The 9 Most Popular Finishes for Machined Parts

Intro

After your precision machined part is fabricated, it can go through a “finishing” process. This adds certain functionality to your part. There are a lot of finishing options to pick from, but the following 9 are the most popular for machined parts. Follow along and see which is right for you.

Black Oxide

A black oxide finish is really popular for off-the-shelf fasteners. If you’re holding a black bolt, it doesn’t mean the original material used was black. Black oxide is a type of conversion coating. You pass through a standard metal like steel, and it turns the surface black. 

This also adds a layer of corrosion resistance and adds a little bit of hardness. A lot of people will black oxide treat a part for appearances and to minimize the reflection of the metal used.

Bead Blast

After machining, it’s typical for a part to have tool marks. If you want a smooth and uniform surface on your part, you can opt for bead blasting. This is a piece of equipment that uses pressurized air and tiny beads of sand, glass, or grit.

The beads remove tool marks and create a satin-like finish on your part. Think of it like a high-tech version of sanding a part.

Electroplating

Electroplating is when you coat a piece with metal. It uses electrical current, which explains the name, and can be done on a number of different materials.

The part is dipped in a tank of chemicals, and electricity forces the metal to stick to the part.

It offers a number of benefits: it adds corrosion resistance, lubricity, abrasion resistance, reflectivity, electrical conductivity, improves the surface finish, and makes for a better appearance. 

Electroless Plating

If you swap out the electrical current in the previous option with a catalyst instead, you get electroless plating. It adds all the same benefits as electroplating, but it creates a much more even layer of metal and removes the electrical conductivity. 

A common example is electroless nickel plating with or without boron. 

Knurling

If you look at an industrial metal handle or knob, there’s a good chance that you’ll find a knurled surface. Knurling is the act of pressing indentations onto a surface. It adds friction so a user can grip the part better.

Passivating

When you passivate a part, you’re changing the surface chemistry of the part. The name is derived from the fact that the part is more passive after this treatment. That means that the environment will affect it less through corrosion or electrical conductance.

The passivation treatment adds a sacrificial layer to your part. It can be done through anodizing the part or adding a chromate conversion. 

Thread Rolling

In a previous blog post, we talked about threading a part using a CNC machine. Well, thread rolling takes the machine out of the equation.

In this finishing method, your blank shaft will get pressed into a die. The die is moved along the shaft and subsequently presses threads into the shaft.

You’re left with a threaded part. If you inspect a thread-rolled part and one that is threaded on a lathe, it’s almost impossible to tell the difference. The only hint will be the very base of the threads, but it takes a skilled eye to know what to look for.

Burnishing

Burnishing is used to create a mirror-like finish. This is achieved by rolling a coated ball across your part repeatedly to shape it to the correct size.

Doing so will change the size, hardness, and finish of your part. On a microscopic scale, this is removing the peaks and valleys in your part. By removing these defects, you’re left with a mirrored surface on your part. It also makes your part less porous.

Burnishing will reduce any visual defects of your part while increasing your parts longevity. It also improves a part’s corrosion resistance, but can’t corrosion-proof a part like carbon steel.

Case Hardening

Case hardening is the act of essentially adding a hardened shell to the outside of your part. The interior is unchanged, so it can still absorb the impact of a stress. The exterior, however, is coated with a hard surface that has high wear resistance. 

A common example is flame hardening. This changes the functionality of the exterior without changing the inside of your part. It’s done by heating up the raw material then rapidly cooling it.

Conclusion

We just reviewed 9 of the most popular finishes for machined parts. If you want a better idea of which one would work for you, reach out to our experts at Rapid Axis. Our fully equipped machine shop can handle the fabrication and finishing of your precision parts.

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