Brief function of a draw die or Forming die

The Draw die is basically a means or tool that is used to shape a sheet metal in to useful articles in a mass production. The main advantage is to produce identical parts in mass volumes, the dimensional stability of well designed tool is excellent, along with high degree of human safety and working ergonomics. The modern cad software offers very user friendly design tools, which offers flexibility to change modify and extremely vivid operations, easy to study manual as well as robotic movement study.

Section view of the assembly

Examples of draw or forming :

Sheet to first Forming

First to second forming

Base and Top housings

The base and top parts are the main parts contributes to the structure of the tool or die, the all parts are fitted / assembled on the both structures. The structure can be made from mild steel, however for bigger parts or panels the shape gets complicated, the custom made gravity castings are designed to lower / optimize the weight of the structure as well as to reduce the excessive machining.

Draw die casting - Front view

Draw die casting - Back view

The U-slots are provided to clamp the die with the bolster of the Power press machine. The mainly two types of press machines are there - 1. Mechanical 2. Hydraulic. The Hydraulic press is preferred for Draw or forming due to it's smooth / Jerk free function. The brief information on press machines are given in the article, refer the link - Brief Comparison of Hydraulic and Mechanical press.

The FG-250 or FG-300 grade is used for the both Base and top castings, used in the both draw or cutting dies.

Draw Punch

The draw punch is the part, around which the material is being wrapped or being formed. It should be enough hard and wear resistant to serve the at least 5 to 10 lac of parts. The main properties of the draw punch should include in order to long last for the desired service life.

Hardness

This is the first and foremost requirement, as it subjected to regular wear and tear due to abrasion of sheet against it. The surface hardness of at least 56~58 HRC is the fundamental requirement, sometimes additional surface coatings are preferred such as Chrome plating or PVD coating. For PVD coating, the vacuum heat treatment is preferred.

Toughness

Toughness is the required to absorb the heavy loads without cracking, which is the inverse function of hardness. Usually, when the material is more hard, it automatically being brittle and more prone to cracks when subjected to sudden loads.

Therefore, the material should be the blend of balance between the hardness and toughness, in order to achieve the desired service life.

Deformation after heat treatment

Sometimes the deformation / shrinkage is observed after heat treatment, which could be the result of unstable micro structure formed during hardening process. Some steels such as OHNS (oil hardened non shrinkable) offers excellent dimensional stability after hardening. The other way is go for vacuum hardening, which is very good for HCHCr material, D2 grade is preferred over D3.

Blank holder

The function of blank holder is to locate and hold material during forming process, again it should also be sufficiently hard to resist abrasion of metal skid over it's surface. The flatness of a blank holder has a vital role in maintaining accuracy of the part throughout the movement, guide pins are eventually provided to guide and maintain it's movement.

Guide Bush and Pins

The role of the guide pins is to maintain and guide the movement of blank holder throughout the process. It should be hard enough to resist regular wear and tear, along with tough enough to absorb the bending moments. The Case hardened materials such as EN-353 or EN-36 are preferred with min. case depth of 0.5 to 1 mm and hardness of 60~62 HRC.

Guide Bush and Pin set

Draw die Ring

Again, the draw die ring is the part, which is used to push / form the material against the walls of the punch, it is female part. It should be enough hard and wear resistant to serve the at least 5 to 10 lac of parts. The main properties of the draw punch should also include - Hardness, toughness and deformation free operation, in order to long last for the desired service life.

Ejector

The purpose of an ejector is to Hit the part, to make it flat. It also includes the letter punch for any additional marking such part number, identification code or batch marking. As it's name indicates, its main purpose is to eject the part with the help of some springs or elastomer, the gas spring is however being used as a preferred one due it's high success rate.

Ejector with Gas spring

Main Guide bush and pillar set

The purpose of main guide pillar and bush is to guide / align the tool throughout the stroke / motion of the press operation. Sometimes it absorb the play in press slides, thereby ensure the accuracy of the operation. The Case hardened materials such as EN-353 or EN-36 are preferred with min. case depth of 0.5 to 1 mm and hardness of 60~62 HRC. The design is the same in many cases, the diameter of the pillar increases with an increase of tool size, the tool size ratio is completely dependent function of the component to be produced, correspondingly the pillar size is required to balance the tool motion.

The adequate experience is thereby required to design a tool while maintaining the balance of human safety and ergonomics along with unmatched performance of service life, the spare parts should be easily exchangeable.

If you want to learn the Metal forming simulation, read the book "Practical guide to forming simulation"