1. Information from stamping scrap
Stamping scrap is essentially the reverse image of the formed hole, that is, the same part in the opposite position. By checking the scrap, you can determine whether the gap between the upper and lower dies is correct. If the gap is too large, the scrap will have a rough, undulating fracture surface and a narrow bright band area. The larger the gap, the greater the angle between the fracture surface and the bright band area. If the gap is too small, the scrap will present a small angle fracture surface and a wide bright band area.
Too large a gap forms a hole with large curling and edge tearing, making the section protrude slightly with a thin edge. Too small a gap forms a slight curling and large angle tearing with a section that is more or less perpendicular to the material surface.
An ideal scrap should have a reasonable collapse angle and a uniform bright band. This keeps the punching force to a minimum and forms a neat round hole with very few burrs. From this point of view, extending the life of the die by increasing the gap is at the expense of the quality of the finished hole.
2. Choice of die gap
The gap of the die is related to the type and thickness of the material being stamped. Unreasonable clearance can cause the following problems:
(1) If the clearance is too large, the burrs of the punched workpiece will be larger and the punching quality will be poor. If the clearance is too small, although the punching quality is good, the die wear will be more serious, greatly reducing the service life of the die and easily causing the punch to break.
(2) If the clearance is too large or too small, it is easy to cause adhesion on the punch material, resulting in material carryover during punching. If the clearance is too small, it is easy to form a vacuum between the bottom surface of the punch and the sheet material, causing waste material rebound.
(3) Reasonable clearance can extend the life of the die, improve the unloading effect, reduce burrs and flanging, keep the sheet clean, keep the hole diameter consistent and will not scratch the sheet, reduce the number of grinding times, keep the sheet straight, and accurately position the punching holes.
Please refer to the table below to select the die clearance (the data in the table are percentages).
III. How to improve the service life of the die
For users, improving the service life of the die can greatly reduce the stamping cost. The factors that affect the service life of the mold are as follows:
Material type and thickness;
Whether a reasonable lower die gap is selected;
Mold structure;
Whether the material is well lubricated during punching;
Whether the mold has undergone special surface treatment, such as titanium plating, carbon titanium nitride;
The centering of the upper and lower turrets;
The reasonable use of adjustment gaskets;
Whether the oblique edge mold is appropriately used;
Whether the machine tool mold base has been worn.
IV. Matters to be noted when punching special size holes
(1) For the minimum hole diameter, please use a special punch for punching holes in the range of φ0.8~φ1.6.
(2) When punching thick plates, please use a larger die than the processing hole diameter. Note: At this time, if a normal size mold is used, the punch thread will be damaged.
Example 1: As shown in the processing conditions in the table below, although the processing hole diameter corresponds to the A-station mold, please use the B-station mold.
Example 2: As shown in the processing conditions in the table below, although the processing hole diameter corresponds to the B-station mold, please use the C-station mold.
(3) The ratio of the minimum width to the length of the punch edge should generally not be less than 1:10.
Example 3: For a rectangular punch, when the edge length is 80mm, the edge width is most suitable to be ≥8mm.
(4) The relationship between the minimum size of the punch edge and the plate thickness. It is recommended that the minimum size of the punch edge is twice the plate thickness.
V. Die grinding
1. The importance of die grinding
Regular die grinding is the guarantee of punching quality consistency. Regular die grinding can not only increase the service life of the die but also increase the service life of the machine. The correct time for grinding must be mastered.
2. Specific characteristics of the die that require grinding
For die grinding, there is no strict number of strikes to determine whether grinding is required. It mainly depends on the sharpness of the edge. It is mainly determined by the following three factors:
(1) Check the radius of the cutting edge. If the radius of the corner reaches R0.1 mm (the maximum R value shall not exceed 0.25 mm), it needs to be sharpened.
(2) Check the quality of the punching to see if there are any large burrs.
(3) Determine whether sharpening is needed by the noise of the machine punching. If the noise is abnormal when the same die is punched, it means that the punch has become blunt and needs sharpening.
Note: If the edge of the cutting edge becomes rounded or the back of the cutting edge is rough, sharpening should also be considered.
3. Sharpening method
There are many ways to sharpen the die. It can be achieved by using a special sharpening machine or on a surface grinder. The frequency of sharpening the punch and the lower die is generally 4:1. Please adjust the die height after sharpening.
(1) The harm of incorrect sharpening method: incorrect sharpening will aggravate the rapid destruction of the die edge, resulting in a significant reduction in the number of blows per sharpening.
(2) Benefits of the correct sharpening method: Regularly sharpen the mold to maintain the quality and accuracy of the punching. The mold edge will be damaged more slowly and have a longer life.
4. Sharpening rules
The following factors should be considered when sharpening the mold:
(1) The sharpness of the edge should be considered when the radius of the edge is R0.1~0.25 mm.
(2) The surface of the grinding wheel should be clean.
(3) It is recommended to use a loose, coarse-grained, soft grinding wheel, such as WA46KV.
(4) The grinding amount (cutting amount) should not exceed 0.013 mm each time. Excessive grinding will cause the mold surface to overheat, which is equivalent to annealing treatment, and the mold will become soft, greatly reducing the life of the mold.
(5) Sufficient coolant must be added during sharpening.
(6) During grinding, the punch and the lower die should be fixed and stable, and a special fixture should be used.
(7) The grinding amount of the die is certain. If this value is reached, the punch will be scrapped. If it continues to be used, it is easy to damage the die and the machine, which is not worth the loss.
(8) After grinding, the edge should be treated with an oil stone to remove overly sharp edges.
(9) After grinding, it should be cleaned, demagnetized, and oiled.
Note: The amount of die grinding depends mainly on the thickness of the sheet being punched.
VI. Things to note before using the punch
1. Storage
(1) Wipe the inside and outside of the upper die sleeve with a clean rag.
(2) Be careful not to have scratches or dents on the surface during storage.
(3) Apply oil to prevent rust.
2. Preparation before use
(1) Clean the upper die sleeve thoroughly before use.
(2) Check whether there are scratches or dents on the surface. If so, remove them with an oil stone.
(3) Apply oil inside and outside.
3. Precautions when installing the punch on the upper die sleeve
(1) Clean the punch and oil its long handle.
(2) Insert the punch into the bottom of the upper die sleeve on the large-station mold. Do not use force. Do not use a nylon hammer. When installing, do not tighten the bolts on the upper die sleeve to fix the punch. The bolts can only be tightened after the punch is correctly positioned.
4. Install the upper die assembly into the turret
If you want to extend the service life of the mold, the gap between the outer diameter of the upper die sleeve and the turret hole should be as small as possible. So please perform the following procedures carefully.
(1) Clean the keyway and inner diameter of the turret hole and oil it.
(2) Adjust the keyway of the upper die guide sleeve so that it matches the key of the turret hole.
(3) Insert the upper die sleeve straight into the turret hole, and be careful not to tilt it. The upper die guide sleeve should slide into the turret hole by its own weight.
(4) If the upper die sleeve is tilted to one side, you can use a soft material tool such as a nylon hammer to gently knock it straight. Repeat the knocking until the upper die guide sleeve slides into the correct position under its own weight.
Note: Do not apply force to the outer diameter of the upper die guide sleeve, only to the top of the punch. Do not knock on the top of the upper die sleeve to avoid damaging the turret hole and shortening the service life of individual stations.
VII. Inspection and maintenance of the die
If the punch is bitten by the material and cannot be removed, please check according to the following items.
1. Re-sharpening of the punch and lower die. A die with a sharp cutting edge can produce a beautiful cut surface. If the cutting edge is blunt, additional punching force is required, and the cross-section of the workpiece is rough, which produces a lot of resistance, causing the punch to be bitten by the material.
2. The gap of the die. If the gap of the die is not selected appropriately relative to the plate thickness, the punch needs a lot of demolding force when it is separated from the material. If the punch is bitten by the material for this reason, please replace the lower die with a reasonable gap.
3. The state of the processed material. When the material is dirty or has dirt, the dirt adheres to the mold, causing the punch to be bitten by the material and unable to be processed.
4. Deformed materials. After punching the hole, the warped material will clamp the punch, causing the punch to be bitten. For warped materials, please flatten them before processing.
5. Excessive use of springs. It will cause spring fatigue. Please always check the performance of the spring.
8. Oiling
The amount of oil and the number of oiling depend on the conditions of the processed material. For rust-free and scale-free materials such as cold-rolled steel plates and corrosion-resistant steel plates, the mold should be oiled. The oiling points are the guide sleeve, oiling port, the contact surface between the cutter body and the guide sleeve, the lower die, etc. Use light engine oil for oil.
For materials with rust and dirt, rust powder will be sucked between the punch and the guide sleeve during processing, resulting in dirt, so that the punch cannot slide freely in the guide sleeve. In this case, if oil is applied, it will make rust and dirt more likely to stick. Therefore, when punching such materials, the oil should be wiped clean instead. Disassemble once a month, use gasoline (diesel) oil to remove the dirt on the punch and lower die, and wipe it clean before reassembly. This will ensure that the mold has good lubrication performance.
IX. Common problems and solutions during mold use
Problem 1: The plate is out of the clamp mouth.
Problem 2: The mold is severely worn.
Problem 3: The punch carries material and the punch sticks.
Problem 4: Waste rebounds.
Problem 5: Difficulty in unloading.
Problem 6: Stamping noise.
10. Precautions for using special molding tools
1. The stroke of the slider of different models of machines is different, so pay attention to the adjustment of the closed height of the molding mold.
2. Make sure that the forming is sufficient, so it is necessary to adjust carefully. The adjustment amount should not exceed 0.15 mm each time. If the adjustment amount is too large, it is easy to cause damage to the machine and the mold.
3. For stretch forming, please use a light spring assembly to prevent the tearing of the sheet or the difficulty of unloading due to uneven deformation.
4. Install a ball-type support mold around the forming mold to prevent the sheet from tilting.
5. The forming position should be as far away from the clamp as possible.
6. The forming process is best to be implemented at the end of the processing program.
7. Make sure that the sheet is well lubricated.
8. Pay attention to the problem of giving way to special forming tools when ordering. If the distance between two forming tools is relatively close, please be sure to communicate with our salesperson.
9. Because the forming tool requires a long unloading time, the forming process must be slow, and it is best to have a delay.
11. Precautions for using a rectangular cutting knife
1. The step distance should be as large as possible, which should be greater than 80% of the entire tool length.
2. It is best to implement jump punching through programming.
3. It is recommended to use a bevel edge die.
12. How to punch holes without exceeding the nominal force of the machine
During the production process, it is necessary to punch a circular hole with a diameter greater than 114.3mm. Such a large hole will exceed the upper limit of the machine nominal force, especially for high shear strength materials. This problem can be solved by punching large holes by multiple punching methods. Using a small die to cut along the large circumference can reduce the punching force by half or more, and most of the dies you already have may be able to do this.
13. Form downwards last
When selecting a forming die, avoid downward forming operations because it takes up too much vertical space and causes additional flattening or bending of the sheet. Downward forming can also get stuck in the lower die and then pulled out of the turret. However, if downward forming is the only process option, it should be used as the last step of the sheet processing.
14. Prevent material distortion
If you need to punch a large number of holes in a sheet and the sheet cannot remain flat, the cause may be the accumulation of punching stress. When punching a hole, the material around the hole is stretched downward, increasing the tensile stress on the upper surface of the plate. The downward punching movement also causes the compressive stress on the lower surface of the plate to increase. For punching a small number of holes, the results are not obvious, but as the number of punching holes increases, the tensile and compressive stresses also increase exponentially until the plate is deformed.
One way to eliminate this deformation is to punch every other hole and then return to punch the remaining holes. Although this produces the same stress on the plate, it breaks down the tensile/compressive stress accumulation caused by punching one after another in the same direction. This also allows the first batch of holes to share some of the deformation effects of the second batch of holes.
15. If the stainless steel flange is deformed
Applying a high-quality forming lubricant to the material before making the flange can better separate the material from the die and move smoothly on the lower die surface during forming. This gives the material a better opportunity to distribute the stress generated by bending and stretching, preventing deformation on the edge of the forming flange hole and wear on the bottom of the flange hole.
16. Some suggestions for overcoming the difficulty of unloading
1. Use a punch with fine core rubber particles.
2. Increase the gap between the lower die.
3. Check the fatigue of the spring.
4. Use heavy-duty molds.
5. Appropriately use oblique edge molds.
6. Lubricate the sheet.
7. Large-station molds need to be equipped with polyurethane unloading heads.
17. The main reasons for waste rebound
1. The sharpness of the edge. The larger the radius of the edge, the easier it is to cause waste rebound.
2. The mold input amount. When stamping each station mold, the mold input amount is certain. A small mold input amount is easy to cause waste rebound.
3. Is the mold gap reasonable? Unreasonable mold gap is easy to cause waste rebound.
4. Is there oil on the surface of the processed sheet?
