Press Tools

9.Fine Blanking   

9.1    FINE-EDGE BLANKING AND PIERCING

Fine-edge Blanking (also known as fine blanking, smooth-edge blanking, or fine-flow blanking) produces precise blanks in a single operation without the fractured edges characteristically produced in conventional blanking and piercing. In fine-edge blanking, a V-shape impingement ring (Fig. 9-1) is forced into the stock to lock it tightly against the die and to force the work metal to flow toward the punch, so that the part can be extruded out of the strip without fracture or die break. Die clearance is extremely small, and punch speed much slower than in conventional blanking.

Fine-edge piercing can be done either separately or at the same time as fine-edge blanking. In piercing shall holes, an impingement ring may not be needed.

No further finishing or machining operations are necessary to obtain blank or hole edges comparable to machined edges, or to those that are conventionally blanked or blanking a simple shape.pierced and then shaved. A quick touchup on an abrasive belt or a short treatment in a vibratory finisher may be used to remove the small burr on the blank.

Specially designed single-operation or compound blanking and piecing dies are generally used for the process.

9.2    PROCESS CAPABILITIES

Holes can be pierced in low-carbon steel with a diameter as small as 50% of stock thickness. In high-carbon steel, the smallest hole diameter is about 75% of stock thickness. Holes can be spaced as close to each other, or to the edge of the blank, as 50 to 75% of stock thickness. Total tolerances obtainable are: 0.0125 mm. on hole diameter and for accuracy of blank outline; 0.025 mm. on hole location with respect to a datum surface and 0.025 mm. on flatness.

No die break shows on the sheared surface of the hole. Blank edges may be rough for a few thousandths of an inch of thickness on the burr side of the part when the width of the part is about twice the stock the stock thickness or less. Finish on the sheared edge is governed by the condition of the die edge and the land within the die. Parts fine-edge blanked from stainless steel will have a surface finish of 1.3 micro-mm. or better. Smooth edges also are produced on spheroidize-annealed steel parts. Burr formation increases rapidly during a run, necessitating frequent grinding of the cutting elements.

Chamfers can be coined around holes and on edges. Forming near the cut edge, or forming offset parts with a bend angle up to 30°, is possible under restricted conditions.

Metals up to 3.2 mm. thick having a tensile strength of 6,000 to 8,000 Kg. / sq. cm. are easily blanked. Parts up to 13 mm. thick can be blanked if press capacity is available. Material thicker than 3.2 mm., especially steel having a carbon content of 0.25% or more, requires an impingement ring on the die so that the corners on the part will not break down. The edges of parts made of 1018 steel work harden as much as 7 to 12 points Rockwell C during blanking.

In tests on 0.60% carbon spring steel with a hardness of Rockwell C 37 to 40, the surface finish on the sheared edges was 32 micro-in. or better, but punch life was only 6000 pieces. The cutting speed for fine-edge blanking is 7.6 mm. to 15.2 mm. per sec.

9.3    WORK METALS

  Low-carbon and medium-carbon steels (1008 to 1035), annealed or half-hard, give good blanked edges and normal tool wear. High-carbon steels in the spheroidize-annealed condition can be blanked easily; blanking of steel with 0.35% carbon or higher is recommended only when it is spheroidize-annealed. Steels quenched and tempered to about Rockwell C 30 are well suited to fine-edge blanking, because they do not require subsequent heat treatment, which could result in deformation.

High-carbon steels and alloy steels such as 4130, 4140, 8620 and 8630 cause considerably higher tool wears than low-carbon plain carbon steels, but surface finish is smoother. Leaded steels are not suitable for fine-edge blanking because of their low deformability.

Parts made of stainless steels of types 301, 302, 303, 304, 316, 416 and 430 in the form of bright rolled fully annealed strip, have good blanked edges, but cause higher tool wear than steels of low and medium carbon content.

Good results have been experienced with aluminum alloys 1100 (all tempers), 5052-O to 5052-H38, 6061-O to 6061-T6 and others having similar yield strength and elongation. Blanked edges on parts made of aluminum alloy 2024 generally are rougher than edges on other aluminum alloys. Brasses containing more than 64% copper are especially suitable. Nickel alloys, nickel silver, beryllium copper and god and silver also are easily fine-edge blanked.    

9.4    BLANK DESIGN  

Limitations on blank size depend on stock thickness, tensile strength and hardness of the work metal, and available press capacity. For example, perimeters of approximately 63.5 cm. can be blanked in 3.2 mm. thick low-carbon steel (1008 or 1010). It is possible to blank smaller parts from low-carbon or medium-carbon steel about 12.7 mm. thick.

Sharp corner and fillet radii should be avoided when possible. A radius of 10 to 20% of stock thickness is preferred, particularly on parts over 3.2 mm. thick or those made of alloy steel. External angles should be at least 90°. The radius should be increased on sharper corners or on hard materials.

Parts with tiny holes or narrow slots to be pierced or with narrow teeth or projections to be blanked may be unsuited to fine-edge blanking. The ratio of hole diameter, slot width, or projection width to metal thickness should be at least 0.7 for reasonably efficient blanking, although a ratio as small as 0.5 has been successful with some parts. The spacing, between holes or between a hole and the edge of the blank should not be less than 0.5 to 0.7 times metal thickness. In order to maintain the quality of hole-wall and blank-edge surfaces, and to avoid distortion.

These limitations have been exceeded. For instance, a 16-mm. dia. hole was pierced in each end of a 1018 steel link 25 mm. wide and 8 mm. thick. Since the part had a 12 mm. radius on each end the wall thickness was 5mm. The part was offset 2.5 mm. in the same die. In a part made of 4 mm. thick aluminum alloy 5052-H34, 3.2 mm. dia. holes were pierced leaving a wall thickness of 0.1 mm. A 15.8 mm. dia. hole was pierced in the same part.

The sheared faces of holes pierced during fine-edge blanking are usually break, provided the maximum hole dimensions are not more than a few times the stock thickness. As in conventional piercing, there is a slight radius around the punch side of the hole, but there are no torn edges on the die side of the blank. A rough sheared surface on the blank may be caused by too great a punch-to-die clearance or improper location and height of the impingement ring for the material being blanked. On parts blanked to a small width-to-thickness ratio, a small rough surface may be noticeable, but may not be detrimental (see Example 1).

9.5    PRESSES

A triple-action hydraulic press or a combination hydraulic and mechanical press is used for fine-edge blanking. The action is similar to that of a double-action press working against a die cushion. An outer slide holds the stock firmly against the die ring and forces a V-shape impingement ring into the metal surrounding the outline of the part.  The stock is stripped from the punch during the upstroke of the inner and outer slides.  An inner slide carries the blanking punch. A lower slide furnishes the counteraction to hold the blank flat and securely against the punch. This slide also ejects the blank.

The stripping and ejection actions are delayed until after the die has opened at least to twice the stock thickness, to prevent the blank from being forced into the strip, or slugs from being forced into the blank. Because loads are high and clearance between punch and die is extremely small, the clearance between the gibs and press slides must be so close that they are separated by only an oil film.

Force requirements for fine-edge blanking presses are influenced not only by the work metal and the part dimensions, but also by the special design of the dies and pressure pads used for fine-edge blanking. Depending on part size and shape, a 100-ton press can blank stock up to 8 mm. thick; a 250-ton press, up 12 mm. thick; and a 400-ton press, up to 13mm. thick.

The total load on the press in fine-edge blanking is the sum of three components: the cutting force (Lc); the lower blank holder force (LLB) or counterforce and the clamping force on the impingement ring (LLR) on the pressure pad. The first two components comprise the total force on the inner slide, and the third component is the force on the outer slide.

The cutting force, in Kgs., is calculated from the equation: Lc = 0.8 S lB t

Where 0.8 is an experimentally determined constant; S is the tensile strength of the work metal, kg/cm.²; lB is the total length of cut (sum of perimeters of blank and holes pierced in blank), cm. and t is the thickness of the work metal, cm.

The counterforce, or lower blank-holder force, in pounds, is calculated from the equation: LLB  =  Pc A

Where Pc is the counter pressure on the lower side of the blank, psi; and A is the area of the blank, sq. in. The counter pressure usually is about 10% of the tensile strength of the work metal. The clamping force on the impingement ring on the pressure pad, in pounds, can be obtained from: L IR = Lr l IR

 

Where LI is the force to embed a 1-in. Length of the impingement ring into the work metal, lb; and l IR is the total length of the impingement ring, in. The force L I for different work metals, as determined by experience in fine-edge blanking, is given in Fig. 9-2. When impingement rings are used on both the pressure pad and the die, the calculation of force is still based only on the pressure-pad impingement ring.

The reduced height of impingement rings when used in pairs allows the use of a lower clamping force, and thereby reduces the over-all load on the press.  This is because the lower impingement ring is impressed into the work piece by the reaction force.

If coining, embossing or other forming is done during the blanking, the additional force required for those operations must be added to the force requirements as calculated above.

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