Virtual condition for an internal feature must have the same condition as an external feature. The feature must have size, a geometric toler- ance, and a modifier. The virtual condition is cal-
13.2 12.3 (LMC) 12.3 (LMC) 0.9 0.9 TOLERANCE TOLERANCE
ZONE GAGEGAGE
0.5 TOLERANCE TOLERANCE ZONE (TOLERANCE ZONE MAY ZONE (TOLERANCE ZONE MAY INCREASE TO 1.5 AT LMC1.5 AT LMC
DATUM AXIS "E" DATUM AXIS "E"
13.5 VIRTUAL 13.5 VIRTUAL CONDITION AN ADJUSTABLE AN ADJUSTABLE CYLINDER TO CYLINDER TO ACCEPT THE RFS ACCEPT THE RFS FEATURE FEATURE GAGE GAGE
culated opposite of an external feature. Virtual condition is the feature MMC size minus the geo- metric tolerance. Figure 7-8 illustrates a feature of size (hole), the geometric tolerance, and a modifier.
The virtual condition of an internal feature is a constant value equal to its Maximum Material Condition size minus its applicable tolerance of form or location. For this hole, the virtual condi- tion is equal to 12.4 mm (MMC) minus 0.2 mm. The virtual condition is 12.2 mm (inner boundary, locus).
The resultant condition of an internal feature is a variable value equal to its actual mating size plus its applicable tolerance of form or location. The virtual condition of this hole is smaller than the specified size limits of the hole. This is
acceptable because the hole at any cross-sectional measurement is within the size limits and the axis may lay any place within the diametral tolerance zone of 0.2 mm at MMC. This hole, like the pin in earlier examples, is permitted to bow, taper, or wave. The shaft that passes through this hole may not be the same shape as the hole. Therefore, some acceptable condition must be established— the virtual condition. Figure 7-9 illustrates how the hole may be produced.
The functional gage used to check this hole would have a 12.2 mm diameter pin (fixed). The pin is not expandable as it would be if the tolerance was specified at RFS. The pin is positioned in the fixture so that it would also check the hole in rela- tion to the datums. Figure 7-10 illustrates how this part might fit a gage for checking the hole. RR 25 BASE RADIUS BASE RADIUS R 50 CREST RADIUS CREST RADIUS 0.5:1.0 Y X 0.5 0.5 A BM X Y 12.5±0.1 0.2 0.2M A 0.5 0.5 A B ( 12.6 12.6 12.2 VIRTUAL 12.2 VIRTUAL CONDITION CONDITION 12.2 12.2 GAGE PIN GAGE
Figure 7-8 Feature of size and modified geometric
tolerance.
Figure 7-9 Acceptable hole conditions for part in Figure 7-8.
Figure 7-10 Functional gage for checking the hole in the part for Figure 7-8.
This gage, then, will accept only parts that are 100% interchangeable if properly designed. The gage accepts holes of any configuration that are within the stated tolerances. This part will need two gages or a gage for the hole and an op- tical comparison for the profile, in order to check and accept the part.
When the controlled feature is also a datum, as is the case with the hole in Figure 7-8, the da- tum feature applies at its virtual condition. The gage-pin axis is considered theoretically exact for the purpose of locating the part profile. When a gage is used to check a datum feature, the gage axis or center line becomes the simu- lated axis.
Summary
Virtual condition is a critical concept of GD&T from design through inspection. It is with proper application of GD&T and interpretation of the virtual condition concept that parts are 100% interchangeable. The concept of virtual condition may best be remembered as an extension of the modifier principles. Virtual condition is always determined to be the MMC size plus geometric tolerance for external features—in other words, the feature’s largest effective actual mating size. For an internal feature, the virtual condition is MMC minus the geometric tolerance. This would be the feature’s smallest effective actual mating size.
Chapter 7 Evaluation
1. Virtual condition is a boundary of perfect form and orientation, generated by the ________ effects of the feature MMC size and applicable geometric tolerances.
2. To determine the virtual condition for an external feature, the feature’s ________ tolerance must be added to the feature MMC size.
3. A feature at virtual condition may ________ the boundary of perfect form as required by Rule One. 4. Virtual condition is the condition in which ________ are made.
5. Virtual condition is the boundary at which features are no longer an ________ part. 6. Functional gaging at virtual condition represents the ________ part.
7. The method for calculating virtual condition for an internal feature is the ________ of that for an external feature.
8. According to Rule ________, the virtual condition of a feature of size is considered the simulated datum.
9. Parts and features made at virtual condition or better are 100% ________.
Introduction
This chapter introduces the principles of tol- erances of location. These tolerances or geomet- ric controls are Concentricity, Symmetry, and Po- sition. The location controls are specified to con- trol the relationships between features or between features and a datum feature. The relationships are toleranced at the axis, center line, or center plane. Positional tolerance then provides the per- missible variation in the specified location of the feature or group of features in relation to another feature or datum. A tolerance of location is ap- plied to at least two features, of which one must be a feature of size (meaning one is a datum).
Because one of the features must be a feature of size, the modifier principles do apply. General Rule Two requires the designer to specify modi- fiers for all features, tolerances, and datums of size. The advantages of the modifiers can be used to their greatest extent with tolerances of location involving part interchangeability and functionality of mating parts. GD&T’s advantages are best real- ized when position and modifiers are specified.
Concentricity
Symbol Definition
Concentricity is the condition where the me- dian points of all diametrically opposed elements
of a figure or revolution (or correspondingly-lo- cated elements of two or more radially disposed features) are congruent with the axis (or center point) of a datum feature.
Tolerance
Concentricity tolerance is always implied and specified Regardless of Feature Size, RFS. The tolerance is a diametral zone in which the axis of the controlled feature must lie. This zone must coincide with the axis (center point) of the datum features. Concentricity is a very restrictive geometric control. A specified tolerance controls the amount of eccentricity error, parallelism of axis, out-of-straightness of axis, out-of-circular- ity, out-of-cylindricity, and any other possible er- rors in the feature axis. This tolerance controls all possible errors at the feature axis. Therefore, it is difficult to verify and may be excessively expen- sive to produce. The actual feature axes must lie within the specified tolerance zone. It may be more effective to use Runout or Position toler- ances.
Application
Concentricity is considered when critical axis-to-axis control is required for dynamically balanced features. This control is selectively specified, because features may be controlled with runout or position. Runout only controls a surface-to-axis at RFS. Position offers all the ad- vantages of GD&T. Concentricity is an axis-to-
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axis control at RFS. Concentricity is normally specified for high-speed rotating parts, rotating mass, axis-to-axis precision, or any other feature critical to function. Figure 8-1 illustrates the proper application of concentricity.
Figure 8-1 Applying concentricity.
Tolerance Interpretation
Concentricity tolerance is always interpreted as RFS. The tolerance zone is diametral around and parallel to the datum axis. Verification is dif- ficult. Concentricity is the determination of the controlled features axes in relationship to the da- tum axis. A radial differential measurement is the most accurate method of determining the actual controlled feature axis. These measurements must be taken opposite of each other. The toler- ance zone for the part in Figure 8-1 is illustrated in Figure 8-2.
Figure 8-2 Concentricity tolerance zone. For concentricity, the feature surfaces must be measured diametrically opposed to each other
to determine the midpoints relative to the axis of the datum.
For coaxial features located with a positional tolerance, the location of the axis of the feature’s actual mating envelope is controlled relative to the axis of the datum feature.
Symmetry
Symbol Definition
Symmetry is the condition where a feature or part has the same profile on either side of the cen- ter plane (median plane) of a datum feature. Tolerance
The tolerance for symmetry is always im- plied to be RFS. The tolerance is applied equally on either side of the controlled feature center line. The implied modifier restricts the tolerance to the specified amount only.
12.5±0.10.1 12.5±0.10.1 0.2 0.2 A-B A B 0.2 TOLERANCE ZONE
DATUM AXIS A-B DATUM AXIS A-B
2.000 2.000±.015.015 3.000 3.000±.015 1.500 1.500±.010 .010 A A
Application
Symmetry is specified for features to be lo- cated symmetrically with respect to the median plane of a datum feature. It may also be specified for a feature in a common plane with a datum plane. This symbol is specified extensively throughout industry. Figure 8-3 illustrates an ap- plication of symmetry.
Tolerance Interpretation
The specified tolerance establishes a toler- ance zone that is the specified width, with half of the tolerance on either side of the datum feature center line. This width zone allows the feature to vary from side-to-side or angularly within the tol- erance zone. The tolerance zone size is not per- mitted to vary with feature size. Figure 8-4 illus- trates how the interior of the part could vary in re- lation to the exterior, which is the datum feature. The controlled feature is permitted a maximum of .005 inch shift to the side in either direction.
Figure 8-4 Symmetry tolerance zone.