1. Information from the waste situation
Scrap is essentially the inverse image of the formed hole. That is, the same parts in opposite positions. By checking the scrap, you can determine whether the gap between the upper and lower molds 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 larger the angle between the fracture surface and the bright zone area. If the gap is too small, the scrap will exhibit a small angle fracture surface and a wide bright band area.
Excessive gaps create holes with large curls and edge tears, leaving the profile with a thin edge sticking out slightly. Gaps that are too small create a band that curls slightly and tears at high angles, causing the profile to be more or less perpendicular to the material surface.
An ideal scrap should have a reasonable slump angle and a uniform bright band. This keeps punching force to a minimum and creates a clean, round hole with minimal burrs. From this point of view, extending the mold life by increasing the gap is at the expense of the quality of the finished hole.
2. Selection of mold gap
The clearance of the mold is related to the type and thickness of the material being stamped. Unreasonable gaps can cause the following problems:
(1) If the gap is too large, the burrs of the stamped workpiece will be relatively large and the stamping quality will be poor. If the gap is too small, although the punching quality is better, the wear of the mold will be serious, which will greatly reduce the service life of the mold and easily cause the punch to break.
(2) If the gap is too large or too small, it is easy to cause adhesion on the punch material, resulting in material stripping during stamping. If the gap is too small, a vacuum will easily form between the bottom of the punch and the sheet material, resulting in scrap rebound.
(3) A reasonable gap can extend the life of the mold, have good unloading effect, reduce burrs and flanges, keep the plate clean, have consistent hole diameters without scratching the plate, reduce the number of sharpening times, keep the plate straight, and position the punching holes accurately.
Please refer to the table below to select the mold gap (the data in the table are percentages)
Selection of gaps (total gaps)
Material
smallest
most
maximum
Copper
8%
12%
16%
brass
6%
11%
16%
Low-carbon steel
10%
15%
20%
Aluminum (soft)
5%
10%
15%
Stainless steel
15%
20%
25%
%×material thickness=mold clearance
3. How to improve the service life of the mold
For users, increasing the service life of the mold can greatly reduce stamping costs. The factors that affect the service life of the mold are as follows:
1. Material type and thickness;
2. Whether to choose a reasonable lower die gap;
3. The structure of the mold;
4. Whether the material is well lubricated during stamping;
5. Whether the mold has undergone special surface treatment;
6. Such as titanium plating, carbon titanium nitride;
7. Centering of the upper and lower turrets;
8. Adjust the reasonable use of gaskets;
9. Whether the inclined edge mold is appropriately used;
10. Whether the machine tool mold base has been worn;
4. Issues that should be paid attention to when punching holes of special sizes
(1) Please use special punches for punching holes with a minimum hole diameter of φ0.8-φ1.6.
(2) When punching thick plates, please use a mold that is one size larger than the hole diameter. Note: At this time, if a normal size mold is used, the punch thread will be damaged.
Example 1: For the processing conditions in the table below, although the processing hole diameter corresponds to the A-station mold, please use the B-station mold.
Material
Plate thickness(mm)
Aperture(mm)
Mild steel (40Kg/mm2)
6.0
8.2-12.7
4.5
11.0-12.7
Stainless steel (60Kg/mm2)
4.0
8.2-12.7
Example 2: For the processing conditions in the table below, although the processing hole diameter corresponds to the B-station mold, please use the C-station mold.
Material
Plate thickness(mm)
Aperture (mm)
Mild steel (40Kg/mm2)
6.0
22.9-31.7
4.5
30.6-31.7
Stainless steel (60Kg/mm2)
4.0
22.9-31.7
(3) The ratio of the minimum width to length of the punch cutting edge should generally not be less than 1:10.
Example 3: For a rectangular punch, when the blade length is 80mm, the blade width should be ≥8mm.
(4) The relationship between the minimum size of the punch cutting edge and the plate thickness. It is recommended that the minimum size of the punch cutting edge be 2 times the plate thickness.
5. Mold sharpening
1. The importance of mold sharpening
Regular sharpening of the mold ensures the consistency of punching quality. Regular sharpening of the mold can not only increase the service life of the mold but also the service life of the machine. The correct sharpening timing must be mastered.
2. Specific characteristics of molds that require sharpening
For mold sharpening, there is no strict number of blows to determine whether sharpening is needed. Mainly depends on the sharpness of the cutting edge. It is mainly determined by the following three factors:
(1) Check the fillet of the cutting edge. If the fillet radius reaches R0.1 mm (the maximum R value shall not exceed 0.25 mm), it needs to be sharpened.
(2) Check the punching quality. Are there any large burrs?
(3) Judge whether sharpening is required by the noise of machine stamping. If the noise is abnormal when the same die is stamped, it means that the punch is blunt and needs to be sharpened.
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 methods for sharpening molds, which can be achieved by using a special sharpening machine or on a surface grinder. The frequency of punch and lower die sharpening is generally 4:1. Please adjust the height of the die after sharpening.
(1) The harm of incorrect sharpening methods: Incorrect sharpening will aggravate the rapid damage of the mold edge, resulting in a greatly reduced number of blows for each sharpening.
(2) The benefits of correct sharpening method: sharpen the mold regularly, and the quality and accuracy of punching can remain stable. The cutting edge of the mold is damaged more slowly and has a longer life.
4. Sharpening rules
The following factors should be considered when sharpening molds:
(1) When the edge radius is R0.1-0.25 mm, it depends on the sharpness of the edge.
(2) The surface of the grinding wheel must be cleaned.
(3) It is recommended to use a loose, coarse-grained and soft grinding wheel. Such as WA46KV
(4) The amount of grinding (cutting amount) each time should not exceed 0.013 mm. Excessive grinding amount will cause the surface of the mold to overheat, which is equivalent to annealing treatment, the mold will become soft, and the life of the mold will be greatly reduced.
(5) Sufficient coolant must be added during sharpening.
(6) When grinding, ensure that the punch and lower die are fixed stably, and use special tooling fixtures.
(7) The sharpening amount of the mold is certain. If it reaches this value, the punch will be scrapped. If you continue to use it, it will easily cause damage to the mold and machine, and the gain will outweigh the loss.
(8) After sharpening, the edges should be treated with a whetstone to remove excessively sharp edges.
(9) After sharpening, clean, demagnetize and oil it.
Note: The amount of mold sharpening mainly depends on the thickness of the plate being stamped.
6. Pay attention before using the punch
1. Storage
(1) Wipe the inside and outside of the upper mold cover with a clean rag.
(2) Be careful not to have scratches or dents on the surface when storing.
(3) Oil to prevent rust.
2. Preparation before use
(1) Thoroughly clean the upper mold cover before use.
(2) Check whether there are scratches or dents on the surface. If there is, use a whetstone to remove it.
(3) Oil the inside and outside.
3. Things to note 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 without using force. Do not use nylon hammer. During installation, the punch cannot be fixed by tightening the bolts on the upper die sleeve. The bolts can only be tightened after the punch is correctly positioned.
4. Install the upper mold into the turret
If you want to extend the service life of the mold, the gap between the outer diameter of the upper mold sleeve and the turret hole should be as small as possible. So please perform the following procedures carefully.
(1) Clean and oil the keyway and inner diameter of the turret bore.
(2) Adjust the keyway of the upper mold guide bush to match the key of the turret hole.
(3) Insert the upper mold sleeve straightly into the tower hole, being careful not to have any inclination. The upper die guide bushing should slide into the turret hole by its own weight.
(4) If the upper mold sleeve is tilted to one side, use a soft material tool such as a nylon hammer to gently knock it straight. Knock repeatedly until the upper mold guide bush slides into the correct position by its own weight.
Note: Do not apply force on the outer diameter of the upper mold guide sleeve, only on the top of the punch head. Do not knock the top of the upper mold sleeve to avoid damaging the turret hole and shortening the service life of individual stations.
7. Mold maintenance
If the punch is bitten by the material and cannot be removed, please check the following items.
1. Re-sharpening of punch and lower die. A mold with a sharp edge can produce a beautiful cut surface. If the edge is blunt, additional punching force is required, and the rough cross-section of the workpiece creates great resistance, causing the punch to be bitten by the material.
2. Mold clearance. If the gap of the mold is not selected appropriately relative to the plate thickness, the punch will require a large demoulding force when breaking away from the material. If the punch is bitten by the material for this reason, please replace the lower die with a reasonable clearance.
3. The state of the processed materials. 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. Materials with deformation. The warped material will clamp the punch after punching the hole, causing the punch to bite. For warped materials, please smooth them before processing.
5. Excessive use of springs. Will cause spring fatigue. Please always check the performance of the spring.
8. Oil injection
The amount of oil and the number of oil injections depend on the conditions of the material being processed. For rust-free and dirt-free materials such as cold-rolled steel plates and corrosion-resistant steel plates, the mold must be filled with oil. The oil filling points are the guide bushing, the oil filling port, the contact surface between the cutter body and the guide bushing, the lower mold, etc. Use light engine oil.
For materials with rust and dirt, the rust powder will be sucked into the space between the punch and the guide bush during processing, causing dirt to prevent the punch from sliding freely in the guide bush. In this case, if oil is applied, the rust will be easier to accumulate. So when punching this kind of material, you should wipe the oil clean instead. Disassemble it once a month. Use gasoline (diesel) oil to remove the dirt on the punch and lower mold, and wipe them clean before reassembly. This ensures that the mold has good lubrication properties.
