Edit Tolerances

You can define or import geometric and dimensional tolerances (GD&T) and surface roughness requirements for component features (GCDs).

Introduction to Tolerances

Tolerances help determine the appropriate manufacturing process for components. Each process is capable of achieving a certain tolerance, so a process may not be possible for a feature if it cannot manufacture to that tolerance. Tolerance values also affect the cycle time to complete an operation; for example, if lower finish machining feed rates are required to achieve tighter (smaller) tolerance values, this increase cycle times.

Some GCDs have associated tolerance values, and each manufacturing process has an associated “best achievable” tolerance. For a GCD to be made by a process, the tolerance value associated with that GCD must be larger than the best achievable tolerance of the process. For example, if a feature has a tolerance of 5mm, then the process which makes that feature must be able to achieve a tolerance of 5mm or less.

Tolerance Ranges

Tolerances on engineering drawings and CAD models are often expressed by recording how much larger or smaller than the designed value a feature may be. For example, if a feature is can be 2mm smaller or larger than the designed value, then this may be expressed as +/- 2mm. This gives the range of values for which the feature is still valid.

Tolerances in aPriori are expressed as a single number which is the total range. For example if a feature has a tolerance of +/- 2mm, the tolerance in aPriori is 4mm. The same is true even if the tolerance is not symmetric. For example, if a feature has a tolerance of +2.5 or –1.5mm, the tolerance in aPriori is still 4mm.

aP Pro and Tolerances

When aP Pro analyzes a GCD, it selects only those processes and operations matching the requirements for that GCD. If the primary process group cannot meet these requirements, aP Pro selects secondary machining operations.

For example, if aP Pro analyzes a component using Sand Casting, but has a roughness tolerances tighter than the best achievable tolerance for the Sand Casting process, then aPriori assigns a Machining process to finish the GCD, as Machining has tighter roughness tolerance capabilities than Sand Casting.

If no suitable secondary machining operations are available, the GCD fails to cost

Important: You do not need to set tolerances for every GCD. For a given GCD, you only need to set tolerances that are close to or tighter than the achievable tolerances of the process aP Pro chooses for the GCD.

Set Tolerance Values

Note: If you do not define any of these values, none are assumed - aP Pro does not set default desired tolerances.

You can set tolerance and roughness requirements for a GCD:

Tolerance Setting Guidance

  • aPriori stores and displays tolerance values as total tolerance. For example, if a CAD system specifies a tolerance of ±0.25, you should enter the total tolerance 0.5. If you import a tolerance from a PMI attribute, aPriori displays it as a total tolerance.

  • For Stock Machined components, only enter total tolerances of 0.004in (0.1016mm) or less.

  • For roughness, identify your main machining process type (milling or turning) in the table below, and enter only roughness values that are within +25% or less of the best achievable roughness for that process type. The following table shows aPirori default roughness capabilities:

    Process Name

    Best Achievable Roughness (µm)

    Roughness (µin)

    Milling

    0.8

    31.5

    Turning

    0.4

    15.75

Tip: For more details about how aP Pro calculates and uses tolerances, see Tolerance Specifications and Calculations.