The various elements of a die are die block, die holder, punch, punch holder, stripper, stock stops and knockouts. We will here consider the design of all these individual elements and from is design for compound, combination and progressive die could also be carried out.
i. Die Block Design:
Overall dimensions of the die block are 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. For its design, generally ‘rule of thumb’ method given below gives quite satisfactory results.
(1) For die block of tool steel, a thickness of 20 mm minimum is used for a blanking perimeter of 75 mm or less, and 25 mm thickness is used for perimeters 75 to 250 mm and 30 mm for larger perimeters.
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(2) There should be a minimum of 30 mm margin around the opening in the die block.
(3) The die opening should be straight for a maximum of 12.5 mm and then it should angle out at 0.25° to 1.5°
(4) In order to secure the die to the die plate or die shoe, the following rules provide sound construction:
(i) On die blocks upto 175 mm square, use two 10 mm alien screws and two 10 mm dowel pins.
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(ii) On sections up to 250 mm square use 3 alien screws and two dowel pins.
(iii) For heavier stock and still bigger blocks, use screws and pins of 12.5 mm dia. Counter bore the die block to accommodate alien heads at least 3 mm deeper than length of heads to compensate for die sharpening.
ii. Punch Design:
The punch length should always be
where d = diameter of punched hole,
fc = Unit compressive stress on punch
E = modulus of elasticity
t = thickness of sheet.
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Methods of Punch Support:
There are a number of methods to mount punches to meet various production requirements.
In the case of blanking punches, the punches are comparatively bigger in size and are, therefore, generally provided with flanges which are mounted into the punch holder directly with the help of dowel pins and alien screws without the use of backup plate.
The other methods for supporting the punch are:
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(i) Headless punches are held in the hole of the ram or punch plate with the help of a set screw which bears on flat portion machined on the bed.
(ii) Peen-head punches are made from drill rod of the size of punch and are left shoulder-less until assembly, when the punch is pressed tightly into a counter-sunk reamed hole in the punch plate and then riveted over.
(iii) Quilled punches are used for piercing very small holes in heavy stock. The punch having a uniform point and shank diameter is enclosed in a quill which is of hardened tool steel with an internal diameter concentric to its outside diameter.
(iv) In the case of large punches, they are provided with heads or shoulders which fit into the recess in the punch plate and are dowelled into the position.
(v) Very thin rectangular punches are mounted with the help of low melting point alloys also.
iii. Stock Stops:
In the simplest form, it 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.
Generally the starting stop is mounted between the die shoe and die block to initial position of the incoming stock. During downward stoke the stop is pressed down. After every die operation is completed, the compression springs return the stop in its position.
In the case of automatic stops, they register the strip at the final die station automatically and the operation has only to keep the strip pushed against the stop in its travel through the die.
iv. Pilots:
When establishing the sequence of operations for progressive dies, piercing operations are done first. Advantage is taken of these holes for piloting so that the blanks formed are concentric to the pierced hole. The piloting is achieved with the help of pilots secured under the blanking punches. These are generally made of suitable tool-steel, heat-treated properly for maximum toughness and hardness. The various ways of fitting pilots are by press fitting, threading or by socket set screws.
v. Stripper:
Its function is to strip the work piece from a cutting or non-cutting punch or die. It also helps in holding down the strip. It is usually of the same width and length as the die block. In the simple die employing fixed type stripper, it is fastened to the die block with the same screws and dowels that fasten the die block and screw heads are counter-bored in the stripper. The stripper thickness should be sufficient to withstand the force-required to strip the stock from the punch.
Other than fixed type of strippers are spring operated or pressure pad strippers. In a simple blanking die the pressure pad contacts the strip ahead of the punch and holds the material flat just before the blank is cut.
The spring operated strippers have the advantages that the spring pad pressure prior to piercing operation removes all the waved irregularities in thin sheets and also the work performed is in full view of the operator and not covered by the stripper plate except at the time of blanking.
vi. Knock-Out:
It is the means of ejecting on the upstroke the cut blank which might, otherwise, be retained, in the die block by friction. A knock-out assembly consists of a plate, a push rod and a retaining clear. This arrangement of ejecting the parts gives positive action, is quite cheap and pressure requirements are low.
Generally the plate is a loose fit with the die opening contour and is capable of moving as the blank is cut. A heavy push rod which slides in a hole in the shank of the die set is attached to the plate by rivets. This rod projects above the shank, and a collar retains and limits the stroke of the knockout assembly. Near the upper limit of the ram stroke, a knockout bar in the press contacts the push rod and ejects the blank-out.