metrologic
Mechanical
- Sep 14, 2021
- 56
Sr GDTP Y14.5-2009 Exam Review Ch-7 Sep2021
Hi Everyone! I'm up to section 7, Tolerances of Location. As you may recall, I'm working through the Y14.5 standard in preparation for upgrading my GDTP certification from the tech level to the senior level. Here are some questions I had from this section:
Q1. 7.2.1.1 Dimensions for True Position. What is the origin of the use of true in GD&T? Wouldn't nominal make more sense?
Q2. 7.4.1 Projected Tolerance Zone. The language in this subsection discusses projecting the tolerance zones associated with positional requirements to ensure the orientation variation of fixed fasteners does not interfere with mating parts. Am I the only one that prefers to think of positional tolerance zones as infinity long, and hence their projection seems kinda strange and unnecessary? In my opinion, the focus should be on identifying the feature axis orientation and relevant length --and then assessing whether that segment is contained by a tolerance zone that is extended axially indefinitely. Fig. 7-20 provides a good example of the problem at hand for fixed fasteners installed after assembly. As can be seen, the segment of projected feature axis that passes through the maximum thickness of the mating part depends on the variation in orientation of the fastener. Projecting a positional tolerance zone is trivial, measuring and projecting a feature axis is not. But maybe I'm thinking about this all wrong?
Q3. 7.4.5.1 Noncircular Features of Size at MMC. Subsections (a) and (c) provide an interpretation of positional tolerances for noncircular features of size. Both paragraphs describe a theoretical boundary that can not be violated. They seem to be redundantly saying the same thing if I'm not mistake. What is the difference?
Q4. Fig. 7-34 Positional Tolerancing, Boundary Concept. So figure 7-34 depicts positional tolerancing for rounded slots. The long axis of the rounded slots run left to right on the page, parallel to datum B. The rounded ends are given a "left right" positional tolerance of 1.5mm at MMC. The long straight sides of the slots are given an "up down" positional tolerance of 0.25mm at MMC. When I try to interpret the situation I see two features of size, a length defined by the opposing rounded ends and a width defined by the straight sides. Each of these two features has its own given positional tolerance, which is defined by two parallel planes running either perpendicular or parallel to datum B. Up to this point, I feel the concepts of feature axis location and virtual condition boundaries have been reasonably well defined. In this case however, what step by step logic will lead one from the feature control frames to the theoretical boundary that is "equal to the MMC size of the internal feature minus its positional tolerance". At first glance, I believed this would be identical in nature to a simple hole or stud that is controlled with bidirectional position tolerancing. But after thinking it over, the rounded slot seems to really be two features, each with their own positional control, while the simple bidirectional toleranced hole is one feature with two complementary positional controls. It seems to me, that the positional requirements shown for the rounded slot in figure 7-34 would conflict with each other and make it quite difficult to derive a theoretical boundary that shall not be violated.
Q5. 7.6.4 Concentricity. Is it allowed to use secondary or tertiary datum feature references when applying a concentricity or symmetry control?
Hi Everyone! I'm up to section 7, Tolerances of Location. As you may recall, I'm working through the Y14.5 standard in preparation for upgrading my GDTP certification from the tech level to the senior level. Here are some questions I had from this section:
Q1. 7.2.1.1 Dimensions for True Position. What is the origin of the use of true in GD&T? Wouldn't nominal make more sense?
Q2. 7.4.1 Projected Tolerance Zone. The language in this subsection discusses projecting the tolerance zones associated with positional requirements to ensure the orientation variation of fixed fasteners does not interfere with mating parts. Am I the only one that prefers to think of positional tolerance zones as infinity long, and hence their projection seems kinda strange and unnecessary? In my opinion, the focus should be on identifying the feature axis orientation and relevant length --and then assessing whether that segment is contained by a tolerance zone that is extended axially indefinitely. Fig. 7-20 provides a good example of the problem at hand for fixed fasteners installed after assembly. As can be seen, the segment of projected feature axis that passes through the maximum thickness of the mating part depends on the variation in orientation of the fastener. Projecting a positional tolerance zone is trivial, measuring and projecting a feature axis is not. But maybe I'm thinking about this all wrong?
Q3. 7.4.5.1 Noncircular Features of Size at MMC. Subsections (a) and (c) provide an interpretation of positional tolerances for noncircular features of size. Both paragraphs describe a theoretical boundary that can not be violated. They seem to be redundantly saying the same thing if I'm not mistake. What is the difference?
Q4. Fig. 7-34 Positional Tolerancing, Boundary Concept. So figure 7-34 depicts positional tolerancing for rounded slots. The long axis of the rounded slots run left to right on the page, parallel to datum B. The rounded ends are given a "left right" positional tolerance of 1.5mm at MMC. The long straight sides of the slots are given an "up down" positional tolerance of 0.25mm at MMC. When I try to interpret the situation I see two features of size, a length defined by the opposing rounded ends and a width defined by the straight sides. Each of these two features has its own given positional tolerance, which is defined by two parallel planes running either perpendicular or parallel to datum B. Up to this point, I feel the concepts of feature axis location and virtual condition boundaries have been reasonably well defined. In this case however, what step by step logic will lead one from the feature control frames to the theoretical boundary that is "equal to the MMC size of the internal feature minus its positional tolerance". At first glance, I believed this would be identical in nature to a simple hole or stud that is controlled with bidirectional position tolerancing. But after thinking it over, the rounded slot seems to really be two features, each with their own positional control, while the simple bidirectional toleranced hole is one feature with two complementary positional controls. It seems to me, that the positional requirements shown for the rounded slot in figure 7-34 would conflict with each other and make it quite difficult to derive a theoretical boundary that shall not be violated.
Q5. 7.6.4 Concentricity. Is it allowed to use secondary or tertiary datum feature references when applying a concentricity or symmetry control?