Surface Roughness Guide For Precision Machined Parts

Any time metal parts are machined, there is a level of roughness on the part surface. The level of roughness varies depending on the manufacturing process used, part design, material, and secondary operations.

Your first thought might be that the part surface finishing isn’t all that important.

That’s not the case at all. The surface finish impacts the part’s corrosion resistance, friction and durability, adhesion of coatings and paints, and the visual appeal.

A smooth finish has benefits like less friction, and less chance of surface irregularities. On the other hand, a slightly rougher finish makes it easier to apply paint or coating to the part.

The surface finish or roughness of a part is measured in Micrometers, which are 1/25400 inches. These very small differences in roughness can have a significant impact on your parts, so it’s important to understand.

How is surface roughness measured?

Because the differences in surface finish can be so small, it’s difficult to measure accurately. You might be able to feel the difference between a smooth and rough finish, but there’s a lot in between.

It’s similar to the different sandpaper grits. You can easily tell the difference between 30 grit sandpaper and 2,000 grit. But the difference between 80 grit and 120 grit is impossible to know by looking at it or feeling it.

The surface finish of a part is usually measured by the roughness, waviness, and lay of the surface.

There are different tools and processes that help get an accurate measurement.

The first tool is called a CNC probe. This is a tool installed inside the CNC machine. The probe helps align the raw material, cutting tools, and can also measure the surface roughness. This eliminates the need to measure the surface roughness after the part is machined.

Another tool used is a handheld surface roughness tester. This has a sensor that measures the surface roughness. The device isn’t the most accurate measuring method, but it’s easy to use anywhere.

There are other ways to measure roughness by using light or sound. The light is shone on the part, and the reflection is used to measure peaks and valleys on the surface.

Why is surface roughness important?

The surface of the part is just as important as the material that’s used for the part. Just like you need to understand the differences between aluminum, stainless steel, or copper, you also need to understand the various surface finishes.

The surface finish of the part impacts the effectiveness of the part and how it works with the other components it touches.

Here are a few reasons why the right surface finish is essential:

The surface roughness impacts corrosion resistance. A rougher finish offers less corrosion resistance because there is more surface area that the chemicals or corrosive material touches.
The adhesion of paint changes. If you need to apply paint or primer to the part, it will stick better to a rougher surface.
The wear resistance is affected. A smooth surface reduces friction and helps moving parts last longer.
The surface roughness changes the conductivity of the parts. A smooth surface offers significantly better conductivity. This study (https://homepages.rpi.edu/~galld/publications/PDF-files/Gall-66.pdf) compared a perfectly flat surface to a rough surface. It found that adding one atom on top of the flat surface to simulate roughness decreased conductivity by 33%.
The surface finish impacts the chances of surface defects. A rougher surface increases the chances of surface defects developing. When a part surface is rougher, there are more tiny indents on the part. Over time, these can become more prominent.

How the manufacturing process impacts surface finish

There are many different ways to manufacture parts. Each manufacturing process is designed to produce a usable surface finish. Each process has an expected surface finish tolerance. For example, broaching offers an expected surface roughness of 0.8-3.2μm. In some cases, a secondary process like grinding or polishing is needed to reduce surface roughness.

In metal forming, when a metal is poured into a die or mold, the surface of the die determines the surface of the part.

When the metal is cut, like in a CNC machine or lathe, the surface finish is determined by the hardness of the metal, the cutting speed, and the cutting tools. A harder metal needs to be cut with sharper tooling, and that needs to be changed out more often.

Usually a smoother surface finish means higher manufacturing costs. But, in cases where a smooth finish is a priority, it means the part will last longer because of reduced friction.

Here are some of the expected surface roughness levels for different manufacturing processes:

Surface Roughness Chart

You might be wondering what surface finish you need for your part. Each project is unique, but this chart can help you decide.

Micrometers Rating (μm)	Microinches Rating (μin)	Applications
25	1,000	This is a low-grade surface from saw cutting or rough forging that shouldn't be used for parts touching other moving componenets.
12.5	500	These surfaces are still considered to have a high level of roughness, resulting from heavy cuts and coarse feeds. These cuts tend to be the result of industrial processes like disc grinding, turning, and milling.
6.3	250	This is a surface finish that occurs from drilling, milling, and disc grinds. This roughness should be used for clearance surfaces that have special design or stress requirements.
3.2	125	The is usually the roughest surface recommended for precision parts. It's also used for parts subject to vibrations, heavy loads, and high stress.
1.6	63	This is a good machine finish when production is done under controlled conditions. It also involves fine feeds and relatively high cutting speeds.
0.8	32	This is a high-grade machine finish, which requires close control. It's relatively easy to achieve with a cylindrical or surface grinder. It's used for products that don't require continuous motion or heavy loads.
0.4	16	This is a high-quality surface produced using buffing, lapping, or coarse honing. This finish is a great option when smoothness is more important.
0.2	8	This is a fine, high-quality surface finish produced by lapping, buffing, or honing. Machinists use this when components need to slide across the part surface grain.
0.1	4	A refined surface that manufacturers use only if there are mandatory design requirements. This is the best finish in gauge and instrument parts.
0.025-0.05	1-2	This is the most refined surface finish produced with the finest buffing, honing, or superfinishing. This is best used for sensitive precision gauge blocks.

Spex is an ISO 9001:2015 certified precision machine shop in Rochester, NY. We machine thousands of unique metal and polycarbonate parts every month for different industries around the world. Reach out to our team to see if we can help with your next project.