CNC Machining Surface Finishes Introductions – CNC Surface Roughness, Texture Topology & Finishing

Surface Finish? Surface Texture? Surface Topology? Surface Roughness? What does it all mean? Let’s find out. Talking about surface finishes can be tricky and often leads to confusion. To make it a little easier for product developers and engineers to understand surface finishes and how to communicate about them, we’ll start by defining some of the terms around this subject. 

#1 Surface Finish

First of all, what do we mean by surface finish? Surface finish is simply the condition in which you find the surface of a part at any point during the manufacturing process. This could be a primary finish, such as you’d get from a pair of rollers in a rolling mill. Or, it could be the finish you get after aluminum is extruded through a die, or the finish on a sand casting before any post-processing. 

Next, there are secondary surface finishes created during CNC milling, turning, grinding, sanding, lapping, or polishing. Each of these processes will impart its characteristic finish to the surface, leaving behind a fingerprint of sorts on the workpiece. Then there are the many flavors of tertiary finishes produced by other treatments. Examples are various chemical passivation techniques like anodizing or galvanizing; electroplating with chrome or nickel; or decorative surface coatings such as painting or powder coating. Surface finishes can also be transferred from one part to another, for example when the machined core of an injection mold tool transfers tool marks onto the inside of a molded plastic part. The plastic part would then have a surface finish that looks like it had been machined, when in fact it hadn’t.

#2 Surface Texture & Surface Topology

In terms of surface texture & surface topology, the term surface finish is pretty much interchangeable with surface texture or surface topology in general usage. Topology means how something is arranged, so in this case, it means how the material is arranged on the surface of a part. In most cases, texture and topology are talking about the same thing. But there is a special usage of surface texture to describe one specific result. If a surface is a bead blasted, or if a piece of metal is “spark eroded” using EDM solid-sink to make a “spark finish” then you end up with a stippled surface. Engineers in the prototyping and low volume industry often just call this, incorrectly, a “textured finish”, so that one is a bit confusing.

#3 Surface Roughness

Surface roughness is a bit different. This is quantifiable using a numerical unit called Ra. It’s derived from measurements made of the surface profile using one of several different techniques and types of instruments. Some of these are contact types, using a very sensitive probe or stylus. Others are non-contact, using optical light, lasers, or even X-rays. In all cases, Ra is a calculated average between peaks and valleys of the surface.

Firstly, a mathematical filter is applied, to lop off very high and low numbers which might indicate random aberrations that don’t represent the true surface roughness. Secondly, to make sure that Ra is accurate, multiple readings are taken so that the measuring instrument captures as much unique data as possible. A low Ra value means little deviation, or a very flat or smooth surface, while higher numbers are rougher. Product developers need to know which Ra value they want so they can communicate with a manufacturer. The costs associated with making finer finishes go up exponentially as Ra numbers get smaller, and this is regardless of the manufacturing process used. This also applies to production lead times, so only specify low Ra values if it’s necessary for your product application, otherwise, you’re just wasting time and money. The standard roughness for CNC machined parts is Ra 3.2 microns, but you can do as fine as, for example, 0.2 microns on a CNC machine, but it might take 10 times longer and cost 10 times as much, so please try to stick to no finer than 3.2 unless you need it.

That’s all for our CNC Machining Surface Finishes Introductions and click HERE to see more!