What are the types of geometric tolerances?

Short Answer:

Geometric tolerances are used in engineering drawings to control the shape, orientation, position, and form of different features of a part. These tolerances help ensure that parts fit and function properly, even if there are small variations during manufacturing. Instead of only controlling size, geometric tolerances focus on how features relate to each other in space.

There are five main types of geometric tolerances: form, orientation, location, profile, and runout. Each type controls a specific aspect of a part’s geometry and is represented by a unique symbol in GD&T. Using these tolerances improves accuracy and product quality.

Detailed Explanation:

Types of geometric tolerances

Geometric Dimensioning and Tolerancing (GD&T) is a modern way of describing and controlling the shape and alignment of part features. It goes beyond traditional size tolerances by controlling the geometry—how flat, round, or aligned a part feature should be. GD&T uses a set of 14 symbols grouped under five main types of geometric tolerances.

Let’s explore each type in simple language.

Form tolerances

Form tolerances control the shape of individual features. They make sure that the feature’s shape is correct even without referring to any other part feature.

  • Straightness: Controls how straight a line or axis should be.
  • Flatness: Ensures a surface stays even and flat.
  • Circularity (Roundness): Makes sure a circle is round across all cross-sections.
  • Cylindricity: Controls the roundness and straightness of a cylinder.

These tolerances do not require a datum and apply directly to the feature itself.

Orientation tolerances

Orientation tolerances control the angle of one feature relative to another. They define how straight, flat, or parallel a surface or axis should be with respect to a reference.

  • Perpendicularity: Keeps a feature at 90° to another.
  • Parallelism: Keeps two surfaces or axes equally spaced.
  • Angularity: Controls features at any specific angle (not just 90°).

These always require a datum reference to define the direction.

Location tolerances

Location tolerances control the exact placement or position of features such as holes, slots, or pins. These ensure proper alignment between mating parts.

  • Position: Most common tolerance, used to control exact location.
  • Concentricity: Controls the center of one feature to be aligned with another.
  • Symmetry: Ensures two halves of a feature are mirrored equally across a center plane.

These tolerances are essential for assembly and require datums for reference.

Profile tolerances

Profile tolerances control the shape or outline of a surface. They can be used to define curves, irregular shapes, or full surfaces.

  • Profile of a line: Controls the shape of a line in a cross-section.
  • Profile of a surface: Controls the shape and size of an entire surface.

Profile tolerances can work with or without datums depending on the situation.

Runout tolerances

Runout tolerances control how much a surface or feature wobbles or deviates while rotating. These are used for rotating parts like shafts or gears.

  • Circular runout: Controls wobble in a single circular cross-section.
  • Total runout: Controls variation over the whole surface while rotating.

Runout ensures that rotating parts work smoothly without vibration or imbalance.

Conclusion:

Geometric tolerances are divided into five types: form, orientation, location, profile, and runout. Each type has its own set of symbols and rules to control the shape, direction, and placement of part features. These tolerances help in maintaining consistency, improving product quality, and ensuring proper fit and function. By using GD&T correctly, engineers can create precise, reliable, and easy-to-manufacture mechanical components.