full form check and two points check

[graceljh]

Hello.
I knew that Full form check at MMC / two points check at LMC.
but I don't know the reason.
Is there anyone knows about it?

if someone knows about figure to related this question, please share it.

 

[drawoh]

graceljh,

Could this be something about go/nogo guages?

A go gauge tests all the features that the gauge must pass through.

A nogo gauge tests one feature only. This makes sense. If your gauge tests more than one feature, only one feature has to conform for your gauge to not pass.

Most of us do not use LMC a lot. I have used it on casting drawings to ensure there is material to be machined off later. I would not want you testing two points only.

--
JHG

 

[Belanger]

It also has to do with Rule #1. Read paragraph 2.7.1 of the ASME Y14.5 standard.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[MechNorth]

Rule #1 leads us to Full Form Check at MMC / 2-Point Check at MMC for RFS and MMC controls; it also tells us to do Full Form Check at LMC / 2-Point Check at MMC whenever the feature control tolerance or datum reference is made at LMC / LMB. Most people aren't aware of that second part as it isn't used terribly often.

As to the why; ASME's Rule #1 gives us that Size Controls Form. In essence, as a default, form deviations of a feature cannot cause the feature to extend beyond a boundary of perfect form when the feature is at its maximum material condition. When asked what this means in a class, I give them this; picture a hole for a 10mm dowel pin. That hole is expected to accept a pin up to max'm 10mm diameter. What happens if the pin at 10mm diameter has a bow to it? It won't fit in the hole now. Rule #1 allows us to do a boundary check to make sure that parts will mate (size-basis at least). ISO does it differently; their default is that size and form are independent aspects of the feature. So, in the ISO system if you you spec'd a 10mm diameter pin but didn't specify straigntness or cylindricity, there would be no restriction on the bowing of the pin and therefore a pin fabricated at its upper limit of size may not fit within the same-sized hole.




Jim Sykes, P.Eng, GDTP-S
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