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Methods to Improve the Accuracy of Stamping Parts

Last updated:
May 16, 2024
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The workpieces obtained by ordinary punching have chamfers, fracture zones, and burrs on the shearing surface, and also have obvious taper, with a surface roughness Ra of 6.3~

12.5μm, and at the same time, the dimensional accuracy of the punched parts is relatively low, generally T10~T11, which can usually meet the technical requirements of the parts.

However, when the shearing surface of the punched parts is used as a reference surface, mating surface, assembly joint surface, or moving surface, higher requirements for the sectional quality and dimensional accuracy of the punched parts are needed. At this time, it is necessary to adopt technological methods to improve the quality and accuracy of the punched parts (see Table 1) to meet the requirements.

Table 1 Several technological methods to improve the quality and accuracy of punched parts

CategoryProcess NameSchematicKey Points of the MethodKey Points of the Method
FinishingOverhaulRemove unclean surfaces, unilateral gap of 0.006~0.01mm or negative gap, determine the amount and frequency of overhaul based on material thickness and shapeHigh precision, low surface roughness, small collapse angles and burrs. High positioning requirements, not easy to remove chips. Efficiency is lower than precision stamping
Extrusion polishingConical concave die extrusion polishing, allowance on one side less than 0.04~0.06mm. The gap between convex and concave dies is generally (0.1~0.2)t (t is material thickness)Lower quality than overhaul and precision stamping, only suitable for soft materials, efficiency lower than precision stamping
Semi-precision stampingNegative clearance stampingConvex die size larger than concave die size (0.05~0.3)t, concave die radius (0.05~0.1)tLower surface roughness, suitable for soft non-ferrous metals and alloys, soft steel, etc.
Small gap rounded corner blade punchingGap less than 0.02mm


Blanking: Die blade rounded corner radius is 0.1t


Punching: Punch blade rounded corner radius is 0.1t
Smaller surface roughness value, larger collapse angle and burrs
Up and down punchingFirst step press convex, punch depth into (0.15~0.30)t, second step reverse punch down the workpieceNo burrs on the upper and lower sides, still has collapse angles and fracture surfaces, complex actions
Up and down punchingPunch cuts into the plate material (0.15~0.35)t, punch ab surface then squeezes the plate material, single-sided gap between punch and die 0.01~0.05mm, blade rounded corner radius is 0.05~0.2mmThe cut surface is smooth, and the surface roughness value is small. It has a great adaptability to the material’s performance and thickness, without the need for specialized precision punching equipment.
Precision punchingToothed ring pressure plate

Precision punching
/
Opposite concave die

Precision punching
/
Opposite concave die

Precision punching
/

The following briefly describes several methods of finishing and semi-precision punching.

I. Refinishing

Refinishing involves using a finishing die to scrape off a thin layer of chips along the outer edge or hole wall of the punched parts, removing the collapse, burrs, and fracture zones left on the cross-section during ordinary punching, thereby obtaining smooth and perpendicular cross-sections and accurately sized parts. Generally, the parts after refinishing can achieve tolerances of IT6 to IT7, and surface roughness Ra can reach 0.4 to 0.8μm.

The refinishing method is shown in Figure 1. The outer shape of the trimmed parts is called edge trimming (see Figure 1a); the inner shape of the punched parts is called inner edge trimming (see Figure 1b), and the mechanism of refinishing is completely different from punching, similar to cutting processing.

Figure 1 Refinishing methods
Figure 1 Refinishing methods

1. Refinishing allowance

The refinishing allowance must be chosen appropriately; too large or too small will reduce the quality of the refinished parts. The refinishing allowance is related to the material, thickness, and shape of the parts, as well as the processing conditions before refinishing. For example, if large clearance punching is used before refinishing, a larger refinishing allowance is needed to cut off the larger taper fracture zone on the cross-section; whereas, with small clearance punching, to cut off the middle rough band and potential cracks formed by secondary shearing, a large refinishing allowance is not necessary.

For inner edge refinishing, if drilling is involved, the refinishing allowance can be smaller than that for punching. If the accuracy of the hole spacing is also required during hole refinishing, then the refinishing allowance should be increased. The refinishing allowance for large clearance punching is shown in Table 2, while for small clearance punching, the refinishing allowance can be found in Figure 2.

Table 2 Refinishing bidirectional allowance y (unit: mm)

Material thicknessBrass, mild steelMedium-hard steelHard steel
MinMaxMinMaxMinMax
0.5~1.60.10.150.150.20.150.25
>1.6 ~3.00.150.20.20.250.20.3
>3.0~4.00.20.250.250.30.250.35
>4.0~5.20.250.30.30.350.30.4
>5.2~7.00.30.350.40.450.450.5
>7.0~10.00.350.40.450.50.550.6

Note:

1. The smallest allowance is used for simple-shaped workpieces, and the largest allowance is used for complex-shaped or sharp-angled workpieces.

2. In multiple repairs, the smallest value in the table is used for the second and subsequent repairs.

3. The repair allowance for titanium alloys is (0.2 ~ 0.3) t.

Figure 2 Repair allowance when using small clearance punching
Figure 2 Repair allowance when using small clearance punching

a) Blanking
b) Punching

According to the size of the die clearance during blanking, the calculation method for the dimensions of the working part of the die is divided into two types. See Table 3 for the calculation of the dimensions of the blanking die, and Table 4 for the punching die.

Table 3 Calculation of dimensions of the working part of the blanking die before repair

Die dimensions and repair allowanceThe first type of repair method
Use large gap blanking
The first type of repair method
Use large gap blanking
Blanking die size

Blanking punch size

Single side gap

Overhaul allowance

Total removal allowance

y refer to Table 2
δ equals 2c plus y

δ, see Figure 2a

Note:

  • c—single side clearance for punching;
  • y—repair allowance, see Table 7;
  • D—basic dimensions of the repair part;
  • t—thickness of the repair part;
  • δ p , δ d – manufacturing deviations of the convex and concave dies, δ p , δ d = (0.8~1.2) (c max -c min );

Table 4 Calculation of dimensions for the working part of the punch mold before repair

Mold dimensions and repair allowanceFirst repair method
Using large clearance punching
Second repair method
Adopt small gap punching
Punch die size

Punch punch size

Single side gap

Repair allowance

Total removal allowance

y refer to Table 2
δ equals 2c plus y

δ, see Figure 2b
δ

Note: d——basic size of the repair hole; 

2. Number of repairs

The number of repairs is related to the material thickness and shape of the workpiece. For workpieces with a thickness of less than 3mm and a simple, smooth shape, generally only one repair is needed; for workpieces with a thickness greater than 3mm, or with sharp angles, two or more repairs are required to prevent tearing. The distribution of the allowance for the second repair is shown in Figure 3. The number of repairs can be determined from Table 5 based on the material thickness and complexity of the workpiece’s shape.

Figure 3 Multiple Repair Allowances
Figure 3 Multiple Repair Allowances

1—First Repair
2—Second Repair

Table 5 Repair Process Frequency

Complexity of the Workpiece ContourMaterial Thickness / mm
<3<3
Smooth Contour without Sharp Angles12
Complex Contour with Sharp Angles23~4

3. Repair Force

The force required for overhaul can be approximately calculated by the following formula

Pz=L(δ+0.1tn)τb

where

  • L – Perimeter length of the overhaul (mm);
  • δ – Total removal allowance (mm);
  • n – Number of parts simultaneously clamped in the die:
  • t – Material thickness (mm);
  • τ b – Material shear strength (MPa).

4. Calculation of dimensions for the working part of the overhaul mold

The formula for calculating the dimensions of the working part of the overhaul mold is shown in Table 6.

Table 6 Calculation of dimensions for the working part of the overhaul mold

Work part dimensionsOuter edge reconditioning (process)Inner edge reconditioning (process)
Reconditioning die dimensionsThe die generally only supports the blank, and the shape and dimensions of the cavity do not need to be strictly specified
Reconditioning punch dimensions

Note:
  • max is the maximum limit size of the reconditioned part (mm);
  • min is the minimum limit size of the reconditioned part (mm);
  • Δ is the tolerance of the reconditioned part (mm);
  • c′—single side clearance of the reconditioning mold, 2c’=0.01~0.025mm;
  • δ p , δ d convex and concave mold manufacturing tolerances (mm), δ p =0.2Δ, δ d =0.25Δ;
  • ε y — shrinkage of the hole after refurbishment
  • For aluminum: ε y =0.005~0.01mm;
  • Brass: ε y =0.007~0.012mm;
  • Soft steel: ε y =0.008~0.015mm.

5. Other refurbishment methods

(1) Burnishing refurbishment

The edge burnishing refurbishment involves forcibly pushing the blank obtained from ordinary punching into a hole with a rounded or conical concave mold (see Figure 4), using surface plastic deformation to achieve a neat and smooth cross-section. The unilateral burnishing allowance is less than 0.04~0.06mm. This process is generally only suitable for soft materials, and its quality is slightly lower than that of cutting refurbishment processes.

Figure 4 Burnishing concave mold
Figure 4 Burnishing concave mold

a) Rounded concave mold
b) Conical concave mold

The determination of the working part size of the concave mold is the same as the refurbishment mold, but since this method involves larger elastic deformation of the workpiece (for workpieces within 30mm, the elastic deformation can reach 0.01~0.025mm), and increases with the thickness of the refurbished workpiece, the concave mold size should be considered accordingly. The convex mold size is larger than the concave mold size by (0.1~0.2)t.

Inner edge finishing using a mandrel or precision-pressed ball (see Figure 5). The process involves using the pressure of a convex mold to force a steel ball (or mandrel) with high hardness (63-66 HRC) through a hole on the workpiece that is slightly smaller than the required size, flattening the surface of the hole. It can not only process circular holes with a ball but also process non-circular holes with notches using a mandrel.

Figure 5 Punching and precision pressing convex mold
Figure 5 Punching and precision pressing convex mold

(2) Laminated finishing

Using general finishing methods, due to the very small gap, high precision in mold manufacturing is required, and there is also the issue of choosing the optimal finishing allowance. Therefore, a smooth surface may not be achieved with a single finishing pass, and laminated finishing can avoid the aforementioned problems.

Laminated finishing involves stacking two blanks together, with the diameter of the convex mold larger than that of the concave mold, and the convex mold presses on the blank being finished through one of the blanks. When the finishing reaches 2/3 to 3/4 of the thickness of the blank plate, the second blank is fed in for the next finishing stroke (see Figure 6).

Figure 6 Laminated finishing
Figure 6 Laminated finishing

1—Convex mold
2—Guide plate
3—Concave mold (with leading angle)
4—Initial blank finishing to 2/3 to 3/4 of the plate thickness
5—Overlapping billets for next adjustment
6—Chips
7—Workpieces

Since the punch does not enter the die during refurbishment, mold manufacturing is easy. The materials suitable for refurbishment and the range of machining allowances are wider than general refurbishment methods. The disadvantage is that after the blank enters in the next stroke, the chips must be removed, so corresponding measures are necessary, such as machining a 10°~15° lead angle or chip-breaking groove on the end face of the die, and using high-pressure compressed air to blow off the chips. Another issue is large burrs.

(3) Vibratory refurbishment

For small, high-precision parts with complex shapes like cams and gears, vibratory refurbishment can also be performed on a special vibratory press equipped with a vibrating slider that has a second motor to ensure that the punch connected to this slider vibrates. The parts placed on the refurbishment die, when the press advances 0.05~0.06mm per stroke, endure 1200~2000 brief impacts per minute.

The deformation in vibratory refurbishment is confined to a smaller volume of the metal being processed, avoiding the extension of lead cracks and the occurrence of tearing. Additionally, due to the vibrating action of the die edge scraping, the shearing surface is smooth and the deformation of the parts is minimized. After refurbishment, the dimensional accuracy of the parts can reach 0.05~0.01mm, with a surface roughness Ra of 0.4~0.8μm.

II. Negative clearance punching

As shown in Figure 7, negative clearance punching is essentially a composite process of punching and refurbishment. Since the size of the punch is larger than that of the die, the direction of the cracks that occur during the punching process is opposite to that of ordinary punching, forming an inverted cone-shaped blank. The punch continues to press down, pushing the blank into the die, cutting off some excess material, and obtaining a higher quality cross-section, equivalent to the refurbishment process.

Figure 7 Negative clearance punching
Figure 7 Negative clearance punching

Generally, the size of the punch is larger than that of the die by (0.1~0.2)t. For circular workpieces, the perimeter by which the punch is larger than the die is uniform. For workpieces with recesses and protrusions, the protruding corners should be twice as large as the rest, i.e., (0.2~0.4)t, and the recessed parts should be reduced by half, i.e., (0.05~0.1)t, as shown in Figure 8.

Figure 8 Distribution of the gap between the punch and die around non-circular workpieces
Figure 8 Distribution of the gap between the punch and die around non-circular workpieces

To achieve a lower surface roughness value on the sheared surface, a radius of 0.1~0.3mm can be made on the edge of the die. Since the punch is larger than the die, the punch should not enter the die hole after punching is completed, but should maintain a distance of 0.1~0.2mm from the upper surface of the die.

At this time, the blank has not been completely pressed into the die, and it needs to be fully pressed in during the punching of the next part. After the workpiece falls from the die opening, its size will increase by 0.02~0.06mm due to elastic deformation. Therefore, when designing the working part of the die, this deformation should be reduced accordingly.

Using this method, the surface roughness Ra of the punched workpieces can reach 0.4~0.8μm, and the dimensional accuracy can reach IT9 to IT11. However, for large-sized thin plates with a thickness t≤1.5mm, significant arching can occur. Additionally, negative clearance punching is only suitable for soft materials with good plasticity, such as soft aluminum, copper, soft steel, etc. It is mainly used for precision blanking of cold extruded plate blanks and some simple flat parts.

The force required for negative clearance punching is much greater than that for normal punching, and the die undergoes greater pressure, making it prone to cracking. Using good lubrication can prevent material from sticking to the mold and extend the life of the mold.

The force for negative clearance punching P can be estimated by the following formula

Pf=CP

In the formula

  • P – Normal punching force (N);
  • C – Coefficient, selected according to different materials: Aluminum: C=1.3~1.6; Brass: C=2.25~2.8; Soft steel: C=2.3~2.5.

III. Small Gap Rounded Corner Punching

During blanking, the die edge has a small rounded or elliptical corner (see Figure 9), while the punch is of a standard form. During punching, the punch edge has a rounded corner, and the die is of a standard form. The double-sided gap between the punch and die is less than 0.01 to 0.02mm and is independent of material thickness.

Figure 9 Small Gap Rounded Corner Die Structure Form
Figure 9 Small Gap Rounded Corner Die Structure Form

a) With elliptical corner
b) With rounded corner

Because the die edge is rounded and uses a very small gap, it increases the hydrostatic pressure in the punching area, reduces tensile stress, and the rounded edge also reduces stress concentration, thus inhibiting crack formation and achieving a bright sheared surface.

Figure 9 shows two forms of dies with elliptical or rounded corners. Figure 9a shows a die with an elliptical corner, where the arc and straight line connection should be smooth and uniform, without any sharp edges.

The radius of the rounded corner R1 is shown in Table 7, which is the result obtained for a workpiece with a diameter of 25mm. Other sizes can choose 2/3 of the values in the table, and increase the rounded corner as needed during the trial punching process. For manufacturing convenience, the die shown in Figure 9b can also be used, where the radius of the rounded corner is generally R=0.1t (t is the material thickness), or selected according to Table 8.

Table 7 Elliptical Corner Die Rounded Corner Radius R 1 Value (Workpiece Diameter ϕ = 25mm) (Unit: mm)

MaterialMaterial conditionMaterial thicknessFillet radius R 1
Mild steelHot rolled40.5
6.40.8
9.61.4
Cold rolled40.25
6.40.8
9.61.1
Aluminum alloySoft40.25
6.40.25
9.60.4
Hard40.25
6.40.25
9.60.4
CopperSoft40.25
6.40.25
9.60.4
Hard40.25
6.40.25
9.60.4

Table 8 Values of round corner concave die R (unit: mm)

MaterialMaterial thickness 1Material thickness 2Material thickness 3Material thickness 4
Aluminum0.250.250. 50
Copper (T2)0.250.5(1.00)
Mild steel0.25(0.5)(1.00)
Brass (H70)(0.25)(1.00)
Stainless steel (0Cr18Ni9)(0.25)(0.5)(1.00)

Note: The data in parentheses are reference values.

Small clearance rounded edge punching is suitable for materials with good plasticity, such as soft aluminum, pure copper, brass, and soft steel (05F, 08F), etc. The workpiece should ideally have a uniform smooth contour, and rounded corners must be used at right angles or sharp corners to prevent tearing. When calculating the punching force, it should be increased by 50% based on the ordinary punching force.

The machining accuracy of the parts can reach IT9 to IT11, and the surface roughness Ra can reach 0.4 to 1.6um. After the part is pushed out from the die hole, due to elastic deformation, its size will increase by 0.02 to 0.05mm, which should be compensated for in the mold design.

IV. Up and Down Punching

The up and down punching process (also called reciprocating punching) is shown in Figure 10. It uses two convex molds to punch the workpiece from above and below, first punching from top to bottom (as shown in a), and stopping when the upper convex mold cuts into the material by 15% to 30% of the material thickness. Then, the lower convex mold is used to punch upwards in reverse (as shown in b, c, d).

Figure 10 The punching process of the up and down punching method

The deformation mechanism of this method is similar to ordinary punching, still producing shear cracks and fracture zones, but because it undergoes two punches, up and down, it can obtain two bright bands, thereby increasing the proportion of bright bands in the entire cross-section, and can eliminate burrs, thus greatly improving the cross-sectional quality of the punched parts (see Figure 11).

Shear section during up and down punching in Figure 11
Figure 11 Shear section during up and down punching
  • 1, 5—Collapse angle
  • 2, 4—Bright band
  • 3—Fracture zone

However, as this method involves a more complex mold structure, it increases the punching time and has special requirements for the stamping equipment, thus it is less used in production currently. To avoid using a specialized press, this method can also be implemented with a three-station progressive die on a single-action press in three steps (see Figure 12).

Figure 12 Three-stage reciprocating punching process
Figure 12 Three-stage reciprocating punching process

1—Upper die holder
2—Lower die holder
3—Embossing die
4—Counter-sinking die
5—Separation punching convex die
6—Embossing concave die
7—Reverse topping convex die
8—Separation punching concave die
9—Embossing pressure plate
10—Separation punching pressure plate

1. In the first step of embossing, the material is not cut but only pressed into a pit in the punching area (see figure 12b).

2. In the second step, the embossed blank is punched back to the still unbroken state in the punching area from the opposite direction of the first step (see figure 12d).

3. In the third step, punching is performed in the same direction as the first step, completely separating the blank (see figure 12f).

The die penetration during the embossing and reverse punching stages is mainly based on the material’s thickness and performance. The depth of die penetration, whether during embossing or reverse punching, must be limited to a level where the punching area is not yet torn.

V. Synchronous Shearing and Squeezing Punching

The working process of the synchronous shearing and squeezing punching method (i.e., step-type punch punching) is shown in Figure 13. When the punch cuts into the sheet metal, the material undergoes shear deformation based on its own plasticity until the ab surface of the punch contacts the sheet surface (see Figure 13a, b). At this point, the sheet metal does not produce shear cracks, and the duration of this phase mainly depends on the material’s plasticity and the condition of the die edge.

Figure 13 The working process of synchronous shearing and squeezing blanking
Figure 13 The working process of synchronous shearing and squeezing blanking

1—Punch
2—Die
3—Sheet metal
4—Workpiece

As the punch continues to press in, the ab surface of the punch presses and squeezes into the sheet metal, and the compressed material establishes a sufficiently large hydrostatic pressure in the shear zone P to enhance the material’s plasticity, suppress the generation of shear cracks, and allow plastic shear deformation to continue throughout the shearing process. When the end face of the punch just enters the die hole, the precise separation of the sheet metal is finally completed (see Figure 13c, d).

In the aforementioned shearing process, the material’s inherent plasticity is appropriately utilized, and then a sufficiently large hydrostatic pressure is timely applied to suppress the generation of shear cracks.

As the ab surface of the punch gradually squeezes in, the hydrostatic pressure will become increasingly large, just compensating for the gradual reduction in plasticity of the sheet metal during the shearing process. In the working part of the punch, the main function of the ao section is to control the timing of the initial increase in hydrostatic pressure, while the main function of the ab surface is to control the magnitude of the hydrostatic pressure. By changing their sizes, they can adapt to sheet metals of various performances and thicknesses.

Using the same principle, precision punching can also be achieved, as shown in Figure 14. At this time, the punching waste must be discharged from above the die.

Figure 14 Synchronous shear extrusion punching process
Figure 14 Synchronous shear extrusion punching process

1—Punch
2—Die
3—Sheet material
4—Workpiece

The technical key to this method is to reasonably determine the shape and size of the working part of the blanking punch (or punching die), and select the recommended values according to the different plasticity of the material.

  • The length of the ao section is (0.15~0.35)t;
  • The horizontal width of the ab surface K=(0.1~0.4)t;
  • The angle between the ab surface and the horizontal plane is 0°~20°;
  • The unilateral gap between the punch (or die) ao section and the die (or punch) is 0.01~0.05mm;
  • The edge radius of the blanking die (or punching punch) is 0.05~0.2mm.

Using this method, experiments were conducted on pure copper, aluminum alloy, brass, 08 steel, 25 steel (hot-rolled steel), and zinc alloy using a guide plate mold on a conventional press, all achieving completely smooth shear surfaces. Additionally, for leaded brass, which is difficult to precision punch, this method allows the sheared surface of the workpiece to nearly reach 0.9t (t being the material thickness).

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