*3.1. Developed Cost Estimation Tool*

The developed cost estimation tool considers the inputs given in the manner described in the previous section to give a production time estimation for parts produced in CNC milling centers, having an interface in MS Excel®, as seen in Figure 7. The interface is divided into six main sections:



**Figure 7.** MS Excel® interface of the developed cost estimation tool.

To input data in the developed interface requires knowledge of machining processes currently being applied, for example, in the "Strategy" section of the interface, a choice for cavity consideration was added. This should be defined according to the total area of the cavities, in relation to the workpiece area. The chosen value will influence the machining time for end-milling operations; however, this input can be left blank.

After filling the input interface, the cost estimation tool will estimate the operation times for each of the mentioned production steps, exhibiting the results in an output sheet, observed in Figure 8.


**Figure 8.** Output sheet of a machined polyethylene part produced for Case Study 1.

The output sheet is divided into five main sections, displaying the filled data regarding the material and project information, the adopted strategy, observations, and the part's technical drawing. Estimated times are displayed in the "Operation times" section. Note that in this section there is an input table for the real machining times, which was added for the validation of the cost estimation tool. These real times were registered by the operator after machining.

As can be observed in Figure 6, there are some deviations that originate from the lack of need to perform the CAD drawings of the part. Furthermore, there are some deviations registered for the machine setup times and finishing operations. This can be attributed to the fact that these operations are performed manually, being harder to estimate correctly (highly dependent on the operator). It is also worthy of noting that, although the finishing times have quite a large deviation from the real times, this is since these operations usually have a short duration. This can also be observed in another part, as depicted in Figure 9. Observing the output sheet shown in Figure 9, it can be noted that the highest percentual deviation, in terms of real machining time, is registered for the calculation of finishing operations (−80%). Although this value is considerably high, the difference between estimated and real times is less than one minute. Again, this lack of accuracy in the estimation of these operations is since finishing operations are performed manually (for the parts produced in these machine types). However, analyzing the deviation from all the other production steps, the maximum deviation is +8%, which is incredibly satisfactory. This was registered for the parts of a similar complexity level, with lower deviations being registered.

