16 Key Points to About Stamping Process [ Practical Experience]

The stamping process is a metal forming process in which sheet metal is given a desired shape. The stamping process may include different other sheet metal operations like punching, bending, blanking, shearing, piercing, etc.

In this article, we will learn about various aspects of the sheet metal stamping process from Shane’s practical experience.

1. Information from waste

stamping process

Scrap is essentially the reflection of the formed hole. That is the same part in the opposite position. By checking the scrap, you can judge whether the gap between the upper and lower dies is correct.

If the gap is too large, the waste will have rough and undulating fracture surfaces and a narrow bright zone. The larger the gap, the greater the angle between the fracture surface and the bright zone area.

If the gap is too small, the waste will show a small angle fracture surface and a wide bright band area. Excessive gaps form holes with large curls and edge tears, resulting in a slight thin edge protruding from the section.

Too small a gap forms a band with slight crimping and large angle tearing, resulting in a section more or less perpendicular to the material surface.

An ideal waste should have a reasonable collapse angle and a uniform bright band. This keeps the punching pressure to a minimum and forms a clean round hole with few burrs.

From this point of view, increasing the gap to prolong the service life of the die is in exchange for sacrificing the quality of the finished hole.

2. Selection of die clearance

stamping process

The clearance of the die is related to the type and thickness of the material being punched. Unreasonable clearance can cause the following problems:

(1) If the gap is too large, the burr of the stamped workpiece is relatively large and the stamping quality is poor.

If the clearance is too small, although the punching quality is good, the wear of the die is serious, which greatly reduces the service life of the die and is easy to cause the breaking of the punch.

(2) If the gap is too large or too small, it is easy to produce adhesion on the punch material, resulting in the material bend during stamping.

Too small gap is easy to form a vacuum between the bottom surface of the punch and the sheet metal, resulting in the waste rebound.

(3) Reasonable clearance can prolong the service life of the die, have a good unloading effect, reduce burr and flanging, keep the plate clean, the hole diameter is consistent, and will not scratch the plate, reduce the grinding times, keep the plate straight and accurate punching positioning.

Please refer to the following table to select the mold clearance (the data in the table is a percentage)

Selection of clearance (total clearance)
Material ScienceMinimumBestMaximum
Red copper8%12%16%
brass6%11%16%
mild steel10%15%20%
Aluminum (soft)5%10%15%
stainless steel15%20%25%
% × Thickness of material = Die clearance

3. 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 affecting the service life of the die are as follows:

  • Type and thickness of material;
  • Whether reasonable lower die clearance is selected
  • Structural form of mold
  • Whether there is good lubrication during material stamping
  • Whether the mold has undergone special surface treatment
  • Such as titanium plating, carbon titanium nitride
  • Alignment of upper and lower turrets
  • Reasonable use of adjusting gasket
  • Whether the inclined edge die is properly used
  • Whether the die base of the machine tool has been worn

4. Problems needing attention in stamping special size holes

  • If the minimum hole diameter is in the range of φ0.8—φ1.6, use a special punch.
  • When punching a thick plate, please use a larger die relative to the machining hole diameter.

Example 1. According to the processing conditions in the following table, although the processing aperture corresponds to the die in station A, please use the die in station B.

Texture of materialPlate thickness (mm)Aperture (mm)
Mild steel (40kg / mm2)6.08.2-12.7
Stainless steel (60kg / mm2)4.08.2-12.7

Example 2. According to the processing conditions in the following table, although the processing aperture corresponds to the B station die, please use the C station die.

Texture of materialPlate thickness (mm)Aperture (mm)
Mild steel (40kg / mm2)6.022.9-31.7
Stainless steel (60kg / mm2)4.022.9-31.7

(3) Generally, the ratio of the minimum width to the length of the punch edge shall not be less than 1:10.

Example 3. For rectangular punch, when the cutting edge length is 80mm, the cutting edge width ≥ 8mm is the most appropriate.

(4) The relationship between the minimum dimension of punch edge and plate thickness.

It is suggested that the minimum dimension of the punch edge should be 2 times the plate thickness.

5. Die grinding

stamping

1. Importance of die grinding

Regular grinding of die is the guarantee of punching quality consistency.

Regular grinding of the die can not only improve the service life of the die but also improve the service life of the machine. It is necessary to grasp the correct grinding time.

2. The specific characteristics of the die need to be sharpened

For die grinding, there is no strict number of blows 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 fillet of the cutting edge. If the fillet radius reaches R0.1mm (the maximum R-value shall not exceed 0.25mm), grinding is required.

(2) Check the punching quality and whether there are large burrs?

(3) Determine whether grinding is required by the noise of machine stamping.

If the noise of the same die is abnormal during stamping, it indicates that the punch has been blunt and needs grinding.

Note: grinding should also be considered if the edge of the cutting edge becomes round or the rear of the cutting edge is rough.

3. Grinding method

There are many ways to grind the die, which can be realized by a special grinding machine or plane grinder.

The grinding frequency of punch and lower die is generally 4:1; Please adjust the die height after grinding.

(1) Hazards of incorrect grinding methods:

Incorrect grinding will aggravate the rapid damage of the die edge, resulting in a significant reduction in the number of blows per grinding.

(2) Benefits of correct grinding method:

The quality and accuracy of punching can be kept stable by grinding the die regularly.

The cutting edge of the die is damaged slowly and has a longer service life.

4. Grinding rules

The following factors shall be considered during die grinding:

(1) When the edge fillet is R0.1-0.25mm, it depends on the sharpness of the edge.

(2) The surface of the grinding wheel shall be cleaned.

(3) A loose, coarse and soft grinding wheel is recommended. Such as WA46KV.

(4) The grinding amount (cutting amount) each time shall not exceed 0.013mm. 

The excessive grinding amount will cause overheating of the die surface, which is equivalent to the annealing treatment, softening the die and greatly reducing the service life of the die.

(5) Sufficient coolant must be added during grinding.

(6) During grinding, the punch and lower die shall be fixed stably, and special tooling fixtures shall be used.

(7) The grinding amount of the die is certain. If it reaches this value, the punch will be scrapped.

If you continue to use, it is easy to cause damage to the mold and machine, and the gain is not worth the loss.

(8) After grinding, the edge shall be treated with oilstone to remove excessively sharp edges and lines.

(9) After grinding, the blade shall be cleaned, demagnetized and oiled.

Note: the grinding amount of the die mainly depends on the thickness of the stamped plate.

6. Pay attention to the punch before use

1. Store

(1) Wipe the inside and outside of the upper mold sleeve with a clean cloth.

(2) When storing, be careful not to scratch or dent the surface.

(3) Oil to prevent rust.

2. Preparation before use

(1) Clean the upper die sleeve thoroughly before use.

(2) Check the surface for scratches and dents. If any, remove it with an oilstone.

(3) Oil inside and outside.

3. Precautions when installing punch on 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 die without force.

A nylon hammer cannot be used.

During installation, the punch cannot be fixed by tightening the bolts on the upper die sleeve. The bolts can be tightened only after the punch is correctly positioned.

4. Install the upper mold assembly into the turret

If you want to prolong the service life of the die, the gap between the outer diameter of the upper die sleeve and the turret hole should be as small as possible.

Therefore, please carefully perform the following procedures.

(1) Clean and oil the keyway and inner diameter of the turret hole.

(2) Adjust the keyway of the upper die guide sleeve to match the key of the turret hole.

(3) Insert the upper mold sleeve straight into the tower hole, and be careful not to tilt.

The upper die guide sleeve should slide into the turret hole by its own weight.

(4) If the upper die sleeve tilts to one side, it can be gently knocked right with soft material tools such as a nylon hammer.

Repeat knocking until the upper die guide sleeve slides into the correct position by its own weight.

Note: do not apply force to the outer diameter of the guide sleeve of the upper die, but only on the top of the punch.

Do not knock the top of the upper die sleeve to avoid damaging the turret hole and shortening the service life of individual stations.

6. Maintenance of mould

If the punch is bitten by the material and cannot be taken out, please check according to the following items.

1. Regrinding of punch and lower die.

The die with a sharp edge can process beautiful cutting sections. If the edge is blunt, additional punching pressure is required. Moreover, the workpiece section is rough, resulting in great resistance, causing the punch to be bitten by the material.

2. Die clearance.

If the clearance of the die is not properly selected relative to the plate thickness, the punch needs a large demoulding force when it is separated from the material.

If the punch is caught by the material for this reason, please replace the lower die with reasonable clearance.

3. Status of processed materials.

When the material is dirty or has dirt, the dirt adheres to the die, so that the punch is bitten by the material and cannot be processed.

4. Deformed material.

After punching the hole, the warped material will clamp the punch and make the punch bite.

Materials with warpage shall be leveled before processing.

5. Excessive use of springs.

Will cause spring fatigue. Please always check the performance of the spring.

7. Oiling

The amount of oil and the number of oil injections depend on the conditions of the processing materials.

For materials without rust and scale, such as cold-rolled steel plates and corrosion-resistant steel plates, the oil shall be injected into the die. 

The oil injection points are the guide sleeve, oil injection port, the contact surface between the knife body and guide sleeve, lower die, etc. Use light engine oil for the oil.

For materials with rust and scale, the rust powder will be sucked between the punch and the guide sleeve during processing, resulting in the dirt, so that the punch can not slide freely in the guide sleeve. 

In this case, if you apply oil, it will make the rust scale easier to stick to. 

Therefore, when flushing this material, on the contrary, wipe the oil, decompose it once a month, and remove the dirt of the punch and lower die with steam (firewood) oil, Wipe clean before reassembly.

This can ensure that the mold has good lubrication performance.

8. Common problems and solutions in the process of mold use

stamping

Problem 1: The plate comes out of the jaw

ReasonResolvent
Incomplete mold unloading1. Use a punch with a slope
2. Apply lubricant on the plate
3. Heavy-duty die is adopted

Problem 2: Serious die wear

ReasonResolvent
Unreasonable die clearance (too small)Increase die clearance
Misalignment of upper and lower die seats1. Station adjustment and alignment of upper and lower dies2. Turret level adjustment
The worn die guide components and turret inserts were not replaced in timeReplace
Punch overheating1. Add lubricant to the sheet2. Ensure lubrication between the punch and the lower die3. Multiple sets of molds with the same specification and size are used in the same program
The improper grinding method will lead to the annealing of the die, which will aggravate the wear1. Soft abrasive grinding wheel2. Clean the grinding wheel frequently3. Small bite4. Sufficient coolant
Step punching1. Increase the step distance2. Bridge stepping is adopted

Problem 3: Punch belt and punch adhesion

ReasonResolvent
Unreasonable die clearance (too small)Increase die clearance
Punch edge passivationTimely grinding
Poor lubricationImprove lubrication conditions

Problem 4: Waste rebound

ReasonResolvent
Lower module problemUse bulletproof material to lower the mold
For small diameter holes, the clearance is reduced by 10%
Diameter greater than 50.00 mm, gap enlarged
Scratches are added on the edge side of the die
PunchIncrease die depth
Install the discharge polyurethane ejector bar
Adopt oblique cutting edge

Problem 5: Difficult unloading

ReasonResolvent
Unreasonable die clearance (too small)Increase die clearance
Punch wearTimely grinding
Spring fatigueReplace the spring
Punch adhesionRemove adhesion

Question 6: Stamping noise

ReasonResolvent
Unloading difficultyIncrease the lower die clearance and good lubrication
Increase discharge force
Discharge plate with soft surface
There is a problem with the support of the sheet on the workbench and in the turretSpherical support die
Reduce working size
Increase working thickness
Sheet thicknessUse oblique edge punch

9. Precautions for using special forming tools

1. The stroke of the slide block of different models of machines is different, so pay attention to the adjustment of the closing height of the forming die.

2. It must be ensured that the molding is sufficient, so it needs to be carefully adjusted. It is best not to exceed 0.15mm each time. 

If the adjustment is too large, it is easy to cause damage to the machine and mold.

3. For stretch forming, please select light spring assembly to prevent tearing of sheet metal or difficult unloading due to uneven deformation.

4. Install a ball 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. Molding is best realized at the end of the processing program.

7. Be sure to ensure good lubrication of the plate.

8. When ordering, pay attention to the yield of special forming tools. If the two forming distance is close, please be sure to communicate with the salesperson of our company.

9. Because the forming tool needs a long unloading time, the low speed must be adopted during forming processing, and it is best to have a delay.

10. Precautions for using rectangular cutter

1. The step distance shall be as large as possible, which shall be greater than 80% of the whole tool length.

2. It is best to realize the jump step through programming.

3. It is recommended to use an oblique edge die.

12. How to punch without exceeding the nominal force of the machine?

Round holes larger than 114.3mm in diameter need to be punched in the production process.

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 holes through multiple punching methods.

Cutting along the circumference of a large circle with a small-size die can reduce the punching force by half or more. Most of the dies you already have may be able to do so.

11. A simple method for punching large round holes

The mold of this convex lens can be made into the radius size you need.

If the hole diameter exceeds the nominal force of the punch, we recommend scheme (A).

Use this die to punch out the periphery of the circle.

If the hole diameter can be punched within the nominal force range of the punch, a radial die and a convex lens die can punch the required hole four times without rotating the die (B).

12. Finally, it is formed downward

When selecting the forming die, the downward forming operation should be avoided, because it will occupy too much vertical space and lead to additional sheet leveling or bending processes.

Downward forming may also fall into the lower die and then be pulled out of the turret. 

However, if downward forming is the only process option, it should be regarded as the last step in the treatment of sheet metal.

13. Prevent material distortion

If you need to punch a large number of holes in the plate and the plate can not keep flat, the cause may be the accumulation of stamping 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 movement also leads to the increase of compressive stress on the lower surface of the plate.

For punching a small number of holes, the result is not obvious, but with the increase of the number of punching holes, the tensile stress and compressive stress also increase exponentially until the plate is deformed.

One way to eliminate this deformation is:

Punch every other hole, and then return to punch the remaining holes.

Although this produces the same stress on the plate, it disintegrates the accumulation of tensile/compressive stress caused by punching one after another in the same direction.

In this way, the first batch of holes shares the deformation effect of the second batch of holes.

14. If your stainless steel flanging is deformed

Apply high-quality forming lubricant to the material before flanging, which can better separate the material from the die and move smoothly on the lower die surface during forming.

This gives the material a better chance to distribute the stress generated when it is bent and stretched, so as to prevent the deformation on the edge of the forming flanging hole and the wear at the bottom of the flanging hole.

15. Suggestions on overcoming unloading difficulties

1. Use a punch with fine rubber particles.

2. Increase the lower die clearance.

3. Check the fatigue of the spring.

4. Use a heavy-duty die.

5. Appropriate use of oblique edge die.

6. Lubricate the plate.

7. Polyurethane discharge head shall be installed for large station die.

16. Main causes of waste rebound

1. The sharpness of the cutting edge. The larger the fillet of the cutting edge, the easier it is to cause waste rebound.

2. Die entry modulus. When stamping the die at each station, the requirements for the entry modulus are certain. The entry modulus is small, which is easy to cause waste rebound.

3. Whether the clearance of the die is reasonable. Unreasonable die clearance is easy to cause waste rebound.

4. Whether there is an oil stain on the surface of the processed plate.

However, you will have to carry out routine and regular maintenance to ensure that coolants and the delivery systems deliver to expectations.


This is a guest post By Shane From the MachineMfg team


machinemfg
Shane Z

Shane has been working in the mechanical engineering industry for more than 5 years. He loves writing and focuses on sharing technical information, guidance, detailed solutions, and thoughts related to metals and metalworking. Through his articles, users can always easily get related problems solved and find what they want.

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