Alessandro Anzalone, Ph.D. Hillsborough Community College, Brandon Campus

Sections: 1. Definitions 2. Material Conditions 3. Modifiers 4. Radius and Controlled Radius 5. Introduction to Geometric Tolerances 6. Exercises

Features and Features of Size A feature is a general term applied to a physical portion of a part, such as a surface, hole, or slot.

A feature of size (FOS) is one cylindrical or spherical surface, or a set of two opposed elements or opposed parallel surfaces, associated with a size dimension. A key part of the FOS definition is that the surfaces or elements must be opposed. A feature of size... Contains opposing elements or surfaces. Can be used to establish an axis, median plane, or centerpoint. Is associated with a size dimension.

Internal and External Features of Size External features of size are comprised of part surfaces (or elements) that are external surfaces, like a shaft diameter or the overall width or height of a planar part. Internal features of size are comprised of part surfaces (or elements) that are internal part surfaces, such as a hole diameter or the width of a slot. Feature of Size Dimensions A feature of size dimension is a dimension that is associated with a feature of size. A non-feature of size dimension is a dimension that is not associated with a feature of size.

Actual Local Size and Actual Mating Envelope Actual local size is the value of any individual distance at any cross section of a FOS. The actual local size is a two-point measurement, taken with an instrument like a caliper or micrometer, that is checked at a point along the cross section of the part. A FOS may have several different values of actual local size. The term, actual mating envelope is defined according to the type of feature of size being considered. The actual mating envelope (AME) of an external feature of size is a similar perfect feature counterpart of the smallest size that can be circumscribed about the feature so it just contacts the surfaces at the highest points. For example, a similar perfect counterpart could be: - a smallest cylinder of perfect form - two parallel planes of perfect form at a minimum separation

The actual mating envelope of an internal feature of size is a similar perfect feature counterpart of the largest size that can be inscribed within the feature so that it just contacts the surfaces at their highest points. A similar perfect feature counterpart could be a largest cylinder of perfect form. It could also be two parallel planes of perfect form at maximum separation that just contact the highest points of the surfaces.

Material Conditions A key concept in geometric tolerancing is the ability to specify tolerances at various part feature material conditions. A geometric tolerance can be specified to apply at the largest size, smallest size, or actual size of a feature of size. Maximum Material Condition (MMC) Maximum material condition is the condition in which a feature of size contains the maximum amount of material everywhere within the stated limits of size for example, the largest shaft diameter or smallest hole diameter.

Least material condition is the condition in which a feature of size contains the least amount of material everywhere within the stated limits of size for example, the smallest shaft diameter or the largest hole diameter.

Regardless of feature size is the term that indicates a geometric tolerance applies at any increment of size of the feature within its size tolerance. An other way to visualize RFS is that the geometric tolerance applies at whatever size the part is produced. There is no symbol for RFS because it is the default condition for all geometric tolerances. Material Condition Usage Each material condition is used for different functional reasons. Geometric tolerances are often specified to apply at MMC when the function of a FOS is assembly. Geometric tolerances are often specified to apply at LMC to insure a minimum distance on a part. Geometric tolerances are often specified to apply at RFS to insure symmetrical relationships.

Modifiers In the language of geometric tolerancing there are a set of symbols called modifiers. Modifiers communicate additional information about the drawing or tolerancing of a part. There are eight common modifiers used in geometric tolerancing. The first two modifiers, MMC and LMC were explained earlier in this chapter. The projected tolerance zone modifier changes the location of the tolerance zone on the part. It projects the tolerance zone so that it exists above the part. The tangent plane modifier denotes that only the tangent plane of the toleranced surface needs to be within this tolerance zone.

The diameter symbol is used two ways: inside a feature control frame as a modifier to denote the shape of the tolerance zone, or outside the feature control frame to simply replace the word, diameter. The radius and controlled radius modifiers are always used outside the feature control frame. The modifier for reference is simply the method of denoting that information is for reference only. The information is not to be used for manufacturing or inspection. To designate a dimension or other information as reference, the reference information is enclosed in parentheses.

A radius is a straight line extending from the center of an arc or a circle to its surface. The symbol for a radius is R. When the R symbol is used, it creates a zone defined by two arcs (the minimum and maximum radii). The part surface must lie within this zone.

A controlled radius is a radius with no flats or reversals allowed. The symbol for a controlled radius is CR. When the CR symbol is used, it creates a tolerance zone defined by two arcs (the minimum and maximum radii). The part surface must be within the crescent-shaped tolerance zone and be an arc without flats or reversals.

Geometric Characteristic Symbols Geometric characteristic symbols are a set of fourteen symbols used in the language of geometric tolerancing. The symbols are divided into five categories: form, profile, orientation, location, and runout. Certain geometric symbols never use a datum reference and other geometric symbols always use a datum reference. Furthermore, some geometric symbols may or may not use a datum reference.

Feature Control Frame Geometric tolerances are specified on a drawing through the use of a feature control frame. A feature control frame is a rectangular box that is divided into compartments within which the geometric characteristic symbol, tolerance value, modifiers, and datum references are placed.

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http://www.thayer.dartmouth.edu/mshop/pdf/introdr.pdf http://www.etinews.com/gdt_glossary.html