General Tolerance on a drawing establishes a default tolerance, thus saving repetitive annotations on the drawing.
Previous Tip (General Tolerance) demonstrated Plus / Minus Dimensions are insufficient to assure clear unambiguous geometric specifications, while Profile can control limits of Surface Form, Orientation, Location and even Size.
This Tip illustrates the Limits established by General Profile control.
Figure 1 is a balloon inspection drawing, note that since part geometric properties limits are controlled by Profile of surface, balloons distinguish the Surfaces to be inspected. The Basic, Theoretical Exact Dimensions define the nominal perfect geometry.
Plate thickness Size is marked STOCK (commercial stock size) therefore the standards for these items govern the surfaces that remain in the as furnished condition on the finished part. STOCK properties and compliance should be reported in plate COC (Certificate of Compliance).
Prescribed Datum Reference Frame:
- Surface 6 tangent to a Planer object (simulating Datum A).
- Surface 1 Tangent to a Planer object (Perpendicular to Datum A simulator).
- Surface 5 & 6 High Points Contacting a Pair of Parallel Planer objects (Perpendicular to both Simulators of Datums A & B) at minimum separation.
Tolerance Zones of surfaces
Line and Color conventions
- Theoretical model surface represented by Black continuous line.
- Real workpiece surface represented by Blue continuous line.
- Extreme Boundaries represented by Blue Dashed lines.
Surface 4 (Detail A)
Shall be within the Tolerance Zone (Extreme Boundaries).
The Tolerance Zone lies between two boundaries 3mm apart, equally disposed about the Theoretical Profile (±1.5).
Theoretical Profile is a surface of 160mm radii, surface axis is perpendicular to Datum A, surface is tangent to a Plane Parallel to Datum B at 100m offset.
Surface 5 (Detail A)
Shall be within the Tolerance Zone (Extreme Boundaries).
The Tolerance Zone lies between two boundaries 3mm apart, equally disposed about the Theoretical Profile (±1.5mm).
Theoretical Profile is a planer surface offset 100mm to Datum C (Width symmetry plane).
Surface 1(Detail B)
At the Datum Feature the distance to the true profile is zero.
only half of the Tolerance Zone (Extreme Boundaries) is available.
The Tolerance Zone lies between Datum B and a 1.5 mm plane offset of it. so in this case the Profile controls only Form and Orientation of the feature.
Surfaces 2 and 3 (Detail B)
Shall be within the Tolerance Zone (Extreme Boundaries).
The Tolerance Zone lies between two boundaries 3mm apart.
Theoretical Profiles are related to Datum Reference Frame |A|B|C| (similar to surface 4 and 5) Profile controls Location as well, therefore Tolerance Zone lies between two boundaries 3mm apart, equally disposed about the Theoretical Profile (±1.5mm).
In summary
In most cases, General Tolerance is intended to be the Common Control, save repetitive annotations on the drawing.
Profile controls the limit of deviation from Nominal. When Profile controls Surface Location default Tolerance Zone is equally disposed about the Theoretical Profile.
Keep in Mind
Geometric Tolerances are a tool to define Geometric Functional Requirements
State Standard & Revision to assure proper Decoding
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