Discussion on the standard of cam evaluation toler

Discussion on the problem of cam evaluation tolerance standard

Figure 2 uses dimensional tolerance to evaluate the tolerance zone and the position relationship of the error zone of the cam (both the tolerance zone and the error zone are radial, and the position of the tolerance zone is fixed)

Figure 4 uses linear profile to evaluate the tolerance zone and the position relationship of the error zone of the cam, (the tolerance zone is normal, the error zone is radial, and the position of the tolerance zone is floating)

Figure 5 uses the concept of radial runout to evaluate the cam

the detection of the cam generally does not directly measure the shape error of the cam, but indirectly evaluates the shape error of the cam by measuring the displacement error of the follower that comprehensively reflects the working quality of the cam mechanism - the cam lift error according to the functional requirements of the cam mechanism

generally, the design drawing of the cam is not marked with the detection reference. For the cam without the detection reference, the position of the actual shape relative to the ideal shape, that is, the detection position of the cam, should be determined according to the requirements of the "minimum condition", that is, as shown in Figure 1, the area between a pair of ideal cams containing the actual cam (shown by the dotted line) should be minimized

referring to the national standard gb/t, the cam lift public electronic tensile testing machine cannot be sloppy in the installation. The shape and size of the difference band depend on the ideal geometry of the tested cam and the functional requirements of the cam mechanism, and the lift error of the cam is evaluated according to this: before establishing the geometric tolerance standard, the cam adopts the dimensional tolerance evaluation After establishing the form and position tolerance standard, Durethan Akv 50 h2.0 is a glass fiber reinforced nylon 66 material with 50% glass fiber content, which is evaluated by the linear profile tolerance. However, the linear profile tolerance controls the normal variation of the cam, while the cam measures the radial variation. The so-called normal direction refers to the direction of the line between the measured point of the cam and its curvature center, and the radial direction refers to the direction of the line between the measured point of the cam and the center of the base circle. Therefore, the evaluation value and measurement value of the two are inconsistent and cannot be directly compared

1 understanding of evaluating cams with dimensional tolerance

cams are evaluated according to dimensional tolerance, and only the lift change of cams is controlled. Although the control method conforms to the actual measurement of cams, the measurement is convenient, and the evaluation is intuitive and simple, according to gb/t 996, the dimensional tolerance does not control the shape error of cams, so the shape tolerance requirements of cams should also be given

cam detection belongs to the detection category of form and position errors. With dimensional tolerance control, the position of the tolerance zone is fixed and does not float with the ideal position of the actual shape of the cam (Fig. 2). According to the requirements of gb/t, it is inappropriate to use dimensional tolerance to control the cam lift error after establishing the geometric tolerance standard

2 views on cam profile tolerance evaluation

line profile tolerance zone is the area between two envelope lines enveloping a series of circles with a diameter of tolerance value, and the center of each circle should be located on the ideal profile. As for the cam, its ideal contour refers to the contour described by the correct size of the cam lift theory (Fig. 3)

it can be seen from Figure 3 that the area controlled by the linear profile tolerance zone is the amount of normal change of the cam in the contact area (Figure 3a), while the measured value of the cam is the amount of radial change (Figure 3B). The linear profile tolerance is used to control the lift error of the cam. Although the position of the tolerance zone can fluctuate with the actual shape of the cam (Fig. 4), the radial value measured by the cam must be converted into the normal value during evaluation, so it can be compared with the tolerance value. Therefore, the measurement and evaluation efficiency is not high, so whether to use the linear profile tolerance to evaluate the cam is questionable

3 it is recommended to evaluate the cam by runout tolerance.

the radial runout tolerance zone is the tolerance value of the radius in any measurement plane perpendicular to the reference axis, and the center of the circle is the area between two concentric circles on the reference axis. For the cam, it is in line with the measurement method of the cam. When measuring, taking the central axis of the base circle of the cam as the reference axis, we can understand the value of the cam lift error in this way: assuming that each lift measuring point on the cam contour is a concentric circle of the base circle (Fig. 5), the radius of the circle is the sum of the lift of the measured point and the radius of the base circle, that is, ri=hi+ro