PRESS2010_EN

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［%］ーーーCompression strength・compression resistance（MPa）Compressionstrength50004000CompressionresistanceDEX20DEX40SKH51SKD1130002000606570硬さ（HRC）Drawing 2　The relationship between hardness and compression resistance Source：Daido Steel（Co., Ltd.）（※DEX40：JIS Symbol SKH40）ExpansionExpansion atthe cut（kgf/mm2）500400300200Working pieceWorking pieceDuctillitySuper-highstrength steelCold-rolledtool steelGO5GO4YK30Drawing 4 Guide for the selection of punch materialSource：Daido Steel（Co., Ltd.）　（※MH51：JIS symbol SKH51）Drawing 5 Expansion and wear on the tip of punchPowder high-speedDRM1tool steelDEX-M1DRM2High strength・high ductillity materialDEX20DRM3SKH40DCXDC53MH51SKD11MH69MH55DEX60GOAMH8DEX80High-speedtool steelStrength・wearabilitySKD11（alloy tool steel）,SKH51（high-speed tool steel）and SKH40（powder high-speed tool steel） are widely used as punch material for press die. And they are selected by customers according to different punching conditions such as the quantity of production and material of working piece. However, shortened lifespan of punch is always caused by low machinability of working piece or high-precision processing. This may trouble customers during the selection of punch material. In order to alleviate this problem, examples of material alteration standard are listed in this catalog.※The examples listed are general cases, and the practical situation may differ from the precision of press die or punching machine.1118Picture 1　BreakageJISTensile strengthσc［kgf/mm2］JISSymbol［kgf/mm2］Tensile elongationSymbolSPCCSPCDSPCESS400S20CS50CSUS301SUS304SUS430C2600P-HC5191P-HC5210P-HTable 1　Mechanical characters of typical materials.33323246405085705348656522201827ーー28263440615544454623ーー645628301630Picture 3　WearHardness［HRB・］HRB49HRB45HRB44Hv172Hv175Hv165Hv156Hv205Hv205■Case 1High punching force causes breakage of punch（Picture 1）The load exerted on punch depends on the hardness or thickness of working piece, and the tip will break if unit load on cross-section （compressive strength） is large. So, it is important to calculate the max. compressive strength during design. The formula is as follows.Compression forceσc［kgf/mm2］＝Workpiece Thicknesst［mm］×Punching profile lengthℓ［mm］　　　　　　　　　　　 ×Shearing resistanceτ［kgf/mm2］÷Section area A［mm2］Shearing resistanceτ［kgf/mm2］≒Tensile strengthσc［kgf/mm2］×0.8The following formula is for the max. compressive strength when a SKD11 punch is applied to process a round hole of φ2.5 on a SUS304 stainless steel plate of 2.0mm thickness.（For tensile strength, refer to Table 1）Compression force＝2.0×（2.5×π）×（70×0.8）÷｛0.25×π×2.52｝＝179.2［kgf/mm2］The compression resistance of different punch materials are listed in Diagram 2 （a 0.2% compression resistance is caused by residual deformation）, and it is directly proportional to material hardness when normal heat treatment is applied. Actually, when the hardness of SKD11 is 60HRC, its compression resistance is 250【kgf/mm²】, and its plastic deformation is 180【kgf/mm²】 （about 70% of the compression resistance）. In order to conquer plastic deformation, material SKH40 is applied to improve the compression resistance.On the other hand, it is important to prevent：breakage of punch shoulder, wear between punch and mounting hole, as well as sunken on the mounting surface of backing plate. Although the backing plate can be hardened through quenching, it must be thick enough to diffuse the punching force. Otherwise, sunken will be found on the mounting surface, and periodical fixing will be required. ■Case 2High punching force expedites the wear of punch（Picture 3） High punching force will expedite the wear of punch when the wearability of punch material is low. And this can be resolved by adopting material of high wearability according to SKD11→SKH51→SKH40.（Diagram 4：Guide for punch material selection）On the other hand, high punching force will also cause plastic deformation （for SKD11 of 60 HRC, it happens at about 180【kgf/mm²】） of punch material, and expand the tip. This will not only expedite the wear of punch, but also enlarge the through hole on working piece （Picture 5）. Especially for punches with coating, tip expansion will disject and peel off the coating. This can be resolved by adopting material of high compression resistance（Diagram 2）.How to Select Punch Material