Stamping die gap analysis and maintenance experience

2021/08/22

I. Information seen from the scrap situation
Scrap is essentially the opposite image of the forming hole. That is, the same part in the opposite position. By checking the scrap, you can determine if the upper and lower die clearances are 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 show a small angle fracture surface and a wide bright band area.


Too large a gap creates a hole with a large rolled edge and edge tears, leaving a thin edge protruding slightly from the profile. Too small a gap creates a hole with a slight crimp and a large angular tear, resulting in a profile 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 pressure to a minimum and creates a neatly rounded hole with very little burr. From this point of view, extending the die life by increasing the gap is at the expense of the quality of the finished hole.

II. Selection of die clearance
The clearance of the die is related to the type and thickness of the material to be stamped. Unreasonable clearance can cause the following problems:
(1) If the clearance is too large, the burr of the punched workpiece will be bigger and the punching quality will be poor. If the clearance is small, the quality of the punched hole is better, but the wear of the die is more serious, which greatly reduces the service life of the die, and it is easy to cause the breakage of the punch.
(2) A gap that is too large or too small tends to produce adhesion on the punch material, thus causing strip material during punching. Too small a gap tends to form a vacuum between the bottom surface of the punch and the plate material and scrap bounce occurs.
(3) A reasonable clearance can prolong the life of the die, unload the material well, reduce burrs and flanges, keep the plate clean and consistent without scratching the plate, reduce the number of sharpening, keep the plate straight and position the punch accurately.

Please refer to the table below to select the die clearance (the data in the table are percentages)
Choice of clearance (total clearance)
Material Min Best Max
Brass 8% 12% 16%
Brass 6% 11% 16%
Low carbon steel 10% 15% 20%
Aluminum (soft) 5% 10% 15%
Stainless steel 15% 20% 25%
% × thickness of the material = mold clearance

Third, how to improve the service life of the die
To the users, improving the service life of the die can greatly reduce the stamping cost. The factors affecting the service life of the die are as follows.
1. the type and thickness of the material.
2. whether to choose a reasonable lower die clearance.
3. the structure form of the die.
4. whether the material is well lubricated when stamping.
5. whether the die has undergone special surface treatment.
6. such as titanium plating, titanium carbide nitride.
7. the alignment of the upper and lower turret.
8. the reasonable use of adjusting shims
9, whether the appropriate use of beveled edge die.
10, whether the machine tool die seat has been worn.

Fourth, punching special size holes should pay attention to the problem
(1) Please use special punches for punching holes in the range of φ0.8--φ1.6 for minimum hole diameter.

(2) When punching holes in thick plates, please use a larger die in relation to the processing hole diameter.
Note: In this case, if you use a normal size die, the threads of the punch will be broken.
Example 1: For the processing conditions in the table below, use a B-station die, although the processing hole diameter corresponds to the A-station die.
Material Plate thickness(mm) Hole diameter(mm)
Soft 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 following table, please use the C-station die although the processing hole diameter corresponds to the B-station die.
Material Plate thickness(mm) Hole diameter(mm)
Soft 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 minimum width to length ratio should not be less than 1:10 for the cutting edge part of the punch.
Example 3: For a rectangular punch with an edge length of 80mm, the width of the edge should 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 edge of the punch should be 2 times the thickness of the plate.

V. Sharpening of die
1、Importance of die sharpening
Regular sharpening of dies is the guarantee of consistency of punching quality. Regular sharpening not only can improve the service life of the die but also can improve the service life of the machine, so it is necessary to master the correct sharpening time.

2、The specific characteristics of the die that needs sharpening
For die sharpening, there is not a strict number of blows to determine whether sharpening is needed. It mainly depends on the sharpness of the cutting edge. It is mainly determined by the following three factors.
(1) Check the rounding of the edge. If the radius of the rounding reaches R0.1 mm (the maximum R value must not exceed 0.25 mm) then sharpening is required.
(2) Check the quality of the punch, is there a large burr produced?
(3) Judge whether sharpening is needed by the noise of the machine punching. If the noise is abnormal when the same die is punched, it means the punch is dull and needs sharpening.
Note: If the edge of the edge is rounded or the rear part of the edge is rough, sharpening should also be considered.

3、Method of sharpening.
There are various methods for die sharpening, which can be realized by special sharpening machine or on surface grinder. The frequency of sharpening for punch and lower die is generally 4:1, please adjust the height of the die after sharpening.
(1) Hazards of incorrect sharpening methods
Incorrect sharpening will aggravate the rapid destruction of the die edge, resulting in a greatly reduced number of blows per sharpening.
(2) Benefits of correct sharpening method
By sharpening the die regularly, the quality and accuracy of the punch can be kept stable. The edge of the die will be damaged more slowly and have a longer life.

4. Sharpening rules.
The following factors should be considered when sharpening the die.
(1) The edge rounding angle in the case of R0.1-0.25 mm size to see the sharpness of the edge.
(2) The surface of the grinding wheel should be cleaned.
(3) It is recommended to use a loose, coarse-grained, soft grinding wheel. Such as WA46KV
(4) The grinding volume (tool eating volume) should not exceed 0.013 mm each time. Too much grinding volume will cause overheating of the mold surface, which is equivalent to annealing treatment, softening the mold and greatly reducing the life of the mold.
(5) Sufficient coolant must be added when sharpening.
(6) When grinding, ensure that the punch and lower die are fixed smoothly and use special fixtures.
(7) The sharpening amount of the die is certain, if the value is reached, the punch should be scrapped. If you continue to use it, it will easily cause damage to the die and machine, which is not worth the loss.
(8) After sharpening, the edge should be treated with oil stone to remove the excessively sharp edges.
(9) After sharpening, clean it up, demagnetize and oil it.
Note: The size of die sharpening depends on the thickness of the sheet to be stamped.

VI. Note before using the punch
1. Storage
(1) Wipe the upper die set clean inside and outside with a clean cloth.
(2) Be careful not to have scratches or dents on the surface when storing.
(3)Oil it to prevent rust.

2、Prepare before use
(1)Clean the upper die set thoroughly before use.
(2)Check if there are scratches and dents on the surface. If there are, use oil stone to remove.
(3)Oil inside and outside.

3、Please note when installing the punch in the upper die set
(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 die without force. Nylon hammer cannot be used. When installing, the punch cannot be fixed by tightening the bolt on the upper die set, and the bolt can be tightened only after the punch is correctly positioned.

4、Install the upper die set into the turret
If you want to extend the die life, the clearance between the outer diameter of the upper die set and the turret hole should be as small as possible. So please execute the following procedures carefully.
(1) Clean and oil the keyway and inner diameter of the turret hole.
(2) Adjust the keyway of the upper die guide bushing to match the key of the turret hole.
(3) Insert the upper die guide straight into the turret hole, being careful not to have any tilt. The upper die guide should slide into the turret hole by its own weight.
(4) If the upper die guide is tilted to one side, tap it gently with a soft material tool such as a nylon hammer. Repeat until the upper die guide slides into the correct position by its own weight.
Note: Do not apply force on the outer diameter of the upper die guide, only on the top of the punch. Can not knock the top of the upper die set, so as not to damage the turret hole, shorten the service life of individual stations.

Seven, the maintenance of the die
If the punch is bitten by the material and cannot be taken out, please check according to the following items.
1. Re-sharpening of punch and lower die. A sharp edge of the die can process a beautiful cutting surface, and a blunt edge requires extra punching pressure, and the roughness of the workpiece section produces a great resistance, causing the punch to be bitten by the material.
2, the clearance of the die. If the clearance of the die is not selected properly relative to the plate thickness, the punch needs a large release force when it is detached from the material. If this is the reason for the punch being bitten by the material, please replace the lower die with a reasonable clearance.
3. The state of the processed material. When the material is dirty, or there is dirt, the dirt adheres to the die, making it impossible to process because the punch is bitten by the material.
4、Material with warpage. The warped material will clamp the punch after punching and make the punch bite. If the material is warped, please make it flat and then process it.
5、Overuse of spring. It will make the spring fatigue. Please check the performance of the spring from time to time.

VIII. Oiling
The amount of oil and the number of times of oiling depend on the conditions of the processed material. For cold-rolled steel, corrosion-resistant steel and other materials without rust and dirt, oil the mold, and the oiling points are the guide sleeve, the oiling port, the contact surface of the tool body and the guide sleeve, and the lower mold. Use light machine oil for oil.

Material with rust and scale, rust micro powder will be sucked between punch and guide bush when processing, producing dirt, making the punch can't slide freely in the guide bush, in this case, if oiled, it will make the rust and scale easier to stain, so when punching this material, instead, wipe the oil clean, decompose once a month, use steam (diesel) oil to remove the dirt from the punch and lower die, and wipe it clean again before reassembling. This will ensure that the mold has good lubrication performance.

Nine, the use of the mold often appear in the process of problems and solutions
Problem 1: The plate comes out from the jaws
Cause Solution
Incomplete unloading of the die 1. Use a punch with slope
2. Apply lubricating liquid on the plate
3. Use heavy-duty die

Problem 2、Severe die wear
Cause Solution
Unreasonable die clearance (small) Increase die clearance
Upper and lower mold seat is not centered 1. Work station adjustment, upper and lower mold centering
2. Turret level adjustment
Not timely replace worn die guide components and turret inserts Replace
Punch overheating 1.Add lubricating liquid on the plate material
2. Ensure lubrication between the punch and the lower die
3、Use more than one set of the same size die in the same procedure
Improper sharpening method causes annealing of the die, thus causing increased wear 1、Use soft abrasive grinding wheels
2、Clean the grinding wheel frequently
3、Small tool eating capacity
4、Adequate amount of coolant
Step punching process 1、Increase the step distance
2、Adopt bridge type step punching

Problem 3: Punch belt and punch sticking together
Cause Solution
Unreasonable die clearance (small) Increase die clearance
Dulling of punch edge Sharpening in time
Poor lubrication Improve lubrication conditions

Problem 4: Scrap rebound
Cause Solution
Lower die problem Use anti-bounce material to lower the die
For small diameter hole clearance is reduced by 10%
Diameter greater than 50.00mm, gap enlargement
Increase the scoring on the side of the concave die edge
For punch, increase the depth of entry
Installation of unloading polyurethane top bar
Adopt beveled edge

Problem 5: Difficulty in unloading
Cause Solution
Unreasonable die clearance (small) Increase die clearance
Punch wear Timely sharpening
Spring fatigue Replace the spring
Punch sticking Remove sticking

Problem 6: Stamping noise
Cause Solution
Difficulty in unloading Increase lower die clearance, good lubrication
Increase unloading force
Use soft surface unloading plate
Problems with the support of the sheet on the table and in the turret Use spherical support for the die
Reduce working dimensions
Increase working thickness
Plate thickness Use beveled punch

X. Precautions for using special forming tools
1、The stroke of slider is different for different types of machines, so pay attention to the adjustment of the closing height of forming tool.
2、Make sure the forming is adequate, so it needs to be adjusted carefully, and the adjustment amount should not exceed 0.15mm each time, if the adjustment amount is too large, it will easily cause damage to the machine and damage to the die.
3、For stretch forming, please use light spring assembly to prevent the tearing of sheet material or the difficulty of unloading material due to uneven deformation, etc.
4、Install a ball type support mold around the forming mold to prevent the sheet from tilting.
5、Forming position should be as far away from the clamping jaws as possible.
6、Forming process is best to be placed at the end of the processing procedure to achieve.
7、Make sure to ensure good lubrication of the sheet.
8、When ordering, pay attention to the problem of giving way to special forming tools. If the distance between two forming is close, please make sure to communicate with our salesman.
9、Because the forming tool needs a long unloading time, so the forming processing must use low speed, preferably with a delay.

Eleven, the use of rectangular cutting knife precautions
1、Step distance as large as possible, to be greater than 80% of the entire tool length.
2、It is best to realize jump step punching by programming.
3、It is recommended to use oblique edge die.

12、How to punch under the condition that the nominal force of the machine is not exceeded
During the production process, it is necessary to punch a round hole larger than 114.3mm in diameter. Such a large hole will exceed the upper limit of the nominal force of the machine, especially for high shear strength materials. This problem can be solved by punching large size holes by multiple punching. Using a smaller die to shear along a larger circumference can reduce the punching force by half or more, and probably most of the dies you already have can do this. The following illustrations show that large hole diameter circles can be punched using round, double D, rectangular with rounded corners, and convex lens shaped dies, respectively. In all three cases, the scrap falls from below and no scrap is left on the table.

XIII. A simple method for punching large round holes
This convex-lens die can be made to your desired radius size. If the hole diameter exceeds the nominal force of the punch, we recommend using option (A). Use this die to punch the perimeter of the circle. If the hole can be punched within the nominal force of the punch, then a radial die and a convex lens die can punch the desired hole in four passes without rotating the die (B)

XIV. Forming downward only at the end
When a forming die is selected, downward forming should be avoided because it will take up too much vertical space and result in additional flattening or bending of the sheet. Down forming can also get caught in the lower die and then be pulled out of the turret, however, if down forming is the only process option, then it should be the last step in the process for the sheet.

Prevent material distortion
If you need to punch a large number of holes in a sheet and the sheet does not stay flat, the cause may be the accumulation of punching stress. When a hole is punched, the material around the hole is stretched downward, increasing the tensile stress on the upper surface of the plate. The downward punching motion also causes an increase in compressive stress on the lower surface of the sheet. For a small number of holes, the result is not obvious, but as the number of holes punched increases, the tensile and compressive stresses increase exponentially until the sheet is deformed.

One way to eliminate this distortion is to punch every other hole and then return to punch the remaining holes. This produces the same stresses in the sheet, but dissipates the tensile/compressive stresses that accumulate as a result of punching one after the other in the same direction. This also allows the first batch of holes to share the partial deformation effect of the second batch of holes.

Sixteen, if your stainless steel flanging deformation
In the manufacture of flanging before the application of high-quality forming lubricant to the material, which can make the material better separated from the mold, in the formation of a smooth movement in the lower die surface. This gives the material a better opportunity to distribute the stresses generated by bending and stretching, preventing deformation and wear at the bottom of the flanged hole in the forming.

XVII. Suggestions to overcome the difficulty of unloading
1、Use punches with fine core rubber particles.
2、Increase the clearance of the lower die.
3、Check the fatigue level of the spring.
4、Use heavy-duty type dies.
5、Minimize wear and tear.
6、Adopt beveled edge die appropriately.
7、Lubricate the plate.
8、Large work station molds need to install polyurethane unloading head.

18、The main reason for scrap bounce
1、The sharpness of the edge. The larger the rounding of the edge, the more likely to cause the rebound of scrap.
2、The amount of mold entry. Each station die stamping, the requirements of the amount of mold entry is certain, the amount of mold entry is small, easy to cause the rebound of scrap.
3、The gap of the die is reasonable. Unreasonable die clearance will easily cause scrap rebound.
4、Whether there is oil on the surface of the sheet being processed.