MANUFACTURING PROCESS - DIE AND PUNCH

DIE AND PUNCH

A TYPICAL DIE AND PUNCH SET USED FOR BLANKING OPERATION. THE SHEET METAL USED IS CALLED STRIP OR STOCK. THE PUNCH WHICH IS HELD IN THE PUNCH HOLDER IS BOLTED TO THE PRESS RAM WHILE DIE IS BOLTED ON THE PRESS TABLE. DURING THE WORKING STROKE, THE PUNCH PENETRATES THE STRIP, AND ON THE RETURN STROKE OF THE PRESS RAM THE STRIP IS LIFTED WITH THE PUNCH, BUT IT IS REMOVED FROM THE PUNCH BY THE STRIPPER PLATE. THE STOP PIN IS A GAGE AND IT SETS THE ADVANCE OF THE STRIP STOCK WITHIN THE PUNCH AND DIE. THE STRIP STOCK IS BUTTED AGAINST THE BACK STOP ACTING AS A DATUM LOCATION FOR THE CENTRE OF THE BLANK. 

THE DIE OPENING IS GIVEN ANGULAR CLEARANCE TO PERMIT ESCAPE OF GOOD PART (BLANK). THE WASTE SKELTON OF STOCK STRIP, FROM WHICH BLANKS HAVE BEEN CUT, IS RECOVERED AS SALVAGED MATERIAL.

THE CLEARANCE ANGLE PROVIDED ON THE DIE DEPENDS ON THE MATERIAL OF STOCK, AS WELL AS ITS THICKNESS. FOR THICKER AND SOFTER MATERIALS GENERALLY HIGHER ANGULAR CLEARANCE IS GIVEN. IN MOST CASES, 2 DEGREE OF ANGULAR CLEARANCE IS SUFFICIENT. THE HEIGHT OF CUTTING LAND OF ABOUT 3 MM IS GENERALLY SUFFICIENT.

CLEARANCE

IN BLANKING OPERATION , THE DIE SIZE IS TAKEN AS THE BLANK SIZE AND THE PUNCH IS MADE SMALLER GIVING THE NECESSARY CLEARANCE BETWEEN THE DIE AND THE PUNCH.

DIE SIZE        = BLANK SIZE

PUNCH SIZE  = BLANK SIZE – 2 X CLEARANCE

CLEARANCE   = K . T . T

WHERE T IS THE SHEAR STRENGTH OF MATERIAL, T IS THE THICKNESS OF SHEET METAL STOCK, AND K IS A CONSTANT WHOSE VALUE MAY BE TAKEN AS 0.003.

IN A PIERCING OPERATION , THE FOLLOWING EQUATIONS HOLD.

PUNCH SIZE = BLANK SIZE

DIE SIZE      = BLANK SIZE + 2 X CLEARANCE

CLEARANCE = K . T . T

TYPES OF DIES

THE COMPONENTS GENERALLY INCORPORATED IN A PIERCING OR BLANKING DIE ARE SHOWN IN. THIS FIGURE SHOWN THE DIE IN THE CONVENTIONAL CLOSED POSITION. THE DIE SET IS MADE UP OF THE PUNCH HOLDER WHICH IS FASTENED TO THE RAM OF THE PUNCH PRESS AND THE DIE SHOE WHICH IS FASTENED TO THE BOLSTER PLATE OF THE PUNCH PRESS.

GENERALLY, THE PUNCH IS FASTENED TO THE PUNCH HOLDER AND ALIGNED WITH THE OPENING IN THE DIE BLOCK. SHOWS ONE TYPE OF STRIPPER PLATE AND PUSH – OFF PINS. THE STRIPPER HOLDS THE SCRAP STRIP SO THAT THE PUNCH MAY PULL OUT OF THE HOLE. THE PUSH – OFF PINS ARE NEEDED TO FREE THE BLANK IN INSTANCES WHERE THE MATERIAL STRIP CLINGS TO THE BOTTOM OF THE PUNCH. THIS MAY BE NECESSARY FOR THIN MATERIAL, OR WHERE LUBRICANTS ARE USED ON THE MATERIAL.

SOMETIMES THE DIE AND THE PUNCH POSITIONS MAY BE INTERCHANGED. THIS MAY BECOME NECESSARY WHEN THE OPENING IN THE BOLSTER PLATE IS TOO SMALL TO PERMIT THE FINISHED PRODUCT TO PASS THROUGH THE BOLSTER OPENING. SHOWS SUCH A DIE.

INVERTED DIE  IS DESIGNED WITH THE DIE BLOCK FASTENED TO THE PUNCH HOLDER AND THE PUNCH FASTENED TO THE DIE SHOE. DURING THE DOWNWARD STROKE OF RAM, THE BLANK IS SHEARED FROM THE STRIP. THE BLANK AND SHEDDER ARE FORCED BACK INTO THE DIE OPENING, WHICH LOADS A COMPRESSION SPRING IN THE DIE OPENING . AT THE SAME TIME THE PUNCH IS FORCED THROUGH THE SCRAP STRIP AND A SPRING ATTACHED TO THE STRIPPER IS COMPRESSED AND LOADED. ON THE UPSTROKE OF THE RAM, THE SHEDDER PUSHES THE BLANK OUT OF THE DIE OPENING AND THE STRIPPER FORCES THE SCRAP STRIP OFF THE PUNCH. THE FINISHED PART (BLANK) FALLS, OR IS BLOWN, OUT THE REAR OF THE PRESS. 

COMPOUND DIE COMBINES THE PRINCIPLES OF THE CONVENTIONAL AND INVERTED DIES IN ONE STATION. THIS TYPE OF DIE MAY PRODUCE A WORKPIECE WHICH IS PIERCED AND BLANKED AT ONE STATION AND IN ONE OPERATION. THE PIERCING PUNCH IS FASTENED IN THE CONVENTIONAL POSITION TO THE PUNCH HOLDER. ITS MATCHING DIE OPENING FOR PIERCING IS MACHINED INTO THE BLANKING PUNCH. THE BLANKING PUNCH AND BLANKING DIE OPENING ARE MOUNTED IN AN INVERTED POSITION. THE BLANKING PUNCH IS FASTENED TO THE DIE SHOE AND THE BLANKING DIE OPENING IS FASTENED TO THE PUNCH HOLDER.

PROGRESSIVE DIES ARE MADE WITH TWO OR MORE STATIONS ARRANGED IN A SEQUENCE. EACH STATION PERFORMS AN OPERATION ON THE WORKPIECE, OR PROVIDES AN IDLER STATION, SO THAT THE WORKPIECE IS COMPLETED WHEN THE LAST OPERATION HAS BEEN ACCOMPLISHED. THEREAFTER EACH STROKE OF THE RAM PRODUCES A FINISHED PART. THUS AFTER THE FOURTH STROKE OF A FOUR – STATION DIE, EACH SUCCESSIVE STROKE WILL PRODUCE A FINISHED PART. OPERATIONS WHICH MAY BE CARRIED OUT IN A PROGRESSIVE DIE ARE PIERCING, BLANKING, FORMING, DRAWING, CUT – OFF, ETC. THE LIST OF POSSIBLE OPERATIONS IS LONG. THE NUMBER AND TYPES OF OPERATIONS WHICH MAY BE PERFORMED IN A PROGRESSIVE DIE DEPENDS UPON THE INGENUITY OF THE DESIGNER.

 FIG 8.5 SHOWS A FOUR – STATION PROGRESSIVE DIE. THE DIE BLOCK IS MADE UP OF FOUR PIECES AND FASTENED TO THE DIE SHOE. THIS PERMITS EASY REPLACEMENT OF BROKEN OR WORN DIE BLOCKS. THE STOCK IS FED FROM THE RIGHT AND REGISTERS AGAINST A FINGER STROP (NOT SHOWN). THE FIRST STROKE OF THE PRESS  PRODUCES A SQUARE HOLE AND TWO NOTCHES. THESE NOTCHES FORM THE LEFT END OF THE FIRST PIECE.

DURING THE UPSTROKE OF RAM, THE STOCK IS MOVED TO THE NEXT STATION AGAINST A FINGER STOP (NOT SHOWN). THE STOCK IS POSITIONED FOR THE SECOND STROKE. THE SECOND STATION IS AN IDLER. THE RIGHT END OF THE FIRST PIECE, THE LEFT END OF THE SECOND PIECE, AND A SECOND SQUARE HOLE ARE PIERCED. 

THE RAM RETRACTS AND THE SCRAP STRIP IS MOVED TO THE THIRD STATION AGAINST AN AUTOMATIC STOP. THIS STOP PICKS UP THE NOTCHED V AND POSITIONS THE SCRAP STRIP. THE THIRD STROKE OF THE RAM PIERCES THE FOUR HOLES AS SHOWN IN. THE FOURTH STROKE, CUTS OFF AND FORMS THE RADII AT THE ENDS OF THE FINISHED PIECE. THEREAFTER EVERY STROKE PRODUCES A FINISHED PART.

PROGRESSIVE DIES GENERALLY HAVE THE CUT – OFF OR BLANKING OPERATION AS THE LAST OPERATION. IT IS PREFERRED TO HAVE PIERCING OPERATION AS THE FIRST OPERATION SO THAT THE PIERCED HOLE CAN BE ADVANTAGEOUSLY USED AS A PILOT HOLE. ALTERNATIVELY, SPECIAL PILOT HOLES ARE PIERCED IN THE SCRAPPED PART OF THE STOCK. IN CERTAIN SPECIAL CASES, BLANKING IS DONE AT THE FIRST STATION, AND THE BLANK RETURNED TO THE DIE BY USING SPRING PLATES AND THEN MOVED TO THE SUBSEQUENT STATION BY MECHANICAL MEANS OR MANUALLY.

PROGRESSIVE DIES ARE USED WHERE HIGHER PRODUCTION RATES ARE DESIRED AND THE MATERIAL IS NEITHER TOO THICK NOR TOO THIN. THEIR USE HELPS IN CUTTING DOWN THE MATERIAL HANDLING COSTS. 

References :- www.nptel.iitm.ac.in