investment casting

Cost Factors

The cost of any part increases in direct proportion to the preciseness of its dimensional tolerance requirements. This is true of castings, as well as fabricated and machined parts.

Investment castings are designed to minimize the cost of producing close tolerance parts.

Maintaining close dimensional tolerance in an investment casting is affected by many factors. Most of these factors can be controlled by the foundry, although minor "lot to lot"

variations will occur. The tolerance bands provided for investment castings are determined by these uncontrollable factors.

While careful machining can achieve closer tolerances than available in an investment casting, a critical design review will often permit minor expansion of tolerances, undercuts,

blind holes, etc. to allow the higher production yields and lower piece costs possible with investment castings. If closer than cast tolerances are necesary, the machining required on

an investment casting will still be substantially less than on conventional cast or fabricated pieces.

Factors affecting close dimensional tolerances in an investment casting include the following:

Variables

Casting size and shape determines the tolerance required to allow for process variables. These allowances will vary by foundry, depending on foundry experience or specialty.

Many process factors affect investment casting tolerances, including:

1. wax or plastic temperature 4. firing temperature

2. die temperature 5. shell composition

3. pressure of injection 6. rate of cooling.

Standard Linear Tolerances

As a general rule . . . normal linear tolerance on an investment casting can be + .010" for up to 1", and + .005" for each additional inch thereafter.

Normal tolerances can be expected for production repeatability of all casting dimensions.

Premium tolerances require additional operations at extra cost and achieve closer tolerances on selected dimensions only, even tighter tolerance than those on the following chart

can be obtained. The tolerance achieved will depend on the alloy and configuration, and should be determined during consultation with your investment casting supplier

Flatness

Flatness and straightness are so closely related that confusion may arise unless the foundry and purchaser clarify the definitions before production.

Mil-Std-8 specifies "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." In measuring, the parallel planes must be the minimum distance apart.

Degree of flatness in an investment casting is almost always determined by the volumetric shrinkage of wax and metal during cooling. Usually in the center of the mass, this

shrinkage is called "dish" (shrinkage, dip, or "out of flat"). Dish can be controlled by specialized techniques, but it will always occur to some extent. General flatness tolerances

cannot be quoted because they vary with configuration and alloy used. The following is a rough guide for areas under 6 square inches.

SECTION POSSIBLE DISH PER

THICKNESS FACE OF CASTING

up to 3" not significant

3" to 1" . 002" to .004"

1" to 1" .004" to .006"

over 1" .006" to .008"

Allowable dishing is in addition to the basic tolerance. Thus on a block 1" + .005" thick, the following would apply:

The method of measuring flatness should be specified by the purchaser.

It may vary from a simple surface plate and feeler gage for normal tolerances to full layout with equalization and dial indicators for premium tolerances.

Straightness

Mil-Std-8 specifies "a tolerance covering the straightness of an axis is the diameter or width within which the axis must lie."

Thus, to correctly measure axial straightness of a shaft, bar, or plate, the tolerance zone within which the axis or axial plane lies must also be measured.

Straightness may be a real problem with certain types of castings. A relatively thin, short part may bend while a long, heavy part may not. Experience tells the foundry a given design

may bend, but experience cannot indicate to what extent. As a rough guide, a constant section will usually have an axial bow of .005" per inch. Ribs and gussets will inhibit warpage,

but also hinder mechanical straightening of whatever warpage occurs.

Parallelism

Casting parts with parallel prongs supported at only one end, and yoke castings, are very specialized problems which should be discussed fully with the foundry before production.

Since Point X is the thickest section, it is the ideal gate point. It is also where the greatest volumetric shrinkage will occur.

Dimension Y, however, will be restrained by the rigid mass of refractory. Parallelism is therefore difficult to maintain. It will be .010" per 1" of L, but can be improved by control

techniques and sizing.

This condition will also affect any through holes usually found in yokes. When specified, such holes should carry considerable finish stock if they are to be finished truly concentric or

line reamed.

Roundness

"Out of round" is defined as the radial difference between a true circle and a given circumference. It is the total indicator reading when the part is rotated 360 half the difference

between the maximum and minimum condition. The latter is usually preferred because it is quicker to determine. The actual inspection method, however, should be specified by the

purchaser.

Out of Roundness

TIR or Difference

Diameter Between Diameters

quot; .010"

1" .015"

1quot; .020"

2" .025"

On larger diameters, linear tolerances apply.

Surface Texture

"Surface texture includes roughness, waviness, lay and flaws," as defined in ASA B 46.1.1962.

Typical investment casting RMS is 125 max. Some of AMT's castings have been measured at between 50 - 60 RMS.

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