Specifying or importing tolerance and roughness values

You can specify geometric and dimensional tolerances (G&DT) and surface roughness requirements for GCDs. You can also automatically import many of these values directly from CAD model Product Manufacturing Information (PMI) attributes. Tolerance and surface roughness requirements can have a large impact on the process or operation selected to manufacture a GCD and the cycle time required to complete it.

aPriori stores tolerance capability data for each manufacturing process in the lookup table tblGtolProcessCapabilities (see * CAD Angularity GD&T callout maps to Bend GCD in Bar & Tube only. Entered manually only.

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 also 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 determines cycle time adjustment factors.

You can specify tolerance and roughness requirements in one of three ways:

• By manually setting default values for all of a component’s GCDs through the Tolerance Policy Editor.

• By manually setting values for specific GCDs either through the Tolerance Editor, or by using the fields in the Geometric Cost Drivers panel.

• By importing semantic PMI annotations directly from the CAD model.

Note: aPriori stores and displays tolerance values as total tolerance. This means, for example, that if a CAD system specifies a tolerance of “±0.25”, you should enter the total tolerance, "0.5”. If you import such a tolerance from a PMI attribute, the aPriori will display it as a total tolerance.