What is the difference between a feature control frame and a tolerance block?

Short Answer:

A feature control frame is a rectangular box used in GD&T to show the type of geometric control, the tolerance value, and the datum references for a specific feature like a hole or surface. It defines how a part must be measured for shape, position, or orientation.

A tolerance block, on the other hand, is a general box found on engineering drawings that gives default size tolerances for linear and angular dimensions when specific tolerances are not written. It provides standard limits but does not define geometric relationships like a feature control frame does.

Detailed Explanation:

Difference Between a Feature Control Frame and a Tolerance Block

In engineering drawings, both feature control frames and tolerance blocks are used to control and guide manufacturing and inspection, but they serve different purposes. Understanding the difference between these two is very important in mechanical design, machining, and quality control.

Let us explain each term clearly and then understand the difference.

What is a Feature Control Frame

A feature control frame is a GD&T symbol box used to define the type of geometric tolerance applied to a specific feature in a drawing. It gives detailed information about:

  1. Geometric characteristic – like flatness, straightness, position, etc.
  2. Tolerance value – how much variation is allowed
  3. Datum references – from where the measurement should be taken

Each feature control frame has one or more boxes arranged in a row. It may look like this:

∘ 0.1 | A | B | C
This tells the inspector that the position of the feature must be within 0.1 mm, and the measurement should be done with respect to datums A, B, and C.

Use: It is used when geometric control (like position, orientation, or shape) is important for function or fit.

What is a Tolerance Block

A tolerance block is usually found in the title box or near the corner of a technical drawing. It gives general tolerance values for dimensions that do not have specific tolerances written next to them.

For example, a tolerance block may look like this:

  • For dimensions up to 10 mm → ±0.1 mm
  • For dimensions between 10 mm and 50 mm → ±0.2 mm
  • For angular dimensions → ±1°

This means that if a linear dimension like 30 mm is written without any tolerance, it should be understood as 30 ± 0.2 mm based on the tolerance block.

Use: It is used to apply default tolerance values to standard linear or angular dimensions.

Key Differences

  1. Purpose:
    • Feature Control Frame: Controls geometry of features (form, orientation, position).
    • Tolerance Block: Provides default size tolerances for general dimensions.
  2. Level of Detail:
    • Feature Control Frame: Very specific and feature-based.
    • Tolerance Block: General for the entire drawing.
  3. Symbols Used:
    • Feature control frame uses GD&T symbols like ∘ (position), ⏤ (straightness), ⏥ (flatness), etc.
    • Tolerance block uses numerical ranges with ± signs.
  4. Location in Drawing:
    • Feature control frame is placed next to the feature it controls.
    • Tolerance block is usually at the bottom or corner of the drawing.
  5. Measurement Reference:
    • Feature control frames use datum references for accurate inspection.
    • Tolerance block has no datum; just size-based tolerances.

When to Use Which

  • Use feature control frame when you need geometric precision—such as in the case of locating holes, flatness of a surface, or alignment between parts.
  • Use the tolerance block when exact geometry is not critical, and standard size variation is acceptable.

Both tools work together in a technical drawing. The tolerance block sets the basic rules, while feature control frames give specific instructions for critical features.

Conclusion

A feature control frame is used in GD&T to describe the geometric accuracy of a specific feature using symbols, tolerance values, and datums. It is feature-specific and controls shape, orientation, or position. A tolerance block, on the other hand, gives general size and angle tolerances for dimensions that don’t have detailed tolerances written next to them. Understanding the difference between these two is important for accurate design, smooth production, and reliable part inspection in mechanical engineering.