[Linear Tolerances¢xFlatness¢xStraightness, Roundness and Positioning]
[Concentricity¢xAngularity¢xBlind Holes¢xParallel Sections]
[Symmetry¢xWall Thickness¢xSurface Finish]
 
These guidelines of investment casting tolerances apply as a general rule of thumb. An investment casting is meant to be an intricately detailed "near net-shape" part that offers the most value and reality. If the buyer and producer continue a completely communication, the design requirement and cost efficiencies will be achievable by processing capability.
 
Linear Tolerances
The general rule-of-thumb for linear tolerance of investment casting is normally expressed as ±.005" per linear inch, of length (± .127 mm per 25 mm). As with any such rule, there are always exceptions required by process capabilities. The table below can be useful for most applications. Optimistic tolerances can be made available for a few selected design characteristics such as holes and slots. One should bear in mind however that tighter tolerances sometimes require special procedures and/or secondary operations such as machining which can increase the cost of the casting and the need for longer production times.
 
U.S.-Inches
Range Standard Optimistic
Up to 1/2" ±.007 ±.003
1/2"-1" ±.010 ±.005
1"-2" ±.015 ±.010
2"-3" ±.020 ±.015
3"-4" ±.025 ±.018
4"-5" ±.030 ±.020
5"-6" ±.032 ±.022
6"-7" ±.035 ±.025
7"-8" * ±.038 ±.028
 
METRIC-Millimeters
Range Standard Optimistic
Up to 12.7mm ±.18 ±.08
12.7-25.4mm ±.25 ±.13
25.4-50.8mm ±.38 ±.25
50.8-76.2mm ±.51 ±.38
76.2-101.6mm ±.64 ±.46
101.6-127.0mm ±.76 ±.51
127.0-152.4mm ±.81 ±.56
152.4-177.8mm ±.89 ±.64
1778.-203.2mm ±.96 ±.71
 
Flatness
Flatness refers to single plane surface and is usually function of volumetric shrinkage:
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 6 sq. inches of surface area.
 
U.S.-Inches
Part thickness Dish per 6 sq.inches of surface
Up to .250" Insignificant
.251" - .500" .004 in. T. I. R.
.501" - 1.000" .008 in. T. I. R.
1.001" - 2.000" 0.016 in. T.I.R.
 
METRIC - Millimeters
Part thickness Dish per 42 sq.cm of surface
Up to 6.5 mm Insignificant
6.5 - 13 mm .102 mm T. I. R.
13 -25 mm .201 mm T. I. R.
25 -50 mm .406 mm T. I. R.
 
Straightness, Roundness and Positioning
Tolerances for axial straightness is commonly expressed as ±.005" per inch. (±.127 mm per 25 mm).
Roundness (solid bar or hollow tube shapes) is a function of normal shrinkage variations in the metal. Shrinkage variation increases with diameter and the tolerance required increases proportionately. Tubular section out-of-roundness may be altered mechanically per to wall thickness and ductility. A general rule of .005" per inch (.127 mm per 25 mm) may be applied.
 
U.S.-Inches
Diameter Tolerances
Up to 1" .006 in. T. I. R.
1" - 1 1/2" .008 in. T. I. R.
1 1/2"-2" .010 in. T. I. R.
2"-3" 0.15 in. T.I.R.
 
METRIC - Millimeters
Diameter Tolerances
Up to 25 mm .152 mm T. I. R.
25 - 37.5 mm .203 mm T. I. R.
37.5 - 50 mm .254 mm T. I. R.
50 - 75 mm .381 mm T. I. R.
-@Tolerance of positioning (holes or bosses) is stated as±.005"per
-@inch. (±.127 mm per 25 mm) form any single reference point.
 
Concentricity
Concentricity of diameters in a shaft is also a function of straightness too. Centers will be concentric within .005" per inch. (.127 mm per 25 mm) of maximum separation.
 
Center of a bore I. D. to O. D. of a part will be concentric within .003" per 1/2" (.076 mm per 13 mm) of wall thickness. This assumes measurements are taken in the same plane and neglect of out-of-roughness tolerance. If the concentricity is being measured in separate planes, the prerequisite of the above paragraph must be included.
 
Angularity
As-cast tolerances of angles is mainly a function of the part design and their location in casting. They range from ±1/2 degree (considered minimum) for well supported positions to ±2 degree where existing distortion could be expected. Inclusion of gussets and ribs normally minimizes distortion and many sections can be mechanically straightened.
 
Blind Holes
These feature are generally cast provided the depth is equal to the width or diameter for small characteristics less than 3/8" (9.40 mm). As the size increases above 3/8" (9.40 mm), the depth can move incrementally to double the size of the opening.
 
Parallel Sections
Controlling relationship is between length and width of the element being checked. To establish a specific tolerance range for all combinations would be impossible, but a general tolerance of .010" TIR per inch (.254 mm per 25 mm) may be used.
 
Symmetry
Reflection across reference line, .005" per inch. (.127 mm per 25 mm).
 
Wall Thickness
Although wall thickness down to .020" (0.50 mm) may be achievable in a small area, the normal minimal wall thickness range lies between .040"(1 mm) and .080" (2 mm) depending on alloy and square inches of surface area.
 
  U.S.-Inches
Minimum Wall Thickness
METRIC-Millimeters
Minimum Wall Thickness
Metal Small Area Normal Small Area Normal
Beryllium Copper .035" .050" .899 mm 1.270 mm
Ductile Iron .035" .050" .899 mm 1.270 mm
Aluminum .040" .060" 1.016 mm 1.524 mm
300's Stainless .040" .060" 1.016 mm 1.524 mm
Cobalt Chrome .040" .060" 1.016 mm 1.524 mm
400's Stainless .045" .070" 1.143 mm 1.778 mm
Carbon Steel .050" .080" 1.270 mm 2.032 mm
 
Surface Finish
Metal MICRO-INCHES
Average RMS value
MICRO-METERS
Average Ra value
Aluminum 60-100 1.50-2.50
Beryllium Copper 60-100 1.50-2.50
Cobalt Chrome 80-100 2.00-2.50
300's Stainless 90-125 2.25-3.20
Carbon Steel 90-125 2.25-3.20
400's Stainless 100-125 2.50-3.20
 
 
 
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