metrologic
Mechanical
- Sep 14, 2021
- 56
Sr GDTP Y14.5-2009 Exam Review Ch-4 Part-III Sep2021
Study review Part-III of III for Section 4, Datum Reference Frames. This thread covers subsections 4.21 thru to the end of the section.
Q1. 4.24.1 Datum Target Symbols. It says here: "Datum targets are designated on the drawing by means of a datum target symbol...." However "The datum feature itself may be identified with a datum feature symbol as shown in Fig. 4-53 or by using the datum reference frame symbol as shown in Fig. 4-54." Using the datum reference frame symbol to identify a datum feature seems quite odd to me. I can see the need to identify the origin of a datum reference frame under many circumstance, and their are many such examples, but that doesn't sound like what's being described here. What is being described here? Can one really identify a datum feature with the datum reference frame symbol?
Q2. 4.24.6 Movable Datum Target Symbol. "Where datum targets establish a center point, axis, or center plane on an RMB basis, the datum feature simulator moves normal to the true profile, and the moveable datum target symbol, though not required, may be used for clarity." OK, let's look at Fig 4-47. This is the one with a V-shaped datum feature simulator established from two datum target lines B1 and B2 on the left of the part. And two datum target points C1 and C2 on the right side. The B target simulator lines are indicated with the regular target symbol and the C target points are clearly indicated as moveable using the movable datum target symbol. There's also a datum reference frame (DRF) shown originating from the center of the hole pattern. Do the B datum target lines establish an axis that could be referenced on an RMB basis? More importantly, could the B target line datum simulators move even though they are not indicated with the movable datum target symbol? B is apparently referenced RMB in this example and I would like to think the B target simulators will move relative to the shown DRF, but perhaps the simulators are meant to be thought of as fixed at basic? How do I know if it's supposed to be fixed or not? Perhaps if the target simulator is not a true geometric counterpart (as is the case here) and can't move normal to the part's true profile at every point along the target line, than the simulator defaults to a fixed basic geometry? What is the pattern that governs the interpretation of fixed and movable simulators?
Q3. 4.24.6 Fig 4-49. The movable datum target example in this figure illustrates some kind of double cam shaft thingy. Two spherical datum target feature simulators labeled A1 and A2 are indicated with phantom lines at either end of the shaft. Target A1 is indicted as movable while target A2 is drawn with the regular datum target symbol. A few geometric tolerances referencing datum feature A are shown controlling a profile and several positions. First off, shouldn't A1 and A2 really be separate datum references? It seems odd to me that both conical features at either end of the shaft are lumped together as one datum feature. Do the cones somehow form a feature of size that is best referenced as one datum feature? If so, how can one target be movable and the other may or may not be? What if A is referenced MMB? Does the movable target symbol override an MMB modifier? -or is it other way around? Lastly, does datum target simulator A2 move? Datum feature A is referenced at RMB apparently, so... does that mean A2 moves normal to the true profile? Or is it treated as a fixed basic? And why is that so?
Q4. 4.24.13 Datums Established From Complex or Irregular Surfaces. "Datum targets may be used to establish a datum from a complex or irregular surface. Where a datum reference frame has been properly established but its planes are unclear, the datum reference frame coordinate axes may be labeled to appropriate extension or center lines as needed. See Fig. 4-54. The datum feature symbol should be attached only to identifiable datum features. Where datums are established by targets on complex or irregular surfaces, the datum may be identified by a note such as DATUM AXIS A or DATUM PLANE A." In Fig 4-54, what value or capability does labeling the DRF in the drawing provide?
Q5. 4.24.14 Datum Features Established From Datum Targets with Fewer Than Three Mutually Perpendicular Planes. "The targets that provide definition for the datums referenced in the feature control frame shall be specified in a note, such as, DATUM FEATURES B AND C ARE INVOKED WHERE ONLY DATUM FEATURE A IS REFERENCED TO RELATE THE TARGETS THAT ESTABLISH DATUM A" Is there a way to do this with a customized datum reference frame instead of a note? It seems like customized datum reference frames should be extended in some manner to handle this scenario.
Study review Part-III of III for Section 4, Datum Reference Frames. This thread covers subsections 4.21 thru to the end of the section.
Q1. 4.24.1 Datum Target Symbols. It says here: "Datum targets are designated on the drawing by means of a datum target symbol...." However "The datum feature itself may be identified with a datum feature symbol as shown in Fig. 4-53 or by using the datum reference frame symbol as shown in Fig. 4-54." Using the datum reference frame symbol to identify a datum feature seems quite odd to me. I can see the need to identify the origin of a datum reference frame under many circumstance, and their are many such examples, but that doesn't sound like what's being described here. What is being described here? Can one really identify a datum feature with the datum reference frame symbol?
Q2. 4.24.6 Movable Datum Target Symbol. "Where datum targets establish a center point, axis, or center plane on an RMB basis, the datum feature simulator moves normal to the true profile, and the moveable datum target symbol, though not required, may be used for clarity." OK, let's look at Fig 4-47. This is the one with a V-shaped datum feature simulator established from two datum target lines B1 and B2 on the left of the part. And two datum target points C1 and C2 on the right side. The B target simulator lines are indicated with the regular target symbol and the C target points are clearly indicated as moveable using the movable datum target symbol. There's also a datum reference frame (DRF) shown originating from the center of the hole pattern. Do the B datum target lines establish an axis that could be referenced on an RMB basis? More importantly, could the B target line datum simulators move even though they are not indicated with the movable datum target symbol? B is apparently referenced RMB in this example and I would like to think the B target simulators will move relative to the shown DRF, but perhaps the simulators are meant to be thought of as fixed at basic? How do I know if it's supposed to be fixed or not? Perhaps if the target simulator is not a true geometric counterpart (as is the case here) and can't move normal to the part's true profile at every point along the target line, than the simulator defaults to a fixed basic geometry? What is the pattern that governs the interpretation of fixed and movable simulators?
Q3. 4.24.6 Fig 4-49. The movable datum target example in this figure illustrates some kind of double cam shaft thingy. Two spherical datum target feature simulators labeled A1 and A2 are indicated with phantom lines at either end of the shaft. Target A1 is indicted as movable while target A2 is drawn with the regular datum target symbol. A few geometric tolerances referencing datum feature A are shown controlling a profile and several positions. First off, shouldn't A1 and A2 really be separate datum references? It seems odd to me that both conical features at either end of the shaft are lumped together as one datum feature. Do the cones somehow form a feature of size that is best referenced as one datum feature? If so, how can one target be movable and the other may or may not be? What if A is referenced MMB? Does the movable target symbol override an MMB modifier? -or is it other way around? Lastly, does datum target simulator A2 move? Datum feature A is referenced at RMB apparently, so... does that mean A2 moves normal to the true profile? Or is it treated as a fixed basic? And why is that so?
Q4. 4.24.13 Datums Established From Complex or Irregular Surfaces. "Datum targets may be used to establish a datum from a complex or irregular surface. Where a datum reference frame has been properly established but its planes are unclear, the datum reference frame coordinate axes may be labeled to appropriate extension or center lines as needed. See Fig. 4-54. The datum feature symbol should be attached only to identifiable datum features. Where datums are established by targets on complex or irregular surfaces, the datum may be identified by a note such as DATUM AXIS A or DATUM PLANE A." In Fig 4-54, what value or capability does labeling the DRF in the drawing provide?
Q5. 4.24.14 Datum Features Established From Datum Targets with Fewer Than Three Mutually Perpendicular Planes. "The targets that provide definition for the datums referenced in the feature control frame shall be specified in a note, such as, DATUM FEATURES B AND C ARE INVOKED WHERE ONLY DATUM FEATURE A IS REFERENCED TO RELATE THE TARGETS THAT ESTABLISH DATUM A" Is there a way to do this with a customized datum reference frame instead of a note? It seems like customized datum reference frames should be extended in some manner to handle this scenario.
