CMM Shortcomings - GD&T - Inability of CMMs to verify the MMC situation

optomist1

A Sea of Statistics
Trusted Information Resource
#1
Hi To All,

I am relatively new to formal GD&T training/indoctrination with a rudimentary understanding of CMMs, I previously read several posts regarding the inability of CMMs to verify the MMC situation. Could someone set straight on this...if this is the case are there other conditions/situations that CMMs cannot verifiy or prove. Is this a function of the CMM manufacturer/software package

Thank you for your assistance..

Marty
 
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Stijloor

Staff member
Super Moderator
#2
Re: CMM shortcomings GD&T

Hi To All,

I am relatively new to formal GD&T training/indoctrination with a rudimentary understanding of CMMs, I previously read several posts regarding the inability of CMMs to verify the MMC situation. Could someone set straight on this...if this is the case are there other conditions/situations that CMMs cannot verifiy or prove. Is this a function of the CMM manufacturer/software package

Thank you for your assistance..

Marty
We have quite a number of Cove Members who are experts in CMM measurements and software applications. "True Position" is one of them..

I am sure they will chime in.

Stijloor.
 
T

True Position

#3
Re: CMM shortcomings GD&T

Hi To All,

I am relatively new to formal GD&T training/indoctrination with a rudimentary understanding of CMMs, I previously read several posts regarding the inability of CMMs to verify the MMC situation. Could someone set straight on this...if this is the case are there other conditions/situations that CMMs cannot verifiy or prove. Is this a function of the CMM manufacturer/software package

Thank you for your assistance..

Marty
It's not a failure of them to calculate MMC on a feature, that works fine. The problem is how some CMMs apply MMC(and LMC) to secondary and tertiary datums. Per the ASME standard, these only apply freedom to move the whole alignment inside their tolerance zones. Some older (and possibly still current?) CMM software just apply additional bonus tolerance to the feature. Zeiss Calypso does this correctly.
 
D

David DeLong

#4
Re: CMM shortcomings GD&T

Hi To All,

I am relatively new to formal GD&T training/indoctrination with a rudimentary understanding of CMMs, I previously read several posts regarding the inability of CMMs to verify the MMC situation. Could someone set straight on this...if this is the case are there other conditions/situations that CMMs cannot verifiy or prove. Is this a function of the CMM manufacturer/software package

Thank you for your assistance..

Marty
CMMs are great machines but are not the best method of checking runouts. Here, the chuck or divider head with a dial indicator is best.

When one gets into positional at MMC with secondary and tertiary datums also at MMC, only some machines can actually pick up the bonus tolerance accrued through the datums at MMC. Most end up in a RFB situation where the CCM could reject a part while the checking fixture might accept the part.

Just look at a positional fixture with 8 holes at MMC. A pin of virtual condition size would enter each hole and confirm that 100% of the hole full depth would meet this boundary. A CMM would would take many readings depending upon the Operator but 100% of the hole? I don't think so.

These are a few thoughts.
 

optomist1

A Sea of Statistics
Trusted Information Resource
#5
Good Day Dave and True Position,

Thank you both for your detailed responses.....during the interim I located a free Zeiss GD&T Y14.5 M - 1994 pdf handout, I'll check this out as well.

Thanks again...

Marty
 
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