Chapter 5

Die Design

Die DesignEffect of center-of-pressure location on tool designCenter of Pressure

Die DesignDie ClearanceDie clearance is the space between the matting members of a die set. Proper clearance between cutting edges enables the fractures to meet and the fractured portion of the sheared edge has a clean appearance. For optimum finish of a cut edge, proper clearance is necessary and is a function of the kind, thickness and temper of the work material.

Die DesignControl of hole and blank sizes by clearance location.Die Clearance Calculation

Die DesignApplication of the die clearance for holes of irregular shape .Die Clearance Calculation

Die DesignDie Block General Design Overall dimensions of the die block will be determined by the minimum die wall thickness required for strength and by the space needed for mounting screws and dowels and for mounting the stripper plate.Wall thickness requirements for strength will depend on the thickness of the stock to be cut. Sharp corners in the contour may lead to cracking in heat treatment, and so require greater wall thickness at such points.

Die DesignDie Block Calculation Rule of Thumb - Assuming a die block of tool steel its thickness should be 20-mm. minimums for a blanking perimeter of 75-mm. or less 25-mm. thick for perimeters between 75-mm. and 250-mm. and 32-mm. thick for larger perimeters. There should be a minimum of 32-mm. margins around the opening in the die block.The die opening should be straight for a maximum of 3-mm; the opening should then angle out at to 1 to the side (draft). The straight sides provide for sharpening of the die; the tapered portion enables the blanks to drop through without jamming.

Die DesignDie Block Calculation To secure the die to the die plate or die shoe, the following rules provide sound construction: On die blocks up to 175-mm square, use two M10 cap screws and two dowels of dia. 10-mm.On sections up to 200-mm. square, use three cap screws and two dowels.For blanking heavy stock, use cap screws and dowels of dia 12-mm. diameter. Counter bore the cap screws 3.2-mm. deeper than usual, to compensate for die sharpening.

Die DesignDie Block Calculation Die thickness is provisionally selected from Table shown. This table takes into account the thickness of the stock and its ultimate shear strength.

DIE THICKNESS PER TON OF PRESSURE* For each ton per sq. cm. of ultimate shear strength.

Die DesignMINIMUM CRITICAL AREA VS. IMPACT PRESSURE Critical distance A must not less than 1.5 to 2 times die thickness.The critical area between the die hole and the die border must be checked against minimum values in Table and die thickness B corrected if necessary.

Impact pressure,

tons Area between die opening border, sq. mm 205075100 3226459681290

Die DesignMETHODS OF PUNCH SUPPORT When cutting punch A is sharpened, the same amount is ground of spacer B to maintain the relative distance. If delicate punches must be grouped closed in hardened guide block with number of holes sliding fitting in stripper plate.

Die DesignMETHODS OF PUNCH SUPPORT A slender piercing punch at left shown in figure may be smaller height than an adjacent large punch. If punches must protrude more than 100 mm long fitted in punch holder. An auxiliary plate may be used to maintain stiffness.

Die DesignMETHODS OF PUNCH SUPPORT Flange width of the punch should be greater than height to provide stability for unguided punches. In a large punch, push-off pins can prevent slugs from pulling up and causing trouble.

Die DesignMETHODS OF PUNCH SUPPORT To avoid cracking a large hardened punch or a punch plate, do not press a small punch directly into either of these members. Instead, use a soft plug or insert. Long slotting punches should be hollow ground so that dimension A equals the metal thickness, so as to put shear on the punch. The ends should be flat for 1/8 in. to avoid bending the stock.

Die DesignMETHODS OF PUNCH SUPPORT A quill is useful for supporting pin punches. A bushing in the stripper plate can guide the quill for increased punch support.

Die DesignMETHODS OF PUNCH SUPPORT Quills need not be limited to a single punch. If prevented from turning, they can be used for pin punches on close centers. Two quills are used for a bit punch one to support the punch, the other to support the inner quill, when a stripper is not used.

Die DesignMETHODS OF PUNCH SUPPORT A dowel can be used to prevent rotation of the punches. For high-speed dies, a flat on the punch head is more positive.

Die DesignMETHODS OF PUNCH SUPPORT In low-production dies, a setscrew is adequate to hold the punch. In low-production dies, a setscrew is adequate to hold the punch.

Die DesignMETHODS OF PUNCH SUPPORT Light drill-rod punches are guided in the stripper plate to prevent buckling. Several punches can be set at close center distances.

Die DesignSTOCK STOPS In its simplest form, a stock stop may be a pin or small block, against which an edge of the previously blanked opening is pushed after each stroke of the press. With sufficient clearance in the stock channel, the stock is momentarily lifted by its clinging to the punch, and is thus released from the stop. Figure in the next slide shows an adjustable type of solid block stop which can be moved along a support bar in increments up to 25 mm to allow various stock lengths to be cut off Finger Stops

Die DesignSTOCK STOPSStarting StopPin Stop

Die DesignSTOCK STOPS Trigger stops: (A) Top stock engagement; (B) Bottom stock engagement

Die DesignSTOCK STOPSAutomatic Stops A illustrates a pin stop suitable for low-to medium-production dies. When the ram ascends, the strip clings to the punch, is stripped, and then is fed until the pin hits the edge of the hole. shows the method of locating the pin stop so that it bears against the blank opening upon an angular edge, so that the strip is crowded against the backstop and accurate piloting is obtained .

Die DesignSTOCK STOPS Automatic Stops If no scrap is left between blanks, as in a double-action blank and-draw die, a bent pin stop is suitable. The sharpened point of the stop faces the incoming strip, when contact is made at the opposite side of the hole. A design in which combinations stop and backup block locates the strip and prevents deflection of the cutoff punch of this two-station die. The part, a flat spring, drops to the punch holder and slides by gravity to the rear of an inclined press.

Die DesignSTOCK STOPS Automatic Stops The part, after cutoff, can drop through a hole in the die set to a box directly beneath. Overhanging stops are useful when the press cannot be inclined, or when the size of the die or press will not allow part removal through the bolster plate.

Die DesignSTOCK STOPSCropping: An unusual stop that requires no moving mechanisms is known as the French stop or Cropping. It operates on the principle of cutting a shoulder in the edge of the stock strip, which acts as stop. A strip wider than necessary is inserted into the strip channel until it contacts the shoulder built into back gage. The first hit of the press performs the first station operation and at the same time punches from the side of the strip a section of metal equal to the length of the pitch. This operation leaves a shoulder in the side of the strip. The strip is then advanced until the strip shoulder contacts the shoulder on the back gage on the return stroke of the ram.Automatic Stops

Die DesignSTOCK STOPS Automatic Stops The advantage of the cropping is its accuracy and speed of operation. It is especially well suited to the light-gage materials that are easily distorted when pushed or pulled against a stop pin. Its main disadvantages are the extra cost of tooling and extra stock scrap.

Types Pilots Press-fit Pilots Press-fit Pilots Transfer dies Spring-loaded pilots Misfeed detector Die Design

Die DesignPress fit pilot: (A) Acorn type; (B) Flattened-point type

Die DesignMethods of retaining pilots: (A) Threaded shank; (B) Screw retained.

Die DesignMethods of retaining pilots: (C) Press fit; (D) Socket grub screw.

Die DesignMethods of retaining pilots: Spring with Socket grub screw. Spring loaded pilot Misfeed detector

Die DesignStrippers are of two types.Fixed strippersSpring-operated strippersThe primary function of either type is to strip the work piece from a cutting or non-cutting punch or die. A stripper that forces a part out of a die may also be called a knockout, an inside stripper, or an ejector. Besides its primary function, a stripper may also hold down or clamp, position, or guide the sheet, strip, or work piece.

Die DesignFixed stripper plate The stripper is usually of the same width and length as the die block. In the simpler dies, the stripper may be fastened with the same screws and dowels that fasten the die block, and the screw heads will be counter bored into the stripper.

Die DesignSpring-operated strippers Where spring-operated strippers are used, the force required for stripping is 35000 times cut perimeter times the stock thickness. It may be as high as 20 per cent of the blanking force, which will determine the number and type of springs required. The highest of these values should be used.

Die DesignStripping Pressure FormulaUsing of Springs for Stripping Pressure

L = KAWhere

L = Stripping force

K = Stripping constant, Kg / cm.2

A = Area of cut surface, cm.2 (stock thickness x length of cut) Approx. values for K, as determined by experiment: 105 for sheet metal thinner than 1.6 mm. when cut is near an edge or near a preceding cut; 150 for other cuts in sheet metal thinner than 1.6 mm; and 210 for sheet metal more than 1.6 mm. thick.

Die DesignUsing of Springs for Stripping Pressure Estimate the stripping force required according to formula.Determine the amount of space available for spring mounting.Select the max. allowable number of springs, which will fit into the available space and total required force, will determine which grade of spring is required.Determine the deflection. The required travel plus preload deflection will be the total deflection and will determine the length of spring required to stop within the allowable percentage of deflection limits. Allowance should be made for punch sharpening, which will increase deflections over a period of time.Select a spring from the lowest (greatest deflection) load rating series from the table. It is important that the spring not be compressed beyond the specific limit for the highest-pressure spring of corresponding length and diameter. Then if the springs prove too light for the stripping force required, each one is replaced with the next higher rated springs. This will provide more stripping pressure without the need changing spring pockets, support rods and the like. Using of Springs for Stripping Pressure

Die DesignUsing of Springs for Stripping Pressure

Die DesignKNOCKOUTS A plain inverted compound die, is of the simplest type. It consists of an actuating plunger, knockout plate and a stop collar doweled to the plunger. Shedder D consists of a shouldered pin backed by a spring, which is confined by setscrew

Die DesignKNOCKOUTS In the ejection of parts positive knockouts offer the following advantages over spring strippers where the part shape and the die selections allow their use:Automatic part disposal; the blank, ejected near the top of the ram stroke, can be blown to the back of the press, or the press may be inclined and the same result obtained.Lower die cost; knockouts are generally of lower cost than spring strippers.Positive action; knockouts do not stick as spring strippers occasionally do.Lower pressure requirements, since there are no heavy springs to be compressed during the ram descent.