Cutting shapes and piercing metal, at the most basic level, is really all the toolmaker is trying to achieve. Like everything, when we learn a new skill or topic, it will have a set of principles and terminology that one should become familiar with. It’s like learning a new language. Similar to the language of medical jargon that a doctor learns or the infamous legalese that a lawyer has to use, you will learn the language of the tool making trade. I hope to give you a basic and meaningful summary of the principles involved in blanking and piercing dies.
Introductory Terminology
Piece Part – A piece part is the product a die produces, it may also be a component of a product.
Stock Material – General term used for the material that will be used to produce the piece part.
Die – Die has many definitions with reference to tooling. A die normally refers to a complete production tool that can make piece parts accurately and consistently, it may also refer to a female part of a complete die.
Punch – A punch can be described as a male component of a complete die that acts in conjunction with the female half of the die to produce a piece part.
A Piece Part Produced by Piercing and Blanking:
When a die is used to make a hole or an opening in the piece part, the operation is called piercing. The unwanted material removed by the piercing operation is called the slug.
The effect of the tool on the stock material is the same whether it is punching out slugs, in order to produce the desired opening in a piece part, or punching out blanks which are the desired product of the die.
When a piece part is produced by blanking, that means the piece part has been made out of a stock strip of material. The distance the stock strip moves through the die is called the advancement. The scrap material left on the stock strip, in between the piece parts, is called the scrap bridge. See figure 1-3, page 2.
Press Cycle When Blanking or Piercing:
When a press is tripped or activated, a clutch will allow engagement with a flywheel. The crankshaft rotates in one full turn. During the first half of the cycle, the ram moves toward the bed of the press, then, during the last half of the cycle, the ram moves back up. The distance travelled by the ram during the first half of the cycle is called the PRESS STROKE.
Actions of Blanking or Piercing Die:
Stripping
Once the punching or piercing operations have taken place and the press moves up, the piece part or slug may remain in the die. The stock material will want to cling to the punch and stick unless another step, known as stripping, is added in the part production. The function of the stripper is to keep the material from travelling with the punch on the return stroke.
The Reaction of the Stock Material:
In tool making, there are 3 important stages:
- The plastic deformation – The press is tripped and the punch is driven toward the die. The punch exerts a force on the stock material. Plastic deformation takes place once the elastic limits are exceeded.
- Penetration – This is the true shearing stage of the cutting cycle. The punch is forced to penetrate the stock material and the blank or slug is pushed out.
- Fracture – The punching force causes fracturing of the piece part/slug. Then the fractures extend and meet to complete the piece part/slug. This is when the greatest stress or pressure is applied to the cutting edges.
Typical Appearance Characteristics
When the piece part is cut under ideal condition, the radius edge is the result of plastic deformation. A highly burnished band as the result of the shearing action is the cut band. The width of the cutting band is approx ⅓ thickness of the stock material. The remainder of the cut is the break, which is the result of the 3rd phase of the cycle.
Cutting clearance is the space between the cutting edge of the punch and the corresponding cutting edge of the die.
Burr Side
The burr side of the piece part is adjacent to the break. The burr should be almost non-existent. The burr side of a blank or slug is always toward the punch and the burr side of a punch opening is always toward the die opening.
Excessive Cutting Clearance
When there is excessive clearance the punch reacts on the material more like a forming tool than a cutting tool.
The edge radius then becomes larger than the piece part specified tolerance and does not blend smoothly into the cut band. The cut band becomes smaller or becomes a thin line. The break begins to show irregularities and may extend into the edge radius. This forces the material into the clearance space of the die causing large fractures and burrs on the part.
Insufficient Cutting Clearance
Objectionable burrs may appear on the piece part if the cutting clearance is insufficient. When there is insufficient clearance, the burr is caused by compressive forces.
Misalignment of Punch and Die
The piece part will show burring to one side and this information is useful for the toolmaker for detecting the misalignment.
Importance of Cutting Clearance
Proper cutting clearance is necessary for the life of the die and the quality part. Excessive cutting clearance results in objectionable piece-part characteristics. Insufficient cutting clearance causes undue stress and wears on the cutting member of the tool because of the greater punching effort required.
There are exceptions to the rule. For example, round cutting punches can operate successfully with less than normal cutting clearance and reduced cutting clearance can also be used to overcome slug pulling.
Determining Cutting Clearance
Optimal clearance must often be determined by experience and actual experimentation. It is always relatively simple to increase the amount of cutting clearance. When in doubt, a toolmaker starts out with less cutting clearance than he estimates to be necessary. Cutting clearance is always expressed as thickness per side.
Relationship of Piece-part and Die Size
The actual cutting of the blank or slug is done by the cutting edge of the die opening. Therefore, the die opening determines the size of the blank or slug. The size of the punch opening is determined by the punch.
Angular Clearance
Angular clearance is a draft or taper applied to the sidewalls of a die opening, in order to, relieve the internal pressure of the blank or slug as it passes through the opening.
Importance of Angular Clearance
The blank-through type of die will not run successfully, unless, there is a relief for the pressure developed within the die opening by the blank or the slugs as they are forced through.
Specifying Angular Clearance
Angular clearance should be expressed in terms of the amount of clearance per side, not as an overall or included angle figure.
Conclusion
This article is a summary of what I feel is the useful information in preparation for my C of Q test. The textbook has a tremendous amount of detailed information that I would encourage you to read further if you are interested. Also, if you found this article helpful please share and subscribe to our email list.