Production potential of a die will vary according to the kind of material which the die components are made of. Generally speaking, the potential production estimates given are conservative, actual production may often be considerably greater.
Punch Life and Die Block Life
When a die is running and producing parts, cutting edges wear and become dull. They must be renewed by sharpening from time to time. Most commonly, cutting components are sharpened by grinding away the cutting face.
Therefore, the die block and cutting height (H and H1) are reduced corresponding amounts each time the components are sharpened. Vertical dimensions are reduced accordingly. After repeated sharpening, the die shut height dimensions and punch and die block dimension will have changed so much that a toolmaker may rework the radius on the base of the punch and die block to further extend die life.
Relative Life: Punches and Die Blocks
Under identical service conditions, blanking punches often show less wear than piercing punches. This is because blanking punches, as a rule, are freer-stripping than piercing punches. Owing to the difference in the proportional area of stock material which must be stripped.
Sidewall Finish
The quality of the sidewall finish has a direct influence on the degree of friction. A smooth finish is essential to satisfactory punch and die function. Even the lay of the finish can be an important factor in punch and die efficiency.
Excessive Wear
Abnormal wear may be caused by any of the following conditions:
- Cutting clearance: insufficient or excessive
- Entry: punches enter too far into the die opening
- Crowding: punches are too close to other punches
- Punch height: vertical punch length is too great in relation to the cross-sectional area of the punch
- Hardness: is component hard enough?
- Finish: are sidewalls of punch or die opening smooth enough? Is the lay of the finish parallel to direction of punch travel?
- Material compatibility: is component made from a material suitable for working with the required type of stock material?
- Mounting: components must be securely mounted, mounting surface must be clean as well as flat and perpendicular to the punching axis, punches must be properly aligned with their mating die opening
- Stripping: is stripping action even?
- Excessive runs: do not attempt to produce too many pieces between sharpening
- Press condition: dies cannot be expected to produce unless presses are in good condition
- Careless setup: check particularly for security and parallelism of bolster and the die must be adequately secured in the press
Characteristic Cutting Wear
Optimal Cutting clearance normally produces a condition where H=¼T to ½T. This is generally the best compromise considering such factors as:
- Quality of piece parts, including the degree of burr
- Slug pulling
- Punch and die life
Newly sharpened cutting edges are dead sharp. The dead sharp condition begins to deteriorate as soon as the die starts to run and after a short time the initial edge breakdown produces a working radius on the cut edges. This is the optimum running condition. Size of working radii is a variable influenced by:
- Type of material from which the component is made
- The hardness of component
- Physical properties of stock material
- The thickness of stock Material T
Working edge radius is normally 0.0002-0.005 thousandths of an inch. As the initial working radius develops the cutting edge attains maximum resistance to wear. It is during this stage that the practical production potential of components is realized.
Eventually, the corner profile begins to assume a parabolic shape. When this stage is reached, component wear accelerates rapidly.
The normally smooth surface of the cut band may assume a rough, striated appearance. Burr effects become more evident, often showing an irregular ridge around the periphery of the opening.
Components should be sharpened before the parabolic wear stage becomes pronounced, a condition which can generally be anticipated by watching the burr on the work. If the components are sharpened before the burr becomes excessive, one can sharpen the cutting edge and can avoid excessive grinding.
If sharpening is put off until burr conditions are severe, an inordinate amount of grinding will be required to renew the cutting edges.
Chipping
Persistent chipping along the cutting edges will seriously detract from the potential life of the components.
Chipping may occur for a number of different reasons:
- Poor housekeeping: foreign material enters the die usually with the stock strip or work-piece. This is mentioned mostly because this occurs too often.
- Grinding cracks: shallow, invisible or nearly invisible cracks are induced by localized grinding heat
- Insufficient cutting clearance
- Hardness (brittleness) of the components
Chipping can often be prevented by eliminating the dead sharp corner condition. This can be accomplished by stoning or lapping the cutting edges to produce a working radius.
Compensating for Differential Wear
Dies are designed to allow shorting of the forming punch simply by grinding its flat base surface as indicated. The forming punch is bolted to the paring punch and is provided with elongated clearance holes for the mounting screws. The elongated holes permit the necessary vertical adjustments between the two punches.
When the cutting height is lowered by sharpening, it is a simple matter to surface grind the same amount from the thickness of the compensator.
Provision for compensation grinding is primarily a maintenance consideration.
Conclusion
Die life is a great topic because it is the culmination of all the efforts of the toolmaker and tooling designer, which is to produce efficient profitable parts, and that is determined by how the die is designed to endure its life and how well it is taken care of. I know at my personal work, many of our dies have a half-life of 200,000 cycles and we will often, with the right maintenance and careful focus on quality, get our tooling to do 300,000 cycles, which can be a huge savings or profit for the die owner. This article is a summary of the text, which I think will be pertinent in preparation for my C of Q exam. If you want to know more detail I do recommend picking up the text. If you like what you reading and want more of it, please subscribe.