最大和最小实体极限边界）
Ф20 +0/-0.021
20
Ф 0.01 M
Ф20.01 Ф20.01
19.979
0.031 0.01
Ф 0.05
Ф 0.09
Ф 50 Ф 49.95 Ф25.04
Datum @ LMC Feature @ LMC Tolerance(max)=0.14
3） Datum 基准
• Datum Feature 基准:
A datum (theoretical) is a plane, axis, or point derived from the true geometric counterpart of a specified datum feature.
Least Material Condition
Regardless of Feature
Symbol
M L
None
Hale Waihona Puke • Regardless of Feature 独立原则 （RFS):
A geometric tolerance or datum reference applies at any increment of size of the feature within its size tolerance
Profile 轮廓度公差
Orientation 定向公差
For Related Features
Location 定位公差
Runout 跳动公差
straightness 直线度 flatness 平面度 circularity 圆度 cylindricity 圆柱度 profile of a line 线轮廓度 profile of a surface 面轮廓度 angularity 倾斜度 perpendicularity 垂直度 parallelism 平行度 position 位置度 concentricity 同心度/同轴度 symmetry 对称度 circular runout 圆跳动 total runout 全跳动
Ф20 Ф20
Ф20 +0/-0.05
Ф 0.0 M
0.05 19.95 20
MMC 最大实体原则：
• The surface of the feature shall not extend beyond a boundary at virtual condition. If the feature is produced at its MMC limit of size. No form variation is permitted. (被测要素实体不得超越实效边界）
a. Point datum target
b. Line datum target
c. Surface datum target
• Datum Targets Symbol 基准目标符号
Ф 15
20
A3
A3
A3
• The datum reference Frame (基准参考体系）
Degree of Freedom • 3 translation direction
▪ External
VC= MMC 尺寸 + Geometric Tolerance
▪ Internal
VC= MMC 尺寸 – Geometric Tolerance
2) Material Conditions
Material Conditions Maximum Material Condition
• The surface of the feature shall not extend beyond a boundary
of MMC. If the feature is produced at its MMC limit of size. No form variation is permitted. (被测要素实体不得超越最大实体 理想边界）
• The surface of the feature shall not extend a boundary of LMC. （被测要素实体不得超越最小实体理想边界）
•Where it is desire to permit a surface of a feature to exceed the boundary of MMC, a local note is directed to a specific dimension as “Perfect form at MMC not required”. （当需要容 许被测要素实体超出最大实体边界， 须加注释于特定尺寸： “不需MMC时的理想形状”）