Basic Die Making

This article is intended to introduce basic tool making knowledge to beginner toolmakers, die maintenance technicians and press technicians. We will first define what is a stamping die and difference type of stamping dies exists in the market. Step by step introduce die component and it function and application. It will also discuss some terminologies use in the stamping industries such as cutting, forming, lancing, ironing operations etc.

What Is a Stamping Die?
Basically, Stamping operation is a cold-forming operation, which mean that, it runs under room temperature and no heat was introduced to the entire operation intentionally. However, heat is still generated from friction during the punching process. As such, proper lubricant needs to be applied during the stamping operation to reduce heat and wear rate of die members.

A stamping die is usually consists of a serious of precision punches and dies that cuts and forms sheet metal into a desired shape or profile within a specified dimension zone. The die members (punch and die) are typically made from special types of hardened tool steel. In some cases, die members are made of special material called carbide to prolong tool life and enable it to runs in much higher speed. ( 800- 1,500 spm in some cases)

A typical die set design can be seen on Figure 1.

Name of Tooling components:-
1. Top Die shoe
2. Top thrust plate
3. Punch holder plate or punch plate
4. Punch
5. Stripper backing plate ( some tools without this plate)
6. Stripper plate
7. Die plate
8. Die backing plate
9. Lower Die shoe
10. Guide pillar and bush

The part that the Die produced is called a piece part (see Figure 2). Usually, a Die can produces 1 part per cycle ( Stroke ) but some Dies can produces more than one piece part per cycle and runs as fast as 1500 strokes per minute with close tolerances up to +/- 0.05mm ( 0.0019 inch ).

Types of stamping Die.
There are many kinds of stamping dies exist in the market, all of which perform a few basic operations, either cutting, forming, bending, drawing or all of them. The Die can be runs manually ( manual loading and unloading of piece part ) or automatically. The automatic Die we called a Progressive Die. Which consists of many operations within a Die.

Cutting operation
Cutting operation is perhaps the most common operation performed in a stamping die. The component to be cut is placing in between the punch and die. Desired shape and form is cut after two members bypass each other. Usually, the Die is bigger than the punch to allow smooth enter of the punch. The sizes difference between the punch and die are called the cutting clearance.
Cutting clearances change with respect to the type of cutting operation being performed, the metal's properties, and the desired edge condition of the piece part. The cutting clearance often is expressed as a percentage of the metal's thickness. Different clearances applied to different material thickness and different type of material to be punched. The most common cutting clearance used is about 5 percent per side of the metal's thickness. In general, the harder the material the bigger the cutting clearance will be applied.

Example 1. . A steel sheet of 1mm thickness ( 0.0393 inch ). Cutting clearance 5%.
Cutting clearance = 1mm ( 0.0393 inch ) x 0.05 = 0.05mm per side ( 0.00196 inch )

Example 2. A Brass sheet of 1 mm thickness ( 0.0393 inch ). Cutting clearance = 4%
Cutting clearance = 1mm ( 0.0393 inch ) x 0.04 = 0.04mm per side ( 0.0015748 inch )

High punching force is needed to perform the cutting operation. In general, the punching force can be calculated as follow:-

F = A X S

Cutting force = Area ( Perimeter of shape to be cut x thickness of material ) x Shear strength of material
Note : Shear strength = 80% of the tensile strength.

Figure 3 show you a typical example of cutting condition.
1. Roll over : Plastic deformation state of material.
2.Penetration : A shinning cut band at the side of cut edge and it is usually 1/3 of the total material thickness.
3. Fracture. Break off.
4. Burr edge.

There are still many different cutting operations, each with a special purpose and application. Some common one are:

Trimming-The outer shape of the piece part is trim away to give the piece part the desired shape.

Notching- In the progressive die, notching is applied on the outer edge of the strip, unwanted part to be cut away to give the desired profile. In some cases, notching is applied in the progressive die to provide a stopper for material feeding. (see Figure 5).

Blanking- The term blanking is a cutting operation in stamping die to cut out the desired piece part from the metal strip. The dimension on the die profile is exactly the dimension of the component wanted. The cutting clearance is applied on the punch. ( subtract 5% / side from die )

Piercing- Cutting a hole onto the sheet metal or component. Some time called perforating. The main difference between piercing and blanking is that in blanking, the slug is used, and in piercing the slug is discarded as scrap. The punch's size is exactly the hole size of component wanted, cutting clearance added on the die side. (see Figure 7).

Lancing-In lancing, the metal is sliced or slit in an effort to free up metal without separating it from the strip.

Shaving-Shaving operation is to remove some small material from the component, usually applied to provide better cut band. (see Figure 9).

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