Why Concentricity Matters for CNC Machined Parts

In manufacturing, every single component contributes to the functionality of the end product. If one sheave is slightly out of place, the resulting vibrations could cause a chain reaction that compromises the integrity of an entire assembly. At Arbiser Machine, we won’t stand for imperfect parts coming out of our Georgia machine shop. 

Making parts that work cohesively and intentionally in their final application is our top priority. It’s one of the major reasons we’re fanatical about concentricity—even when many other shops try to avoid this notoriously difficult measurement. 

What Is Concentricity? 

Three indicators showing the runout of a piece of raw bar stock and two diameters that area concentric within .001

Concentricity is a complex tolerance that refers to the median points of spherical or cylindrical parts. When a part is concentric, it indicates that the tube wall thickness between the outside diameter (OD) and the inside diameter (ID) is consistent.

Concentricity ensures that a part fits precisely in its intended application—and that it functions smoothly without adding friction or vibratory movement. 

Concentricity vs. Total Runout

Concentricity is often captured using a measurement known as total runout—but the two aren’t exactly the same. 

Both concentricity and total runout are challenging to calculate and focus on axial alignment. Concentricity is measured by plotting median points in a reference circle around the axis, whereas total runout is measured by fixing a datum and rotating the part around it to ensure all points fall within the tolerance zone. 

Concentricity is more complex to measure than total runout, and more often than not, engineers don’t call it out on their drawings. 

Arbiser Machine’s Approach to Concentricity

Even when we don’t see concentricity called out on the part drawings we receive, we’re committed to machining concentric parts every single time. 

Yes, making a part concentric can require extra steps, but the way we see it, it’s always better to make a part correctly the first time than to have it fail in its application. 

At our machine shop in Georgia, the first way we ensure concentricity is by using high quality materials. A high-quality piece of bar stock that has been turned, ground, and polished is more likely to be perfectly circular to begin with, whereas lower quality bar stock often has more of an oval shape. Machining from a perfect circle is always preferable because when we perform work on either end, we’re more likely to have a concentric outcome. We can still achieve concentricity with standard bar stock; it just requires some additional effort.

Some machine shops are content to jam a piece of bar stock into a lathe a few times, turn it, and call it a day. But not us. We create fixtures as needed to hold parts perfectly in place. That additional step helps us ensure concentricity every time. 

Want to work with a shop that goes above and beyond for your CNC machined parts? Let’s work together. Request a quote today!