Critical Operations

There can be two types of critical areas on a work piece

 (a) Process Critical Area

 (b) Product Critical Area

Process critical area is used to provide location areas for the work piece in subsequent machining operations. Product critical area is identified by one or more demanding functional specifications on the part print. A surface on the workpiece can qualify as both a product and process critical area.

The important dimensional relationships of the workpiece can be maintained throughout the process sequence by identifying the critical process areas first.

 Product critical areas may originate as a result of close surface finish, specifications or tolerances which cannot be maintained in normal machining operations on these areas and may have to be completed by a more refined or special process. The positions of such operations within the sequence are determined by several conditions as given below:

(a) The succession of machining operations which must take place normally before the operation in question can be performed must be taken into account. For example, a hole must be drilled before it can be tapped.

(b) The degree of accuracy that must be maintained between the related surfaces may dictate that several less refined operations be performed before the final operation on the surface can take place.

(c) The introduction of auxiliary processing operations into the major operation sequence may dictate how the product critical surface can be accomplished.