Choosing the Low Cost Method for Manufacturing



Choosing the Low Cost Method for Manufacturing

Robert Mueller, Ph.D., CLSO

NuTech Engineering Inc.

Milton, ON

Email: rob.mueller@nutech-

Introduction

When the first lasers were built over 50 years ago, they were described as “a solution looking for a problem”. In the years since, lasers have solved many problems, and laser cutting, welding, drilling, and other laser processes are now accepted manufacturing methods for a number of applications. But there are still many more applications where lasers are not yet considered to be viable process options. For some of these, laser processes may never be technically or economically feasible, but for many applications, laser processes are becoming viable alternatives to traditional processing methods.

Adoption of any new technology requires the demonstration of merit in two areas: technical proficiency and cost. Technical proficiency means that the process performs a function that meets or exceeds all technical requirements; everything from accuracy and reproducibility to meeting cycle time. Cost implies the lowest possible cost to produce the product, including capital costs, and all operating and material costs.

Laser manufacturers continue to advance the technical capabilities of the lasers themselves, making more powerful and energy-efficient lasers with better beam quality. These new lasers are also simpler to operate and maintain, and often cost less than older lasers. Increasing the technical capability of lasers and lowering the purchase and operating costs moves the boundary of technical and economic viability, and in many cases, makes laser processing the low cost method of manufacturing.

This presentation will describe a method for calculating the cost of manufacturing, and compare the cost of several laser-based methods with traditional methods to determine if, and at what production volume, the laser method becomes the low cost method of manufacturing. The case studies to be reviewed are: remote laser welding vs. resistance spot welding, laser blanking vs. press blanking, and laser hybrid laser welding vs. multi-pass MIG welding.

Cost of Manufacturing

There are several ways to calculate the cost of manufacturing. For this study, we will use a cash flow method, and not consider the depreciation of capital assets.

The cost of manufacturing is composed of the following cost elements: purchase of the equipment, labour, building costs, electricity usage, and process consumables. A spreadsheet may be used to record and calculate the relevant costs for all methods under consideration, and calculate the production capacity of each system. Several assumptions must be made, and applied uniformly to the competing processes. These assumptions include: the number of working hours per shift, the number of shifts in a day (1, 2, or 3), the number of operating days in the year, and the expected production efficiency or up-time. All these factors affect labour, electricity and consumables costs, and the total production capacity of the machines.

For the equipment costs, the purchase price for each machine, and its tooling, is estimated. This cost is amortized over an assumed loan period, with two or three years being typical, at a typical commercial interest rate, and the annual cost of these loan payments is included in the annual cost for the system. Note that after the loan is paid off, this cost element drops to zero. For labour costs, we consider the number of operators that each system requires per shift (usually one), and that all the operators are paid at the same rate. We also factor in a fraction of the maintenance personnel cost, and consider the amount of work required to be performed on each type of machine. A factory building costs a certain amount per year, to rent or lease, and to keep the lights on and the space heated. This cost should be divided up among all the operations taking place in the building, in proportion to the floor space occupied by each operation. We therefore assume a reasonable annual building cost of $50/sq.ft., and multiply this by the cell area for each case. The cell area includes space for the equipment and a safety enclosure, operator workspace and space for racking or other dunnage for in-process parts. Warehousing of incoming and completed parts is not included. For electricity utilization, we list the rated electrical consumption of the major components of each cell, factor in the usage fraction of each component, derive a total consumption, and multiply by a uniform price for electricity. The consumables category captures all the items, with cost and lifetime, required to keep a system operational for a year. It includes items such as spot welding tips, laser gasses, protective windows, even water filter cartridges. The cost of each item is multiplied by the number of service lifetimes in an operating year. The total costs for each system are totalled, along with the production capacity.

Other costs could be considered in the analysis, such as rework or the cost of poor quality, and inspection costs. These costs may be more difficult to quantify, especially for a new or proposed process. In some cases, these costs can be significant, and a process that minimizes rework and allows for in-process inspection (such as laser welding) can have an advantage. Rework and inspection costs are not considered in the examples below.

Note that some cost elements are fixed, such as the cost of the equipment and floor space, while the other costs are variable, and depend on the usage of the machine. You have to pay for the equipment, and the floor space it takes up, whether it is used for half a shift per day, or 3 shifts per day. On the other hand, operators can be re-assigned, power can be turned off, and consumables last longer when the machine is not fully utilized.

The production capacity is an important consideration in these calculations. Competing processes often have different production capacities, making direct cost comparisons misleading. A more useful approach is to calculate the cost per part, or per length of process. Comparing cost per part gives a better indication of the low cost process, but only if you can use the production capacity of the machine. A more thorough analysis requires determining the production cost for a range of production volumes. At low volumes, a machine may not be fully utilized, while at high production volumes, some processes may need several machines to meet demand. Plotting production cost against production volume for competing processes clearly indicates the low cost manufacturing method for each product volume range.

Remote Laser Welding vs. Resistance Spot Welding

Resistance spot welding is a mainstay of sheet-metal manufacturing. The equipment is relatively inexpensive and reasonably productive. If higher productivity is required, additional units can be installed to work in parallel. A robotic spot welder typically performs one spot weld every 3 seconds.

Remote laser welding is a relatively new process that is capable of performing laser stitch welds very quickly, typically at a rate of 3 or 4 welds per second. A high-power laser beam is directed by galvo mirrors and focussed onto the workpiece anywhere within the work envelope of the remote welding head. The process works on the same concepts as galvo-head laser markers, just using about 100 times more laser power. A typical remote welding head and application are shown in Figure 3. One remote laser welder can replace up to 9 robotic resistance spot welders.

In order to compare the costs of remote laser welding and resistance spot welding, we will consider three cases: 1) a simple resistance spot welding cell with one robotic spot welding gun, 2) a larger resistance spot welding cell with four robotic spot welding guns, and 3) a remote laser welding system using a near-IR fiber-delivered laser, with the remote welding head on a robot, as shown in Figure 3. For each case, we consider the following cost elements: capital equipment (the cell, including robot, power source, end effector, and tooling), manpower, building costs, electricity usage, and process consumables. A spreadsheet was constructed to record and calculate the relevant costs for each case, and calculate the production capacity of each system. Table 1 summarizes these costs and production capacities. Note that the remote welding system uses 2 operators, to keep up with the productivity of the welder.

Table 1. Production Capacity and Annual Costs for Resistance and Remote Welding Systems.

|Process |Units |RSW - Single Gun |RSW - 4 Gun Cell |Fiber Remote Laser |

|Productivity |  |  |  |  |

|  |Weld rate |Spots/s |0.33 |1.33 |4 |

|  |Hours/shift |hr |7.5 |7.5 |7.5 |

|  |Shifts/day |shifts |2 |2 |2 |

|  |Days/Year |days |240 |240 |240 |

|  |Machine efficiency |  |70% |70% |70% |

|Hourly Production |spots/hr |840 |3360 |10080 |

|Annual capacity |spots/yr |3,024,000 |12,096,000 |36,288,000 |

|  |  |  |  |  |  |

|Cost Summary |  |  |  |  |

|  |Equipment Cost |$ |232500 |667500 |1035000 |

|  |Loan Period |Years |3 |3 |3 |

|  |Annual cost of Capital inc Financing |$/yr |86000 |245000 |381000 |

|  |Floor space |$/yr |6500 |10500 |8100 |

|  |Number of operators per shift |  |1 |1 |2 |

|  |Worker costs |$/yr |125000 |125000 |203000 |

|  |Consumables |$/yr |2500 |9800 |3400 |

|  |Electricity |$/yr |1600 |4900 |3700 |

|Total Cost |$/yr |221600 |395200 |599200 |

|Cost/Spot |$/spot |0.0730 |0.0327 |0.0165 |

Figure 4 shows the cost per spot weld for the three configurations described above, and running at capacity on 2 shifts. For all configurations, capital and manpower are the largest contributors to the total cost. The total costs per spot for the remote laser welding system is significantly less than for the resistance welding systems, due mostly to the increased productivity of the remote welding system. The remote welding systems has a somewhat higher annual cost than the 4 gun resistance welding cell, but that cost is spread over three times the production capacity of the resistance cell.

Figure 5 shows the result of a calculation of total annual cost (capital and operating) against production volume. We assume that capital, manpower, and building costs are fixed, and the rest of the costs vary with machine usage. For example, an idle machine does not use consumables or electricity. We also assume no overtime, and if a system reaches its production capacity, another machine must be purchased in order to increase capacity. The data shows that, at low volume, a single robot resistance welding cell is the low cost option. But as production demand increases, one must add cells (and operators), and the single robot cell configuration becomes uncompetitive. At moderate volumes, the 4 robot resistance cell is the most cost-effective, until this configuration reaches capacity, and a second cell is required. At that point, the cost of the resistance system takes a step increase, and is no longer competitive with remote welding. This defines the production volume threshold for cost-effective remote laser welding. In this case, remote laser welding would be the most cost effective production method if you require more than 15 million spots to be produced per year.

Due to the flexibility of remote welding, this volume threshold of 15 million spots/year could be made up from more than one part. By changing tooling, or interleaving tools in a flexible manufacturing system, several assemblies could be run through the remote welding cell to achieve the economic production threshold. Also note that the remote cell is the most economical method of production, even if the cell is only utilized at half its capacity.

Laser Blanking vs. Press Blanking

Press blanking using cutting dies is the standard method for cutting automotive blanks from coil stock prior to stamping in forming dies. Blanks are cut to a defined size and shape to provide the material to draw into the stamping die during deep drawing and to prevent wrinkling from excess material. Presses and dies are expensive, and cutting dies are subject to wear and maintenance.

Flat sheet laser cutting is commonly used at the prototype stage to cut blanks, before the blank shape and die design are finalized and the dies can be manufactured. Flat sheet laser cutting uses no tooling, and the cutting machine is programmed directly from the part CAD drawing. This allows designs to be changed and produced quickly for testing.

It has been suggested that laser cutting could be cost competitive with press blanking for moderate volume production, up to about 60,000 parts per year, if a coil-fed laser blanking line were to be constructed. Laser blanking would allow tighter nesting of parts, increasing coil utilization and reducing scrap, and would completely eliminate hard tooling, with its cost and maintenance requirements. Laser blanking would not be as fast as press blanking while in production, but change-over time between part runs would be reduced to the time required to call up a new program.

Performing an analysis similar to the previous example, with the assumption that each machine is run near capacity, and filled with jobs of equal size, we get the result shown in Figure 6. For small job sizes, press blanking has high costs, due to the cost of many tools for all the jobs that fit into the machine capacity. Laser blanking has no hard tooling cost, only some programming for each part path. In this example, laser processing is the low-cost manufacturing solution for low to moderate volume production, and traditional press blanking technology has the lowest cost at high volumes.

Laser Hybrid Welding vs. Multi-Pass MIG Welding

Multi-pass MIG welding is the standard method for fusion welding carbon steel plate structures. Examples of this type of weld would be joining steel pipeline sections, or rolled plate sections for a large oil storage tank. In both cases, we would expect the butt joints to be about 1/2“ thick, and these would require several MIG welding passes to fill the joint groove.

Laser hybrid welding combines the welding speed and penetration of high power laser welding with the stability, filling and gap-bridging capability of MIG welding. Using high powered lasers, up to 15 kW, welds up to 3/4” deep have been full-penetration welded in a single pass. Single pass laser hybrid welding does not require any special joint preparation, like a V-groove, and laser cut plate edges are usually sufficient.

For this example, we will consider manual MIG welding, robotic MIG, robotic twin-wire MIG, and laser-MIG hybrid welding. The analysis will be carried out using the same procedure as the remote welding case, with the addition that the cost of edge preparation for the weld was considered. The results are shown in Figures 7 and 8.

Laser hybrid welding is a high productivity process; single-pass welding joints that would traditionally take 3 or 4 passes to fill. Hybrid welding also does not require any special edge preparation; perpendicular laser cut edges are sufficient. Being a single-pass, narrow fusion-zone process, hybrid welding imparts much less heat into the part, reducing distortion. Hybrid welded parts often require less post-weld rework to straighten the parts. This cost saving has not been included in this analysis.

Laser hybrid welding of thick parts requires a powerful laser to achieve full penetration, which requires a substantial capital investment. Figure 9 shows that the capital investment required for hybrid welding only makes sense if you require over 100,000 feet of weld length per year. Many small shops would not approach this value, but large shops should consider hybrid welding, and the benefits that it brings.

Discussion and Conclusions

The cost models shown here use values for individual costs deemed appropriate and representative. Each user’s actual costs may vary, and prospective users should perform this type of study using their own data before selecting the appropriate manufacturing method for a specific situation.

Laser processing is a viable technology for a wide range of industrial applications. The productivity, efficiency and price of lasers now make laser processes economically competitive in many areas, including applications that have not, up until now, been considered for laser processing. Companies that can identify a novel low-cost laser processing application can gain a significant competitive advantage in the global market.

-----------------------

Figure 1. A typical breakdown of operating costs for a capital-intensive process. The major cost components are the capital equipment and worker costs.

Figure 2. A resistance spot welding line for automotive production.

Figure 8. Plot of cost vs production for several MIG welding methods and laser hybrid welding. The low cost manufacturing method depends on the production demand.

Figure 4. Breakdown of weld costs per spot, for a system running at capacity.

Figure 3. A Fiber laser remote welding system, using a 2 mirror galvanometer scanning head. Cut-away view courtesy of HighYAG GmbH.

Figure 6. Comparison of cost and production volume for laser and press blanking, assuming each system is running at capacity with jobs of the sizes indicated. At low volume/job, tooling costs for many jobs hurts the press blanking cost structure. Laser blanking is efficient for multiple small jobs.

Figure 7. Breakdown of weld costs per length of weld, for systems running at capacity. Laser hybrid welding has a low cost due to its productivity and lack of preparation requirement.

Figure 5. Plot of annual cost vs. production volume for resistance and remote welding systems. The low cost method of manufacturing varies with production volume.

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download