Specifying or importing tolerance and roughness values

You can specify geometric and dimensional tolerances (GD&T) and surface roughness requirements for GCDs. You can also automatically import many of these values directly from CAD model Product Manufacturing Information (PMI) attributes.

aPriori stores tolerance capability data for each manufacturing process in the lookup table tblGtolProcessCapabilities.(For more information, see How aPriori calculates achievable tolerance ranges .)

  • When aPriori costs a GCD, it selects only those processes and operations which can meet the specified tolerances and roughness requirements for that GCD.
  • If the primary process group cannot meet the tolerance or roughness requirements, aPriori selects secondary machining operations.
  • If no secondary machining operations are available or capable of achieving the specified tolerance and roughness, the GCD fails to cost.
  • If no tolerance or roughness values are specified by either importing or manually entering the desired value, aPriori assumes all processes can achieve the desired tolerance and roughness; aPriori does not set default desired tolerances.
  • Tolerance and roughness specifications affect the cycle time required to complete an operation. For example, lower finish machining feed rates are required to achieve tighter (smaller) tolerance values, which results in increased cycle times. aPriori machining cost models account for this best practice. See How aPriori adjusts cycle times based on tolerances for more information.

You can specify tolerance and roughness requirements using one of the methods:

Note: 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.

Important: You do not need to specify tolerances for every GCD. For a given GCD, you only need to specify tolerances that are close to or tighter than the achievable tolerances of the process aPriori chooses for the GCD. aPriori recommends the rules of thumb listed below.

The following are useful rules of thumb:

  • 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

Some aspects of tolerancing are not directly supported and require special workflows. See:

  • Tolerance Relations Between Features and Datums

  • Dimensional Tolerances

How tolerances map to GCDs

Each tolerance and roughness type applies only to certain GCD types. The following table which shows what types of tolerance apply to each GCD type, and whether they can be imported as PMI from CATIA, Creo, NX™, and SOLIDWORKS™.

 

Symbol

Curved Surface

Curved Wall

Keyway

Planar Face

Simple Hole

Complex Hole

MultiStep Holes

Edge
/Edge-Segment

Bend (B&T)

Lance

Import from PMI?

Tolerance (coordinate)

X

X

X

X

X

X

 

X

 

X

N

Roughness Ra

X

 

 

X

X

 

 

 

 

 

Y, except  SOLIDWORKS™

Roughness Rz

X

 

 

X

X

 

 

 

 

 

Y, except SOLIDWORKS™

Diam Tolerance

X

X

 

 

X

 

 

 

 

 

Y

True Position

X

X

 

X

X

 

 

X

 

X

Y

Bend Angle Tolerance

 

 

 

 

 

 

 

 

X*

 

N*

Circularity

 

X

 

 

X

 

 

 

 

 

Y

Concentricity

 

X

 

 

X

 

 

 

 

 

Y

Cylindricity

 

X

 

 

X

 

 

 

 

 

N

Flatness

 

 

 

X

 

 

 

 

 

 

Y

Parallelism

 

X

 

X

X

 

 

 

X

 

Y

Perpendicularity

 

X

 

X

X

 

 

 

 

 

Y

Profile of Surface

X

X

 

X

 

 

 

 

 

 

Y

Runout

X

X

 

X

X

 

 

 

 

 

Y

Total Runout

X

X

 

 

X

 

 

 

 

 

Y

Straightness

 

X

X

X

X

 

 

 

 

 

Y

Symmetry

X

X

X

X

X

 

 

 

 

 

Y

* CAD Angularity GD&T callout maps to Bend GCD in Bar & Tube only. Entered manually only.