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can we apply LMB at primary datum , is this correct way of applying gd&t pls suggest 1
- Thread starter Goppinath
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1. Whenever an LMC modifier follows the tolerance value, a physical virtual condition functional gage is not possible. You could work according to the "axis interpretation" (a tolerance zone for the axis including bonus tolerance when available), or make the CMM generate a virtually simulated virtual condition boundary inside the material, if possible.
2. Whenever an LMB modifier appears after the datum reference in the feature control frame, a physical datum feature simulator for this datum reference is not possible. You could virtually simulate a theoretical true geometric counterpart using the CMM. A physical fixture gage will not be used. If this is not possible or not desired, the LMB modifier could be disregarded at the expense of potentially rejecting some good parts when simulating the datum at RMB.
3. LMC and MMB can be combined, but for what purpose? Typically, LMC is combined with LMB when you want to provide allowance for additional variations as long as wall thickness between the datum feature and the controlled feature is maintained, or as long as there is some stock for subsequent machining operations is kept available. You combine MMC and MMB when you want to allow additional variations as long as parts assemble together at their worst case tolerance conditions.
2. Whenever an LMB modifier appears after the datum reference in the feature control frame, a physical datum feature simulator for this datum reference is not possible. You could virtually simulate a theoretical true geometric counterpart using the CMM. A physical fixture gage will not be used. If this is not possible or not desired, the LMB modifier could be disregarded at the expense of potentially rejecting some good parts when simulating the datum at RMB.
3. LMC and MMB can be combined, but for what purpose? Typically, LMC is combined with LMB when you want to provide allowance for additional variations as long as wall thickness between the datum feature and the controlled feature is maintained, or as long as there is some stock for subsequent machining operations is kept available. You combine MMC and MMB when you want to allow additional variations as long as parts assemble together at their worst case tolerance conditions.
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- #5
Goppinath,
It is not about whether you need both modifiers or one is "enough" to ensure minimum wall thickness. As a matter of fact, neither LMC nor LMB provides you with tighter control over wall thickness. The smallest wall thickness will occur with the largest datum feature bore per its size tolerance, and the largest hole in the pattern per its size tolerance produced with the maximum position error according to the specified position tolerance in the feature control frame. You will have exactly the same worst case situation for wall thickness whether you use the LMC and LMB modifiers or not. As I mentioned in my previous reply, the LMC and LMB modifiers are used in order to provide additional variation - it is additional (bonus) tolerance for position of holes in the pattern when they are produced smaller - due to modifying the tolerance by LMC, and a possible displacement of the entire pattern relative to the datum feature when the datum feature bore is produced smaller. This way the minimum wall thickness remains the same as when the tolerance is RFS and the datum feature is referenced RMB, but there is potential to reduce scrap.
The method of tolerancing depends on what the holes are for - if they are there just for weight reduction, it is probably a good idea to apply both LMB and LMC. Otherwise you may want to consider the other extreme case condition - which is when the holes are produced the smallest and with the maximum possible position tolerance due to bonus ("Resultant Condition" per the standard for the LMC case), and the entire pattern shifts in location relative to the bore due to datum shift. What would be the functional implications of that?
It is not about whether you need both modifiers or one is "enough" to ensure minimum wall thickness. As a matter of fact, neither LMC nor LMB provides you with tighter control over wall thickness. The smallest wall thickness will occur with the largest datum feature bore per its size tolerance, and the largest hole in the pattern per its size tolerance produced with the maximum position error according to the specified position tolerance in the feature control frame. You will have exactly the same worst case situation for wall thickness whether you use the LMC and LMB modifiers or not. As I mentioned in my previous reply, the LMC and LMB modifiers are used in order to provide additional variation - it is additional (bonus) tolerance for position of holes in the pattern when they are produced smaller - due to modifying the tolerance by LMC, and a possible displacement of the entire pattern relative to the datum feature when the datum feature bore is produced smaller. This way the minimum wall thickness remains the same as when the tolerance is RFS and the datum feature is referenced RMB, but there is potential to reduce scrap.
The method of tolerancing depends on what the holes are for - if they are there just for weight reduction, it is probably a good idea to apply both LMB and LMC. Otherwise you may want to consider the other extreme case condition - which is when the holes are produced the smallest and with the maximum possible position tolerance due to bonus ("Resultant Condition" per the standard for the LMC case), and the entire pattern shifts in location relative to the bore due to datum shift. What would be the functional implications of that?
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