6.5 Choosing a machine
Tim A. Bacon
A machine will be selected for its ability to efficiently remove material, and its ability to repeatedly achieve a consistent tolerance. When “roughing out a part,” the majority of material will be removed in the most productive manner possible. To illustrate, grinders are designed to remove around .01 inch of material at the most. This would not be a good machine choice if .5 inches of material needed to be removed. A hobbyist might enjoy just the process of removing material without consideration of timeliness. A paid machinist, however, needs to attend to time being careful to work within project deadlines.
Referencing the picture of the bench block in Section 6.0, you’ll note that it is round. The outside shape of the part being made is called the profile of the part. The sample bench block we are creating is made of 1018 mild steel. This material comes in a flat bar that is band sawed into smaller squares so the machinist can more easily handle the part during production and the material isn’t wasted. If the outside of the profile is cut first, it will be more difficult to hold on to because a circle requires special tooling for workholding. This means if you immediately start cutting a circle out of your material, you will create more work, and the part will take more time to produce. From what we see of the bench block, the vertical mill is the best tool for this project. Again, job planning that includes tool choice leads to greater efficiency. And, efficiency equals profitability.
Machine capability is the level of accuracy a machine provides without user influence. Machine capability varies from one machine to the next. A band saw might be used for the first step in cutting a block of material to +/- .06. But, if the requirements are +/- .01, a table saw might be a better choice because it offers greater accuracy. In making these decisions, machinists need to consider machine capability. When deciding on the machine to use, consider the tolerance requirements and identify a machine that is capable of achieving those results. When machine capability is aligned with technical requirements, productivity increases.
In addition to machine capability, a profitable operation will factor in material usage and conservation. For large quantity jobs, the project plan should include optimization strategies. These strategies are identified after the steps are outlined.
Making 100 of the same part is different from a single part. When the number of parts increases, repeatability and reliability need to be considered. The plan may also shift to one that has batch processing, where a small number of features are cut on all parts before continuing to the next step. This approach is instead of making a complete part before making the next one.
Choosing the right machine for the job requires a comprehensive understanding of the machining process, materials used, tooling, equipment, and the ability to comprehend technica