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SPECIALIZED TOLERANCES FOR PRECISION ALUMINUM FORGINGS

INTRODUCTION

Precision aluminum forgings are aluminum components plastically deformed to a finished part shape, engineered and toleranced to require little, if any, subsequent processing. They are characterized by 0° and 1° draft angles, thin sections, small radii and excellent surface condition, and often feature multiple parting lines, permitting optimum grain flow control.

TOLERANCES

The final exactness of a precision forging is the result of the actual dimensional condition of the die cavity at the onset of production, and the interaction of natural variation of the forging processes. The combination of these factors result in practical limitations of dimensional control-tolerances.

The tolerances set forth herein represent what the Forging Industry Association believes to be the prevailing levels within the industry, as determined by actual measurements of specimens precision forged under normal operating conditions on production equipment.

The experience of producers and purchasers of precision forgings indicates that these tolerances are comparable to similar processes used for the same intended applications.

When less restrictive tolerances are acceptable maximum economy is achieved. This should be noted and confirmed by buyer and seller in advance of production.

Consultation between the purchaser and the producer is advisable, should more restrictive tolerances be required. Where special conditions require more restrictive dimensional tolerances, special provisions are generally confirmed by buyer and seller in advance of production.

UNITS AND METHODS OF MEASURE

Precision forgings are measured using instruments such as coordinate measuring machines (CMM), micrometers, dial indicators, calipers, checking fixtures, and templates. The accuracy of measurements is limited by the characteristics of such instruments. Units of measure of one one-thousandth of an inch, or metric equivalents generally are found to be consistent with such limits.

Tolerances are expressed as units of one one-thousandth of an inch or one one-hundredth of a mm.

In the field of precision forging, a dimension will carry a different tolerance depending upon the form in which it is expressed. In the decimal inch system, a two place decimal (6.30 in.) will carry a tolerance of ±0.03 in. For greater precision a three place decimal should be used (6.300 in.) and will carry a tolerance of +0.020, -0.010 in. In the metric system a one place decimal (160.1 mm) will carry a tolerance of ±0.8 mm. For greater precision a two place decimal should be used (160.10 mm) and will carry a tolerance of +0.60 mm, -0.30 mm.

ADVANTAGES OF PRECISION ALUMINUM FORGINGS

  • PART CONFIGURATION _ Back drafts, lateral protrusions or undercuts can frequently be made without machining.
  • WEIGHT SAVINGS _ Precision Forging technology provides opportunities for economic weight control through techniques for reduced draft, scalloped edges and near net shape configuration.
  • TOLERANCES _ Precision forgings require considerably less tolerance than conventional forgings. Advances in forging technology allow for identified key characteristic tolerances to approach machine tolerances ±0.010 for specified critical areas.
  • MATING SURFACES _ Zero degree draft is available on specified mating surfaces. In most cases this is achievable on forged surfaces through cooperative part design, but can be achieved through permissible machining in other cases.
  • GRAIN FLOW _ Proper placement of the parting line allows utilization of the most desirable grain flow and metallurgical characteristics. End grain exposure is minimized and its location can be controlled through the design process.
  • COST SAVINGS _ Precision forgings can provide savings over conventional forgings and machined parts through reduced material requirements and elimination of machining operations.
  • SINGLE SOURCE CONVENIENCE _ Precision forging companies can provide raw forgings, finished parts or complete assemblies. This can provide lower cost and reduced lead time.

DEFINITIONS

  1. THICKNESS _ The amount of material confined between two parallel surfaces, and measured normal to the surfaces.
  2. WEB _ Thin panel member usually parallel to the plan view of forging.
  3. WALL _ Members that create the periphery of the forging and are usually perpendicular to webs.
  4. RIB _ Thin gusset type internal members usually perpendicular to the web.
  5. FLASH EXTENSION _ Excess material remaining on a forging after normal trimming, usually present at all parting line locations.
  6. MATCH _ Is the alignment of feature on a forged part formed by opposing segments of a die.
  7. DRAFT _ A taper applied to selected surfaces of a forged part to aid its removal from the die Draft normally is larger on internal surfaces, and smaller on external surfaces, where features are formed by more than one piece of the die.
  8. NO-DRAFT _ Refers to external surfaces on forgings that are free of draft but are controlled by the implied angular tolerance of ±0 degree 30 minutes. This usually is specified in the drawing title block.
  9. PARTING LINE _ The location on the forging where excess material in the form of flash is allowed to exit from the forging during the forging operation.
  10. SEAM LINE _ A line that may be visible on finished precision forgings, indicates a junction of mating die components in segmented die construction.
  11. PLAN VIEW AREA _ Is the surface area that the press must apply pressure to; it is express in square inches.
  12. FORGING DIRECTION _ The direction in which the forging press is applying pressure to produce the part.
  13. DIE CLOSURE _ Refers to the function of the closing together of the upper and lower members of a forge die during the process of actually producing a forging. The features of the forging that will be affected by die closure will be all web thicknesses and wall heights.
  14. SEAMLESS OR FLASHLESS FORGING _ Refers to method of forging in which the part material if forged into a closed die at a predetermined area only and is restricted from escaping the cavity area in the form of flash. The result is a forging that has superior grain flow characteristics and has no parting line on the critical part surfaces, thus eliminating transverse end grain exposure on these surfaces. lack of parting line increases the mechanical properties in this area.

RECOMMENDED DESIGN PROPORTIONS

Defining specific parameters to apply to all precision forgings is extremely difficult. Often flexibility in technique and tooling concepts enables specific part geometries to be produced to even closer dimensions and tolerances to meet customer needs. On the other hand, a few configurations cannot be economically or technically produced to the parameters indicated in the data published.

Although many or all of the precision forging characteristics discussed in this booklet can be incorporated in a particular forging, the lowest cost per part can be produced when such criteria as minimum thicknesses and tolerances are specified only where they are actually required.

RECOMMENDATIONS:

  1. 1. DRAFT
    (a) external; 0° + 30' -30'
    (b) internal; 1° +0° -1° or 0° ±30' if accompanied by machining permissible.
  2. 2. EDGE (CORNER) RADII
    mm 1.6 +0.8 -1.6
    inches 0.06 +0.03 -0.06
  3. 3. FILLET RADII
    * mm 3.3 ±0.8
    * inches .013. ±0.03
  4. 4. WEB THICKNESS
    Web thickness is one of the most difficult dimensions to obtain in a precision forging. "Lightening Holes," 2.5 in. dia. and larger, in webs will usually permit forging to a thinner web gage.

*IMPROVEMENT OVER PREVIOUSLY PRINTED GUIDELINES

WEB THICKNESS GUIDELINE

Notes:

1) Required for designs that are approximately rectangular and/or shapes with exterior walls.
2) Limited to shapes meeting one or more of the following:
    a. Long, narrow shapes.
    b. Parts not confined by exterior walls.
    c. Parts with "Lightening Holes."
3) Designs over 400 pva requiring minimum web gage will require vendor coordination prior to release.
4) Reduced web thickness may be achieved by chem-milling or by machining.

5. WALL OR RIB THICKNESS
Figure 2 illustrates the recommended relationship between rib thickness and the height of the rib from the web or the adjoining surface.

WALL/RIB HEIGHT THINNESS GUIDELINE

Wall/Rib Height (inches)
Figure 2

6. SURFACE FINISH

Surface finish of a precision aluminum forging commonly equals, or is better than, a 125 RMS finish.

7. GRAIN DIRECTION AND GRAIN FLOW
Grain direction corresponds to the location of the starting stock in the die cavity and as specified on the customer drawings. Grain flow usually follows the general part configuration and is dictated by part shape and die design.

LINEAR AND THICKNESS TOLERANCES

SCOPE
1. Linear tolerances represent dimensional variations of specified feature sizes.

TOLERANCE

2. METRIC
      The tolerance for 1 place decimal is ±0.8 (.X ±0.8 mm)
  The tolerance for 2 place decimal if +0.6 (.XX +0.6 mm)
     -0.3  

INCH
      The tolerance for 2 place decimal is ±0.3 (.XX ±0.3 inches)
  The tolerance for 3 place decimal if +0.6 (.XXX +0.020 inches)
     -0.3    0.010

This includes allowances for temperature variations, die sinking, wear, polishing, and subsequent processing of the forging.

Tighter tolerances are achievable when machining permissible is allowed.

QUALIFIERS OR ADDITIONS

3. For Length, Width, and Height dimensions in excess of 10 inches (254 mm), the following additional tolerance applies.

* ±0.002 mm/mm or in/in

*IMPROVEMENT OVER PREVIOUSLY PRINTED GUIDELINES

STEP DIMENSION TOLERANCES

SCOPE

1. Step dimension tolerances represent variations in dimensions of offsets or "steps" where such incremental dimensions are contained within and controlled by a single die.

TOLERANCE

2. Step dimensions tolerances are ±0.010 (0.025 mm) per step. (This does not include straightness.

MEASUREMENT

3. Step dimensions are typically checked at the tangent point of the step fillet and corner radii, or at the mold point depending on step depth.

(See Figure 3.)


Figure 3

MATCH TOLERANCES

SCOPE
1. (a) Match tolerances relate to displacement of a point in one die from the related point in the opposite die in any direction parallel to the fundamental forging plane. Out of match (mis- match) is included within dimensional tolerance.

ANGULARITY TOLERANCES

SCOPE

1.   Angularity tolerances relate to variations in relationships between features of the forging described by angles rather than dimensions. (Note: Coordinate dimensions rather than angular specifications are recommended.)

TOLERANCE

2.   Angularity tolerance is ±0° 30'.

  DRAFT AND TOLERANCES

SCOPE
1.   Draft angle tolerances apply to all draft angles and relate to variation from draft angle specifications.

TOLERANCES

2.  

External draft angle tolerance is 0°+30'-30'.
* Internal draft angle tolerance is 1°+0°-1° or 0°+30'-30' if accompanied by machining permissible.

When tooling points fall on draft surfaces the draft is added through the tooling point as shown in Figure 4.


Figure 4

* IMPROVEMENT OVER PREVIOUSLY PRINTED GUIDELINES

FILLET RADII TOLERANCES

SCOPE
1.   Fillet radii tolerances relate to variations from specified fillet radii.

TOLERANCE
2.   * Fillet radii tolerance is (1.6 mm) .03±.030 mm ±0.030 in.

CORNER RADII TOLERANCES

SCOPE

1.   Corner radii tolerances relate to variations from specified corner radii.

TOLERANCE

2.   Corner radii tolerances are described by a range from plus 0.030 in. (0.8 mm) to square condition with no sharp edge. 0.06+0.03-0.06.

 

FLATNESS TOLERANCES

SCOPE

1.   Flatness tolerances relate to deviations of surfaces from the specified configuration as caused primarily by heat treatment and die deflection.

TOLERANCE

2.   *The flatness tolerance is 0.016" up to 10 and 0.016 for each additional 10" dimension.

*IMPROVEMENT OVER PREVIOUSLY PRINTED GUIDELINES

PROFILE TOLERANCES

SCOPE

1.   Profile tolerances relate to variations from nominal contours.

TOLERANCE
2.   *Profile tolerance is ±0.010 in. up to 10" inches in length. ±0.015 over 10" inches in length.

FLASH EXTENSION TOLERANCES

SCOPE

1.   Flash extension is excess material left on the forging after trimming.

TOLERANCE

2.   *Flash extension tolerance is 0.015 in

*IMPROVEMENT OVER PREVIOUSLY PRINTED GUIDELINES

MECHANICAL PROPERTIES OF PRECISION FORGINGS

Aluminum precision forgings are ordered to the same specifications, quality assurance provisions and mechanical property levels that apply to conventional forgings.

However, many users feel that precision forgings used without machining have better mechanical properties, fatigue characteristics and resistance to stress corrosion cracking. This superiority is attributed to the high degree of work during forging, the grain orientation, parting line location and

metallurgical advantages retained when the "as forged" surfaces are not removed. In fact, studies have shown that when precision forgings are compared to machined parts of the same configuration, fatigue life is significantly increased.

Precision forgings are available in all aluminum alloys used for conventional forgings.

The tempers usually specified for these alloys can be produced in precision forgings except that T4, T652, and T7352 are rarely specified since the precision forged parts are not intended to be machined before installation. Most precision forgings are used in the T6, T73 or T74 type tempers.

 

Minimum Mechanical Properties of
Aluminum Alloys Commonly Used for Precision Forging

Alloy and Temper Longitudinal Transverse
Tensile Strength
(ksi)		 Yield Strength (ksi) Elongation (%) Tensile Strength
(ksi)		 Yield Strength (ksi) Elongation (%)
2014-T6 65 55 6 64 54 3
2219-T6 58 38 8 56 36 4
2618-T61 58 45 4 55 42 4
6061-T6 38 35 7 38 35 5
7049-T73 72 62 7 71 61 3
7075-T6 75 65 7 71 62 3
7075-T73 66 56 7 62 53 3
7175-T74 76 66 7 71 62 4
7175-T66 86 76 7 77 66 4
7050-T74 72 62 7 68 56 5
NOTE: FOR SPECIFICATIONS OF OTHER ALUMINUM ALLOY FORGING MATERIALS,
CONTACT YOUR PRECISION FORGING SUPPLIER,

 

FINISHED PARTS CAPABILITY

Although the purpose of this document is to present recommended tolerance guidelines for precision forgings, it is also intended to introduce others areas in which the industry can improve both service and product. One major area is that of finished parts products.

A growing trend among major manufacturers is that of stressing capability to produce a forged product which meets finished part requirements and can be delivered ready for assembly by the customer.

To meet the requirements for a part ready for assembly, the precision forging industry has been steadily developing capabilities for full machining and other post processing. Today, finished parts offered by leading forging suppliers come complete not only with painting and anodizing, but also can be delivered with bushings, bearings, nut plates, and other sub-assembly hardware.

The advantages of purchasing finished parts from precision forge vendors are many. Those most important for customers are:

- Reduced cost
- Entire job/single contact
- Single manufacturing/quality control system
- Elimination of multiple purchasing channels
- Elimination of in-stream parts movement
- Better control of delivery schedules - Reduced lead times

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