Inspection and Judgment of Working Accuracy of Bending Machine

At present, many enterprises do not pay attention to the daily management of equipment. Improper use and maintenance of equipment. Or excessive use of equipment, the operator does not use the equipment according to the equipment operating procedures, resulting in serious loss of equipment accuracy. Some small and medium-sized enterprises will purchase some second-hand equipment, but they lack the detection technology and methods of equipment accuracy. They usually judge the sound condition of the equipment based on the service life, efficiency, and appearance of the equipment, ignoring the mastery of the processing accuracy of the equipment.
The bending machine is one of the key equipment for forming sheet metal products and ensuring the processing accuracy of sheet metal products. The accuracy of bending equipment will directly affect the processing accuracy of sheet metal products. The impact of bending equipment with insufficient precision on product deviation is a long-term objective existence. If the accuracy status of a bending equipment can be detected, it is very necessary to provide the accuracy data of the equipment for the company’s acceptance and maintenance of the bending equipment.
The national standard GB/T14349-2011 “Bending Machine Accuracy” stipulates the accuracy inspection, inspection allowance and detection method for the accuracy of the bending machine. The inspection items of the bending equipment include the inspection of the geometric accuracy and the working degree of the bending machine, which are explained as follows:

Test

1. Geometric accuracy testing items◆Lateral and longitudinal flatness of the machine table

◆Parallelism between the horizontal supporting surface of the upper mold fitting surface and the plane of the machine

◆Perpendicularity of slider to working platform

2. About the detection of geometric accuracy

The author believes that the purchaser of the machine tool can ask the supplier to provide the geometric accuracy data, and there is no need to do the geometric accuracy self-test. It is only necessary to detect the working accuracy of the machine tool to verify the purchase accuracy of the machine tool.
The user of the machine tool can plan to arrange the annual working accuracy inspection of the machine tool. If the working accuracy exceeds the allowable range of machine tool accuracy, then consider the geometric accuracy test of the machine tool.
Considering that the vast majority of enterprises do not have the large and medium repair capabilities of machine tools, they are generally accepted by professional maintenance organizations. This article does not introduce the geometric accuracy detection of bending machines in detail. If you need to master the detection geometric accuracy, you can refer to the relevant content of the national standard GB/T14349-2011 “Bending Machine Accuracy”.

Influence on working accuracy

1. The influence of the flatness (longitudinal) of the worktable of the bending machine on the straightness of the bending parts① Influence of workpiece angle accuracy :

When there is a longitudinal flatness error in the workbench, it will affect the depth of the upper die entering the notch of the lower die. The depth deviation △H is equal to the plane error ɑ. Accuracy impact diagram

②Influence of workpiece straightness accuracy

When there is a flatness error on the work surface, the straightness of the bent part is equal to the flatness error. The accuracy impact is shown in the figure below:

③The influence of the flatness (horizontal) of the worktable of the bending machine on the angle and straightness of the bending parts

The mechanism of the flatness (transverse direction) of the bending machine worktable on the angle and straightness accuracy of the bending parts is the same as the “longitudinal influence” mechanism.

2.  The effect of the slider stroke on the verticality of the working surface on the bending accuracy

① Influence of workpiece angle accuracy:

Work Accuracy Inspection

1. Inspection conditions

①Requirements for test piece:

◆Length requirements ( length deviation is ±3mm )

Table length/mmSpecimen length/mm
≤2000l
>2000~32002000
>3200~50003000
>50004000
◆Width requirements: the width of the test piece is not less than 100mm. (*Width deviation is ±0.2mm.)◆Thickness requirements:

Nominal force/KNSpecimen thickness t
≤10002
>1000~25003
>2500~63004
>63006

◆Material requirements: The material of the test piece is Q235A steel plate, and the tensile strength of the material is σb≤450MPa.

◆Number of pieces: The number of test pieces should not be less than 3.

◆Mould requirements: The opening size of the lower mold for the test piece is 8~10 times the thickness of the material.

◆Position requirements: The specimen should be placed in the middle of the workbench.

◆Bending angle requirements: The bending angle of the specimen is 90°.

◆Measuring position: measure the test piece at 100mm from the end of the test piece.

②Checking tool requirements

Qualified flat ruler, universal angle ruler and feeler gauge.

③ General requirements

Before the working accuracy test, the equipment needs to be leveled, and the horizontal and vertical directions should not exceed 0.2/1000

2. Detection of the bending angle of the test piece

① Supplement

The bending angle of the test piece is ±10 (over the full length)

② Detection method

Place the universal angle ruler against the outer surface of the bending test piece (see the figure below), and measure at multiple places in sequence (at least 3 places per meter). The error is calculated by the difference between the measured maximum angle, minimum angle and the specified angle 900.
3. Detection of the discount rate of the test piece① Supplement

0.05mm on a length of 1000mm .

② Detection method

Put the 1000mm flat ruler detection surface against the edge of the bending test piece (see the figure below), and measure the gap δ between the two with a feeler gauge. The error value is calculated as the maximum gap value within 1000mm.

4. Secondary bending test piece

If the material stress of the test piece is large, it is allowed to bend the test piece twice (see the figure below). Check the working accuracy of the bending machine. The detection method shall comply with the contents of 3 and 4.

 

Conclusion

According to the analysis of a set of reference test data provided by the bending machine manufacturer, the influence of the longitudinal error of the working surface of the bending machine on the bending accuracy is about 3 times that of the lateral error. The influence value of the bending angle deviation is about 3 times of the flatness error of the workbench.
The bending angle deviation is not greatly affected by the stroke of the slider on the verticality of the working surface and can be ignored. The straightness of the table surface will directly reflect the influence of the straightness deviation of the workpiece, and the two are almost equal. The bending straightness deviation is less affected by the slider stroke on the verticality error of the working surface. The worktable mentioned in this article includes two, one is the machine table, and the other is the influence of the geometric accuracy of the horizontal support surface of the upper mold fitting surface. for example:
The length of the working surface of a bending machine is 3200mm. After measuring the working accuracy of this bending machine, the following data is obtained: the straightness deviation of the workpiece is 0.06mm, and the bending angle deviation is 0.1720. From this, it can be inferred that the flatness of the worktable of this bending machine is about 0.06mm.

 

According to the national standard GB/T14349-2011 “Bending Machine Precision”, the flatness of the bending machine with a table length of 3200mm is 0.08mm, so it can be judged that the flatness of the bending machine is qualified. In fact, as a user of a bending machine tool, he only needs to pay attention to the working accuracy of the machine tool.
Because the working accuracy of the machine tool after long-term use is not only influenced by the factors mentioned in this article, but also the transmission clearance of the machine tool and the influence factors of mechanical wear on the accuracy of the workpiece should be taken into account to comprehensively evaluate and consider the machine tool. That is to say, through the inspection of the working accuracy of the bending parts of the machine tool, combined with the national standard of the accuracy of the bending machine, it is possible to inspect and judge the pros and cons of the comprehensive precision processing capability of a bending machine.

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