By Stuart Croxford
You don’t need a sledgehammer to crack a nut.
A sledgehammer can be used to crack a nut but is not optimal. Compared to nut crackers, it is large, heavy, difficult to operate, can easily cause a quality defect and relatively expensive. Both will work, but one is more optimal than the other.
The same can be said for manufacturing processes, or to be more accurate, the combined sequence of different manufacturing processes that is required to complete a product. Selecting the correct materials and manufacturing processes is best done in the very early stages of design, to optimise overall product and component cost.
Each individual process type has a different cost. This is driven by the direct labour needed to complete the process, power usage, equipment depreciation, tooling amortisation, factory overhead, set-up time, consumables, waste material (scrap) and cycle time.
The process cost is usually stated as an hourly rate and will often have overhead applied to it. This can be expressed as a “machine hour rate”, or “direct labour cost per hour”.
Therefore, the cost to complete a single processing operation is the machine hour rate + the direct labour rate multiplied by the time it takes for that particular operation to be completed. So, by extension, the total process cost for a complete product, is the sum of all labour costs + all machining costs + the raw material cost.
The above may be rather obvious but, in a “perfect market”, every processing element of every component, will impact cost. Therefore, thought should be given, at the earliest stages of product design, as to the best way to manufacture the product. The best design is the cost optimal compromise between form, function and manufacturability, all are interrelated.
Another significant driver is the batch quantity in which the product will be manufactured. High volume production is best suited to relatively complex, expensive equipment with a long setup time. Small volume manufacturing may well have a large labour content and simple equipment which is quick to set up but slower, and more expensive to operate. Think of an automotive plant, where a large, complex sequence of operations essentially manufactures the same vehicle day in, day out. You would not consider using this same production line to manufacture a small batch of custom vehicles due to the huge setup cost, jig, fixture, and tooling cost involved.
Consequently, right from the initial concept, the designer should have an appreciation of the volumes in which the product will be manufactured, and the types of process that will be employed to manufacture it. Processes like injection moulding, transfer pressing, die casting, robotic assembly, robotic welding/spraying and other mainly automated machining process, arranged in a dedicated production line, are well suited to high volume products. In contrast, manual assembly, fabrication, CNC machining (not auto-fed), hand welding, punch pressing, bending, and forming on discrete machines, managed through the factory on a process-by-process basis, is usually more efficient for lower volumes. Get this wrong at the design stage, by specifying a complex injection moulded part, for which the tooling alone can cost tens of thousands of dollars, all to make a hundred or so parts, and there is no way the product cost, after tooling amortization, will be optimal.
Materials are also a key consideration, and the need to select the most cost-effective material for strength, durability, cosmetics, and cost, is somewhat obvious but, also, how that material may limit the choice of process should be understood and taken into consideration. Taking an automotive gearbox casing as an example. These could be made from many different materials, but most common would be Steel or Aluminium. The cost of Steel is lower than Aluminium pound for pound but, Aluminium has a higher volume per pound (and so goes further). Steel is stronger and more wear resistant than Aluminium, and so can have smaller wall thicknesses, where Aluminium may need bushes on wear surfaces, but is light weight, and so more fuel efficient over the life of the vehicle. All the above are typical design compromises that should be considered when designing a gearbox casing.
However, other, process related considerations are just as important. Machining Steel (high cost) is harder and slower than machining Aluminium (low cost). Steel can be cast (usually sand cast), but this is a relatively slow process adding cost (either in labour or expensive automated casting lines, high cost) and requires relatively simple tooling (low cost) but is not suitable for very thin wall thicknesses (design constraint). Aluminium, on the other hand, lends itself well to die casting, an easily automated injection casting process (medium cost) that has short cycle times (low cost) but requires complex tooling (high cost).
Once these additional factors are taken into consideration, as a generalization, for very low volumes, a Steel gearbox casing would be machined from solid or sand cast & machined and an Aluminium gearbox casing would be machined from solid. For medium volumes a Steel casing would still be sand cast and machined and an Aluminium casing may be gravity die cast (simple tool, medium cost) and machined. For high volume, a Steel casing will be sand cast on at automatic line and machined, while an Aluminium casing will be die cast and machined. The part design and tolerancing must reflect the capabilities and limitations of the processes employed, as well as design features included on the product to enable robotic handling, and the like, for automated, high volume production lines.
To summarise, as well as the usual design considerations associated with form and function, selecting the most cost-effective material, and resulting process steps appropriate to the anticipated manufacturing quantities, in the very early stages of design, is critical to the eventual cost optimization of the component or product.
If your business is planning to outsource manufacturing, partnering with an experienced sourcing company in the very early stages of product design would be highly beneficial. A company with many years of product development experience, a broad knowledge of the supply base, related processes and their respective costs, can help to influence design decisions and ultimately, optimise product/component cost.
SureSource is deeply experienced in product development and manufacturing and provides full-service sourcing capabilities. With its in-house engineering, project management, sourcing, quality, and logistics teams, together with an extensive supply base of pre-qualified factories, SureSource will work in partnership with your organization. From the earliest stages of design, through sample approval, validation, certification, mass production, quality and finally, delivery, including to your customers’ dock, SureSource can add significant value to your business, by variablising overhead, shortening time to market, and optimizing cost.
SureSource, your One-Stop Shop, from Concept to Dock.
