How to really shine with the right polish
Polishing is the proverbial finisher among the grinding methods. To the untrained eye, it seems no more than a finish machining process that leaves no trace. But your many years of experience in the field have taught you that polishing is about one thing, and one thing only: increasing a component’s contact ratio. Adding shine is no more than a pleasant bonus.
What definitely matters is that polishing must never change surface geometry, regardless of whether you’re working on metals or plastics, on soft or fibre-reinforced material. In simple terms, polishing means removing less of what you want to keep.
Read on to find out what industry sectors value the shiny properties of good polishing and why. We’ll also tell you how to best go about your polishing jobs and what common mistakes to avoid.
Everything goes – if you want. Polishing as the finisher
How intense to polish and make the finish also depends on what industry we’re talking about. We can generally say that reducing surface roughness is the focus wherever contact adhesion is of interest, in other words where the contact ratio needs to be increased. Conversely, if it’s a component’s aesthetic that's important – like with car trim parts, for example – roughness is more of a secondary concern. In your everyday work, you’ll be just as likely to encounter metal dies used as injection moulds for plastic or as blow moulds for PET bottles. Particle adhesion on these surfaces is out of the question – the polish needs to be perfect. We have a similar situation with medical technology components. These undergo complex certification procedures and may even have to operate inside human bodies.
Sealing surfaces and races literally need to keep things moving smoothly – the rougher the surface, the rougher your time polishing it. Perfection is the name of the game. Hygienic design is another field where metal parts and surfaces demand the utmost attention to detail. After all, this is where medicines and foodstuffs are produced – the polish grade indicates purity. Maximum material smoothness ensures that sterilising, steam blasting or hot-steam cleaning components and interiors become a wash.
To get the smoothest-possible results for die and mould construction in your workshop, we recommend using felt tools for polishing containers and moulds. Felt or CaraFin polishing tools are also ideal if you’re a master craftsman looking to optimise sliding properties in the automotive or mechanical engineering sector. Combine them with diamond polishing paste or silicon carbide polishing paste to get roughness down to precisely where you want it. If you lean more towards the softer side of things and your workshop is working on copper or aluminium rather than on (stainless) steel, you can get matching pastes to apply on cloths or discs to give your jobs that final finish.
Polishing – the three absolute essentials
For a metalworker like yourself to really sell your polishing results, you need to watch out for three things:
roughing, roughing and, of course, roughing! No matter whether you’re looking to polish steel, aluminium, coated aluminium, brass or stainless steel, or even plastic: the better your roughing, the more flawless your polishing. To get optimal results – regardless of material – always keep relative motion in mind. The polishing tool’s sense of rotation should match the path you’re travelling: if you’re moving the workpiece surface to coincide with the polishing disc’s sense of rotation, the polishing grit will apply lower resistance to the machined surface. Plastic interlocking of the crystalline matrix won’t be impeded. If the workpiece surface moves contrary to the polishing disc’s sense of rotation, however, the polishing grit will resist the workpiece surface more. The result is a more burnished look.
You also need to take into account what type of metal you’re working on. Here are some useful tips:
- When polishing stainless steel, take particular care to rough very finely and that your workpiece does not get too hot when you polish it. Avoid maintaining prolonged contact to do so.
- Keep on checking and remove any particle and adhesions from your stainless steel component to optimise your polishing result.
- If mirror-smooth sheen is what you need, make sure to work in multiple separate polishing steps – and never increase grit size by more than double from one step to the next.
- With aluminium, you can really dial up speed: set cutting speeds between 30 and 60 m/s but make sure to err on the side of smaller grit sizes.
- Caution: if the temperature gets too high, metallurgical structures may be destroyed. Make sure cooling is sufficient and pause polishing frequently. Optionally, you might want to use pleated polishing rings, which will allow more air to reach the workpiece’s surface.
- Not all is gold that glitters – but polished properly, brass comes pretty close. Flannel discs or what you call polishing cloth rings and high-sheen polishing pastes are useful grinding media.
Just keep moving – any standstill will leave its mark
Proceed as follows:
- Fasten your component in a clamp or vice.
- Begin with sandpaper and a grit size matching material and processing condition.
- Hard metals, like tool steel for instance, in milled starting condition will require grinding media with a grit size of 600 or more.
- Softer metals, like aluminium, in milled starting condition will require a grit size of 220 or more.
- Use a soft cloth or compressed air to remove any polish or particle residue between each step. Foreign particles can completely wreck a successful polish.
Note: When you’re polishing soft materials like aluminium or plastic, brush your polishing tool clean every now and then. You’ll get better removal rates if there’s no swarf clogging your tool. Wet machining requires only a few drops of fluid. Remember: only one polishing body per workpiece. Don’t switch back and forth between workpieces.
- Keep on refining the result until there’s no trace of the previous polishing step left.
- Begin with back-and-forth movements on the component, later switch to criss-crossing movements.
- If you’re polishing using a machine, begin by testing pressure and increase it slowly.
- In terms of contact pressure, less is more here. What’s more important than pressure is that movement remains uniform – it’s essential to avoid stopping the tool at any specific point.
Tip: Successively reduce contact pressure on the workpiece surface so that it is at a minimum just before the polishing process ends.
Note: Polishing swirls up microscopic dust and particles that may become suspended in the air around you. Keep yourself and your team safe by wearing personal protective equipment.
Does your work day revolve around finish machining or polishing different kinds of surfaces and components?
Perhaps you’re not satisfied with the results you’re getting? Either way: our experts will be happy to help out.