Concentricity (concentricity) is the tolerance that controls the central axis of a reference feature to the datum axis. The datum axis and the axis of the reference feature are derived from the midpoint of the part or feature. Concentricity is a very complex characteristic because it relies on measurements taken from derived axes rather than tangible surfaces or features.
Coaxiality is the positioning tolerance, and the theoretically correct position is the reference axis. Since the difference between the measured axis and the reference axis may appear in all directions in space, the tolerance zone is a cylinder with the reference axis as the axis, the tolerance value is the diameter of the cylinder, and the total sum before the tolerance value is Note the symbol "Φ".
The coaxiality label indicates the coaxiality of the central cylinder axis relative to the common reference axis at both ends.
The workpiece drawing marked with public datum, this kind of public datum marking, must be used for workpieces with center holes at both ends, the marked journals are equal in size, and bearings need to be installed. The processing of this kind of parts needs to be clamped one by one when turning, and the two ends are clamped by the top when grinding. The processing method is sufficient to ensure the coaxiality of the marked journal, so the marking is no problem, so this public datum marking method is not suitable for all shaft parts, as described above, one end is turned over and the other end is turned. Definitely can't use such a labeling method. If the tolerance can be marked, it should be able to be checked by the simplest and most reliable method.
Concentricity mark
Marking requirements:
(1) The coaxiality tolerance value of the inner hole axis of Φ128 relative to the inner hole axis of Φ85 is Φ0.015.
(2) The radial circular runout tolerance of the outer cylindrical surface of Φ160 relative to the axis of the inner hole of Φ85 is 0.03.
(3) The perpendicularity error of the right step surface relative to the outer circle axis of Φ160 is 0.03.
1. Determine the content of the geometric tolerance code
(1) Determine the number of frames required
In general, positional tolerances require 3 sashes, while form tolerances require 2 sashes.
(2) Determine the feature symbol of the geometric tolerance item
(3) Determine the tolerance value. If there is Φ in front of the tolerance value, it must not be omitted.
(4) Determine the datum symbol (if it is a shape tolerance, there is no such item)
When the datum feature is a prime line or surface, the datum symbol should be marked close to the contour line of the feature or its lead line, and should be clearly staggered from the dimension line.
When the datum feature is an axis or center plane, the datum symbol should be aligned with the dimension line.
1. Determine the position of the leader arrow
When the element to be measured is a line or a surface, the arrow of the leading line should point to the contour line of the element or its extension line, and should be clearly staggered from the dimension line.
When the element to be measured is an axis or center plane, the arrow of the leader line should be aligned with the dimension line of the element.
How to measure coaxiality
The coaxiality measurement must be a revolving part, such as a bolt hole and a countersunk hole on a base. Since the base is not a revolving part, the coaxiality cannot be applied to the bolt holes and countersinks on it.
1. Two benchmark types of coaxiality
(1) Designated benchmarks
Take the axis line of a given cylindrical surface on the part as the reference, as shown in the values of pA to B and pB to A.
(2) The public axis line as the reference
There are two holes A and B on the part. When measuring the coaxiality error, neither hole A nor hole B is used as the reference, but the common axis line of the two holes A and B is used as the reference. The coaxiality errors of the two holes A and B to the common axis are pB and pA respectively.
2. Measurement of coaxiality
(1) Measurement of the coaxiality error of the specified datum
Measure the concentricity of hole B to hole A with the axis of hole A as the reference.
Measurements must be taken both horizontally and vertically.
(2) Measurement of the coaxiality error based on the common axis line
Measure the coaxiality error of the axis lines of A and B holes to the common axis line.
When measuring, first fix the measured part on the platform, and measure the full length of the measured axis of the two holes A and B respectively. The maximum reading difference between the measured axis line and the common axis line is the coaxiality error.
How to mark the coaxiality datum
1. Coaxiality tolerance requirements for a single datum element
It means that the reference element is a single axis element, and the intermediate gear seat is used to support the intermediate gear. In order to ensure the correct meshing between the intermediate gear installed on it and the relevant gear of the gear train, the installation position must be correct. The φ30mm cylindrical surface on the part is used as the installation positioning surface, and the φ25mm cylindrical surface is the center surface of the intermediate gear rotation, so the distance between the two axes must be kept
Only when it is coaxial can its correct meshing position be guaranteed, so the coaxiality tolerance is given.
When marking, first mark out the datum symbol in alignment with the dimension line of the datum element to determine the datum axis A, and mark out the corresponding datum letter code in the tolerance box, and add "φ" before the tolerance value, and then put the box The grid guide line arrow is aligned with the measured element dimension line φ25mm, and the measured axis is indicated. The coaxiality tolerance of the φ25mm cylindrical axis to the φ30mm cylindrical axis is 0.03mm, and the tolerance zone is centered on the φ30mm axis, and the tolerance value is 0.03 mm is the area defined by the cylindrical surface of the diameter.
2. Coaxiality tolerance requirements with auxiliary datum
It means that when the length of the reference axis is very small and it is difficult to determine its axial orientation, the second reference is used as an auxiliary reference to determine the orientation of the reference axis. The functional requirements of the shaft degree of the intermediate gear, but its structure is different. The main difference is that the length of the φ30mm positioning cylinder is very small, which can only play the role of positioning the center point, and the axial direction is difficult to determine. In actual application, it is fixed by the middle flange and positioned on the left side of the flange. Therefore, the direction of the positioning journal is determined by the positioning surface. For this reason, when the coaxiality tolerance is given, the positioning surface can be selected as the second datum to determine the orientation of the datum axis.
When labeling, only the second reference element y is added. The axis of the φ30mm cylindrical surface is used as the center, and the line perpendicular to the reference plane is used as the reference axis. The concentricity tolerance of the φ25mm axis to the axis is 0.03mm. The tolerance zone is the area defined by a cylindrical surface with a diameter of 0.03mm and a straight line passing through the center of φ30mm and perpendicular to the surface as the axis.
3. Coaxiality tolerance requirements based on the common axis
It refers to taking the common axis of two or more axis elements as the datum element. When the transmission shaft is working, the φ25mm journals at both ends are supported on the box bearings, and the transmission part installed on the middle φ30mm rotates around it. The center of rotation should be the common axis formed by the axes at both ends. In order to ensure the normal operation of the transmission parts and prevent vibration due to eccentricity, the coaxiality tolerance is given.
When marking, the two datum elements of A and B should be marked first, and the common datum letter code "A-B" should be marked in the tolerance grid, and "φ" should be added before the tolerance value, and then the grid leader line arrow Aligns to the dimension line of the feature being measured. Requirements indicate: the concentricity tolerance of the axis of the φ30mm cylinder to the common reference axis of the two cylinders A and B is 0.02mm. The tolerance zone is the area defined by a cylindrical surface whose diameter is a tolerance value of 0.02mm and whose axis is coaxial with the "A-B" common reference axis.
Contact Person: Mr. Frank Li
Tel: 86-15989457573