In order to improve the bending accuracy of elevator sheet metal parts, the bending radius (R-angle) of several commonly used materials such as SPC, SPHC, SUS304 and 804-GG etc. bent at 90° on the CNC press brake in the sheet metal workshop was accurately measured with an optical measuring instrument, and the bending coefficient was calculated using vernier calipers.

The test results provide reference and data support for the selection of suitable bending tooling, improvement of bending R-angle accuracy and calculation accuracy of bending dimensions.

**Test Significance**

**Test Significance**

The bending radius (inside R) and bending coefficient are important factors that affect the quality of workpiece bending.

The bending radius is related to the bending tool, material thickness and performance factors, while the bending coefficient is determined by the material thickness, bending radius and bending angle, and the bending coefficient affects the unfolding dimension of the billet.

The existing formula for calculating the 90° bending factor is α = 1.36* t* + 0.43R (

*is the material plate thickness), and the main errors in the usual calculation of the bending factor are as followed:*

*t*(1) The difference between the t value and the true thickness of the material.

(2) The deviation between the actual bending R inside and the drawing required R inside (R inside is usually taken according to the drawing when calculating α);

(3) The existing bending R using R gauge measurement (R gauge: index value below R3 is 0.25, above R3 is 0.5), lower accuracy.

(4) The influence of the material and bending method on the bending R is not considered.

When the workpiece is bent several times, the error of the bending coefficient will accumulate, resulting in poor or unqualified dimensional accuracy of the drop material.

Based on the above error analysis and considerations, this experiment measured the actual thickness of several bending materials, used an optical measuring instrument to more accurately measure the bending inner and outer radius, measured and calculated the actual bending coefficient of the workpiece, and calculated the value with the formula A comparison was made.

This will help to select the appropriate bending die, improve the bending forming R accuracy and expand the dimensional calculation accuracy.

**Test scheme**

**Test scheme**

**Test material**

**Test material**

The test materials are SPCC, SPHC, SUS304, 804-GG purchased by our company, and their thickness specifications are shown in Table 1.

Table 1 Test materials and thickness (mm)

Thickness t/mm |
1.0 | 1.2 | 1.5 | 2.0 | 2.3 | 2.5 | 3.0 | 3.2 | 4.5 | 6.0 |

SPCC |
√ | √ | √ | √ | √ | √ | ||||

SPHC |
√ | √ | √ | |||||||

SUS304 |
√ | √ | √ | √ | √ | |||||

804-GG |
√ |

**Test specimen**

**Test specimen**

The sample size is 100mm×100mm, which uses laser cutting and blanking, and the dimensional accuracy can reach 0.1mm.

**Test equipment**

**Test equipment**

The test bending machine is a CNC press brake in the elevator production sheet metal workshop.

The V-groove has FASTI-50 and Beyeler, and the scimitar upper die is selected, as shown in Figure 1.

Fig.1 V-groove bending die

The three-point bending machine is 3P250, and the straight knife upper die (pointed cutter R7 and round cutter R9) is selected, as shown in Figure 2.

Fig.2 Three-point bending die

Table 2 The parameters of press brake, punch & die

Die opening (Bv/mm)Press brake & punch type |
7 | 8 | 10 | 12 | 16 | 24 | 32 | 40 | |

V opening(Gooseneck punch) |
Beyeler | √ | |||||||

FASTI-50 | √ | √ | √ | ||||||

Three-point(straight punch) |
3P250 | √ | √ | √ | √ | √ | √ | √ |

**Test method**

**Test method**

The true thickness of the specimens was measured with a micrometer and four pieces were averaged for each specification.

The specimens were bent on different bending dies at a bending angle of (90 ± 1) °, which try to ensure that the length of one side was 50mm, as shown in Figure 3.

Figure 3 Test Bending Diagram

Each specification should be repeated 5 times.

After the bending was completed, the bending angle contours were scanned with an optical measuring instrument to calculate the bending outer angle R outside and inner angle R inside, as shown in Figure 4.

Fig. 4 Optical measuring instrument and bending R-angle measurement

It uses a vernier caliper to measure the length of both sides to calculate the bending coefficient,

Each specification should be repeated 5 times and take the average value.

**Test results and analysis**

**Test results and analysis**

The attached table is a collated test result.

The data shows the true thickness of the test material, the inner and outer radius of the 90° bend, the bending coefficient and the thinning of the bend.

**Actual material thickness**

**Actual material thickness**

The actual thickness of the specimen measured with a micrometer is compared with its nominal thickness, as shown in Table 3.

Table 3 The actual thickness of the test materials (mm)

Nominal thickness |
1.0 | 1.2 | 1.5 | 2.0 | 2.3 | 2.5 | 3.0 | 3.2 | 4.5 | 6.0 | |

Real thickness |
SPCC |
1.00 | 1.18 | 1.48 | 2.01 | 2.50 | 2.97 | ||||

SPHC |
3.13 | 4.20 | 5.91 | ||||||||

SUS304(Remove film) |
0.93 | ||||||||||

804-GG |
2.26 |

It can be seen from the table that the difference between the actual thickness of SPCC and the nominal thickness is within 0.03 mm, the actual thickness of uncoated SUS304 is about 0.07 mm thinner than the nominal thickness, and the actual thickness of the 4.5 mm hot rolled plate SPHC is 4.2 mm.

**Bending inside angle R**_{inner}

**Bending inside angle R**

_{inner}

Comparing the R_{inner} under different bending conditions, it can be seen that the R_{inner} is influenced by the material, plate thickness, bending method, and bending tooling.

Of these four factors, the situation of three other factors being equal:

- R
_{inner}(SUS304) > R_{inner}(SPCC).

If the width of the V-groove B* v* = 12 mm, the R

_{inner}in SPCC with the thickness of 1.2 mm and SUS304 is 1.85 mm and 2.09 mm, respectively.

- When the bending die is the same, for the same material, the plate thickness of R
_{inner}influence is less.

For example, when B* v* = 12mm in three-point bending, R

_{inner}in 1.0 ~ 2.0mm thickness of SUS304 is 2.33 ~ 2.51mm, the difference is not significant.

- R
_{inner}(three-point) > R_{inner}(V-groove).

Comparing the same slot width bending lower die (B* v*=7mm, 12mm and 16mm) shows that the bending R

_{inner}in three-point is slightly larger than that in V-groove.

- The larger the slot width B
, the larger the R*v*_{inner}, and the larger the corresponding R_{inner}

Figure 5 is a comparison of the three-point bending R angles for the lower die slot width Bv is 24mm, 32mm and 40mm, corresponding to about 4.0mm, 4.7mm and 5.9mm of R_{inner}, respectively.

Fig. 5 Comparison of the inside radius of 4.5 mm SPHC bending with different slot widths (three-point acute punch)

Therefore, in addition to the bending die slot width B* v*, the material, bending method (V-slot and three-point) also affects the bending R

_{inner}, which should be paid attention to.

**Reducing ratio and outside bending angle R**_{outer}

**Reducing ratio and outside bending angle R**

_{outer}

In the test, the difference between R_{outer} and R_{inner} is used to represent the average thickness near the bending angle, i.e., t’ = R_{outer} – R_{inner}.

Thus, reducing ratio is η = (t – t’)/t.

From the data in the attached table, it can be seen that thickness reduction occurred in all cases in this test case.

Most of the reducing ratio is within 6%-15%, and the influence of material thickness, bending mode and slot width on the thinning rate is more complicated, and the rule is difficult to identify.

However, it can be seen that the SPHC thinning rate is lower, about 4% to 6%.

The test’s R_{inner} uses an optical measuring instrument to scanning calculate, while the roundness value can be calculated.

(1) When Bv = 7 ~ 16mm, the roundness value of R_{inner} and R_{outer} is very small, most of which is ≤ 0.05mm, indicating that the bent inner and outer corner contours highly match the degree of roundness.

(2) When Bv = 24mm, 32mm and 40mm (all three-points type), the roundness values of R_{inner} and R_{outer} are slightly increased, exceeding 0.1mm, which means that after the groove width Bv of the lower bending dies increases to 24mm, the degree of arc of the inner and outer contours of the bending decreases.

**Bending coefficient α**

**Bending coefficient α**

The schedule also gives the measured and calculated values of the bending coefficient test (currently method used to calculate the bending coefficient, calculation formula: α = 1.36t + 0.43R_{inner}).

For comparison, the difference is not large (in the calculation, the thickness t and R_{inner} are both brought into the calculation by the actual test value), which indicates that the current bending coefficient formula α = 1.36t + 0.43R_{inner} is universal, the bending coefficient depends on two parameters in the actual thickness t of the material and the actual bending R_{inner}.

The R_{inner} is influenced by the material, plate thickness, bending method and bending tool, the actual R_{inner} is the simplest and most effective method.

For new materials or bent parts with other thicknesses, it is necessary to actually measure the true thickness and the bending R_{inner} of the bending tool.

**Conclusion**

**Conclusion**

Based on the above analysis, several conclusions can be obtained:

(1) The test results show the bending R_{inner}, R_{outer} and bending coefficients of several commonly used thickness sheet of SPCC, SPHC, SUS304, 804-GG in the sheet metal workshop CNC press brake machines like Beyeler, FASTI-50 and 3P250;

(2) R_{inner }is not only related to the bending die but also related to the material;

The test shows that the R_{inner} of SUS304 is slightly larger than that of SPCC under the same bending parameters;

(3) When the other bending parameters are the same, the R_{inner} of the three-point bending is slightly larger than the V-groove bending, so the bending work center should be considered when selecting the bending coefficient;

(4) The bending coefficient calculation formula α=1.36t+0.43R_{inner}is universal.

Accumulating the real thickness of the commonly used bending materials in the workshop and the corresponding bending mold forming R_{inner} can calculate a more accurate bending coefficient.