Introduction to GD&T Part 2

In my last blog, we highlighted a basic overview of what GD&T is, the different categories, and the number of different characteristics that can be used in a part drawing. For more information on my previous blog, follow this link. This blog, GD&T Part 2, will focus on defining a feature control frame and breaking down how to understand what’s happening within it.

Feature Control Frames

One item that’s needed no matter which category is used in a print is a feature control frame. This is defined as an enclosed area used to describe tolerances and the conditions on the feature of a part. Figure 1 shows a basic feature control frame with arrows describing what can be added to each cell. The number of items that can be placed within this frame can be anywhere from 2 cells up to 3+ depending on how many datums are called out on the print. A general rule is that the more information within the feature control frame, the tighter the tolerancing will be. The reason for including the “IF APPLICABLE” note is that those are not needed for each different geometric symbol.

Figure 1: Feature Control Frame

·         Geometric symbol – this will always be filled out with any of the 14 symbols

·         Diameter symbol – only used in conjunction with certain geometric symbols

·         Tolerance – the amount of extra wiggle room that can be added to a part feature

·         Tolerance modifiers – symbols notifying

·         Primary Datum – the main datum used to constrain the part

·         Secondary Datum – the next datum after the main datum to constrain a part

·         Tertiary Datum – the datum after the secondary datum used to constrain a part

Figure 2: FCF Example

Above is a basic example of a feature control frame in use. The GD&T being used is calling out the location of the 4 holes in the corners of the part. Assuming no general tolerance is being applied, the holes will each have a diameter of ø1.000. The feature axis of each hole must be within the .005 tolerance zone called out in the control frame. The position of this tolerance zone is when the hole is at its Maximum Material Condition (MMC) in relation to datums A, B, and C.

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