Selection of materials for precision machined parts should be held to a higher standard than just “cheapest price per pound.”
Here are 4 questions to help decide if a grade of steel (or other material) is appropriate:
- Is it economical in assuring a satisfactory end product?
- Does it provide sufficient safety factor for the properties called upon in the design?
- Does it provide the most economical means of production?
- Is it the lowest cost raw material?
The order in which these questions are asked is critical. Answering number 4 first puts the entire company in jeopardy for product liability lawsuits if the first three questions are ignored in the buyer’s holy quest for cheapest raw material price.
In steel, with many possible carbon and alloy grades, qualities, and finishes (cold drawn, turned and polished, ground and polished, or combinations of these) the end use is of particular importance in arriving at the grade, type, and quality of the steel.
Human safety critical components (airbags, anti lock brakes, climbing equipment) require different thinking than parts for less critical applications where failure is merely inconvenient, not life threatening.
Components for expensive machines and production equipment also fall into this category, where the failure of a part purchased under the assumptions of “false economy” result in extensive downtime of a very high value production asset.
Once the suitability for the end use and safety factor as designed has been determined, then the suitability of the material for the production method becomes the next selection criteria. In high volumes of relatively simple parts, for example, very low carbon, plain carbon steel is the appropriate choice using cold heading. If the volumes are not there, attempting to use this same steel on screw machines would result in inferior finishes and far more expensive parts than if a free machining grade of steel were chosen. Selecting for Manufacturability can help lower the total cost over the entire supply chain as well as for the final consumer.
The final criteria then becomes transactional cost. But even this is more than just dollars and cents- it is both dollars and sense! Is the supplier a legitimate source? Do they have statistically controlled systems? Do they have a mature quality system that has demonstrated it’s strength over time? Do they limit their number of suppliers so that you will not be subjected to the full range of variability of inputs possible in an increasingly global, interconnected world?
If buying for mere cheapness was the point, we could replace all purchasing agents with third graders. By third grade, most kids know which number is larger, and which is smaller.
The professional value that purchasing adds is by establishing and following a process that assures an optimum outcome for the entire value chain, not just one part of it.
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