LINEAR TOLERANCES
The chart below can be used for most applications.

  • Grade MD 1: for dimensions with no tolerances required.
  • Grade MD 2: for functional dimensions which require specified tolerances.

The general rule for linear tolerancing of investment castings is usually expressed as ± 0,5 % of the nominal dimension. Tighter tolerances can be made available for a few selected design characteristics. However, one should know that tighter tolerances usually require special procedures and/or secondary operations which can increase the cost of the casting.

DIMENSIONS (mm)
PRECISION GRADE
MD 1
MD 2
from
to
deviation
+ or -
tolerance
range
deviation
+ or -
tolerance
range
6
± 0,10
0,20
± 0,08
0,16
6
10
± 0,12
0,24
± 0,10
0,20
10
14
± 0,15
0,30
± 0,12
0,24
14
18
± 0,20
0,40
± 0,14
0,28
18
24
± 0,25
0,50
± 0,17
0,34
24
30
± 0,30
0,60
± 0,20
0,40
30
40
± 0,36
0,72
± 0,25
0,50
40
50
± 0,42
0,84
± 0,30
0,60
50
65
± 0,49
0,98
± 0,35
0,70
65
80
± 0,58
1,16
± 0,42
0,84
80
100
± 0,68
1,36
± 0,48
0,96

FLATNESS
A flatness tolerance is the total deviation permitted from a plane and consists of the distance between two parallel planes between which the entire surface so toleranced must lie.

Degree of flatness in an investment casting is almost always determined by the volumetric shrinkage of wax and metal during cooling. This problem is typical of the casting process, and can be controlled but not avoided.

General flatness tolerances cannot be quoted because they vary with configuration and alloy used. Wall thickness of the plane is, for instance, a significant parameter: with the same surface extension, the higher the wall thickness, the larger the flatness error.

Morover, the flatness error of an uninterrupted plane will be higher than the flatness error of a plane with holes or other things which interrupt its extension (basically, to calculate the tolerances, an interrupted plane must be considered not only one plane, but as many planes as those definited by the interruptions). The following tab reports a general indication:

Areas under
2500 mm2
Areas over i
2500 mm2
Section thickness
Allowed dish
Allowed dish for every 3800 mm2
over 2500 mm2
Up to 6 mm
Not significant
Not significant
from 6 to 13 mm
0,25 mm
0,10 mm
from 13 to 25 mm
0,25 mm
0,20 mm
over 25 mm
0,25 mm
0,40 mm

PERPENDICULARITY
When specifying perpendicularity, use the longest plane for reference: the allowed deviation to be specified will be the deviation of the shortest plane against the longest one.

Perpendicularity tolerance of a plane is generally 0,8% the plane lenght. Basically, when A is the dimension of the shortest plane and B the dimension of the longest one, plane A will be perpendicular to surface B with a tolerance of 0,008 x A.

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Tolerances
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