Similarity of LMB (Least Material Boundary)

[Ivan Silva]

I'm venturing on learning GD&T alone. When reading about Least Material Boundary I've concluded that least material boundary is equal to the outer boundary for a hole (internal feature)and equals to the inner boundary for a shaft (external boundary). Is that rigth? Could I consider the least material boundary scenario as the scenario where there is the biggest clearence?

 

[Belanger]

Not quite, because the datum in question (the datum being modified with LMB) might be a secondary or tertiary datum. So you may have to consider (or not consider) other relationships.

 

[AndrewTT]

The below is written with respect to the ASME standard (ASME Y14.5).

If you are just starting to study GD&T then I recommend you make it a priority to understand the terms and their definitions, as written in the standard. Learn the difference between LMB and LMC, and the difference between MMB and MMC. While these definitions are provided in the ASME standard, I do not recommend using ASME-Y14.5 as a text book for learning. I recommend reading a book written to teach GD&T, per ASME Y14.5, and then reading the ASME Y14.5 standard itself.

You can take seminars and watch you-tube but I have found that there is no substitute for reading the standard, slowly and thoughtfully, once you have a base understanding of the concepts that the standard contains.

Don't be dis-heartened if you need to read the teaching text book and the standard more than once before the light starts to come on.

 

[drawoh]

Ivan Silva,

Watch out for Rule[ ]1, which states that the feature must have perfect form at MMC. At LMC, most of the time, you can have maximum angle and other form errors, as long as they don't violate your tolerances. This is particularly true if you have applied [±][ ]tolerances to stuff.

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JHG