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How aPriori Analyzes Tolerance and Surface Roughness Requirements

To help you optimize tolerance and surface roughness requirements, aP Design calculates the range of tolerances that the selected process can achieve for each GCD, whether the specified tolerance is within the achievable range, and the effect that meeting the requirement has on cycle time.

Achievable Tolerance Ranges for GCDs

The range of tolerances that a process can achieve for a GCD depends on the size of a GCD. The achievable tolerance range helps determine operation feasibility. The boundary tolerance required for the GCD is the best achievable tolerance for the parent process of the operation that is required to make the GCD. The achievable tolerance range also helps aP Design adjust cycle times based on required tolerance; see Cycle Time Adjustment Factors.

To determine the best and worst tolerance that a given process can achieve for a given GCD, aP Design:

  1. Determines the best and worst achievable tolerance grade for the process  based on International Tolerance (IT) Grades for the process. IT Grades are independent of feature size but can be used with size to obtain a literal tolerance range.

  2. Determines the GCD size using the criteria and equations in this table.

    GCD GTOL Category GCD Size
    Simple Hole Position Tolerance Part length/2
    Simple Hole All other GTOL categories Diameter
    Curved Wall Diameter Tolerance Diameter or bend diameter (if diameter is null. If both are null, use Sqrt(Finished area)
    Curved Wall Position Tolerance Part length / 2
    Keyway Symmetry Max of GCD length, width, and height
    Remaining GCDs Remaining Categories Sqrt(Finished area)

  3. Calculates the best or worst achievable tolerance by substituting the GCD size (as obtained in a previous step) and the best or worst achievable tolerance grade (as obtained in a previous step) into this equation:

 Tolerance = 10^(0.2 * (Grade - 1)) * (0.45 * Size ^ (1 / 3) + 0.001 * Size) / 1000

Unlike achievable tolerances, the achievable roughness for a process and GCD does not depend on the size of the GCD.

Usage Tips

  • Although the distance between the GCD and some relevant datum feature can also affect the achievable tolerance, it is not considered.
  • If you encounter a roughness specification with no Rz or Ra label, it is Ra. By default, if no Roughness Rz value is specified, the default “Best Achievable”/”Worst Achievable” values for Roughness Rz are 4.5 times the value of the specified Roughness Ra.
  • The Injection Molding process allows for significantly tighter best achievable surface roughness than machining processes. This difference occurs because the best surface finishes in Injection Molding are achieved by hand polishing the mold surface. Mold hand polishing is not applicable for other processes.

Cycle Time Adjustment Factors

For a given GCD and process, the cycle time is adjusted with a compensation factor that is based on the:

  • Tolerance that you specify (or import) for the GCD
  • Tolerance range for the process and the GCD
  • Compensation factors that can be edited by your system administrator. The default values are:
    • Worst achievable compensation factor –  0.75 in out-of-box Digital Factories
    • Best achievable compensation factor –  1.5 in out-of-box Digital Factories

The position of the specified tolerance in the tolerance range determines the position of the compensation factor within the compensation factor range. If the tolerance specified for a GCD is at the midpoint of the tolerance range, then the compensation factor is 1. If the tolerance specified is in the bottom half of the tolerance range, the compensation factor is between the worst achievable compensation factor and 1. If the tolerance specified is in the top half of the tolerance range, the compensation factor is between 1 and the best achievable compensation factor.

In particular, if the tolerance  specified is a certain fraction of the way from the worst achievable tolerance to the midpoint of the tolerance range, the compensation factor is that same fraction of the way from the worst achievable compensation factor to 1.

Similarly, if the tolerance  specified is a certain fraction of the way from the midpoint of the tolerance range to the best achievable tolerance, the compensation factor is that same fraction of the way from 1 to the best achievable compensation factor.

If a tolerance for a GCD has not been specified, the compensation factor is 1.