The surface finish of your part should be specified on every drawing you submit to a machine shop. It can determine how well your part works and how expensive it is to make. In this quick guide, we’ll explain everything you need to know about surface finishes in manufacturing.
What Is a Surface Finish?
The surface finish of a part refers to how rough the part is. It has to do with irregularities on the face due to machining, fabrication processes, exposure to the elements, handling, and stock material characteristics.
When a CNC milling machine cuts a piece of metal, you’re left with a series of tiny ridges and teeth marks on the material. This is the side-effect of machining with bits that have cutting teeth on them.
It’s impossible to create a perfectly smooth part after using a machine to cut or make the part. However, a machinist can do finishing steps after fabrication to smooth out the part.
The surface finish will determine how smooth the part has to be before the machinist can ship it to the customer.
If there’s no surface roughness spec, then the customer says the part is okay as-is from the machine. With a very small surface roughness spec, then the machinist will have to polish the part after.
Why Surface Finish in Manufacturing Is Important
Even though you might not be able to see the surface roughness of a part, it can have a dramatic impact on the part. In certain chemical or cleanroom environments, any level of surface finish can make the part unusable.
When it comes to mating parts, the surface finish will determine how easy the parts are to install, and what kind of gap is left. For leak-tight parts, you need a very small surface finish in order to get a good seal.
Another benefit of smooth surface finishes revolves around aesthetics. Parts with very low surface roughnesses can look almost mirror-like. They will be perfectly smooth to the touch and look highly professional.
With high surface roughness, the part will look as if it were corroded — there will be large bumps all over the surface.
Another reason why surface finish matters has to do with paints and coatings. Getting the right surface roughness will ensure the coating applies evenly and the paint sticks for longer.
The bottom line? Surface finish comes up a lot when it comes to engineering.
How Surface Roughness Gets Measured
We mentioned that you often can’t see the roughness of a part without a microscope. Surface roughness specs are usually given in micrometers, or 0.00004” — that’s tiny. So, how do machinists measure surface roughness to make sure they hit the spec?
We have a few options that we can use. The most common option is to use a stylus and run it against the face of your part. The stylus will go up and down along the peaks and valleys of the surface. These features are the surface roughness.
As the stylus moves, it calculates the average roughness based on how much it moved up and down along the length of travel. This results in a surface roughness value that we can then compare to your spec.
Another option is to use instruments that use sound or light. These measuring instruments will bounce either soundwaves or light waves against the surface of your part and then measure the exact distance. It creates a profile of the part, determines the total roughness, then boils it down to an average roughness value.
How to Pick the Right Surface Finish
The surface finish you spec will be given in terms of microinches (one millionth of an inch).
A 1000 microinch rating is a rough, low-grade surface. This is used when you don’t care about the surface finish. It can be the result of forging or saw cutting without any finishing steps.
A 250 microinch rating is the result of milling, grinding, or drilling. It means that you don’t want any finishing done after the machining process is completed.
A 63 microinch rating requires one of two things. Either the shop will have to polish and clean up the part after machining, or they’ll have to use slow feeds and high speeds during the fabrication process. In either case, this is one of the most popular options that we see in our shop. It should be used when roughness needs to be minimized while optimizing the cost of the part (since you don’t have to overpay for this surface finish).
A 16 microinch rating is a high-quality surface finish. It can only be done by buffing or honing the part after fabrication. This part will be smooth to the touch and tells the machinist that you care about the surface roughness a lot.
A 1 microinch rating is the most extreme version of surface finish. It is the most refined surface finish that requires heavy-duty buffing and superfinishing (not offered by every shop). You might see this surface finish on gauge blocks that precisely measure something. It’s a surface finish reserved for highly sensitive, high-precision surfaces.
The surface finish of your part can make or break your project. With the right surface finish, you’ll get great results. It can determine how reactive your part is, how well it fits, and the overall precision of each measurement. At Rapid Axis, we can measure the surface finish and hit your spec. We are a full-service machine shop that has a number of finishing steps to help you get the best results. To get started today, get a quote from us.