Dimensions and Tolerances
Proper dimensioning is the key to obtaining what is wanted at optimum cost. Proper dimensioning will assure a precise product, speed production and permit accurate measurement. It is probably the most important phase in preparing a drawing.
Dimensions serve two important functions. First, they limit the size of the part shown on the drawing. Second, they define the limits within which the parts will be acceptable. Precise dimensions correctly applied aid the user as well as the producer.
Tolerances should be noted for each dimension, and a tolerance block should note those that apply to all other dimensions. When noting tolerances or dimensions, it is preferred that fractions not be used and that a decimal system be used to show all tolerances. Specify tolerances only as close as necessary for part functions — but clearly state a tolerance for all lineal and radial dimensions and angles.
Surface texture designations should conform to ANSI/ASME Y14.36M, Surface Texture Symbols. Unless a maximum and minimum value is shown, the value shown will be the average surface texture value for that surface as defined in this standard.
A dimension shown only as a maximum implies no minimum. A corner, for example, shown as break “.005 maximum” could be supplied razor sharp and be to specification. If this is not what is wanted, a minimum must be shown.
Always dimension with this question in mind — “How can this be measured?” Try to take advantage of readily available stock size materials, tools and gages.
Wherever possible, use a common reference datum point or plane for all dimensions. Be sure all distances are clearly dimensioned. Ambiguity leads to misinterpretation and trouble.
Show all dimensional relationships for concentricity, squareness or control of another dimension. Use standard symbols.
Dimension in a flat plane view and use as many views as necessary to show each detail. Avoid perspective drawings.
Threads are best dimensioned in the standard accepted manner; e.g. 1/4-20 UNC-2A. If special or restricted major, minor or pitch diameters are required, the maximum and minimum limits should be dimensioned in decimals to three or more places.
A chamfer at the end of a thread should be dimensioned at least .015″ below the thread root diameter.
See Drawing #2 below for a sample of the dimensioning and tolerance call outs mentioned in this section.