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coaxial bore holes parttern - repetitive datum
- Thread starter bxbzq
- Start date
pmarc
Mechanical
- Sep 2, 2008
- 3,248
bxbzq,
I would not be afraid too much about presence or absence of "8X" prefix. I think the intent is quite clear here.
I have, however, some additional questions:
1. With the current state of the print, if you were to verify composite positional requirement for dia. 16, dia. 7.6 and dia. 20, what size of maximum material boundary (MMB) of datum feature F would you use?
2. Is reference to tertiary datum E in both segments of composite positional callout for datum feature pattern F needed? Does it add any value? Does it constrain any degree of freedom that has not been already constrained by higher precedence datums?
Just some things to consider.
I would not be afraid too much about presence or absence of "8X" prefix. I think the intent is quite clear here.
I have, however, some additional questions:
1. With the current state of the print, if you were to verify composite positional requirement for dia. 16, dia. 7.6 and dia. 20, what size of maximum material boundary (MMB) of datum feature F would you use?
2. Is reference to tertiary datum E in both segments of composite positional callout for datum feature pattern F needed? Does it add any value? Does it constrain any degree of freedom that has not been already constrained by higher precedence datums?
Just some things to consider.
PaulJackson
Automotive
Frank,
I was simply suggesting an alternate specification that will provide a pattern of feature tolerance zones "similar" to the given "complex" feature specification but without the "local" individual datum feature designations.
I do not know the mating feature's, the function, or the assembly constraints so I really have no applicable "good" advice for bxbzq's question... but with what I do know of pattern constriants and measurement stability I thought "there is a simpler way to detail that."
I do not need to understand the actual process used to install these feature tolerance zones... but it very helpful if the design enables simplified processing. It is however critical that I understand the function and assembly to apply a good tolerance strategy. That strategy being reflective of functional liberties and constraints and simultaneously being stable and repeatable.
Paul
I was simply suggesting an alternate specification that will provide a pattern of feature tolerance zones "similar" to the given "complex" feature specification but without the "local" individual datum feature designations.
I do not know the mating feature's, the function, or the assembly constraints so I really have no applicable "good" advice for bxbzq's question... but with what I do know of pattern constriants and measurement stability I thought "there is a simpler way to detail that."
I do not need to understand the actual process used to install these feature tolerance zones... but it very helpful if the design enables simplified processing. It is however critical that I understand the function and assembly to apply a good tolerance strategy. That strategy being reflective of functional liberties and constraints and simultaneously being stable and repeatable.
Paul
- Thread starter
- #7
pmarc,
Datum surface E is definitely needed because if no datum E the bores will tilt at any angle to it (while staying parallel to datum surface A). This is not what I want. Your questions made me think more of the DRF though. Now I would do 2 single segment position control on datum feature F to constrain them more to the axis B while being perpendicular to surface E:
pos|ø0.8(mmc)|E|B|A|
pos|ø0.1(mmc)|E|B|
Then I think the MMB of datum feature F to verify composite position of rest of the bores would be ø10.8mm.
Paul,
I like simple, clean and clear callouts much more than complex ones. I need to think more of the way you call out the hole patterns, in terms of functional requirements.
Datum surface E is definitely needed because if no datum E the bores will tilt at any angle to it (while staying parallel to datum surface A). This is not what I want. Your questions made me think more of the DRF though. Now I would do 2 single segment position control on datum feature F to constrain them more to the axis B while being perpendicular to surface E:
pos|ø0.8(mmc)|E|B|A|
pos|ø0.1(mmc)|E|B|
Then I think the MMB of datum feature F to verify composite position of rest of the bores would be ø10.8mm.
Paul,
I like simple, clean and clear callouts much more than complex ones. I need to think more of the way you call out the hole patterns, in terms of functional requirements.
PaulJackson
Automotive
Bxbzq,
If you use the small circular ring surfaces "[E] Individually" as the primary datum feature... then each surface will stop three degrees of freedom (two rotation and one translation) seperately. The shallow cylinder then would attempt to arrest one remaining rotation and just one of the two translations that it is capable of constraining since [E] has already constrained one of them. That leaves [A] to constrain the one remaining translation. Think of how suseptable that coordinate system is to measurement error and ask yourself if you could ever trust the measurement values derived from it. Ask youself if inspectors will not use the datum features in an alternate prescedence unknowingly to derive their measurements. Compare all of that with what you can see of assembly constraints in terms of "degrees of freedom" and "if you have captured it" then keep what you have and manage the measurement problem that I see you are going to experience. If that is not the way the assembly is constrained the spend your energy determining how those degrees of freedom are constrained.
Paul
If you use the small circular ring surfaces "[E] Individually" as the primary datum feature... then each surface will stop three degrees of freedom (two rotation and one translation) seperately. The shallow cylinder then would attempt to arrest one remaining rotation and just one of the two translations that it is capable of constraining since [E] has already constrained one of them. That leaves [A] to constrain the one remaining translation. Think of how suseptable that coordinate system is to measurement error and ask yourself if you could ever trust the measurement values derived from it. Ask youself if inspectors will not use the datum features in an alternate prescedence unknowingly to derive their measurements. Compare all of that with what you can see of assembly constraints in terms of "degrees of freedom" and "if you have captured it" then keep what you have and manage the measurement problem that I see you are going to experience. If that is not the way the assembly is constrained the spend your energy determining how those degrees of freedom are constrained.
Paul
- Thread starter
- #9
- Thread starter
- #11
PaulJackson
Automotive
bxbzq,
Not knowing what you do now about the function it is difficult to answer "better now?" but by changing the position of the coaxial patterns [F] to a single segment to you have tightened the location-constraint of those coaxial patterns to the axis 800%. If that captures the functional liberty then I would say "yes" better now.
Paul
Not knowing what you do now about the function it is difficult to answer "better now?" but by changing the position of the coaxial patterns [F] to a single segment to you have tightened the location-constraint of those coaxial patterns to the axis 800%. If that captures the functional liberty then I would say "yes" better now.
Paul
pmarc
Mechanical
- Sep 2, 2008
- 3,248
bxbzq,
1. I understand that you decided to tighten location of both holes F to B by using two single segment positional callouts instead of composite callout as in your initial post. That is fine, if it is line with functional requirements.
2. Tertiary datum reference F(M) in profile callout for surface previously assigned as E is redundant. All DOFs that could be constrained by referencing to F(M) have already been constrained by datums A and B. If orientation of this surface to axis F is not important, simply get rid of F(M) from profile FCF and rely on simultaneous requirement - position of holes F to |A|B| and profile of surface to |A|B| will make these features tied to each other as well as it will keep them together in their rotation axis B (the only DOF that has not been constrained at all here).
3. Counterbore symbol use - first, I do not think this symbol is needed in this view. Second, it is hard for me to understand why the depths of all bores originate at former surface E. In other words, why the bottom diameters 11 and 20 have their depth measured from the former surface E. Is it how they will be bored? I don't think so. Why not to locate each of the steps from former surface E with basic dimensions and profile callouts to E, if this is the funtional requirement?
4. Out of curiosity, what will be fitting into these bores? Some kind of pins? Perhaps two pins - one from the top and second from the bottom of bores?
1. I understand that you decided to tighten location of both holes F to B by using two single segment positional callouts instead of composite callout as in your initial post. That is fine, if it is line with functional requirements.
2. Tertiary datum reference F(M) in profile callout for surface previously assigned as E is redundant. All DOFs that could be constrained by referencing to F(M) have already been constrained by datums A and B. If orientation of this surface to axis F is not important, simply get rid of F(M) from profile FCF and rely on simultaneous requirement - position of holes F to |A|B| and profile of surface to |A|B| will make these features tied to each other as well as it will keep them together in their rotation axis B (the only DOF that has not been constrained at all here).
3. Counterbore symbol use - first, I do not think this symbol is needed in this view. Second, it is hard for me to understand why the depths of all bores originate at former surface E. In other words, why the bottom diameters 11 and 20 have their depth measured from the former surface E. Is it how they will be bored? I don't think so. Why not to locate each of the steps from former surface E with basic dimensions and profile callouts to E, if this is the funtional requirement?
4. Out of curiosity, what will be fitting into these bores? Some kind of pins? Perhaps two pins - one from the top and second from the bottom of bores?
- Thread starter
- #14
1. Yes. The bores position relative to datum axis B should be tighter than to datum A. The tolerance values in my sketch are just illustrative, not the ones in actual design.
2. Agree with you on the Simultaneous Requirement. This rule always slips away from my mind.
3. I don't have drafting background, many times I struggle with how to put callouts on drawings. Fig.7-26 in '09 std shows a simpler but similar case. The counterbore symbol is put to a cross section view. About your second question, I want a simple way to specify the bores while not detrimental to function req'ts.
4. There are two shafts go into the bores. One takes the ø20 bore, the other takes the rest. There are bushings fitting the pattern F. By the way, I exaggerated the difference between bore diameters. The diameters on the shaft fitting the Bushings are a little bit smaller than than the ø7.6 able to slide in.
2. Agree with you on the Simultaneous Requirement. This rule always slips away from my mind.
3. I don't have drafting background, many times I struggle with how to put callouts on drawings. Fig.7-26 in '09 std shows a simpler but similar case. The counterbore symbol is put to a cross section view. About your second question, I want a simple way to specify the bores while not detrimental to function req'ts.
4. There are two shafts go into the bores. One takes the ø20 bore, the other takes the rest. There are bushings fitting the pattern F. By the way, I exaggerated the difference between bore diameters. The diameters on the shaft fitting the Bushings are a little bit smaller than than the ø7.6 able to slide in.
pmarc
Mechanical
- Sep 2, 2008
- 3,248
As for counterbore symbol, I do not think it adds something useful either to fig. 7-26 or to your print.
If two separate pins go into the bores, is it really functionally reasonable to choose both bores of dia. 11 for derivation of datum axis F? Just asking...
If two separate pins go into the bores, is it really functionally reasonable to choose both bores of dia. 11 for derivation of datum axis F? Just asking...
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