Machining Tolerances

The dimensional tolerances which are attainable on secondarily machined surfaces differ from those on as-formed surfaces. The level of precision that can be achieved is dependent on the dimensional stability of the formed part, the type of machining to be done, its location relative to the rest of the formed part, and the machining process used. Most secondary maching operations can be classified as being either manual, numerical or punched or stamped.

Manual machining is the most common and refers to an operator drilling, sawing or routing the formed part using simple fixtures.

COMPUTRIMSM refers to computer controlled machining wherein the formed part is held in a precision stationary fixture while the cutter moves through predetermined computer controlled machining paths.

Punching and stamping refers to the use of fixed cutters such as steel blanking dies or punch and die sets. Table 1 shows the tolerances which can be expected with these three machining procedures on the formed part shown in Figure 19.

The attainable tolerance on a given part is a very individual thing that is dependent upon many variables. Each combination of part design, material, die and process must be considered individually. Establishing tolerances is another one of those ill-defined details which should be reviewed with an experienced Thermo Pressure Former before releasing the part for production tooling.