CAST SURFACE SMOOTHNESS AND MACHINE FINISH ALLOWANCE

Casting drawings show the casting prior to machining. They should have the finish allowance indicated below included in the dimensions shown and no finish symbols added. It is recommended that an undimensioned phantom line be used to represent surfaces after machining.

Generally only surfaces marked with machining symbols on the drawing will have machine stock added to them.

It is imperative that the designers intentions be clearly stated on the drawing to avoid costly misunderstandings. Dimensions that are toleranced more tightly than casting dimensions can be held will be assumed to be machined even though no finish symbol is shown. If the finish symbol is used on the field of the drawing, stock will be allowed for machining. To indicate desired casting smoothness a note like the one at left should be provided in the list of drawing notes.

The surface smoothness required shall be expressed by a numerical surface "roughness-height-rating" (RHR) in micro inches. The value of the number is maximum as specified and indicates the maximum permissible sustained height rating. Any lesser rating is acceptable. A micro inch is one millionth (0.000001) inch.

A range of smoothness normally obtained with sand castings is shown on the cast surface comparator shown at left. Values shown apply to major portion of the casting surfaces. Areas of gating, risering, fillets and parting lines are not included.

Cast surface comparator blocks for visual and touch inspection are available from the Aluminum Association, 818 Connecticut Ave., Washington, D.C. 20006. It is necessary that inspections be made against the actual comparator block.