1. Numerical relationship between shape tolerance and dimensional tolerance of CNC machined parts
When the dimensional tolerance accuracy of the CNC machined parts is determined, the shape tolerance has an appropriate value, that is, generally about 50% of the dimensional tolerance value is used as the shape tolerance value; about 20% of the dimensional tolerance value in the instrument industry is used as the shape tolerance value ;Heavy-duty industry takes about 70% dimensional tolerance value as shape tolerance value. This shows that. The higher the dimensional tolerance accuracy, the smaller the proportion of the shape tolerance in the dimensional tolerance. Therefore, when designing the size and shape tolerance requirements, except for special circumstances, when the dimensional accuracy of the CNC machined parts is determined, the 50% dimensional tolerance value is generally used as the shape Tolerance value, which is not only conducive to CNC manufacturing but also conducive to ensuring quality.
2. Numerical relationship between shape tolerance and position tolerance of CNC machined parts
There is also a certain relationship between the shape tolerance and position tolerance of CNC machining parts. From the perspective of the cause of the error, the shape error of the CNC machined parts is caused by machine tool vibration, tool vibration, spindle runout, etc.; while the position error is caused by the non-parallelism of the machine tool guide, the tool clamping is not parallel or perpendicular, and the clamping force From the perspective of the definition of tolerance zone, the position error includes the shape error of the measured surface. For example, the parallelism error contains the flatness error, so the position error is much larger than the shape error. Therefore, under normal circumstances, when there is no further requirement, the position tolerance is given, and the shape tolerance is no longer given. When there are special requirements, the shape and position tolerance requirements can be marked at the same time, but the marked shape tolerance value should be less than the marked position tolerance value, otherwise, the CNC machined parts cannot be manufactured according to the design requirements during production.
3. The relationship between the shape tolerance of CNC machined parts and the surface roughness
Although there is no direct connection between the numerical value and measurement between the shape error of the CNC machined parts and the surface roughness, there is a certain proportional relationship between the two under certain cnc processing conditions. According to experimental research, the surface roughness The degree occupies 1/5～1/4 of the shape tolerance.
4. It can be seen from this that in order to ensure the shape tolerance of CNC machining parts, the maximum allowable value of the corresponding surface roughness height parameter should be appropriately limited.
In general, the tolerance values among the dimensional tolerances, shape tolerances, position tolerances, and surface roughness of CNC machined parts have the following relationship: dimensional tolerance>position tolerance>shape tolerance>surface roughness height parameter.
It is not difficult to see from the numerical relationship between the size, shape and position of the CNC machined parts and the surface roughness, the numerical relationship of the three should be coordinated during the design. When marking the tolerance value on the drawing, it should be followed: given the roughness of the same surface The value should be smaller than its shape tolerance value; and the shape tolerance value should be smaller than its position tolerance value; the position differences should be smaller than its size tolerance value.
Otherwise, it will bring all kinds of troubles to manufacturing. However, the most involved in the design of CNC machining parts is how to deal with the relationship between dimensional tolerances and surface roughness and the relationship between various matching accuracy and surface roughness.
Under normal circumstances, it is determined according to the following relationship:
1. When the shape tolerance is 60% of the dimensional tolerance (medium relative geometric accuracy), Ra≤0.05IT;
2. When the shape tolerance is 40% of the dimensional tolerance (higher relative geometric accuracy), Ra≤0.025IT;
3. When the shape tolerance is 25% of the dimensional tolerance (high relative geometric accuracy), Ra≤0.012IT;
4. When the shape tolerance is less than 25% of the dimensional tolerance (super high relative geometric accuracy), Ra≤0.15Tf (shape tolerance value).
The simplest reference value: the dimensional tolerance of CNC machining parts is 3-4 times the roughness, which is the most economical.
Selection of the shape and position tolerance of CNC machining parts
The functions of comprehensive control items should be fully utilized to reduce the geometric tolerance items and corresponding geometric error detection items given on the drawings of CNC machining parts.
On the premise of meeting the functional requirements, the items of CNC machining parts that are easy to measure should be selected. For example, the coaxiality tolerance is often replaced by radial runout tolerance or radial runout tolerance. However, it should be noted that radial circle runout is a combination of concentricity error and cylindrical shape error, so when replacing, the runout tolerance value given should be slightly larger than the concentricity tolerance value, otherwise it will be too strict.
Selection of tolerance principles for CNC machining parts
According to the functional requirements of the tested elements, the tolerance function and the feasibility and economy of adopting the tolerance principle should be fully exerted.
The principle of independence is used in occasions where there is a big difference between the dimensional accuracy and the accuracy of the shape and position accuracy of the CNC machined parts, and the requirements must be met separately or the two are not connected to ensure the motion accuracy, sealing, and no tolerances.
Containment requirements are mainly used in occasions that require strict assurance of the nature of cooperation.
The largest entity requirement is used for the central element, and is generally used in occasions where the CNC machining parts require assemblability (no requirements for matching properties).
The minimum entity requirement is mainly used in occasions where the strength and minimum wall thickness of CNC machined parts need to be guaranteed.
The combination of reversible requirements and maximum (minimum) entity requirements can make full use of the tolerance zone, expand the actual size range of the measured element, and improve efficiency. It can be selected under the premise of not affecting the performance.
Selection of benchmark elements
1、Selection of reference parts
(1) Select the bonding surface of the CNC machined parts positioned in the machine as the reference part. For example, the bottom plane and side of the box, the axis of disc parts, the supporting journal or supporting hole of rotating parts, etc.
(2) The reference element should have sufficient size and rigidity to ensure stable and reliable positioning. For example, using two or more axes that are far apart to form a common reference axis is more stable than one reference axis.
(3) Choose the more accurate surface processed by cnc as the reference part.
(4) Try to make the assembly, cnc processing and testing standards unified. In this way, errors caused by inconsistent benchmarks can be eliminated; the design and manufacturing of fixtures and measuring tools for CNC machining parts can also be simplified, and the measurement is convenient.
2. Determination of the benchmark quantity
Generally speaking, the number of benchmarks should be determined according to the orientation and positioning geometric function requirements of the tolerance items of the CNC machining parts. Orientation tolerances mostly require only one datum, while positioning tolerances require one or more datums. For example, for parallelism, perpendicularity, and coaxiality tolerance items, generally only one plane or one axis is used as the reference element; for the position tolerance items of CNC machining parts, it is necessary to determine the position accuracy of the hole system, and two may be used. One or three benchmark elements.
3、Base order arrangement
When selecting two or more reference elements, the order of the reference elements must be clarified and written in the tolerance box in the order of first, second, and third. The first reference element is the main one, and the second reference element is the second.
4. Selection of shape and position tolerance value
The general principle: select the most economical tolerance value under the premise of satisfying the functions of CNC machining parts.
◆According to the functional requirements of CNC machining parts, considering the economy of machining and the structure and rigidity of CNC machining parts, determine the tolerance values of the elements according to the table. And consider the following factors:
1. The shape tolerance given by the same element should be less than the position tolerance value;
2. The shape tolerance value of cylindrical CNC machining parts (except for the straightness of the axis) should be less than its dimensional tolerance value; if on the same plane, the flatness tolerance value should be less than the parallelism tolerance value of the plane to the datum.
3. The parallelism tolerance value should be less than its corresponding distance tolerance value.
4. The approximate proportional relationship between the surface roughness of the CNC machined parts and the shape tolerance: Generally, the Ra value of the surface roughness can be taken as the shape tolerance value (20%~25%).
For the following situations, considering the difficulty of CNC machining and the influence of other factors besides the main parameters, to meet the requirements of the functions of CNC machining parts, appropriately reduce the selection of 1 to 2 levels:
○The hole is relative to the shaft;
○Slim and larger shafts and holes; shafts and holes with larger distance;
○The surface of parts with large width (more than 1/2 length);
○Tolerance of parallelism and perpendicularity of line-to-line and line-to-face relative to face-to-face.
5. Provisions for shape and position without tolerance
In order to simplify the drawing, the shape and position accuracy that can be guaranteed by the general machine tool CNC machining, it is not necessary to inject the shape and position tolerances on the drawings of the CNC machining parts, and the shape and position without tolerance shall be implemented in accordance with the provisions of GB/T1184-1996. The general content is as follows:
(1) Three tolerance levels of H, K, and L are specified for straightness, flatness, perpendicularity, symmetry and circular runout without marking.
(2) The uninjected roundness tolerance value is equal to the diameter tolerance value, but cannot be greater than the uninjected tolerance value of radial circle runout.
(3) The tolerance value of uninjected cylindricity is not specified, and it is controlled by the injection or uninjected tolerance of the roundness tolerance, the straightness of the element line and the parallelism of the relative element line.
(4) The unmarked parallelism tolerance value is equal to the larger of the dimensional tolerance between the measured element and the reference element and the unmarked tolerance value of the measured element shape tolerance (straightness or flatness), and take two The longer of the elements serves as the benchmark.
(5) The tolerance value of coaxiality is not specified. If necessary, the unmarked tolerance value of coaxiality can be equal to the unmarked tolerance of circle runout.
(6) The tolerance values of uninjected line profile, surface profile, inclination, and position are all controlled by the injection or uninjected linear dimensional tolerance or angle tolerance of each element.
(7) The total runout tolerance value is not specified.
6. Graphic representation of the shape and position without tolerance value
If the unmarked tolerance value specified in GB/T1184-1996 is used, the standard and grade code should be noted in the title column or technical requirements.
: “GB/T1184-K”.
There is no “tolerance principle in accordance with the working tolerance of GB/T 4249” on the drawings of CNC machining parts, and the requirements of “GB/T 1800.2-1998” should be implemented.