There has been
a lot of discussion over the years between various
suppliers of investment castings regarding tolerance
that is appropriate for the process. While there continues
to be some debate, what follows is meant to be a general
guideline. An investment casting is meant to be a
"near net-shape" part that offers the most value.
If the buyer and producer maintain a high level of
communication, the design requirements and cost efficiencies
will be met by processing capabilities.
 Straightness, Roundness, &
Positioning (holes or bosses)
Axial straightness, roundness (bar or tube shapes)
and positioning of holes or bosses is generally tolerance
as +/-.005 per inch.
Concentricity
The center of a bore to the outside diameter
of a part will be concentric within .003" for each
1/2" of wall thickness. Note, this assumes measurements
are taken in the same plane and does not include out
of roundness tolerance.
Angularity
As is the case with other dimensional tolerances for
castings, the as-cast angle tolerance is largely a
function of the part design. In general, a tolerance
of +/-1/2 degree is considered minimum.
Blind Holes and
Slots
These features are generally cast provided the depth
is equal to the width or diameter for small characteristics
less than 3/8". As the size increases above 3/8",
the depth can move incrementally to double the size
of the opening.
Flatness
This feature of casting is measured as the distance
between two parallel planes, within which, the entire
surface of the part must lie. Once again, the actual
shape of the part will largely influence the cooling
behavior of the wax pattern and the part as it solidifies;
and this, in turn, tends to govern process capability.
This requirement is frequently expressed in terms
of six (6) square inches of surface area.
| Part Thickness |
Flatness
T.I.R. |
| .000" to .500” |
.004" |
| .051" to 1.000” |
.008" |
| 1.000" to 2.000” |
.15” |
Wall
Thickness
Although wall thicknesses down to .020 may be achievable
in a small area, the normal minimal wall thickness
zone lies between .040 and .080 depending upon alloy
and square inches of surface area. |
