Design of Typical Powder Metallurgy Parts
Although powder metallurgy is used in many different applications, most parts typically fall into one of the four categories shown below, each of which has its own unique design considerations:
Gears are well suited to PM production:
- Carbide dies provide long life and accuracy.
- Residual part porosity tends to dampen sound.
- PM gears can be made with blind corners, thus eliminating the need for undercut relief.
- PM gears can be combined with other configurations such as cams, ratchets, other gears, and various components.
- Helical gears are possible; copper infiltration sometimes used to improve teeth densities.
- Since tooth shape is not a problem, true involute gear forms are easier to produce through PM than through other fabrication methods.
Here are some things to keep in mind when designing PM gears:
- Note that the center hole location relative to the gear shape is affected by the running tolerances of the various tool members. This makes it more difficult to hold close TIRs obtainable with arbor-cut gears, and hubs or pinions that increase the number of concentric tool members increase TIR tolerance needed. TIRs can be reduced by grinding gear IDs true to the gear pitch diameter.
- As the AGMA Class of gear increases, so does the cost of the gear because of the secondary operations required to meet the tighter tolerances.
- To avoid having very thin tool members, hear hubs or pinions should be located as far as possible from gear root diameters (see figure).
Cams are well suited to PM production:
- The process provides excellent surface finish and part-to-part consistency.
- The natural finish of a self lubricating PM cam will often outwear a ground cam surface.
- For radial cams, the cam shape is formed in the die; for face cams, the shape is formed in the punch faces.
- Two or more PM parts can often be joined to form a unit that is difficult, if not impossible to make as a single structure.
- Capitalizing on PM’s flexibility, it is feasible to make assemblies of very different materials: a bronze bearing in a ferrous structural part, for example, or a heat-treated part with a non-heat-treated part.
- PM parts can be joined by conventional methods such as staking, press fitting, brazing, soldering, welding, epoxying, riveting, bolting, and also by sintering together materials of appropriately different size change characteristics.
- Copper infiltration during sintering can be used to bond steel parts.
- Bearings are a natural product for PM because of its controlled porosity and the resulting self-contained lubrication.
- Plain bearings, flanged bearings, spherical bearings, and thrust washers are commonly produced.
- The operating environment should be carefully considered: external lubrication, cooling, and hardened or chromium-plated shafts tend to increase permissible loads. Repeated start-stop operation, oscillatory or reciprocating motion, high speeds, shock loads, and temperature extremes tend to decrease permissible loads.
- For detailed information on permissible loads, press-fits, and tolerances, see the engineering section in MPIF Standard 35, Material Standards for PM Self-Lubricating Bearings.
Sizes and Shapes
Wide parts are possible if perpendicular dimensions are reduced so that the maximum projected area is not exceeded. At higher press tonnages, 560 MPa (40 tsi), for example, the projected die/punch area will shrink, and a 200-ton press will have a maximum round die diameter of 64.0 mm (2.52 in.).
- Since compaction occurs in the vertical direction, using only top and bottom motions, part lengths in the pressing direction are limited.
- Compression ratio—the ratio of the height of the loose powder filling the die to that of the compacted part—also tends to limit vertical part lengths.
|Press Capacity (tons)||Round PartMax. Diameter (in.)|
|—30 tsi (410 MPa) compaction—|
|25||1.03 (26.2 mm)|
|200||2.91 (73.9 mm)|
|500||4.61 (117.1 mm)|
- Shapes with uniform dimensions in the pressing direction are easiest to produce and eject from the press.
- Cams, gears, and sprockets are readily made.
- Thin walls and projections should be considered carefully, since they may require fragile tools.
- Face forms on upper or lower punches can provide bosses, pads, lettering, countersinks, and other features.
- More complex, multi-motion tooling is required to maintain consistent density throughout parts with more than one level.
- Both mechanical and hydraulic presses are available for making parts with five or more levels.
- Parts weighing from a few grams to 11.8 kg (26 lbs.) or more are possible.