Form tolerances control the shape of a feature without considering its orientation or location. They do not require datums and ensure the geometry itself is correct.
Straightness
Controls how straight a line or axis is
- Can apply to a surface element or an axis
- Tolerance zone: two parallel lines (for surfaces) or a cylinder (for axis)
Used for shafts, edges, and long features to prevent bending or waviness
Flatness
Controls how flat a surface is.
- Applies to an entire surface
- Tolerance zone: two parallel planes
Ensures proper contact between mating surfaces (e.g., sealing faces).
Circularity (Roundness)
Controls how close a cross-section is to a perfect circle.
- Measured at any cross-section
- Tolerance zone: two concentric circles
Cylindricity
Controls the overall 3D shape of a cylinder.
- Combines straightness + circularity + taper
- Tolerance zone: two concentric cylinders
Ensures smooth rotation and even wear in cylindrical parts.
Ensures the entire cylindrical surface is uniform and precise.
Leave a Reply