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GD&T Basics: CNCCookbook Beginner’s Guide


Like almost anything else, GD&T basics are simple concepts that we can use to group all of the other information into. We’ll discuss the following key GD&T Basics:

– GD&T Symbols

– Basic Dimensions

– Feature Control Frames

– Datums and Features

– Material Condition Modifiers

If we think about engineering drawings, a basic drawing provides size and shape descriptions. If we add some tolerances, they affect the description of size. Adding GD&T affects the description of shape in a very quantitative way, and it also provides a more comprehensive way to think about tolerances. In fact, the specification of shape includes tolerance as well. For example, if we use GD&T to suggest a surface must be flat, that describes its shape. But we will also include a tolerance that describes exactly how flat the surface needs to be–tolerances for flatness, in other words.

Let’s take a more detailed look at each of GD&T’s basic concepts.

GD&T Basic: Symbols

The Symbols, or Geometric Symbols, are what most people think of first when they hear about GD&T. There’s a collection of symbols that are used to specify each shape characteristic or tolerance that GD&T can describe. Here is a typical example, the Concentricity Symbol:

GD&T Concentricity Symbol…

Like most GD&T graphical symbols, Concentricity looks like what it is–two circles are shown concentrically. This makes it easier to remember the symbols.

This course includes a detailed Quick Reference Chart for the GD&T symbols, and we’ll put a link to that chart at the bottom of every page for your convenience. You may find it convenient to keep the chart open on a separate browser window for reference when working with GD&T.

GD&T Basic Dimensions

When you see a number in a box, that’s a Basic Dimension.

A number in a box denotes a Basic Dimension. Image Source

Given that there is just a number in a box, Basic Dimensions have no tolerances.

GD&T Basic: Feature Control Frames

A Feature Control Frame describes the conditions and tolerances of a geometric control on a part’s feature. A feature control frame contains four pieces of information:

  • GD&T symbol or control symbol
  • Tolerance zone type and dimensions
  • Tolerance zone modifiers
  • Datum references if the GD&T symbol requires them

Each Feature Control Frame will typically (it is optional) have a leader arrow pointing to the feature it describes. Consider the Feature Control Frame in the drawing above:

A Feature Control Frame…

The Leader Arrow points to the hole feature. It has a Geometric Symbol, in this case, a Position Symbol. Given that this is a round hole, the tolerance could be a diameter, in which case we’d have a diameter symbol ahead of the tolerance. Instead, we just have a tolerance value, so this is a positional tolerance for the center of the hole. We could follow the tolerance with a Tolerance Modifier. The “A” and “B” are the primary and secondary datum. If we look to the original drawing (top of page) we can see that these tell us where to measure the hole’s center from. In this case, “A” denotes the top edge of the part and “B” denotes the left edge.

This is a lot to take in, and we’re not trying to completely describe every aspect of Feature Control Frames here. Rather, we are introducing a concept we will elaborate on more fully later in the tutorial.

GD&T Basic: Datums and Features

As mentioned above, Datums tell us where to measure from. Calling them out clearly and labelling each Feature Control Block with the applicable Datums makes it completely clear how things are to be measured. While a datum may make sense on a drawing, it is important to keep in mind that parts are physical. You could specify a datum in such a way that it is impossible to take the measurement on a real physical part, so always visualize how a datum could be used to inspect the part.

Features, meanwhile, are the real geometric shapes that make up the physical part. Examples include holes, screw threads, profiles, and faces or slots.

GD&T Basics: Material Condition Modifiers

Material Condition Modifiers are used to refer to a feature in its largest or smallest condition, or to refer to it regardless of feature size. There are three Material Condition Modifiers:

– Maximum Material Condition (MMC): MMC might be used, for example, to refer to the largest pin or the smallest hole.

– Least Material Condition (LMC): LMC could refer to the smallest pin or the largest hole.

– Regardless of Feature Size (RFS): RFS could refer to any increment of feature size of any feature within its size tolerance.

Basic Rules of GD&T

The proper application of GD&T requires that these rules be followed:

  • All dimensions must have a tolerance. Since all real physical objects are subject to variation, we must specify the limits of allowable variation. We can do this either by applying plus and minus tolerances directly to dimensions or by using a tolerance block or note. For Basic Dimensions, GD&T applies tolerances using a related Feature Control Block.
  • Dimensions define the nominal geometry and allowable variation. It is not allowed to derive that information through measurement or scaling of the drawing except in certain cases. This is important as casual CAD users are quite used to just adding dimensions to a model or measuring the model directly, which is not allowed under GD&T.
  • Every dimension and tolerance required to define the finished part shall be shown on the drawing. If additional dimensions would be helpful, but are not required, they should be marked as reference.
  • Dimensions should be applied to features and arranged in such a way as to represent the function of the features. Additionally, dimensions should not be subject to more than one interpretation–they must be unambiguous.
  • Descriptions of manufacturing methods should be avoided. The drawing should strictly limit itself to describing the geometry and not the method of manufacture.
  • If certain sizes are required during manufacturing but are not required in the final geometry (perhaps due to shrinkage) they should be marked as non-mandatory.
  • All dimensioning and tolerancing should be arranged for maximum readability and should be applied to visible lines in the drawing.
  • When geometry is normally controlled by gage sizes or by code (e.g. stock materials), the dimension(s) shall be included with the gage or code number in parenthesis following or below the dimension.
  • Angles of 90 degrees are assumed when lines (including center lines) are show at right angles, but no angular dimension is explicitly shown. (This applies to any other orthogonal angle of 0, 180, 270, etc.)
  • Dimensions and tolerances are valid at 20 degrees C and 101.3 kPa (standard conditions) unless stated otherwise.
  • Unless explicitly stated, all dimensions and tolerances are only valid when the item is in a free state.
  • Dimensions and tolerances apply to the length, width, and depth of a feature including form variation.
  • Dimensions and tolerances only apply at the level of the drawing where they are specified. It is not mandatory that they apply at other drawing levels, unless the specifications are repated on the higher level drawings.

The basic rules tell us how to apply GD&T to a drawing.

Next Article: GD&T Datums

GD&T Table of Contents          GD&T Symbols

CNCCookbook Beginner’s Guide to GD&T: Basic Concepts
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