Design Guide
Our goal is to work in partnership with the customer to produce high quality investment cast components that will function in the intended manner, while maintaining the greatest value versus other more costly manufacturing processes. As with any process, there are specific limitations to what can be expected, and an investment casting should be designed with these limitations in mind.
We therefore offer the following general guidelines for consideration when designing an investment casting.
For further assistance, we are pleased to meet with you or have a discussion over the phone to go over your specific requirements in greater detail.
Linear Tolerances
The investment casting process is capable of excellent reproducibility but there are some sources of variation to be considered:
Process Capability: Due to normal fluctuations in process parameters, variances of ± .003" to ± .004" per inch can be expected.
Part Geometry: Most parts are complex in shape and are subject to variation in nominal size due to asymmetry of form and the resulting variable shrinkage rates in different areas of the part. This variation is totally dependent on part configuration and whether it is possible to compensate in the tooling, but it is not uncommon for additional tolerance requirements of ± .004" per inch due to part shape. (An example of this is a cylinder closed on one end and open on the other. This shape will always "bellmouth" to some degree.)
Tooling: Prediction of part shrinkage factors and die makers' tolerances can add as much as ± .002" per inch to the total process tolerance requirements. This is especially true when dies require loose pieces and/or mechanically operated pulls.
Normal linear tolerances reflect these three sources of variation, from nominal dimensions and are as follows:
Normal Linear Tolerances
English
Up to 1/2" ±.007"
Up to 1" ± 010"
Up to 2" ± .015"
Each additional inch: ± .005"
Metric
Up to 15 mm ± 0.20 mm
Up to 25 mm ± 0.25 mm
Up to 50 mm ± 0.40 mm
Each additional 25 mm: ±0.13 mm
Apply Normal Linear Tolerances only to those dimensions that are important to part function.
Premium Linear Tolerances
All three sources of variation can be reduced by:
i ) Part redesign, including addition of tie bars, ribs, and gussets to certain shapes.
ii) Tuning of wax injection tooling after the first sample to meet the nominal dimensions.
iii) Straightening
iv) Gauging and hand fitting
v) Machining
vi) Other secondary operations.
These factors can assist in obtaining tighter than normal tolerances. Johnson Matthey staff will work with the customer to meet all design requirements in the most economical and practical manner. Additional costs may be associated with items 2 - 6 in this list. Premium tolerance capability must be considered on a part by part, dimension by dimension basis.
Noncritical dimensions that are put on a part drawing simply to describe the shape can be most economically produced by applying General Linear Tolerances.
General Linear Tolerances
English
Up to 2" ± .020" Each additional inch: ± .010"
Metric
Up to 50 mm ± 0.50 mm Each additional 25 mm: ± 0.25 mm
Tolerances- Other
Flatness and Straightness
Parts will be held flat and/or straight to .005" per inch of length.
Heavy Part sections may also be dished up to .010".
Unless otherwise agreed, feeler gauge and surface plate (or straight edge) method will be used to inspect flatness and straightness.
Roundness
The hole profile will fall within Normal Linear Tolerances. Apply Premium Linear Tolerances when necessary. Apply General Linear Tolerances when possible.
Concentricity
Two cylindrical surfaces with common center axes will have centre axes separated by no more than .005" times the difference between the diameters in inches. If the length of the cylinder is greater than two times the diameter, add the straightness tolerance above to concentricity.
Concentricity is not a simple characteristic to measure. Consider changing the control to a runout or position notation.
Angularity / Perpendicularity
Normally ± 0.5° or ± .008"/inch of length is a good production tolerance. Some shapes can be held to ± 0.25°; others require ±1.0° Some surfaces will be drafted up to .005"/inch of length, but draft is not normally needed if it will interfere with part function.
Cast Features- General Guideline
Through Holes
Through holes may be cast to:
Size Max. Depth
.060"-.090" 2 x hole dia.
.091" -.200" 3 x hole dia.
.201" -.400" 4 x hole dia.
.401 " + 6 x hole dia.
Preformed ceramic coring may allow hole depths up to 30 x diameter, at additional cost.
Blind Holes
Blind holes may be cast to:
Size Max. Depth Blending Corner Radii
.040" -.120" 5 x hole dia. .5 x hole dia.
.121" -.400" 1 x hole dia. .060- .090
.401" + 2 x hole dia. .090- .180
In blind hole design, large corner radii blending from the part surface to the hole are necessary to provide adequate core strength. Bottoms of blind holes should be full round or generously radiused. Preformed ceramic cores can be used to allow greater blind hole depths.
Countersinks / Counterbores
Incorporate as needed with holes.
Slots
Through slots may be cast to:
Size ->Max. Depth
.040" -.080" 2 x width
.081" -.200" 3 x width
.201 " -.400" 4 x width
.401 " + 6 x width
Blind slots may be cast up to 1.5 x width. Corner radii blending from the part surface to the slot are necessary.
Wall Thickness- Corner Radii
Minimum wall thickness and corner radii are very dependent upon part configuration and size. Small investment castings may in some cases have walls cast to .030" thickness. Corner radii may be cast to .005" R. Alloy selection will play a significant role here.
Surface Texture
Cast surfaces of investment castings are typically in the range of a 125 RMS texture. Larger castings may exhibit a slightly rougher finish. If this is important for part function, be sure to make these needs known. Secondary smoothing operations can be used to improve surface texture.
Gate Witness
Each part is gated into one or more heavy sections. Normally a .010" to .025" high gate witness left on the part will allow best manufacturing economy. Advise if this is objectionable. Gates can be removed flush to the adjacent surface or ground to a specific dimension. Often this results in a higher manufacturing cost.
Radii
Normally, large fillet and corner radii reduce stress in the part and improve appearance. Design with the largest fillet radii that are practical. Allow outside corner radii where possible. Sometimes outside corners must be tooled sharp, but this is avoided whenever possible.
Letters, Numbers, Logos
Normally, raised letters or numbers in protective depressed pads are most economical to manufacture. Specify exact text and figure dimensions. Logos can be investment cast to almost any design.
Metallurgical and Visual Ouality Requirements Well defined material, heat treatment and visual standards should be agreed upon between the customer and Johnson Matthey Limited.
Special Requirements
The information contained in this guide is intended to be a good starting point for designers of investment castings. Many parts have special requirements that fall outside these guidelines or are not discussed. For more specific requirements please consult with qualified Johnson Matthey technical personnel who are available to offer advice and assistance regarding these needs.
