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.
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 = 1.36t + 0.43R (t is the material plate thickness), and the main errors in the usual calculation of the bending factor are as followed:
(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.
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)
The sample size is 100mm×100mm, which uses laser cutting and blanking, and the dimensional accuracy can reach 0.1mm.
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
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
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
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)
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 Rinner
Comparing the Rinner under different bending conditions, it can be seen that the Rinner is influenced by the material, plate thickness, bending method, and bending tooling.
Of these four factors, the situation of three other factors being equal:
- Rinner(SUS304) > Rinner (SPCC).
If the width of the V-groove Bv = 12 mm, the Rinner 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 Rinnerinfluence is less.
For example, when Bv = 12mm in three-point bending, Rinner in 1.0 ~ 2.0mm thickness of SUS304 is 2.33 ~ 2.51mm, the difference is not significant.
- Rinner(three-point) > Rinner (V-groove).
Comparing the same slot width bending lower die (Bv=7mm, 12mm and 16mm) shows that the bending Rinner in three-point is slightly larger than that in V-groove.
- The larger the slot width Bv, the larger the Rinner, and the larger the corresponding Rinner
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 Rinner, 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 Bv, the material, bending method (V-slot and three-point) also affects the bending Rinner, which should be paid attention to.
Reducing ratio and outside bending angle Router
In the test, the difference between Router and Rinner is used to represent the average thickness near the bending angle, i.e., t’ = Router – Rinner.
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 Rinner uses an optical measuring instrument to scanning calculate, while the roundness value can be calculated.
(1) When Bv = 7 ~ 16mm, the roundness value of Rinner and Router 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 Rinner and Router 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
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 + 0.43Rinner).
For comparison, the difference is not large (in the calculation, the thickness t and Rinner are both brought into the calculation by the actual test value), which indicates that the current bending coefficient formula α = 1.36t + 0.43Rinner is universal, the bending coefficient depends on two parameters in the actual thickness t of the material and the actual bending Rinner.
The Rinner is influenced by the material, plate thickness, bending method and bending tool, the actual Rinner 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 Rinner of the bending tool.
Based on the above analysis, several conclusions can be obtained:
(1) The test results show the bending Rinner, Router 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) Rinner is not only related to the bending die but also related to the material;
The test shows that the Rinner of SUS304 is slightly larger than that of SPCC under the same bending parameters;
(3) When the other bending parameters are the same, the Rinner 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+0.43Rinneris universal.
Accumulating the real thickness of the commonly used bending materials in the workshop and the corresponding bending mold forming Rinner can calculate a more accurate bending coefficient.
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