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.