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GD&T: MMC & LMC Explained!
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🙋♂️ Interview Practice Question of the Week
*Company: Relativity Space
What is the difference between MMC and LMC, and when would you use each?
“A brown bear bear in glasses sitting at a drafting table doing an engineering drawing, digital art.” Graphic is generated using OpenAI’s Dalle2.
✅ The Answer
Geometric dimensioning & tolerancing (GD&T) is another category of questions asked in technical interviews for design, production or manufacturing engineering positions with fair regularity, and as such we will be covering a few questions related to it over the coming months.
Most mechanical engineering students received an extraordinarily brief introduction to GD&T through coursework, and a lecturer probably spent all of 1-3 hours covering all of drafting. If you were lucky, perhaps you received a homework assignment. If that sounds familiar, it was not adequate. Hobbyists can get away with FDM printing an .stl file, saying it warped a bit, and either reprinting or accepting the defect. Professionals cannot.
Any machined, cast, DMLS, or printed part is guaranteed to have a corresponding drawing provided to the manufacturer calling out critical features (if multiple machining steps are needed, an intermediate drawing is provided for before and after each operation). As a hardware engineer, being able to create and/or interpret these drawings is necessary for clear communication of what exactly is needed. Tight tolerances mean higher cost, but loose tolerances can result in non-functional parts (also increasing cost). It is our job to determine exactly how tightly a component needs to be controlled, and effectively communicate that. As such, one function of GD&T is to form a shared language between designers and manufacturers so that the functional aspects of a design can be focused on.
As you can imagine, one of the most common design features is the hole. The maximum material condition (MMC), is extremely aptly named, and refers to the condition at the end of the tolerance zone where the most material remains on the part (think of it as the condition where the part has the most volume or mass). For a hole, this would be when it is the smallest, such that the remaining component is as large as possible. The opposite case is the least material condition (LMC). This is when the remaining component is as small as possible, and usually refers to when a hole is the largest it can be while remaining within its tolerance band.
In most cases, some form of pin, shaft, rod or bolt goes through the aforementioned hole. Given that assumption, if I’ve designed a hole to be between 9 and 10 mm, the larger hole must have some form of “slop” or “looseness” since a 9 mm hole is also functional. As such, a 10 mm hole can have a looser positional tolerance given its larger size suggests unnecessary slop in the system. This looser positional tolerance is referred to as a “bonus tolerance,” and is applied when an appropriately called-out feature departs from MMC.
What that means is that if the previous hole is larger than MMC by 0.5 mm (for a total diameter of 9.5 mm), assuming a positional tolerance at MMC of 0.5 mm, the total positional tolerance of that specific part becomes 1 mm (0.5 mm at MMC + 0.5 mm bonus tolerance due to departure from MMC). Use of MMC in this fashion allows us to accept parts using dimensioning based on functionality rather than regular dimensions, and leads to more parts accepted / lower production cost.
MMC’s far less common cousin, LMC, is typically used in situations where an interference fit is desired or where a hole lies close to the edge of a component. In this case, as the hole gets smaller, its positional tolerance loosens to allow the hole closer to the edge without concern of wall thickness reducing to dangerous levels.
In summation, MMC refers to the maximum material condition of a part, when it occupies as much volume as possible while remaining in specification. It is most often used to guarantee fitment when some form of rod or shaft goes through a hole. LMC refers to the least material condition of a part, when it occupies the least possible volume while remaining in specification. It is most often used to guarantee an interference fit or to prevent damage/encroachment onto a thin wall when a hole is placed close to the edge of a part. In both cases, as hole size departs from the material condition, a “bonus tolerance” is added to the tolerance, effectively loosening it.
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