I. What is Concentricity?

Concentricity is a critical aspect of CNC turning that determines the quality and precision of machined parts. It refers to the alignment of the central axis of a cylindrical or spherical part with its geometric center. Achieving perfect concentricity ensures that parts function correctly and meet design specifications. This article will explore measuring and ensuring correct concentricity tolerance.

Concentricity Definition

Concentricity is the measure of how well the central axis of a cylindrical or spherical part coincides with its theoretical central axis. A concentricity symbol and tolerance zone typically represent it in engineering drawings. The tolerance zone defines the acceptable deviation of the measured axis from the theoretical axis. The smaller the tolerance zone, the higher the precision required in manufacturing the part.

What is Concentricity VS Eccentricity?

Eccentricity refers to the degree to which an object’s center deviates from its true center, while concentricity refers to the quality of having centers that coincide. It is like comparing a wobbly tire and a well-aligned one.

Concentricity vs. Total Runout

Concentricity is often confused with total runout, but they’re not the same. Total runout measures the circular runout of a part’s central axis. It determines the amount of wobbling or vibration of the part during rotation. It’s represented by two concentric circles and a tolerance zone. While total runout is essential, achieving perfect concentricity is even more critical in ensuring that the part functions correctly.

II. Importance of Concentricity

Applications of Concentricity

Here are some examples of CNC machined parts where concentricity plays an important role:

1. Bearings: Bearings require precise concentricity to ensure smooth rotation and reduced friction. The inner and outer races of a bearing must be concentric with each other to maintain an accurate gap for the balls or rollers to rotate freely.

2. Shafts: Shafts need to be concentric to prevent vibration, wobbling, or bending. In a CNC machined shaft, the central axis must perfectly align with the rotational axis to ensure smooth rotation and power transfer.

3. Pistons: Pistons in internal combustion engines require precise concentricity to avoid vibration and to maintain a tight seal with the cylinder wall. The piston head and skirt must be concentric to prevent damage to the engine.

4. Gears: Gears must be precisely concentric with each other to ensure smooth and accurate transmission of power. In a CNC machined gear system, concentricity ensures that the teeth mesh properly and don’t cause excessive wear or noise.

5. Housings: Housings are often used to enclose other parts or components. The accommodation must be concentric on the parts inside it to ensure proper alignment and function. In a CNC machined housing, concentricity ensures that the interior diameter is consistent and accurately fits the components inside.

Why is Concentricity So Important?

Concentricity plays a crucial role in the performance and reliability of various parts and products. It ensures that the parts rotate smoothly without wobbling or shaking, minimizing friction, wear and tear, and premature failure.

For example, For turbine engine parts such as shafts, rotors, and bearings in the aerospace industry, any deviation from the specified tolerance can cause vibration, imbalance, and reduced efficiency, which can compromise the safety and performance of the aircraft.

Similarly, for automotive engine components such as pistons, crankshafts, and camshafts, if these parts are not perfectly round or centered, they can cause excessive wear, noise, and reduced engine performance, leading to costly repairs and recalls.

Concentricity is vital for precision medical instruments such as catheters, needles, and implants. Any deviation can affect the accuracy and reliability of the medical procedure, risking patient safety and health.

Measuring and controlling concentricity can improve the quality, performance, and lifespan of CNC machined parts and products, leading to customer satisfaction, cost savings, and competitive advantages in the market.

III. How to Ensure Correct Concentricity

In Production Control

To ensure concentricity, it is essential to use proper machining techniques and to inspect the part using appropriate tools. When programming the machine, we should consider the tubing wall thickness, maximum wall thickness, and varying part diameters and use the proper tooling to replace concentricity. An optical comparator or coordinate measuring machine can verify that the part meets the specified concentricity.

Here are some steps and considerations for achieving optimal concentricity

1. Design considerations: Proper design of the parts can ensure that the critical features, like the central axis, are well-defined and easy to measure. For example, specifying a single datum axis for all measurements related to the central axis can simplify the measurement process.

2. Inspect raw materials: Before machining begins, we must inspect the raw materials to ensure they meet the specifications for concentricity. This includes checking the diameter and roundness of the material.

3. Set up the machine correctly: This includes properly aligning the lathe, securely fastening the chuck, and selecting the correct cutting parameters. Also, the part should be securely held in the lathe with the right fixtures and clamping mechanisms to ensure it does not move during machining.

4. Check tooling condition: Check the condition of the cutting tool, insert, and holder to ensure they are sharp and in good condition.

5. Check runout: Runout measures how much the part deviates from its true center. Checking runout is a critical step in ensuring proper concentricity. Use a dial indicator to measure runout and adjust the lathe accordingly.

6. Monitor feed rates: Monitor feed rates during machining to ensure they are consistent and appropriate for the machined material.

5. Check finished dimensions: Once the part is finished, check the finished dimensions using a coordinate measuring machine or optical comparator to ensure that the part meets the required specifications. This involves placing the part in a fixture and measuring the deviation from the theoretical central axis.

Let’s consider a cylindrical part with a diameter of 50mm and a length of 100mm. The design requires a concentricity tolerance of 0.05mm between the part’s inside diameter (ID) and outside diameter (OD).

To ensure correct concentricity, the machinist uses a fixture to hold the part in place during machining. The fixture will position the part’s datum axis, a theoretical central axis, in a precise location relative to the cutting tool.

Then the machinist uses a dial indicator or CMM to measure the part’s concentricity. The measurement is taken by comparing the part’s axis, derived from the median points of the part’s cross sections, to the theoretical central axis.

Suppose the measurement shows that the part’s concentricity is not within the specified tolerance range; the machinist can adjust the cutting tool’s or fixture’s position to improve the concentricity.

By controlling the runout of the cutting tool and ensuring that the fixture positions the part’s datum axis correctly, the machinist can achieve the required concentricity of the part’s ID and OD, which ensures that the part will function properly in its intended application.

How to measure Concentricity

Preparations:

When measuring a part’s concentricity, it is important to ensure that preparations have been made beforehand. Here are some key steps to follow:

1. Clean the part to ensure debris and contaminants are free.

2. Secure the part using a fixture or clamp that does not interfere with the measurement process.

3. Verify that the measuring instrument, such as a CMM or dial gauge, is calibrated and functioning correctly.

4. Establish a datum axis or plane as a reference point for the measurement, either by aligning the part with a reference feature or using a separate datum element.

5. Determine the specified tolerance zone for the concentricity measurement based on design requirements or industry standards.

Measurement Tool

We have various measurement tools, like CMM, height gauges, hardness gauges, projectors, and dial gauges. Let’s see how these measurement tools work for concentricity.

1. CMM

CMM stands for Coordinate Measuring Machine, a device used for measuring the geometry of objects with high accuracy and precision.

Working Principle

Fix the part on the machine’s measurement stage. A probe precisely measures the part’s features at various points along the rotational axis. These measurements are then used to calculate the deviation of the part’s centerline from its ideal position, indicating its concentricity.

Limitation:

1. If the CNC-turned parts’ datum plane is too small, it may not provide enough support for the part, leading to inaccuracies in the measurement. In such cases, it may be necessary to use a more extensive datum feature or a different measuring technique altogether.

2. CMMs can be expensive and require specialized training to operate effectively. As such, they may not be a practical solution for small-scale manufacturing operations or those with limited budgets.

2. Optical Comparator- Projector

Optical comparators or projectors use a magnified image of the part to precisely measure features such as diameter, height, and position.

Work Principle:

The operator places the part on the stage, measures the diameter of the cylindrical feature at several points, calculates the mean diameter, and compares it to the theoretical diameter. The part is considered concentric if the deviation falls within the specified tolerance zone.

Limitation:

Optical comparators are accurate and can measure various part types. Still, they have limitations like a limited field of view and manual measurement, which can be time-consuming and prone to errors.

3. Dial Gauge

• 

Work Principle:

The gauge is placed against the part, and the operator can read the deviation from the theoretical center of the part.

Limitation：

Dial gauges are a popular and affordable choice for measuring the concentricity of CNC machined parts. Still, they have limitations, including lower accuracy, susceptibility to external factors, potential for human errors, and the inability to capture the complete picture of a part’s concentricity. They may not be suitable for high-precision applications.

4. Height Gauges

• 

Work Principle:
Set the gauge’s reference surface to be in contact with the reference surface of the workpiece, adjust the indicator to zero, and carefully move the indicator along the cylindrical surface to record the distance between the surface and the reference point.

Limitation:

Height gauges have operator error, material deformation, surface finish, gauge calibration, and limited range limitations. So, it’s important to consider these limitations and use other measuring instruments for more complex or precise measurements.

Different measuring instruments have different functions and limitations, and using the most suitable instrument for different parts is important. In STCNC, we use the proper measurement tool for all your parts.

Some Calculation Formulas for Concentricity:

1. Concentricity between two cylindrical surfaces:

Concentricity = (TIR) / D

TIR is the total indicator reading, the difference between the maximum and minimum distances between the two cylindrical surfaces, and D is the diameter of one of the cylindrical surfaces.

2. Concentricity between a cylindrical surface and a datum axis:

Concentricity = (TIR) / (2 * D)

where TIR is the total indicator reading, which is the difference between the maximum and minimum distances between the cylindrical surface and the datum axis, and D is the diameter of the cylindrical surface.

3. Concentricity as a radial runout:

Concentricity = (Rmax – Rmin) / (Rmax + Rmin)

Rmax (the maximum radius of the cylindrical surface),

Rmin(the minimum radius of the cylindrical surface)

Please note that these formulas approximate concentricity and may not be accurate in all cases. The accuracy of the measurement depends on the precision and accuracy of the measuring instrument used and the operator’s skill. Therefore, it’s essential to use appropriate measuring instruments and ensure that they are calibrated and operated correctly.

Difficulties in Measuring Concentricity

1. The varying diameters of cylindrical or spherical parts. This can lead to difficulties in finding the theoretical central axis and derived axis, as well as controlling runout and circular runout.

2. The wall thickness of the part can vary at different points along the length of the region. Measuring concentricity requires finding the perfect center points of the part, which can be difficult when the part is not perfectly round or has opposing elements that create an uneven surface.

3. Verifying concentricity requires careful attention to geometric and positional tolerances and ensuring that the measuring instrument, such as a coordinate measuring machine, dial gauge, or dial indicator, is calibrated and functioning correctly.

4. It is essential to establish the datum axis or plane, which serves as a reference point for the measurement. This can be challenging when the part has a varying wall thickness compared to the cylindrical tolerance zone or has two concentric circles with different diameters.

IV. Forecasting

New techniques for controlling concentricity in CNC turning are continually being developed to improve machining accuracy. These techniques include using advanced measuring instruments such as CMMs and optical comparators and integrating software programs to analyze data in real-time and adjust the machining process. Other techniques include using specialized tooling and fixtures and implementing advanced toolpath strategies and cutting parameters. They allow for more precise control of the machining process and can help reduce errors in concentricity, resulting in higher-quality parts.

STCNC uses both CMM, optical comparators(we called projectors), height gauges, and dial gauges to check concentricity. We are making the round parts with high precision based on our capacity. For special parts, we don’t have standard fixtures; we produce the fixtures to fix the parts for production and measuring. Welcome to contact us for communications.