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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.
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