What is GD&T?
Geometric Dimensioning and Tolerancing (GD&T) refers to the symbols, dimensions, and other manufacturing notes that accompanies a mechanical drawing or schematic. These are critically important because they tell the manufacturer exactly how much variance is allowed in the manufacturing process as well as other insight into assembly, part fits, allowed modifications for manufacturing, and anything else the person making or checking the part may need to know.
A good engineering drawing should clearly and concisely convey all necessary dimensions and tolerances while leaving out all unnecessary information and clutter. Below are some basic rules for creating a good drawing (adapted from PLTW Introduction to Engineering Design)
Rules for a good drawing:
- Dimensions or information should never be given twice on the same drawing
- Unnecessary dimensions should be left out of the drawing
- Placing dimensions:
- Place dimensions on finished edges and features or important centerlines whenever possible
- Dimensions should be placed in such a way that no numbers need to be assumed or determined
- Always place dimensions on the view that best shows that feature
- Unless necessary, avoid dimensioning to hidden lines
- Avoid placing dimensions on the part view – these should always lay beside the part
- Formatting dimensions:
- Never cross dimension lines
- Never cross extension lines
- The largest dimension should be placed furthest from the part view so smaller dimensions can be nested without crossing extension and dimension lines
- Center lines may be used as extension lines
- Leader lines should have a slope of 30, 45, or 60 degrees
- Dimensions should be centered between arrowheads of dimension line whenever possible (staggering is allowed for stacked dimensions)
- The dimension should ALWAYS give the actual size of the object, not the scaled size
- Dimensioning circles:
- Typically circles are given by their diameter and ars/fillets by their radius
- Hole location should be given by their center point, not by their tangent distance
- Holes should be located/dimensioned on the view showing them as a circle (see Placing Dimensions rule 3)
See below for a few drawing examples! One is an example of a good drawing, another is a bad drawing. Before you take a look at the cheat sheet, see if you can figure out what is wrong with the dimensions in the bad drawing based on the above rules!
Take a look at the below drawings: notice how with the bad drawing, the part could be designed and made – the drawing is just missing critical tolerance information, and while all the dimensions are present in one form or another it is done in a much less clear manner than the good drawing. The key to a good drawing is concisely and clearly relaying the information without clutter and unnecessary information.