Forming forces and process time

Force requirements and production time are determining factors in the selection of a suitable technology.

Hole flanging by conventional press working requires heavy machinery and dedicated dies and punches for each flanging size. Assuming that the press is available, the high cost of these accessories renders the process acceptable only large production batches.

An incremental hole flanging operation requires considerably less force than a conventional operation, which enables the operation in a standard CNC machine.

In this case, the auxiliary structure and forming tools are substantially less expensive and more versatile than conventional hole flanging dies, which renders incremental sheet forming processes profitable for small production batches. However, their main weakness is the high processing time required for a conventional operation.

This comparison could be completed by quantifying the level of force and process time that is required for performing the hole flanging tests by single-stage SPIF and conventional press presented in this study. The optimization of these process variables was not the aim of this study; thus, the differences listed in Table 5-3 are estimates of the results expected in industrial operations.

Table 5-3 Summary of maximum forming force in z-direction and processing time in both hole flanging operation for tests with a 65 mm pre cut hole diameter and 8

mm tool radius

Hole flanging Operation

Max. forming force (kN) Processing time (min) Hole milling and deburring Set-up time Hole flanging Single-stage SPIF:

feed rate: 1000 mm/min, 0 rpm

stepdown: 0.2 mm/rev, 8 mm tool 1.5

15

3 30

Conventional press working:

cylindrical punch - 8 mm edge radius punch speed: 0.1 mm/s

30.5 1 1

Table 5-3 summarizes the maximum forming force in the z-direction (vertical) and

the processing time required to perform a hole flanging test with a 65 mm pre cut hole diameter and 8 mm tool radius by both technologies. As described in previous chapters, the feed rate and stepdown per revolution used for single-stage SPIF was 1000 mm/min and 0.2 mm/rev, respectively, and the punch speed for press working was 0.1 mm/s.

Figure 5.10 Evolution of forming force vs. tool travel in z- direction in single-stage incremental and conventional hole flanging for tests of 65 mm pre cut hole

diameter and 8 mm tool radius

Figure 5.10 depicts the evolution of the tool registered in the z-direction as a

function of the tool travel in both forming operations. As expected, the maximum force in conventional forming is one order of magnitude higher than the

incremental one. As shown in Table 5-3, the maximum forces are approximately

30.5kN and 1.5kN, that is, a difference of approximately 20 times between operations.

Regarding the processing time, the trend reverses. The processing time can be split into three parts: (1) the time for milling and deburring of the initial hole, which is a common operation in both forming processes, (2) the set-up time of the test, that is, the time required to place the specimen, apply the tribological system, and select the process parameters or CNC program, and (3) the hole flanging time, i.e.,

the time required by the tool to form the flange. Table 5-3 shows the time spent in

each of these phases. The flanging time is critical; it is one order of magnitude higher in incremental forming than conventional forming for these tests. This difference can be considerably modified in an industrial context.

5.6. Conclusions

Two hole flanging processes conventional press working and the single-stage incremental forming have been critically compared in terms of flangeability, geometrical features of the finished part and required forming force and time. The main conclusions drawn are summarized as follows:

 Regarding the strain in the flange, the main difference between both processes

is derived from the evolution of the meridional strain. In single-stage SPIF, this strain displays a high positive value around the middle of the flange, which is responsible for a longer and thinner flange thanthe flange in the conventional process.

 The analysis based on the FLD satisfactorily reflects the mechanisms that

control the flangeability in both processes. In press working, the failure is

triggered by necking at the flange edge and controlled by the FLC. In

incremental forming, the failure initiates by ductile fracture near the middle of the flange wall and is then controlled by the FFL.

 The traditional LFR is not an appropriate parameter for measuring the flangeability in hole flanging by SPIF as it does not capture the physics of failure of this process. A LFR value of approximately 1.6 is obtained for both processes.

 Alternatively, the non-dimensional flange height (ℎ⁄ ) and the average

thickness ratio ( ̅⁄ ) are more suitable for evaluating the flangeability in both ₃

processes. In terms of these parameters, the single-stage SPIF exhibits a significant enhancement in formability regarding the press working. A gain in

flangeability of approximately 40% and 30% is quantified using ℎ⁄ and

̅ ₃

⁄ , respectively.

 Regarding the flange geometry, the single-stage SPIF is able to perform higher

and thinner flanges for a given HER parameter. The thinner area locates near the middle of the flange.

 For the tests analysed in this study, the maximum force in conventional forming is one order of magnitude higher than that in single-stage SPIF. This this trend reverses when the processing time is considered.

results conclude that conventional press working is preferred to produce shorter, thicker and more homogeneous flanges, whereas single-stage SPIF is better for manufacturing longer and thinner flanges with more flexibility in shape.

6. CONCLUSIONS AND FUTURE