Sheet metal processing lines: how to calculate productivity, availability and efficiency

Sheet metal production lines nowadays are more and more automated, with increasing number of processing modules. Knowing how to balance a production system has become a strategic skill for the production manager in order to optimize the line and increase the productivity.

Production systems consist of a series of machines, each one performing one part of the process. In sheet metal processing lines, for example, a system can include uncoiling, punching, laser cutting, forming, roll forming, welding and packaging.
To calculate the productivity and efficiency of a given production system, it is essential to understand the performance of each machine/module of the line, and how these modules are connected.

This guide illustrates a simple methodology to calculate productivity, availability and efficiency of complex production systems, given the main parameters of each module.

Defining the line and the processing modules

From now on I will define a line as a series of processing modules, each one characterized by the parameter “cycle time” or productivity P: expressed in seconds per part, is the amount of time the module is engaged to complete its process, including loading, processing and unloading time.
Another parameter that characterizes a module is the availability A, defined by this formula:

A = (Mean Time Between Failures)/((Mean Time Between Failures) + (Mean Time To Repair))
Each module is defined by the previous two parameters, and between each module we can have a buffer (parallel processing) or not (serial processing).

The previous parameter will be used to estimate, together with other considerations, the line efficiency E. Expressed as a percentage, this is the expected productivity over an extended period of time, including the preventive maintenance and setup time. For example, a line with

85% efficiency means that it will be active in production 85% of the time.

Calculating the Productivity of the line

When we know the Productivity of each module, we can calculate the Productivity of the line in three different cases.

Simultaneous processing

In the first case, the modules have buffers inbetween, so each module carries on its operation on simultaneously on different parts down the line stream.
The Productivity P will be in this case the maximum value of the Productivities P of each module. This module is the bottleneck of the system.

P = MAX (P1, P2, P3…, Pn)

For example, in a processing line for venetian blinds, we identify three modules:

Roll forming and punching, P1 = 10 seconds per part
Module for head pin positioning, P2 = 5 seconds
Module for hooks insertion and loop tape assembly, P3 = 8 second
The Productivity of the complete line will be, in this case 10 seconds per part.

Serial processing

In the second case, the modules are tightly connected and work without a buffer, i.e. in serial processing.
The productivity P will be in this case the sum of the Productivities P of each module.

P = P1 + P2 + P3 + … + Pn

Some lower productivity lines for venetian blinds have this kind of operation.
With the same example of the previous venetian blind line, the productivity becomes:

P = P1 + P2 + P3 = 10s + 5s + 8s = 23 seconds – a 130% increase in the cycle time.

It is clear that adding buffers can lead to a significant increase of the productivity, and of the availability of the system as well. In large plants, the buffers can be very large and if one of the modules has a function problem, they sometimes have the possibility to divert the production flow to a second backup module.

Serial + Simultaneous processing

In the third case, some modules are positioned in simultaneous processing and others in serial processing. In this case, the serial modules will be seen as a new module with Productivity P equal to the sum of the two productivities.

For example, if the line has:

A first module with performance P6 = 8 seconds, followed by a buffer
The following modules with P7 = 7 seconds and P8 = 12 seconds positioned in serial processing configuration.
The productivity of the line will result in:

P = MAX (P6, (P7+P8)) = MAX (8s, (7s + 12s)

P = MAX (8s, 19s) = 19 seconds

Calculating the Availability

Each module, as any machine, is characterized by an availability that can be expressed in percentage. Availability is defined as:

A = (Mean Time Between Failures)/((Mean Time Between Failures) + (Mean Time To Repair))
Also written as:

A = MTBF / (MTBF + MTTR)
In production lines, the availability of each module has a direct influence on the availability of the complete line. A conservative calculation for the line availability is the following.

A = A1 * A2 * A3 * … * An
Consider three modules with different availabilities:

A1 = 99% = 0,99
A2 = 99,9% = 0,999
A3 = 98,5% = 0,985
A = 0,99 * 0,999 * 0,985 = 0,974 = 97,4%

I consider that the same calculation is valid for serial or simultaneous configuration, since a malfunction in one of the modules, stops the complete line in both cases.

Defining the Efficiency

The calculated productivity indicates the cycle time per part in continuous production. The line, however, can be non productive for a number of reasons:

  • raw material change – for example a coil change
  • change of tooling or configuration – for example at the end of one production batch
  • for preventive maintenance

These times depend on the machine design, of course, but also on the organization of the production, number of material changes per week or day (for example in case of frequent color changes) and on the technicians’ skills.

I call these times as Setup Times or ST and they have to be either measured or estimated. Since efficiency is defined as expected productivity over an extended period of time, I suggest estimating the ST value in the production of a large enough batch of N parts – for example the quantity that is expected to be produced in one week.

Efficiency E can be calculated conservatively as follows:

E = N * P / (N * P + ST) * A

I use the availability value to take into account any stop caused by the modules.

For example, if we have:

  • N = 10000 parts
  • P = 12 seconds per part
  • ST = 7 hours
  • A = 97,4%

E = 120000 / (120000 + 25200) * 0,974 = 0,805 = 80,5%

The production manager can use this Efficiency value – knowing it is an estimation – to calculate the line Gross Productivity GP and simplify his calculations.

GP = P / E

In the previous example, with P = 12 seconds and E = 80,5%:

GP = 12 / 0,805 = 14,9 seconds per part

This means that, even if the machine produces one part every 12 seconds, we have to consider 14,9 seconds to take into account the setup times and the availability of the system.

In one week, with 144000 seconds totally available, the line will be able to produce:

N = 144000s / 14,9s = 9660 parts

 

Notes

The calculations have been presented in a conservative way, but two additional factors should be considered.

  1. If the machine or system relies to an operator feeding or discharging the line, the productivity will have to consider the efficiency of the operator.
  2. We assumed the line producing 100% good parts: if the line is less efficient, the percentage of scrap will reduce the calculated efficiency of the line.

Conclusions

The formulas I proposed, represent a simple methodology that can be applied to a number of sheet metal working systemspunching machines, laser cutting systems, FMS, roll forming machines and packaging systems.
In the article I pointed out how the Efficiency depends also on the manufacturer’s organization. In fact, larger batches reduce the impact of Setup Times ST in the calculation. However, in today’s competitive markets, sheet metal manufacturers need to be able to react quickly to customers’ requests: this turns out in smaller batches, increased setup time and reduced efficiency.
Even in this case, improving the efficiency is possible with a careful organization of the production and, of course, with the use of modern and flexible lines with lower setup times.

Author
Dallan – C.E.O

Waterjet, oxycut, plasma or laser, which cutting technology should I use?

 

There is significant competition in the market between different cutting technologies, whether they are intended for sheet metal, tubes or profiles. There are those that use methods of mechanical cutting by abrasion, such as waterjet and punch machines, and others that prefer thermal methods, such as oxycut, plasma or laser.

 

However, with recent breakthroughs in the laser world of fiber cutting technology, there is technological competition taking place between high definition plasma, CO2 laser, and the aforementioned fiber laser.

Which is the most economical? The most accurate? For what kind of thickness? How about material? In this post we will explain the characteristics of each, so that we are best able to choose the one that best suits our needs.

Waterjet

This is an interesting technology for all those materials that might be affected by heat when performing cold cutting, such as plastics, coatings or cement panels. To increase the power of the cut, an abrasive material may be used that is suitable for working with steel measuring greater than 300 mm. It can be very useful in this manner for hard materials such as ceramics, stone or glass.

Punch

Although laser has gained popularity over punching machines for certain types of cuts, there is still a place for it due to the fact that the cost of the machine is much lower, as well as its speed and its ability to perform form tool and tapping operations that are not possible with laser technology.

Oxycut

This technology is the most suitable for carbon steel of greater thicknesses (75mm). However, it is not effective for stainless steel and aluminum. It offers a high degree of portability, since it does not require a special electrical connection, and initial investment is low.

Plasma

High-definition plasma is close to laser in quality for greater thicknesses, but with a lower purchase cost. It is the most suitable from 5mm, and is practically unbeatable from 30mm, where the laser is not able to reach, with the capacity to reach up to 90mm in thickness in carbon steel, and 160mm in stainless steel. Without a doubt, it is a good option for bevel cutting. It can be used with ferrous and non-ferrous, as well as oxidized, painted, or grid materials.

CO2 Laser

Generally speaking, the laser offers a more precise cutting capability. This is especially the case with lesser thicknesses and when machining small holes. CO2 is suitable for thicknesses between 5mm and 30mm.

Fiber Laser

Fiber laser is proving itself to be a technology that offers the speed and quality of traditional CO2 laser cutting, but for thicknesses less than 5 mm. In addition, it is more economical and efficient in terms of energy usage. As a result, investment, maintenance and operation costs are lower. In addition, the gradual decrease in the price of the machine has been significantly reducing differentiating factors in comparison to plasma. Due to this, an increasing number of manufacturers have begun to embark on the adventure of marketing and manufacturing this type of technology. This technique also offers better performance with reflective materials, including copper and brass. In short, the fiber laser is becoming a leading technology, with an added ecological advantage.

So then, what can we do when we are carrying out production in thickness ranges where several technologies might be suitable? How should our software systems be configured in order to obtain the best performance in these situations? The first thing we must do is to have several machining options depending on the technology used. The same part will require a specific type of machining that ensures the best use of resources, depending on the technology of the machine where it will be processed, thus achieving the desired cutting quality.

There will be times when a part can only be executed using one of the technologies. Therefore, we will require a system that uses advanced logic to determine the specific manufacturing route. This logic considers factors such as the material, the thickness, the desired quality, or the diameters of the internal holes, analyzes the part that we want to manufacture, including both its physical and geometric properties, and deduces which is the most suitable machine to produce it.

Once the machine has been selected, we may encounter overload situations that prevent production moving forward. Software that features load management systems and allocation to work queues would have the capacity to choose a second machining type or a second compatible technology to process the part with another machine that is in a better situation and that allows manufacturing in time. It may even allow for work to be subcontracted, in the event that there is no excess capacity. That is, it will avoid idle periods and will make manufacturing more efficient.

As we can see, the cutting specialization and the use of different cutting technologies for each particular case also involves having CAD/CAM software that is able to address the use and combination of these machines within a single system. In addition, it must include the possibility of assigning and managing the ideal machine, combining both technology and the workload situation. It should also always allow us to manufacture with the quality that is needed, in the most economical manner possible, and respecting delivery times.

Digital Transformation, Cad/Cam,Lantek

 

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