Proper LMC Callout
Proper LMC Callout
(OP)
So I have a boss with a blind hole going into it and want to ensure a minimum wall thickness if either the hole or boss moves or changes diameter but want to give bonus tolerance so that the part isn't rejected.
It seems like the LMC callout is what I need for both the location as well as datum B (assuming datum B is the hole going into the boss) to convey the condition that when both the hole and boss are both at their LMC then the parts have to be dead nuts on one another but anything other than that you get bonus tolerance for the boss to move around. Is what I have drawn right now conveying that correctly or is it supposed to be called out differently? I wasn't able to find anything exactly like this online or in the ASME book that describes something like this.
It seems like the LMC callout is what I need for both the location as well as datum B (assuming datum B is the hole going into the boss) to convey the condition that when both the hole and boss are both at their LMC then the parts have to be dead nuts on one another but anything other than that you get bonus tolerance for the boss to move around. Is what I have drawn right now conveying that correctly or is it supposed to be called out differently? I wasn't able to find anything exactly like this online or in the ASME book that describes something like this.





RE: Proper LMC Callout
Certified Sr. GD&T Professional
RE: Proper LMC Callout
The LMC concept can communicate the design intent - wall thickness - you described. But be aware, the tolerance zone (mating envelope) is "in the metal" and requires calculations based on surface analysis. This demands careful planning from a QA standpoint to confirm requirements.
Certified Sr. GD&T Professional
RE: Proper LMC Callout
RE: Proper LMC Callout
RE: Proper LMC Callout
Certified Sr. GD&T Professional
RE: Proper LMC Callout
There are two sets of patterns as you seem to have guessed. The two sets of holes are on different planes which is probably more evident with the new picture I attached. I believe the INDIVIDUALLY is valid here since the datum feature is repeated but I have only used the callout a couple of times before so I might be using it wrong.
Is there a better way to achive the same result or is it something our Quality department has to accept will involve some work? when I talked about what I wanted to do they seemed like they would be able to do it, granted they weren't the most excited.
The title block has standard tolerances of +/-.005" so the LMC should be .287" Attached is a view that shows datums B and C as well.
RE: Proper LMC Callout
RE: Proper LMC Callout
Based upon size limits of the hole and the boss and the LMC FCFs you are able to calculate what the minimum wall thickness allowed is. This is what QA needs to verify (along with correct size). If you were not using GD&T there would most likely be a note stating the minimum allowable wall thickness for these features. Although that note might lead to the measurement being taken with calipers. So, the LMC FCF is superior to a wall thickness note, in that it clearly describes how that minimum thickness needs to be thought of.