Keeping It Smooth: Why Surface Roughness Can Make or Break Your Project

Like many things, prototype casting is not as straightforward as it may seem. There are many factors to consider before and after a piece leaves its cast. One example that raises a lot of questions is surface roughness. How is it measured? What should you aim for? Why does it matter? And how do you control it? 

We’re going to go ahead and answer those questions for you right here.

While there are multiple methods to measure surface roughness, the two most common in the casting world are Ra and Rz. Both have their place and are related. However, there is no real conversion between one and the other.

Ra is the mean roughness value taken from all amounts of a profile value. Unlike Rz, it does not differentiate between peaks and valleys. This option works well for regular surface patterns that can be found after milling or laying.

Rz, on the other hand, considers the average of the five most extreme roughness values — the highest peaks to the lowest valleys. This type of measurement is most useful for random surface patterns, such as those found on raw castings.

At ACTech, we take measurements using a 3D Profilometer VR-5000 from KEYENCE. This is an optical measuring device for contour, flatness, and roughness with a resolution of 0.1 µm, a very fine and high-resolution GOM (general outcome measure). Unlike standard techniques that can only measure roughness in a single line, the 3D Profilometer can measure an entire predefined surface. In components such as turbochargers, a silicone-like mass is pressed in for this purpose, and this negative imprint is then optically measured. 

Surface roughness can vary depending on a number of factors, but it is particularly important for those casting a prototype. Unlike serial production, which casts using shooting cores, prototype casting often uses printed cores. Due to the manufacturing process, these cores have construction stages (the staircase effect) at certain points and thus a rougher surface, resulting in a correspondingly rough surface in the casting. Like every other quality feature of a part, this can profoundly influence both price and function. Finding the right balance is important.

From a financial point of view, a general rule is that the rougher the surface of your prototype, the cheaper it is to finish. For example, a piece at Rz 100 (micrometer) takes far less effort to produce than one at Rz70, which requires a mold coating. Not only does that extra work cost money, but it could also have a negative effect on the casting as the coating traps air in the mold. The same goes for machining: a smoother surface means more machining time, and those extra hours end up on your bill. 

However, your desired surface roughness should really depend on its potential to impact how your prototype functions. This is particularly true for internal channels, as a rough surface can negatively impact whatever needs to flow through it, such as air, water, or oil, by causing turbulence, resistance, or deposits. 

In that case, it is definitely worth considering factoring in the extra hours - the added performance of the final part will be well worth the additional cost.

Luckily, smoothing out and polishing the surfaces of your casting isn’t as complicated as it once was. We can reach the desired finish as part of the "all in" experience through a technique known as abrasive flow machining. In this process, a chemically inactive and non-corrosive media is pressed through the components, grinding the channels and smoothing the surface for a perfect surface finish and edge conditions.

"For our requirements at ACTech, conventional abrasives only worked to a limited extent," explains Alexander Seidel, an employee specializing in raw part finishing at ACTech. "With AFM, we can finally grind internal complex contours and barrels and make the surfaces in them better. The bottom line for the customer is that the media - air, water, oil, etc. - can flow better through the component." 

So, how exactly does the process work?

It begins with a choice of abrasive - coarse, medium, and fine - all made of silicon carbide, a viscoelastic mass with abrasive particles of different strengths and sizes. This choice depends on the nature of the part that needs to be machined: the smaller the opening, the finer the abrasive. 

The process begins with a one-time setup, lasting anywhere between 60 and 90 minutes, depending on the part. To do this, the compound is poured in and made pliable by pushing it back and forth. Afterward, the part is screwed onto the necessary adaptor and prepared for the actual AFM process. 

"This takes only a few minutes and is carried out at between 20 bar for a fine abrasive and 36 bar for a coarse abrasive," adds Alexander.

To complete the process, we remove the abrasive with a wire, compressed air, or spatula with as little loss as possible. The part is then washed with a combination of a basic alkali and 50°C water, which dissolves any residual abrasive and removes the oily film that has formed.

Since mid-2020, ACTech has been happy to offer this service in-house. Doing so allows us to shorten the lead time of each project by up to ten days, guarantee a level of quality that meets our demands, and allows us to react much faster if we did not obtain the desired surface roughness the first time around.

Time is money in prototype casting, after all, and we’ll do everything we can to make sure your project runs smoothly — in every sense of the word.

Would you like to learn more about prototype casting or ask for more information about starting a project with ACTech? We'd love to have a conversation.

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