Design and analysis of an industrial, progressive die for cutting and forming

Nowadays, many components which were earlier cast or machined have now been replaced by steel metal stampings. Material economy and the resultant reduction in weight and cost, high productivity and a high degree of possible precision have made press-work essential for many mass-produced products such as electronic appliances, utensils and car parts. Although, laser-cut technology is widely developed and more flexible in terms of variety of produced components, it cannot reach the extremely high productivity rates of a progressive die. Progressive die can perform a sequence of operations, in different stations at a single stroke of press. In this work, an innovative progressive die consists of two stations was designed, in order to produce a complex metal part with three different manufacturing processes. The components of the die have been calculated by mathematical formulas and empirical data, designed with Computer Aided Design software and analyzed by Finite Element Analysis tool.


Introduction
A progressive die performs a series of fundamental sheet-metal operations at two or more stations during each press stroke in order to develop a workpiece as the strip stock moves through the die. Each working station performs one or more distinct die operations, but the strip must move from the first station through each succeeding station to produce a complete part.
The workpiece in the investigation case is a part of the trigger mechanism for a speargun and it is build in 316L stainless steel, which has high corrosion resistance in very corrosive enviroments like sea .
This progressive die consists of two stations. To the first station are performed the operations of piercing and notching while to the second the operations of bending and blanking.
For the design of the die and the simulation of the manufacturing process, the commercial Computer Aided Design software Solidworks was used and for the F.E analysis, the commercial code Ansys Workbench.

Research Objectives
The aim of this work is to design a progressive die in order to produce a complex metal part with four different manufacturing processes, piercing, notching, blanking and bending, with as few as possible stations. So, a two-stations die was designed with manual feeding in which a stripper plate is also helping the operator with the guidance and alignment of the metal sheet.  The thickness of the material is 2mm.

Calculations
For the workpiece a 316L stainless steel, was used as shown in Table 1. The components of the die have been calculated by mathematical formulas and empirical data, as presented in the next sections.

A. Cutting Force
During punching operations ( Fig.3) such as piercing and blanking, the cutting force applied in the punch can be calculated using formula (1) [12,13].
where,  fs = shear strength of material (N/mm 2 )  C = cut length (mm)  t = sheet thickness (mm) So, in our case we have:

Fig.3 Part cut outline
Cut length calculation:

C. Total Force
Due to friction between various components of the die, the cutting force must be increased by 20% [14].

Definition of pressure center point
During the press working process of the shearing-cut and bending progressive die, the position of the die's pressure center has a direct impact on whether the die can work accurately in balance.
In Fig. 4, are presented the distances from the zero point (left corner).

Springback
Springback occurs when a metal is bent and then tries to return to its original shape. After a bending operation, residual stresses will cause the sheet metal to spring back slightly.
Due to this, it is necessary to over-bend the sheet an amount to achieve the desired bend radius and bend angle.
The springback radius can be calculated by formula (6) The springback angle can be calculated by formula (7). where, So, in our case: The springback factor, commonly denoted by Ks, is the relation between the initial and final angles. A springback factor of Ks = 1 means there is no springback, where a value of 0 means total springback.
The springback factor can be calculated by formula (8). where, So, in our case:

Die Design
The die design was made with the help of commercial software SolidWorks, in which was also made the assembly of the die and the motion study (Fig.5a,b). Also, in Table 2, die components and materials are presented.
Die block back plate 1.0050

Die block
The die block is the most important part of a progressive die and it defines the design of all the other components.

A. Active surface
Our die has two stations and constant width, so: B = active width = 70 mm A = active length = 2 x V = 180 mm

B. Thickness
The thickness of the die block can be calculated by formula (9).
where,  F = total force (N)

C. Margin
Margin is the solid cross-section around the die cutting edge. The fixing screws and dowels should be placed outside the margin to prevent weakening of the die.

D. Die Clearance
The intentional gap between the punch and the cutting edges, depends upon the physical properties of the sheared material. Our material is stainless steel and the clearance is 20% of the sheet thickness. So:

E. Width and length
The total width of the die block can be calculated by formula (12). where,

Stripper plate
The stripper plates guide the punches through the sheet and also helps the operator with the manual feed of the strip.

Top plate
The top plate is mounted on the press and it is standardized. Its dimensions depend on the width and length on the die block.

Bottom plate
The bottom plate is mounted on the bed of the press and it is also standardized. Its dimensions depend on the width and length of the die block.

Guide bushes
The guide bushes guide the pillars so the whole assembly is perfectly straight and also lubricate the pillars. They are standardized.

Guide pillars
The guide pillars guide with the help of the bushes the whole die. They are press fitted on the bottom plate. Their height depends on the height of the die. They are also standardized.

Punches
In this die assembly there are four kinds of punches. There two standardized circular punches, a custom-made cutting punch and a custom-made bending punch.
First, we should calculate the critical length of the punches. Τhe lengths of the punches are calculated from Euler's formula (15).
where,  During the production, all the punches must have the same, safe length. So, for our case we choose 40 mm.

Finite Element Analysis
The analysis of the parts was made with the commercial code Ansys Workbench. The type of the analysis we chose is Static Analysis and every part had a fixture at one end and the other end was free. The right load for each part was applied at the free end.
In Fig. 6-15, are presented the results of the F.E analysis for the most important components of the die.
All the parts can withstand the load that is applied to them without failure according to Von-Mises criterion of failure. The results of the FE analysis are given in Table 3. In conclusion according to the Fig. 6-15 and Table 3, the results of Von-Mises stress and deformation of the most important parts of the die specifically the punches and the die, can withstand the load of the procedure.

Conclusions
In this work a progressive die was developed, in order to produce a complex metal part with four different manufacturing processes, piercing, notching blanking and bending, with as few as possible stations.
The components of the die have been calculated by mathematical formulas and Finite Element Analysis tools.
The proposed approach is based on real manufacturing data, in order to be easily produced.

Ethical Approval
For this type of study formal consent is not required.

Consent to Participate
Informed consent was obtained from all individual participants included in the study.

Consent to Publish
The participants have consented to the submission of the case report to the journal.

Author contribution
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding
No funds, grants, or other support was received.

Competing Interests
There are no relevant financial or non-financial competing interests to report.

Availability of data and materials
All the data (numerical, figures, diagrams, tables, etc.) used to support the findings of our study are included within the article. Thus, data sharing regarding the aforementioned paper is totally allowed and any reader can access the data supporting the conclusions of the study.