Dimensional and Tool Size Determination

When determining tooling sizes and process control limits, one must begin with a consideration of the final product, then work backward through the process. As parts move from compaction, to sintering, to secondary operations, the sizes usually change significantly. Additionally, the variation generally increases with each step in the process (Fig. 5). Notable exceptions are sized or machined parts, as these secondary processes are intended to improve tolerances and thus reduce variation.

 \ After \ compaction y ! V After heat treatment

Fig. 5 Typical change in size distribution after sintering and heat treatment of a P/M compact. The distribution widens after additional processing steps.

The tool designer must draw upon material and processing knowledge to determine the size of the compaction tools. Commonly, the tooling designer begins with the finished part print and then "factors" the dimensions of all tooling members. For example, for a part with a 1.000 in. outside diameter and 0.800 in. inside diameter that shrinks through processing from compacting tooling size, an appropriate factor may be 1.005. The designer thus would size the die at 1.005 in. (1.000 in. * 1.005) and the core 0.804 in. (0.800 * 1.005) as shown schematically in Fig. 6.

0.1000

Finished part Core Die

0.1000

Finished part Core Die

Fig. 6 Example of using tooling factor for die sizing. Factor used: 1.005; tooling not shown: upper punch, lower punch, various adapters, and so forth

The determination of the appropriate factor depends on the material, the processes used, and the parameters of the processes used. Powder manufacturers offer a great deal of baseline data for tooling size determination, but the best method is to draw upon previously tooled parts of similar processing and geometric configuration. When this previously generated data are not available, a pilot run using prototype tooling is an appropriate method of determining sizes. This reduces the likelihood of having to retool production tooling and can decrease the lead time for the first production run.

Complicated geometries can require the use of different factors for different portions of the tooling. For example, raised hubs formed by the top punch tend to have lower densities than the balance of the part. Lower densities have a tendency to shrink more, which means that the factor used should be higher than for the rest of the part geometry.

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