TOPIC: Least Material Condition (LMC) GD&T
Least Material Condition (LMC)
It refers to a feature of size.(The feature of size is nothing but the set of parallel surfaces associated with the dimension.)
This is a condition where there exists a minimum amount of material within the given dimension tolerance zone.
Least Material Condition is one side of the dimensional limits on a part. The other side of the tolerance range would be the Maximum Material Condition.
LMC is fairly rare inGD&T. The reasons to call out the LMC is close/interference between the two mating parts.
See the below LMC conditions for Pin and Hole
LMC condition for pin with No-GO gauge
The pin will have the minimum diameter within the tolerance limit. (i.e Ø9.5)
Gauging Least material condition for pin
A pin always tight fit into a hole, when the pin is at the LMC condition. So that we need to design the Hole Gauge for the pin. so this hole gauge controls the Least material condition for the pin. this gauge is called No-GO GAUGE.
The Hole gauge is having the diameter same as the Pin maximum diameter(Ø).
The formula to find the Hole Gauge Diameter(Ø)= Minimum Diameter(Ø) of LMC Pin
The No-Go Gauge would be designed ~2µm smaller to make up for any error.
We can not use GD&T with the No-Go Gauge.
LMC condition for Hole with No-Go Gauge
The Hole will have the maximum diameter within the tolerance limit. (i.e Ø10.5)
Gauging Least material condition for Hole
a pin always tight/press fits into a hole when the hole is at LMC So that we will design the Pin Gauge for the HOLE. so this pin gauge controls the Least material condition for the hole.
The Pin gauge is having the diameter same as the Hole maximum diameter(Ø).
The formula to find the Pin Gauge Diameter(Ø)= Min Diameter(Ø) of LMC
The No-Go Gauge will be designed ~2µm larger to make up for any error.
Where can we apply Least Material Condition?
LMC is rarely used and will not combine with the GD&T tolerances, due to Unable to accurate gauge for size and geometry tolerances.
Feature of Size Callouts in GD&Ts
Regardless of feature of size
When there is no call out to Maximum Material Condition or Least Material Condition, the part, by default, is measured regardless of feature size (RFS).