Blanking and Piercing Dies by the Numbers

About this book

A blanking and piercing die is a pair of matched cutting edges that a press drives through sheet metal, hundreds of times a minute if need be, to cut out the same flat part again and again. The part is only as good as the die: its size comes from the die’s openings and punches, its edges from the clearance between them, its flatness and burr from the die’s condition, and its cost from the material the strip layout wastes and the hours the die runs between sharpenings. Every one of those can be worked out before steel is cut, and checked afterwards.

This book works through a die with numbers. It starts with what happens when a punch cuts, then estimates the cutting force, chooses the clearance and sizes the punches and die openings, checks small punches for crushing and buckling, sizes the stripper springs, lays out the strip, chooses between die types and finds the centre of load, sets the die in the press, and finishes with wear, sharpening and the cost of the tool against laser cutting. Each chapter follows one running case, includes a plausible mistake and how it was found, and ends with an exercise and its worked answer.

It is for toolmakers, press setters and apprentices, for production engineers who buy and run dies, and for designers who want to draw parts that dies make well. It assumes school algebra and some familiarity with sheet metal. The press itself, its force curve, flywheel energy, frame, feeding and guarding, is the subject of the Library’s companion book, Power Presses by the Numbers.

The running case

Every chapter follows one fictional part. This is an illustration. A manufacturer of electrical enclosures needs a mounting plate, called MP-40 here, for its DIN-rail brackets: a flat plate 80 × 50 mm with R6 corners, two Ø8.5 mm holes on 60 mm centres and a 16 × 8 mm obround slot between them, cut from 2.0 mm low-carbon cold-rolled steel. Demand is 60,000 plates a year, made in batches of 5,000. For the calculations the sheet’s tensile strength is taken as 350 MPa and its shear strength as 280 MPa, illustrative values that a real job would take from the material certificate and trials.

The part is made in a three-station progressive die: the holes and slot are pierced at the first station, pilots locate the strip in those holes at the second, and the outline is blanked at the third. The manufacturer, the part, the prices and the mistakes are invented. None describes KEVOS® work or a real business.

How to use the book

Read Chapters 1 and 2 first: they set out the cutting action and the force, which every later chapter uses. Chapters 3 to 5 size the cutting parts, Chapters 6 and 7 plan the strip and the die, and Chapters 8 and 9 deal with running and keeping the die. Values are given to more figures than a shop needs so that the arithmetic can be followed; every calculation was checked by running it.

Source limits and professional responsibility

The relationships in this book are standard estimates, and the material properties, clearances, spring rates, tool lives and prices are illustrations. Real dies are designed with the material supplier’s data, the die component maker’s ratings, trial strikes and the toolmaker’s experience. Presses and dies can cause amputation and crush injuries: setting, trial strokes and maintenance are done by trained people, with guarding, isolation and the controls that Australian work health and safety law and the regulators’ guidance require. This book teaches how dies work and how to check them. It is not a design for any tool, and qualified review governs real work.