Do you know the difference between “finish” and “finishing” in precision machining?
If you aren’t embedded in the world of contract machining like we are, it’s easy to confuse these two terms. However, engineers must understand the distinction so that they can communicate their precise needs and avoid issues regarding costs, lead time, and quality.
We’re happy to help you navigate these order callouts by offering this primer and working with your team as your trusted contract machining partner.
Defining Surface Finish vs. Finishing
When a machinist refers to the “finish” of a part, they’re talking about the surface finish, which is the texture of the raw material’s surface after the machining process alters it.
We use many systems to track this measurement, including direct measurement, non-contact, comparison, and in-process methods.
Surface finish is an important consideration because it impacts part quality, durability, and performance. The ideal surface finish for your part is dictated by its final application.
If you hear a machinist talk about a part’s “finishing,” they’re referring to a part’s final surface preparation, such as plating, powder coating, anodizing, or electropolishing, to name a few. Finishing occurs after precision machining.
How to Call Out Your Desired Surface Finish
While a part’s finishing is called out simply by specifying the desired finishing process (and color, if required), surface finish callouts are more nuanced and can be confusing.
A part’s surface finish is a microunit measurement (microinch Ra in the US; micrometer Ra in metric) that averages the peaks and valleys on the surface of a part. A common misconception is that the higher the surface finish number is, the smoother the finish, but the opposite is true. The smaller the average, the smoother the finish.
To give you an idea of the measurement sizes we’re talking about, consider that 256 Ra is 0.0002”, which is smaller than a blood cell. However, even that seemingly tiny measurement is detectable by the skin of your fingers, as are all surface finishes down to 63 Ra.
Reata has exceptionally high standards for quality when it comes to surface finish. In fact, our standard surface finish (125 Ra) is a higher quality (lower finish number) than what many customers’ prints specify, and we often average closer to 40 Ra.
How Surface Finish and Finishing Processes Work Together
Now that we’ve established how surface finish and finishing differ, let’s examine how they affect one another.
When considering surface finish requirements, we always account for the part’s intended finishing process, which can affect the part’s surface. For example, if you need a 32 Ra surface finish for an aluminum part that will be anodized, we’ll make the part smoother than 32 Ra to accommodate the texture this plating process will add.
In contrast, we’ll prepare a stainless steel part for electropolishing by giving the part a rougher surface finish to accommodate for the smoothing that will occur during electropolishing.
Working With Reata as Your Contract Machining Partner
At Reata Engineering, we take special care to provide parts that meet your precise specifications. If we’re making a part we suspect would benefit from a smoother surface finish, such as a part with a sealing or bearing surface, we might recommend a lower Ra than the one you specified.
If you don’t call out surface finish requirements, we’ll provide a default surface finish of 125 Ra (and a surface finish of 250 Ra for holes). But it’s best not to leave a specification as critical as your part’s surface finish up to interpretation.
When you send your parts to us, please call out the following:
- Required surface finish
- Required finishing processes
- If any sides of your part require a different surface finish
We are happy to provide our expert recommendations. Think of us as your contract machining consultants.
Working with Reata Engineering means working with a precision machine shop that prides itself on being a trusted partner. We take customer service seriously. If we can save you time and money by recommending a different surface finish, we will.Request a quote to get started on your project today!