The impact of decisions made before polishing on costs and results.
In many cases, the cost and outcome of polishing are determined not by the polishing itself, but by the decisions made beforehand. Thoughtful planning and creating a suitable starting surface often save more polishing cost and time than the additional machining time and cost required.
- The requirements of polishing should ideally be considered and planned as early as product development, or at the latest during tool design and process planning. This is where I aim to provide practical guidance.
What Should Be Taken into Consideration?
- 1. Selecting the Right Material
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Naturally, the primary consideration is always that the insert material meets the required mechanical and chemical properties. Within these requirements,
however, there are usually several suitable material options.
From a polishing perspective, a material that polishes well is not necessarily one that polishes quickly. For high-quality surfaces (typically finer than approximately 10 microns), the price of the material—although certainly not insignificant—should no longer be the primary deciding factor.
It is always advisable to consult the material manufacturer rather than selecting a grade solely by its designation. Material standards define permissible tolerances, meaning that a particular producer may not recommend one of its own materials for a specific surface finish requirement. This can lead to unpleasant surprises, especially where fine, high-gloss surfaces are required.
It also makes a difference whether a manufacturer specialises in materials intended for high-quality polished surfaces or primarily serves other industrial sectors while producing the same material grades—or perhaps both.
- 2. The Machining Process
- The first step is selecting the appropriate manufacturing process itself, such as turning, milling, grinding or EDM (Electrical Discharge Machining). Within each of these processes, however, choosing the correct machining parameters is equally important.
- General but Important Considerations:
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• To obtain a surface free from machining marks, material must be removed from the entire surface down to the bottom of the deepest machining mark. It is therefore easy to see that if the deepest mark is, for example, 0.1 mm deep, then 0.1 mm of material must be removed from the entire surface using tools that are much finer—and therefore considerably slower—than those used for machining. Moreover, the greater the amount of material to be removed, the coarser the tools that are generally required. The marks left by these tools must then be removed with progressively finer ones, adding further processing time and additional finishing stages.
• Furthermore, the more material that has to be removed and the larger the surface area, the more difficult it becomes to remove exactly the same amount of material everywhere using hand tools. As a result, maintaining the intended shape, geometry and dimensional accuracy becomes increasingly challenging.
• For these reasons, it is usually worthwhile paying greater attention to the quality of the machined surface, as this can reduce the manual polishing effort by as much as 30–50%.
- About Individual Machining Processes:
- • Turning
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In turning, even the choice of insert grade should not be underestimated. A higher-quality insert may produce a sufficiently better surface finish for its additional cost to be recovered during the polishing process. For example, it is often worthwhile to perform the final finishing pass with a dedicated insert used exclusively for that purpose, avoiding unnecessary wear during rougher machining operations. The same applies to feed rate selection: the additional machine time is almost certain to pay for itself during manual polishing, for the reasons explained above.
- • Milling
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For non-rotational components, milling is clearly the preferred choice wherever it is technically feasible. Modern machining centres and high-precision cutters are capable of producing surfaces that can often be polished almost immediately, eliminating one, two or even more abrasive finishing stages and saving a considerable amount of time. Furthermore, milling generally produces machining marks of relatively uniform depth that are easy to identify, making it the machining process that offers the best conditions for maintaining the intended geometry during subsequent manual finishing.
- • Grinding
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In most cases, grinding is not the most effective solution. Although it appears to produce a very uniform surface, achieving a sufficiently fine base would require an excessively fine grinding wheel, making the process uneconomical in terms of both time and cost. By its very nature, grinding creates millions of tiny scratches with an elongated triangular profile, while detached abrasive grains and metal particles inevitably become embedded in the surface. Because these marks vary in depth and are relatively deep, removing them can be surprisingly difficult. For this reason, the general principles discussed above apply even more strongly in the case of grinding.
- • EDM (Electrical Discharge Machining)
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EDM is often an unavoidable manufacturing process and, in many situations, the best available choice. At the same time, however, it can make subsequent polishing considerably more difficult.
- Where a surface can be machined by milling, using EDM is generally not worthwhile from a polishing perspective.
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This is partly because producing a sufficiently fine EDM surface is time-consuming, and partly because EDM hardens the surface layer. Another fundamental characteristic of the process is that it creates countless tiny craters of varying depth. A perfectly polished surface can only be achieved once even the last of these microscopic craters has been removed. Unfortunately, some of them become visible only when the surface is already close to its final finish, making it necessary to rework the surface several times. The rougher the EDM process, the deeper these craters will be.
- For high-quality polished surfaces, the machined surface should ideally be no rougher than VDI 16, and in any case not rougher than VDI 20.
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Another potential disadvantage is the risk of arcing in areas where flushing is insufficient. This may create deep craters that can render a component unusable because too much material has to be removed, resulting in dimensional deviation.
- It is also important
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that whenever a sink EDM surface is intended for polishing, a copper electrode should preferably be used, and its working surface should itself be finished by hand (essentially polished). Milling marks left on the electrode are reproduced on the steel surface, where removing them takes considerably longer than removing them from the electrode itself. This effect is multiplied if the same electrode is used to produce several workpieces.
With wire EDM, it is also almost unavoidable that a witness line or a slight step will remain at the point where the cut begins and ends. This can be particularly problematic on rotational parts and in bores, because either material must again be removed from the entire surface down to the depth of that line, or removing it locally will result in ovality.
- 3. Accessibility of the Surfaces
- • Wherever possible, polished surfaces should be designed to provide at least a 45° viewing angle, and ideally a direct 90° line of sight.
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• This is not simply a question of whether a polishing tool can physically reach the surface.
• Since visual inspection is the primary means of controlling the polishing process, it is equally important whether the surface can be observed directly or whether, for example, reflections from a textured surface interfere with its inspection. In such cases, evaluating the quality of the finished surface may only be possible after moulding the plastic part, making additional polishing work potentially necessary.
• It is also important to consider which tools can reach the surface and from what angle. This directly influences the pressure that can be applied during polishing and therefore affects both processing time and cost.
• For rotational geometries, it is generally advisable to design the insert itself as a rotational component that can be clamped in a lathe chuck.
• For partly rotational geometries, a well-designed parting line or the addition of an extra insert (where mechanical requirements allow) can reduce polishing costs by as much as 50%, for the same reasons discussed earlier under the general considerations.
- 4. Timing
- When product prototypes are being developed, it is not always possible to take polishing requirements into account. However, once the mould is being designed and the product model is available, it is always worthwhile considering surface accessibility from the very beginning.
- Accurate quotation of polishing work is only possible once the mould models and drawings are available, precisely because all of the factors discussed above have a direct influence on the machining time.
- It should also be considered whether, for example, unpolished sample parts need to be produced first, or whether polishing should be carried out in two stages. This allows the trial parts to be evaluated while avoiding the need to repeat the entire polishing process if design modifications become necessary.
- Overall, polishing is always one of the final stages of tool manufacturing, where delays from previous operations have often accumulated. At the same time, rushing the polishing process is rarely beneficial, as correcting mistakes at this stage is usually time-consuming and may even require inserts to be manufactured again. For this reason, errors caused by unnecessary haste can have the greatest consequences.
- The beauty of toolmaking lies in the fact that every tool is different and presents new challenges. Achieving the best possible result therefore requires close cooperation, combining knowledge, experience and an understanding of each stage of the manufacturing process.