5.2 Relationship to manufacturing processes

In any machining process, the tolerance that can be achieved will depend upon two things. Firstly, the variability caused by the vagaries within a manufacturing process such as vibrations, discontinuities, inconsistencies, etc. These will produce a deviation about some mean value. Secondly, there is the variation that occurs when the tool wears. This will be progressive. Thus, in any accuracy graph or table, there will be two factors: an increasing trend with wear and variability scattered around this trend. This is shown in the graph in Figure 5.2. The nominal diameter was 10mm and the manufacturing process was gun-drilling. The graph shows that there is a general trend produced by wear and variability given by the error bars essentially equi-spaced about the mean. In this case the variability about the mean value represents the out-of-roundness. This is the deviation of the hole from a perfectly circular hole. The out-of-roundness refers to random as well as systematic errors.

Figure 5.2: Gun-drill wear against hole diameter showing wear trend and out-of-roundness

An example of a systematic error is shown in the picture in Figure 5.3. This is a photograph of a 6mm-diameter hole in a 3mm thick aluminium sheet. The hole is clearly of a triangular form. The halo round the edge of the hole is where it has been chamfered to remove the burr. The reason the hole is triangular is because of a lack of stability of the drill caused mainly by the fact that...