Have you ever wondered why some CNC machined components have very visible toolpaths on the surface while others have a much smoother appearance? The answer lies in how the tools move across the material. Choosing the right toolpath is critical, as it affects surface finish, tool marks, and whether the part meets both appearance and functional requirements. In this article, we’ll show how different toolpaths affect surface finish, using real examples from parts manufactured at our factory.
The two main factors that affect toolpath appearance are (1) the tool used in the final finishing step and (2) the programmed tool movement.
Before machining any product, our engineers carefully select the most appropriate type of tool for each process, such as face mills, endmills, drills, reamers and T-slot cutters. Each tool serves a different function and produces a different surface finish as shown in the table below:
| Tool type | Typical Application | Roughing Machining (Ra µm) |
Finishing Machining (Ra µm) | Toolpath Visibility | Notes |
| Face Mill | Machining flat surface | Ra 6.3~25 |
Ra 0.8~1.6 | Low | Smooth, uniform finish. Stepover lines are still visible but less pronounced compared to endmill machining. |
| Endmill | Machining pockets, vertical walls, tight corners, narrow grooves | Ra 6.3~25 | Ra 1.6~3.2 | Medium-high | Visible stepovers |
| Ball endmill | Contouring complex geometries or curved surfaces | Ra 12.5~25 | Ra 3.2~12.5 | Medium-high | Visible stepovers, surface is rougher for contouring machining compared to planar machining |
| Drills | Holes with no tolerance | Ra 12.5~25 | N/A | N/A | Not for surface finish |
| Reamer | Precision holes | N/A | Ra1.6~3.2 | Very low | High accuracy |
Example of endmills used at Yumoto Vietnam. Endmills vary in diameter, length, and material compatibility.
Example of face mill mounted to Yumoto Vietnam's Fanuc Robodrill CNC milling machine. Available diameters range from 20 mm to 80 mm.
After engineers have selected the appropriate tools, they program the movement of these tools based on multiple factors, including the cutting direction (parallel or circular), feed rate, stepover distance (the spacing between successive passes), and other machining conditions. The slower the feed rate and the smaller the stepover and pass depth, the finer the machining lines, resulting in a smoother surface. However, doing so can increase the machining time, so engineers must take into account what is the required surface finish and what is the best machining set up to ensure cost-effective production.
For example, in contour machining of curved surfaces, a ball endmill is typicall used. Let's say we select a ball endmill of diameter 10 mm. If we set the stepover to 0.25 mm, the resulting surface roughness is about Ra12.5µm. If we reduce the stepover to 0.05 mm, we can achieve a surface roughness of about Ra3.2µm. However, machining time will increase by up to five times, significantly raising production cost. If functionality of product is not affected by the surface roughness, it is better to select the larger stepover setting.
Let’s take a closer look at how these toolpaths appear on real components we have manufactured.
Material surface vs. visible endmill toolpaths on A5052 aluminum component
The outer edges have no machining line as we kept the surface of the material as is, but the endmill toolpath is visible on the inner part.
Rectangular pocket and stepover patterns on aluminum plate component
This image is zoomed into the pocket of a larger aluminum plate product. It showcases a rectangular tool path created using an endmill. The successive stepover passes and corner transitions are clearly visible on the surface, but the surface is still smooth to the touch.
Face-Milled Aluminum Surface
This is not a completely finished product - it has just finished one of the machining steps - but illustrates a face-milled surface. A 20 mm diameter face mill was used to create this smooth surface finish.
Comparing face-milled vs. pocket surfaces on hard-to-machine hastelloy component
The surface surrounding the pockets and the flat surface on the opposite side have been face milled. While you can still see some lines on the face milled surface, they are much finer than the endmill lines seen inside the pocket.
Planar milling vs side milling using an endmill on SS400 steel component
You may notice that the lines on the top surface are more pronounced than those on the sides. Both surfaces were machined using an endmill, but the top surface was machined with planar passes while the outer contours were done via side milling, which produces surfaces that are smoother with less visible lines.
Lathe machining vs. milling surface finish on turned aluminum component
This is an example of an aluminum component machined using a lathe machine (turning process). Compared to milling, lathe machining produces smoother surfaces with less visible machining lines.
Our sales representatives can recommend the most suitable surface finish for your CNC machined components. We work closely with our engineers to ensure appropriate tool and toolpath selection to achieve the required surface finish while ensuring cost-effective production. Contact us here for quotations or specific questions about our CNC machining service in Vietnam.
You can also explore other articles in our Surface Finish Guide series:
1. As-Received Surface Finish Explained – Learn when to retain the raw material surface and which materials are suitable.
2. Face Milling Finish – Learn how face milling is applied to material blocks before CNC machining.
