Form and Positional Tolerances of Forged Products

 Formand Positional Tolerances of Forged Products

The "ideal shape" of a forging designed by engineerson technical drawings is the form intended to achieve the desiredfunctionality. However, due to manufacturing process influences, the"actual shape" of the forging inevitably deviates from the drawing.

Causesof Form and Positional Errors in Stamped Parts
The reasons for form and positional deviations in stamped parts are as follows:

1.     Dimensionalerrors caused by tool adjustment, wear, cutting forces, or heattreatment.

2.     Formerrors (e.g., roundness or flatness deviations) caused byclamping forces, cutting forces, vibrations, or residual forging stresses.

3.     Positionalerrors (e.g., parallelism relative to an axis or surface) causedby cutting pressure, clamping forces, or machine tool positioning inaccuracies.

In components such as crankshaft drive mechanisms, dimensionalaccuracy, roundness, and axial alignment are critical for bearing clearance andload-bearing ratios at bearing support surfaces. Compared to surface quality,dimensional and form errors have a greater impact on the mating capability offorged parts.

Magnitudeof Form and Positional Tolerances

·        If form and positional tolerances are not specified on thedrawing, all deviations must remain within the dimensional tolerance limits.

·        Even if unspecified, tolerances critical to the forging’sfunctionality must not exceed half of the dimensional tolerance.

InspectionRequirements
If inspections do not follow the "datum features" and"toleranced features" marked on the drawing, they are invalid. Forexample:

·        Testing the shape of a shaft on a gauge without referencing thedatum.

·        Clamping positions in tolerance cylinders, but measuring radialrunout errors on shaft journals.

Typesof Tolerances
Tolerances are categorized into:

1.     PositionalTolerances:

o    Directional tolerances (e.g.,parallelism of guide rails or perpendicularity of a milling machine spindle tothe worktable).

o    Angularity tolerances (e.g.,a 60° inclined hole relative to datum A must lie between two parallel planesspaced 0.1 mm apart).

o    Location tolerances (e.g.,positional deviation of a hole).

o    Coaxiality tolerances (e.g.,a tubular tolerance zone around a datum axis).

o    Symmetry tolerances (e.g.,slots or holes symmetrically aligned with a central plane).

All positional tolerances are datum-dependent. For example,runout tolerances (radial/axial) require rotating the forging about a datumaxis.

2.     FormTolerances:

o    Flatness (e.g.,straightness of a cylinder or planar surface).

o    Circularity (e.g., roundnessof cylindrical or conical surfaces with annular tolerance zones).

3.     ProfileTolerances:

o    Surface profile (e.g.,limiting shape errors of an entire wing or car roof).

o    Line profile (e.g.,contour accuracy of a linear feature like an airfoil edge).

http://www.forging-shafts.com/Tolerances-of-Forged-Products.html