The material from which the part on the drawing will be machined is as important as the piece itself. The right material does the job best.

Specify and identify all materials using standardized and accepted designations, such as those published by the American Iron and Steel Institute, the Copper Development Association, the Aluminum Association, the Society of Automotive Engineers, etc.

If a trade name material is specified for a special characteristic, the producer or source of acceptable equivalent materials should be listed on the drawing. When a company identification must be used to specify a material, a copy of the specification should be included, or a listing of alternate standard materials provided.

Wherever possible, free machining grades or alloys should be specified consistent with the end use of the part. Usually the ductile grades or alloys are not specified for precision machined products; e.g. AISI 1016,1018, etc.

Stock Size and Part Size

Stock sizes and tolerances often can be used to advantage when designing a component. The largest diameter on the component can be the stock size of the material when the material's stock tolerances are acceptable. Examples of components designed using stock sizes and tolerances are shown in the drawing below.

CAUTION: The surface of bar stock may have shallow seams, laps, pits or be decarburized. In many cases, these minor imperfections are not detrimental to the function of the component. If they are detrimental to the function or appearance of the component, sufficient material must be removed to produce a defect free surface. Consult your material supplier since the amount of material to be removed varies by grade and material.

When a turned diameter, without flats showing, is required on a component from square, hexagonal or octagonal bar stock, the tolerance on distance between flats for that material must be considered when specifying the diameter's size.

Tubing sizes can have three tolerance zones, the OD, the ID and the wall thickness. These should be considered when tubing can be used for a hollow cylindrically shaped component. Only two of the three tolerances should be used when dimensioning a component from tubing.

Not all materials use the same tolerances. A study of material suppliers' handbooks will reveal which metals have tolerances above and below the nominal size, and which have the tolerance range only below the nominal size. This condition is shown in the examples in the drawing below.

Cold drawn carbon steels are produced to nominal specified sizes. The tolerance is on the minus side. Nonferrous metals and stainless steels are produced to nominal sizes with the tolerance range both plus and minus.

Plastics can be obtained to any specified size, but the size can vary as ambient conditions change.

CAUTION: Subsequent processing of a part should be considered when specifying the kind and grade of material to be used.

Materials and Finish

Surface finishes on precision machined products depend to a great extent on the grade of material used. For example, soft ductile grades of steel do not machine to as fine a finish as do the free machining grades. On soft ductile grades of material, it is often necessary to add a grinding or polishing operation to obtain a fine finish. This adds to cost. In steels, the free machining grades in the 1100 and 1200 series and the leaded grades are preferred. Half hard or harder annealed copper alloys are preferred. The best aluminum finishes are produced on the harder tempered aluminum alloys.