GDT CHAPTER 3- Form Tol.pdf

form tolerance

1. GDT
ANSI Y14.5M
1994




CHAPTER 3: FORM TOLERANCE

2. Objective:
1. Interpret the flatness control

2. Interpret straightness control.

3. Interpret the circularity control

4. Interpret the cylindricity control

3. FORM TOLERANCES
Definition

Control the form of features without a datum reference
and relation to another features

The form tolerances includes :
• straightness
• flatness
• roundness or circularity
• cylindricity
O 0.2

4. FORM TOLERANCES

5. STRAIGHTNESS

Condition where one line element of a surface or axis
is in a straight line

Does not control the flatness of surface
STRAIGHTNESS TOLERANCE

Represents two parallel planes or diameter tolerance
zone in which the surface or axis of a feature must lie

Must less than the size tolerance

6. TYPE OF STRAIGHTNESS TOLERANCE

1. Straightness of surface element
- Tolerance zone represent two parallel planes

2. Straightness of axis element
- Tolerance zone represent diameter

3. Straightness of center plane
- Tolerance zone represents two parallel planes

10. A. Straightness applied to a surface
1)1. Tolerance value must be less than the size
2) tolerance
3)2. Tolerance zone applies to each line element
4)3. Rule #1 applies to the FOS dimension.

B. Straightness applied to an axis
 1. Part must also be within size limit
 2. Tolerance value may be greater than
 size limit
 3. Rule #1 overridden

13. STRAIGHTNESS OF SURFACE ELEMENT - CYLINDRICAL PART
Bench Centre

14. STRAIGHTNESS OF SURFACE ELEMENT - NONCYLINDRICAL PART

15. FLATNESS
• Condition of a surface where all elements are in one
plane and not related with a datum

• Applicable only for flat surfaces

• Cannot be gage but must be measured


FLATNESS TOLERANCE
• Represent by tolerance zone of two parallel planes
within which the surface must lie

• Must less than the size tolerance

• MMC principle is not applicable

16. FLATNESS OF SURFACE ELEMENT

17. ROUNDNESS
• Condition of a cylindrical surface at any cross-sectional
measurement where all points of the surface are perpendicular to
and equal distance from a common axis during one complete
revolution of the feature

• Applicable only for cylindrical part and not related with datum

• Must be measured and cannot be gaged

ROUNDNESS TOLERANCE
• Represent radial tolerance zone which defined by two perfectly
concentric circles in which the radii are stated tolerance apart

•MMC is not applicable

18. ROUNDNESS OF SURFACE ELEMENT
Surface periphery at any cross section perpendicular to the feature axis must
be within the specified tolerance of size and must be lie between two
concentric circles.

V-Block

19. CYLINDRICITY
• Condition of an entire cylindrical surface of one revolution in
which all points of the surface are equidistant from a common axis

• No datums relationship

• Simultaneously controls roundness, straightness and parallelism

CYLINDRICITY TOLERANCE
• Represent radial tolerance zone defined by two perfectly
concentric cylinders in which the radii are stated tolerance apart

• MMC is not applicable

• Cylindrical tolerance cannot exceed the size tolerance limits

20. CYLINDRICITY OF SURFACE ELEMENT

21. SUMMARIZATION OF FORM CONTROLS