What Is Concentricity & How To Measure It

Concentricity is the measure of how true a geometric shape is to its ideal form. The concentricity value can be calculated by using two diameters — one for the hole and one for the shaft that goes through it.

Let’s look at what concentricity means in CNC machining as well as how it’s measured in both imperial units (inches) and metric units (mm).

The Need to Measure Concentricity

Concentricity provides assurance that no part of a manufacturing cycle will land outside manufacturing tolerances while also guaranteeing perfect spacing inside those limits.

As such, concentricity ensures quality and precision during machining and after production when finished parts fit into their intended slots without issue. It’s a vital part of many manufacturing processes, including CNC machining.

Concentricity measurement is the amount of variation from perfect symmetry that may occur when pushing a workpiece through a machine or vice versa.

This deviation can result in waste material, increased costs, and variations in quality for parts coming out on the other end. Concentricity will be measured either axially or radially to determine how much this error occurs along these specific dimensions. However, the measurement process is complicated and, therefore, only gets used in certain circumstances.

When Is Concentricity Used?

In general, concentricity is only used for parts that demand a significant degree of precision to ensure proper functioning. When determining if concentricity is required, the primary question lies in the end-use of the product.

For example, if a tube is required to fit into an opening and a second tube is required to fit within the first, the concentricity is critical to ensure a proper fit and working end product.

On the other hand, if a liquid or a gas is the only thing that will fill the tube, then concentricity is not required as the liquid or gas can conform perfectly to the inside of the tube regardless of any slight deviations.

However, this is not to say that concentricity isn’t crucial for parts that don’t require extremely tight tolerances. It can be beneficial to know how far out of tolerance a part may be, as parts that aren’t machined to tolerance could lead to waste material, increased costs, and variations in part quality.

Other parts will still require a minimum wall thickness to ensure the safe and proper flow of liquids and gasses. If a wall is too thin, high-pressure flow can cause a break or crack in a thin spot, leading to significant issues including improper flow, lost material, dangerous working conditions, and even loss of life.

Concentricity Symbol And Its Interpretation

As shown in Figure 1, this is how concentricity is shown in a drawing. As you can see, the left side shaft diameter has a concentricity tolerance of .030 with respect to datum A. What that means is that the axis of the measuring shaft can have a circular tolerance zone of .030

Concentricity Symbol
Figure 1: Concentricity Symbol

Below figure shows what it means if we interpret the concentricity tolerance that is shown in figure 1.

Concentricity Tolerance Explanation
Figure 2: Concentricity Tolerance Explanation

Position and runout tolerance can be used instead of concentricity tolerance as measuring concentricity tolerance accurately is very difficult and may cost more. That is why concentricity tolerance is generally used for very critical parts. If concentricity tolerance is not met, you will notice wobbling.

How To Measure Concentricity Tolerance?

The concentricity value for a hole or shaft diameter is calculated using two diameters: one for the hole (outer boundary) and one for the shaft (inner line). The larger diameter will represent the outer boundary, while the smaller diameter will represent an inner line to get an accurate reading on surface deviation. Measuring with imperial units gives summarized measurements in inches, where metric measuring has its results displayed in millimeters.

There are several ways to calculate concentricity, but only three methods matter when considering CNC machined parts:

  • Radial error
  • Axial error
  • Overall accuracy (AX+RA)

These values can either be found pre-calculated or must be measured empirically. All measurements should always come from machine centerlines.

Radial error is the difference in measurement between the center of the feature on one side and that same point on the other.

Axial error is measured by subtracting distance from machine zero to a datum line, then measuring deviation from this line at two points along its length.

Overall accuracy can be calculated with radial and axial errors added together, or it can come pre-calculated as some machines are equipped for complete concentricity verification.

After the concentricity is calculated, it should be checked for quality by comparing it with what was specified in your manufacturing design and inspection procedures.

For example, if you have a specification of ±0.00025 inches (or 0.06 mm), then at least two measurements are required on each side to check that the diameter difference does not exceed this limit. Alternatively, one measurement can be used provided there has been no significant wear because of usage since manufacture.

If the concentricity exceeds the manufacturing limits then further corrective measures would need to be taken as needed, such as:

  • Adjusting keyways so they align better for accurate machining
  • Replacement of bearings which may require re-machining with updated tolerances

Here is a general of how to measure the concentricity of a typical shaft. Industries use CMM to scan data points instead of any mechanical method that was used before to get more accurate results.

how to measure concentricity
Schematics of Concentricity Measurement

We can follow the below steps in order to measure the concentricity of the shaft.

  1. Lock all Degrees of Freedom of the part using fixtures except the circular motion.
  2. Plot control surface outer profile using a CMM ( Preferred)
  3. Determine the center point of the plotted surface at various cross-sections
  4. Verify if all those center points fall under the specified tolerance zone or not.

All center points should fall under the tolerance zone for qualifying the part as specified on the drawing.

Challenges Of Measuring Concentricity

One way to measure concentricity is using a dial indicator. The measurement process must be done in both directions, with an offset on one side and perpendicular to the bore axis of 0.00025 inches.

The challenge with this method, however, lies in having enough room. You need at least 18 inches (460 mm) from the spindle tip for measuring concentricity. There’s also the concern of not accidentally deflecting or breaking any rotating components such as gears when pushing against them during the measuring process.

Another challenging aspect of this procedure is that it can be successfully achieved only if there has been no significant wear since manufacture because of usage, requiring re-machining with updated tolerances.

Tubing With Accurate Tolerances

Manufacturing tubing with accurate tolerances is critical for the sustainability of your machining operations and the reliability of the end product you produce. Though slightly complicated, measuring concentricity is one method for ensuring an accurate end product for your customers on products with especially tight tolerances.

Conclusion: Concentricity Tolerance

It’s a fact that the metrology team always tries to avoid measuring concentricity as it is challenging to measure and get an accurate result. Better to switch to position or run-out tolerance. If a part is perfectly round, the runout will be equal to the concentricity.

I hope this article helped you to learn the basics of concentricity tolerance. If you still have any questions, please write in the comment section, and I will try to help you out.

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This is a guest post By Christine Evans From the Fictiv team


Guest-Author_1
Christine Evans

Christine Evans is the Director of Product Marketing & Content Strategy at Fictiv, an on-demand manufacturing company. Over the past six years, Christine has grown Fictiv’s popular Hardware Guide and Digital Manufacturing Resource Center, with over 2,000 teardowns, DFM guides, and mechanical design articles to help democratize access to manufacturing and hardware design knowledge.

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