Calculation of Face-Milling Time

For this kind of operation, face-mills with diameters between 44 and 64 mm are typically used, using a value of the width of cut (*ae*) corresponding to 70% of the tool's diameter. The number of facing passes (*No. FM.P.*) is calculated by dividing the material's width by the chosen *ae* value, and the result should be rounded up, as for Equations (1) and (2).

With the knowledge of the number of facing passes and knowing the length of the part (*Plength*) and the tool's diameter (*Øtool*), the facing length (*LFacing*, in mm) can be calculated, as shown in Equation (4).

$$L\_{Facing} = (P\_{length} + \mathcal{Q}\_{total}) \times \text{No.}\_{FM.P.} \tag{4}$$

The value chosen for *ap* is usually 1 mm, however, in some cases, face-milling must be performed on the opposite side of the part, requiring its clamping. For the second facing operation the value for *ap* is 5 mm. The machining time for the first (*M.T.* <sup>1</sup>*st Facing*) and second machining operation (*M.T.* <sup>2</sup>*nd Facing*) is determined by Equations (5) and (6).

*<sup>M</sup>*.*T*. <sup>1</sup>*st Facing* <sup>=</sup> *LFacing Vf* (5)

$$M.T.\_{\text{2nd Factor}} = \frac{L\_{\text{Facing}} \times \left(\frac{5}{ac}\right)}{V\_f} \tag{6}$$

Calculation of End-Milling Time

End-milling time estimation is performed based on the value of *ae*, dependent on the tool diameter (40% of this value). The value for end-milling distance per depth increment (*lE.M.*, in mm) needs to first be calculated, and this is dependent on the length and width of the machined cavity. This value is then multiplied by the number of increments (in depth) that will be performed to machine the cavity, obtained by dividing the depth of the cavity (*Dcavity*) by the depth of cut value (*ap*). The obtained value is the total end-milling distance of the operations (*LE.M.*).

The total machining time for the end-milling operations (*M.T.E.M.*, in minutes) can be calculated, by using Equation (7), essentially dividing the *LE.M.* by the feed rate value (*Vf*).

$$M.T.\_{E.M.} = \frac{L\_{E.M.}}{V\_f} \tag{7}$$

Regarding finishing operations, these are performed on the interior cavity walls and can be calculated in the same way as the side-milling operations.

Calculation of Drilling Time

Drilling time calculation depends on tool diameter (*Øtool*), chosen according to the desired hole diameter, the depth of the hole (*Dhole*, in mm), the value for feed per rotation (*f*, in mm/rotation), and the rotational speed (*N*, in RPM) employed during drilling operations. During the drilling process, the machine performs plunges, quickly retracting and then resuming drilling for a few more millimeters, after retracting again, performing this cycle until the operation is concluded. This promotes a correct chip evacuation from the cutting zone. The number of plunges (*No.plunges*) that need to be performed during the process is calculated by Equation (8).

$$\text{No.}\,\text{plungs} = \frac{D\_{\text{hole}}}{\mathcal{Q}\_{\text{total}}}\tag{8}$$

The value for the number of plunges should always be rounded up; next, to this number, one more plunge should be added. After determining the value for *No.plunges*, the total drilling length (*Ldrilling*, in mm) is next. The calculation of this value is determined based on the *No.plunges* needed for the operation and the tool diameter. Equation (9) shows the calculation process for total drilling length.

$$L\_{\text{drilling}} = \sum\_{n=1}^{No\_{\text{plunges}}} n \times \mathcal{O}\_{\text{tool}} \times 2 \tag{9}$$

The machining time for drilling operations (*M.T.Drill*, in minutes) is calculated according to Equation (10).

$$M.T.\_{Drill} = \frac{\left(\frac{L\_{drilling}}{f}\right)}{N} \tag{10}$$

Calculation of Boring or Threading Times

The method for calculating the machining time of boring and threading (*M.T.Thread*, in minutes) operations is shown in Equation (11), where the depth of the hole (*Dhole*), the feed per rotation (*f*), and rotational speed (*N*) values are considered.

$$M.T.\_{Thread} = \frac{\left(2 \times \frac{D\_{\text{hole}}}{f}\right)}{N} \tag{11}$$

Influence of Production Quantities on the Times

The quantity of requested parts is also considered in the estimation of machining times, as it was found that this factor had an influence on the total production times of machined parts. A larger quantity implies that the worker is more familiar with the production procedures of a certain part. This familiarity causes a slight increase in the production rate. As such, an inflation factor was created for certain quantity levels, which should be multiplied by the estimated machining times, as shown in Table 5.

**Table 5.** Inflation factors applied to machining operation estimated times for different part quantities.

