Parallel Cylinders (Dowels or Pins) As Datum Features

optomist1

A Sea of Statistics
Super Moderator
Hi To All,

I have a question regarding the use of a pattern of features (dowels or pins) as a Datum Feature. Specifically when using a set of these features as a datum feature and referencing them in datum portion of a feature control frame, the material condition for the construction of the gage must be specified; MMC or LMC. To specify the set of features as RFS is next to impossible; tough to create a repeatable datum axis

How does Y14.5M -2009 standard address this? Is there a difference between 1994 and 2009?

Thank you...

Marty
 

Stijloor

Leader
Super Moderator
Hi To All,

I have a question regarding the use of a pattern of features (dowels or pins) as a Datum Feature. Specifically when using a set of these features as a datum feature and referencing them in datum portion of a feature control frame, the material condition for the construction of the gage must be specified; MMC or LMC. To specify the set of features as RFS is next to impossible; tough to create a repeatable datum axis

How does Y14.5M -2009 standard address this? Is there a difference between 1994 and 2009?

Thank you...

Marty

Marty,

Looking at ASME Y14.5-2009, Fig 4-35.

Four examples are provided consisting of a 3-pin pattern.
  1. The axis of an inscribed cylinder (inside the 3 pins simultaneously)
  2. The axis of a circumscribed cylinder (outside the 3 pins simultaneously)
  3. The centerplane of a distance (within) between 2 pins and 1 pin
  4. The centerplane of a distance (outside) between 2 pins and 1 pin

This is different from the 1994 version, where a centroid is developed within a pattern of features of size at MMC. This centroid (the mathematical midpoint within the pattern) becomes the datum axis.

Hope this helps.

Stijloor.
 

optomist1

A Sea of Statistics
Super Moderator
Hi Stijloor,

I do not have access to the 2009 version, more specifically, I noted on another website that the RFS condition is an acceptable material condition for use when using a pattern of features to establish a Datum Axis and construct a gage, am I of the mark?

Thanks again for your insight!!
Marty
 

Stijloor

Leader
Super Moderator
Hi Stijloor,

I do not have access to the 2009 version, more specifically, I noted on another website that the RFS condition is an acceptable material condition for use when using a pattern of features to establish a Datum Axis and construct a gage, am I of the mark?

Thanks again for your insight!!
Marty

ASME Y14.5M-1994 provides an example in Fig 4-22, and is explained in 4.5.8. The text only makes references to patterns of features of size at MMC.

Alex Krulikowski in his "Fundamentals of GD&T" book on pages 162 and 163 provides an example of a 4-pin pattern where the datum axis is a centroid of the pattern. In the feature control frame, this datum feature (the pattern) is referred to at MMC.

The datum feature simulator consists of a block with four pins at their virtual conditions from which the centroid (the datum axis) is developed.

Stijloor.
 
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