Powered Metal Parts Recommended Mechanical Tolerances

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Powered Metal Parts Recommended Mechanical Tolerances

Design for Manufacturing
Mechanical Tolerances Specifications

Economical Recommended Mechanical Tolerances for Powered Metal Parts per. MIL-HDBK-727

Powder metal parts are made by compacting metal powders in a precision die. Subsequently, the compacted part is ejected and then sintered in a controlled atmosphere to develop its mechanical and physical properties.

Virtually every metallurgical composition used is available in one or more powder formulations, including iron, carbon steel, stainless steel, nickel steel, copper steel, nickel silver, copper, brass, aluminum, bronze, and even the refractory and reactive metals. Each is available in a number of different compositions or special blends that give the designer a wide choice of properties and enable him to select the optimum material for his particular application.

The mechanical property of tensile strength is commonly used in the evaluation of powder metallurgy materials although other properties may also be of prime importance; this depends upon intended applications. Powder metallurgy parts, such as bronze bearings, can be produced with high porosity while structural parts can have high density, minimum porosity, and tensile strengths ranging from 1034.2 to 1241.1 MPa (150,000 to 180,000 psi) - even approaching 1379 MPa (200,000 psi) in special circumstances. In many cases, properties of powder metallurgy parts either equal those of wrought materials or exceed them; however, ductility and resistance to impact are often much lower.

Through selective compacting, parts can be produced with multiple densities. This feature, available only with powder metallurgy, enables the design engineer to specify, for example, a hard, dense, wear resisting surface and a porous, oil-impregnated running surface.

Information relative to shapes and sizes follows:

1. Shapes. Powdered metal parts can be compressed only in the direction of punch movement. Parts with threads, holes, or undercuts at angles to the direction of pressure, reentrant angles, and reverse tapers are either impossible to press or restrict the ejection of the part from the die. These design limitations can frequently be overcome by secondary machining. Inserts should not be molded into powdered metal parts. Tables 1 - 4 summarizes economical as-manufactured tolerances for powdered metal parts - post manufacturing processes such as machining is often facilitated to to create tapped holes, gasket flat surfaces, etc.

2. Sizes. The available press stroke and the compression ratio of the material determine the practical sizes that can be produced by the powder metallurgy process. Parts weighing as much as 23 kg (50 lb) and having a compacting area of 25,800 mm2 (40 in.2) can be produced with modern presses.

Table 1

Diameter or Length vs Tolerance
Diameter or Length		Length Tolerance ±		Diameter Tolerance ±
mm	(in.)	mm	(in. )	mm	(in. )
< 2 5	(<1 )	0.13	(0.005)	0.05	(0.002)
> 25 to 38	(> 1 to 1.5)	0.19	(0.0075)	0.05	(0.002)
> 38 to 51	(> 1.5 to 2)	0.38	(0.015)	0.08	(0.003)
> 51 to 64	(> 2 to 2.5)	0.38	(0.015)	0.10	(0.004)
> 64 to 76	(> 2.5 to 3)	0.38	(0.015)	0.13	(0.005)

Table 2

Flange Diameter vs Tolerance
Diameter ±		Tolerance ±
< 2 5	(< 1)	0.10	(0.004)
> 5 to 38	(> 1 to 1.5)	0.15	(0.006)
> 38 to 51	(> 1.5 to 2)	0.20	(0.008)
> 51 to 64	(> 2 to 2.5)	0.25	(0.01)
> 64 to 76	(> 2.5 to 3)	0.36	(0.014)
76 to 102	( 3 to 4)	0,41	(0.016)

Table 3

Flange Thickness vs Tolerance
Thickness		Tolerance ±
< 6. 4	(< 0.25)	0.10	(0.004)
> 6.4 to 9.52	(> 0.25 to 0.375)	0.15	(0,006)
> 9.52 to 12.7	(> 0.375 to 0.5)	0.20	(0.008)

Table 4

Concentricity Tolerance
Diameter		Total Indicator Reading
< 2 5	(<1 )	0.08	(0.003)
> 25 to 38	(> 1 to 1.5)	0,10	(0.004)
> 38 to 51	(> 1.5 to 2)	0.127	(0.005)
> 51 to 64	(> 2 to 2.5)	0.152	(0.006)
> 63.5 to 76	(> 2.5 to 3)	0.178	(0.007)

Source

MIL-HDBK-727 - Design for Producibility Handbook
Department of Defense
Army Materials and Mechanics Research Center