CNC machining almost always leaves tool marks on the surface – such imperfections are visible on milled or turned parts. The reason is simple: it is the result of that type of material removal and is visible even after the finishing machining. Sometimes these machining marks are visible on the surface; often – tactile.
There are many situations when parts do not need perfectly smooth surfaces, but in situations when these elements should work in bigger systems (e.g. gears or sleeves) – a spotless finish is required. In addition – during such bettering of the surface parts can be improved mechanically, eg. during anodization aluminum parts gain a better finish, but also – improved corrosion resistance.
What are the types of surface finishes used in contemporary machining – traditional (automatic) and numerically controlled?
What are the finishing processes for CNC machined parts?
To improve the quality of the surface parts made on the CNC machines can be subjected to several additional bettering processes. Most of them are mechanical, some – electrochemical. The goal of each process is simple – to improve the high-quality finish and smooth surface to decrease friction of parts in bigger systems.
Such processes can be divided into:
- mechanical – often executed with CNC machines (eg. grinders), but can also leave some tool marks, but – as a part of the machining process – can deliver high precision and the best parts tolerances,
- basic finishing, such as sandblasting, that changes the mechanical structure of the surface and smoothes it significantly. Can cause marginal changes in dimensions but provide much better surface quality,
- secondary finishes, such as anodization or powder coating, that can change the dimensions of the part. For electrochemical processes (eg. mentioned anodization) – minimally; for painting – significantly. In addition, such processes can improve parts resistance against environmental factors and achieve a perfectly smooth finish. Another advantage of that finish is the possibility to change surface color.
Each of these methods provides other properties of the achieved surface and has its advantages and disadvantages. Also – these processes differ in price: the service itself (e.g. anodization), applications (not every material can be processed with a particular method), and often – required time to achieve expected results.
Types of surface finishes after a CNC machining
The division mentioned above can be refined by the description of particular processes of surface bettering, especially after machining processes (CNC and traditional).
Among these, it is worth mentioning:
CNC Grinding
Although it is a part of machining, that method of removing excess material allows a smooth finish, often without tool marks. Such results can be achieved with higher abrasive height (over P240) and wet grinding. It is a good method for flat surfaces or parts with regular dimensions that also helps to better the surface of parts without altering their final dimensions,
Very precise machining
Professional CNC machining (lathes or mills) allows a perfectly smooth surface during manufacturing parts. It requires very high-quality devices and cutting tools – sometimes can leave tool marks but barely noticeable. Such surface finish is executed as an additional step in machining and has negligible impact on the tolerances,
Sandblasting / bead blasting
Is a popular form of parts finishing. Allow to effectively remove imperfections from the surface (tool marks, discoloration, etc.). That form of finish is executed with specialized fine particles in a high-pressure stream of air.
The smoothness of the achieved finish can be classified with the SA scale, where SA3 is the lowest, and SA1 is the highest. Mentioned abrasive agents are basically fragmented materials – sand, glass, or specialized particles (eg. a proprietary agent called Blast Grid). That type of finishing surface of CNC machined parts is very popular due to its high efficiency but should be executed in a special chamber. On top of that, all of the part's holes should be secured to not damage their internals (eg. threads). Well-performed sandblasting provides a smooth surface finish, that can be shiny or matte. The final effect strongly relies on the used abrasive agent,
Anodization (aluminum or titanium)
It is a very popular and highly valued type of surface finish. Anodization is an electrochemical process of bettering the surface by creating a thin layer of aluminum oxide. In addition to bettering its quality, anodization also improves abrasion and corrosion resistance, and mechanical properties and removes tool marks. That type of surface bettering is expensive, especially hard anodization, but provides a (very) hard surface. The big disadvantage of that process is its limitation to aluminum and titanium alloys, but on the other hand – has minimal impact on the final dimension of the part (that can be included in the specification of parts manufacturing due to high repeatability and precision created coating thickness),
Malowanie proszkowe
Kolejna forma obróbki wtórnej, która pozwala na uzyskanie gładkiej i wytrzymałej powierzchni. Sam proces jest stosunkowo prosty – po nałożeniu specjalnego podkładu, poprawiającego odporność na korozję, część jest pokrywana drobnym granulatem („farbą w proszku”), która utrzymuje się na powierzchni dzięki elektrostatyczności, a następnie – umieszczana w specjalnym piecu (rozgrzanym do ok. 200 stopni Celsjusza) w celu roztopienia.Aowder painting
Another form of processing that can provide a smooth and wear-resistant finish. The process itself is pretty simple – after the application of primer and powdered paint (attached to the surface by electrostatic) part is put in a special oven (heated up to 200 °C / 392 °F) to melt the paint. The achieved coat is smooth and has good mechanical parameters, but can impact the final dimensions of the part. In addition – that method is often very cost-effective. Each surface finish has its advantages and disadvantages but for sure – can impact the final price of the whole manufacturing process.
Impact of the finishing processes on the unit price
Questions such as 'what is the cost of aluminum anodization' or 'How to decrease the cost of CNC manufacturing' are popular and their high frequency (especially in Google) is justified.
But still – the quality and type of selected surface finish have a direct impact on the final price of the project and single unit.
It is caused by the following reasons:
1. Additional machine time required to achieve expected finish quality (eg. lower machining speeds),2. The additional cost of particular processes and their types – for example, hard anodization is more expensive than 'soft', but provides a very hard surface,
3. Additional time to prepare for selected processes.
When choosing the right type of CNC machined parts surface finish it is important to remember the system in which part will work. Such an approach helps to select the best process.
Another aspect of surface bettering is acceptable tolerances – if a part will work in a complex system, its tolerances should be as low as possible.
For such demanding applications the best solution could be mechanical finish or anodization.
If such precision is not very important – powder coating can be a good solution, due to acceptable protection against the corrosive agents and smooth surface.
On the other hand – low abrasive resistance can be problematic in some applications.
Cost-wise:
- the most efficient is powder coating,
- soft anodization (type I and II) are more expensive,
- the most expensive type of surface finish remains hard anodization but provides very good mechanical parameters and perfect tolerances.
One order, countless advantages – this is how we work at RADMOT
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