In the field of precise measurement, concentricity inspection is a crucial step for ensuring the assembly accuracy
and operational stability of parts. Many engineering technicians have doubts about whether dial indicators can be used
for concentricity inspection. From a professional perspective, dial indicators can indeed be used for concentricity
inspection, but they require the use of specialized fixtures and the correct measurement method to obtain accurate
results. Generally, when combined with a measuring frame, dial indicators can detect concentricity errors ranging
from 0.01 to 0.02 mm. For more demanding precision requirements, additional auxiliary equipment needs to be
combined to enhance measurement accuracy.
I. The basic principle of using a micrometer to measure concentricity
1. The essence of concentricity measurement is to detect the radial offset of the axis of the measured cylindrical
surface relative to the reference axis. The dial indicator indirectly reflects the concentricity condition by
measuring the radial runout. When the workpiece rotates around the reference axis for one full turn,
the maximum swing amplitude of the micrometer pointer is the radial runout value. Theoretically, half of the radial
runout value is approximately equal to the concentricity error, but this method has certain measurement errors.
2. The measurement accuracy of the micrometer directly affects the results of the concentricity detection.
The graduation value of the commonly used mechanical micrometer is 0.01mm, while the digital indicator can reach
an accuracy of 0.001mm. For parts with a concentricity requirement of within 0.05mm, an indicator with a 0.01mm
accuracy can basically meet the measurement needs. However, for more precisely machined parts with stricter
concentricity requirements, it is recommended to use a measuring device with a higher accuracy.
3. The determination of the reference axis during the measurement process is a crucial step.
Usually, the main mating surface of the part or the design reference is taken as the measurement benchmark,
and the reference axis is established using a three-jaw chuck or a special fixture. The accuracy of establishing
the reference axis directly affects the final measurement result. If the reference deviation exceeds 0.005mm,
it will significantly affect the measurement accuracy.
II. Specific Method for Measuring Centrifugal Degree with Micrometer
1. The V-block measurement method is suitable for detecting the concentricity of the outer circle.
Place the workpiece to be tested on a precision V-block. The angle of the V-block is usually 90° or 120°,
and the accuracy grade should reach 0 grade or 1 grade. Install the micrometer on the measuring frame, with the
measuring head vertically touching the cylindrical surface of the workpiece. Rotate the workpiece manually for one
full turn and record the maximum and minimum readings of the micrometer. The concentricity error is approximately
half of the difference between the readings, but the geometric error of the V-block needs to be taken into account.
2. The top-notch measurement method is commonly used for the concentricity inspection of shaft-type parts.
The workpiece is installed between the spindle of a lathe or the top of a dedicated measuring device,
ensuring that the top hole aligns with the reference axis. The dial indicator probe contacts the cylindrical surface
to be measured, and the motor or manual rotation of the workpiece is performed to observe the swing range
of the dial indicator pointer. The advantage of this method is that the reference axis is established relatively accurately,
but it requires that both ends of the workpiece must have standard top holes.
3. The chuck clamping method is suitable for hole-type parts or short shaft-type parts. Use a precision three-jaw chuck
or four-jaw chuck to hold the reference part of the workpiece. The radial runout of the chuck should be controlled within
0.005mm. The dial indicator is fixed on the machine tool spindle or a dedicated bracket. During measurement,
slowly rotate the spindle and record the changes in the dial indicator reading. It is important to note that the clamping
force of the chuck should be moderate to avoid deformation of the workpiece and affect the measurement results.
III. Key Factors Affecting Measurement Accuracy
1. The installation posture of the micrometer has a significant impact on the measurement results. The measuring tip
of the indicator should be perpendicular to the surface being measured. Any deviation angle exceeding 5° will result
in a noticeable cosine error. The contact pressure of the measuring tip should be controlled within the range of 1-3N.
Excessive pressure will cause the workpiece to deform, while insufficient pressure will affect the measurement stability.
The spherical radius of the measuring tip is generally selected to be 2-3mm. For small-diameter parts,
a smaller measuring tip can be chosen.
2. Changes in environmental temperature can affect the measurement accuracy. The standard measurement temperature
is 20℃. A 1℃ change in temperature will cause the size of steel parts to change by approximately 0.01mm/m.
When measuring in a workshop environment, it is necessary to wait until the temperature of the workpiece and the
measuring equipment is stable before conducting the measurement, or temperature compensation measures should
be adopted to correct the measurement results.
3. The surface quality of the workpiece also affects the measurement accuracy. The roughness of the measured surface
should be controlled within Ra 1.6 μm, and the surface waviness should not exceed 10% of the concentricity tolerance.
If there are burrs, scratches or oil stains on the surface, it will cause the reading of the micrometer to be unstable.
Appropriate surface treatment must be carried out before measurement.
IV. Precautions during the measurement process
1. The preparatory work before measurement is of utmost importance. Firstly, check the indication error and return error
of the micrometer to ensure they are within the allowable range. The standard requires that the indication error should
not exceed ±0.01mm and the return error should not exceed 0.005mm. The pre-compression amount of the
indicator should be set at 1/3 - 1/2 of the measurement range, which not only ensures the measurement sensitivity
but also avoids damage due to exceeding the measurement range.
2. The operational norms during the measurement process directly affect the reliability of the results. The rotational speed
of the workpiece should be slow and uniform, generally controlled at 5-10 r/min. If it is too fast, centrifugal force will
affect the measurement. Each measurement section should be measured at least three times, and the average
value is taken as the final result. For long-axis parts, measurements should be conducted at multiple sections to
comprehensively assess the concentricity condition.
3. Data recording and processing must be standardized. Establish measurement record forms to meticulously document
measurement conditions, environmental parameters, and measurement data. When concentricity errors approach
the tolerance limit, it is recommended to increase the number of measurements or adopt a more precise measurement
method for verification. The assessment of measurement uncertainty is also an important requirement of the quality
management system.
