Punching or stamping? How to calculate the most efficient production system based on your production mix

Manufacturing a sheet metal product requires several of stages: punching and laser cutting, forming, welding and painting. A variety of technologies can be employed in each one of these phases.

In this article I will focus on the metal cutting process, comparing sheet metal punching and press stamping, and also calculate when it is convenient to punch a product and when to build a punching hard tool.

Here are the technologies I will compare and their main characteristics:

• CNC from sheets: high flexibility, lower productivity, material waste up to 15%.
• Press stamping: lower flexibility, high productivity, material waste less than 3%
• CNC Punching from coils in the flexibility/productivity graph it is positioned right in the middle, and keeps material waste less than 3%

In fact, sheet fed CNC punching is usually employed where the production mix is high, with products with low quantities and materials changing continuously (small batches).

Press stamping is a good choice for mass production of a part with high quantities, and using the same material. We find less of these productions today: the market requires more customization for the products, continuous improvement of the design and smaller production batches. Here is where coil fed punching machines come in play, as I will highlight in this article.

Following these base indications, the sheet metal technologist will in most cases identify the correct processing cycle that completes the process at the lower total cost.

In other cases the choice is not so obvious, therefore comparing the total production cost is important to take the right decision and maximize profit.

Follow this link to discover more on coil fed punching lines.

Comparison of the total production cost

The cost of the production is the sum of the following factors:

(Total production cost) = (Total material cost) + (Total process time) * (Machine and labor hourly cost) + (Tooling cost)

The formula can also be written with the following acronyms:

(TPC) = (TMC) + (TPT) * (MLHC) + (TC)

Let's read this formula for each of the three technologies.

Sheet fed punching machine

In sheet fed CNC punching machines, Total Material Cost is higher than with the other two technologies, due to the fact that parts are generally nested into a skeleton, and the scrap rate can reach over 15%.

Total processing time is higher than with punching presses or coil fed punching machines, since the machine stops its operation for loading and unloading the sheet.

Machine and labor hourly cost depends on the level of automation, investment and operating hours of the plant in the year; I will cover the calculation of a sheet metal working machine hourly cost in one of my next articles.

Tooling cost is low: in fact punching machines use standard thick turret tools that can be purchased from several suppliers, and one tool can be used several times on different parts of the sheet. New tools can also be available in very short time, generally from one to three weeks.

Stamping press

Stamping presses operate from coils, and material is usually optimized to the coil width. It is common to have material waste reduced to less than 3%. This material cost has a great influence on Total Production Cost, especially for very large productions.

In punching presses then, the total process time will be the smallest, with the machine usually producing one part with every hit; the system is also producing continuously and automatically.

On the other hand the tooling cost will be highest (see picture in the header). Clearly, with very large numbers the tooling cost will have a small influence on the result; on the opposite case of smaller annual quantities, a hard tool will take very long for its payback.

Coil fed punching machine

In coil fed punching machines, the material is usually optimized to the product width and the material waste is reduced to 3% or less.

Production time for a part is longer than in the stamping press but faster than the sheet fed punching machine, since coil fed systems work continuously and automatically and the machine does not stop for the loading and unloading of the sheet.

These machines use standard thick turret punching tools, therefore tooling cost is lowest as in the case of sheet fed punching machines.

Moreover, thanks to the flexibility of these systems, introducing a change in the product is simple and it can often be done in real time (with hard tools it would require an additional investment for the modification of punches and dies).

Payback of a hard tool

In comparing the production costs of press stamping and coil fed punching, with the previous formula, it is possible to find the number of parts N that will payback the hard tool cost with just two assumptions.

Here is the formula of the total production cost, written for punching machine and for the coil fed punching machine.

(TPC) = [(TMC) + (TPT) * (MLHC) + (TC)] press = [(TMC) + (TPT) * (MLHC) + (TC)] coil punching machine

The total material cost can be considered identical in the two machines.

We assume that the tooling cost in a coil punching machine is zero, and that the processing time in the stamping press is negligible compared to the punching time.

The formula becomes:

[(TC)] press = [(TPT) * (MLHC)] coil punching machine

The total processing time is expressed as:

(TPT) = (Cycle time per part) * (Number of parts)

And with a couple of steps we find the formula that identifies the number of parts that will payback the die:

(Number of parts) = 3600 * (TC) / ((MLHC) * (Cycle time per part in seconds))

For example, if the cost of the tooling is 20000 Euro, the machine and labour hourly cost is 100 Euro and the cycle time in coil fed punching machine is 20 seconds, we can calculate:

N = 3600 * 20000 / (100 * 20) = 36000 parts

Additional factors

In the choice of the right production system for a part, the lowest production cost is one of the elements in play.

The technologist will have to consider also other factors such as the lead time for the production start (time to market) and likelihood of modifications of the product during the production.

In fact, at the beginning of the production, a new sheet metal product will go through different stages of optimization from the prototype phase to large production. The ability to speed up the prototyping phases relies on technologies that have to be as flexible as possible.

At the same time, more and more companies have seen the dimensions of their production batches reduced, towards a customized production: a trend towards the “batch one” production.

For this reason, coil punching machines are more and more popular being right in the middle of the productivity/flexibility graph: more flexible than a stamping press, more productive of a sheet fed punching machine, and with a very high material utilization.

Coil fed punching and simultaneous laser cutting systems

In the past two years, coil fed punching machines with simultaneous laser cutting have expanded both the flexibility and the productivity of the coil fed punching machines: in fact, the laser cutting unit makes it possible to produce and nest parts just like in a standard flatbed laser.

The most unique feature of these lines is the separation of the laser module from the punching unit. In this way the two modules process simultaneously the strip, and this can lead to a production speed increase up to 200%.

Conclusions

The thoughtful choice of the right production system for a given production improves the economic results for the sheet metal manufacturer, in terms of minimal production costs and, as we have seen, in terms of reduction of accessory costs such as the ones connected to the modifications of the product during the production.

In planning a new production, or considering the investment in a new processing system, it is important to have a complete picture of total material costs and operating costs of the plant.

This new flexibility-productivity graph shows the position of punch presses and sheet fed punching machines: the new coil fed punching and laser cutting machines have improved both flexibility and productivity and become an interesting alternative to traditional manufacturing technologies, by keeping the maximum material utilization and allowing the manufacturer to introduce customizations and improvements into the product during its life cycle.