GD&T (ANSI Y14.5) Perpendicularity Callouts

M

mattm1958

#1
We have a number of parts that have a short bore (1-1.5") bored thru a large face (10-15"diameter). The perpendicularity callout is typically about .002.

Here's the problem; parts have checked OK for years, but they are always written up as non-conforming because the CMM area insists on checking it both ways; 1) short bore as the major alignment and then 2) large plane as the major alignment. Reported results are typically .0040 and .0005 respectively, so they always fail one way or the other.

So the question is; Does ANSI Y14.5 perpendicularity really call out a precedence depending on where the symbol (on bore or face) is placed, or can we use common sense here and discard the highly variable reading?

Seems we're spinning wheels and spending time for nothing. Thanks in advance.
 
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#2
We have a number of parts that have a short bore (1-1.5") bored thru a large face (10-15"diameter). The perpendicularity callout is typically about .002.

Here's the problem; parts have checked OK for years, but they are always written up as non-conforming because the CMM area insists on checking it both ways; 1) short bore as the major alignment and then 2) large plane as the major alignment. Reported results are typically .0040 and .0005 respectively, so they always fail one way or the other.

So the question is; Does ANSI Y14.5 perpendicularity really call out a precedence depending on where the symbol (on bore or face) is placed, or can we use common sense here and discard the highly variable reading?

Seems we're spinning wheels and spending time for nothing. Thanks in advance.
The feature block should show what the datum is, usually the large plane
 

Stijloor

Staff member
Super Moderator
#3
We have a number of parts that have a short bore (1-1.5") bored thru a large face (10-15"diameter). The perpendicularity callout is typically about .002.

Here's the problem; parts have checked OK for years, but they are always written up as non-conforming because the CMM area insists on checking it both ways; 1) short bore as the major alignment and then 2) large plane as the major alignment. Reported results are typically .0040 and .0005 respectively, so they always fail one way or the other.

So the question is; Does ANSI Y14.5 perpendicularity really call out a precedence depending on where the symbol (on bore or face) is placed, or can we use common sense here and discard the highly variable reading?

Seems we're spinning wheels and spending time for nothing. Thanks in advance.
The precedence is determined by where the datum feature symbol is placed.

First of all. What GD&T Standard is referred to? ASME Y14.5M-1994? ASME Y14.5-2009? An older GD&T standard perhaps?

Next. Look at where the datum feature symbol has been placed. Most likely the large surface. That will be your datum feature from which the datum will be simulated..

Next, look at the feature control frame. The axis of the bore will then be perpendicular within .XXX" relative to the datum feature. The tolerance zone will likely be cylindrical (look for the diameter symbol).

Checking the perpendicularity of the large surface relative to the bore is "bass ackwards.." :nope:
 
M

mattm1958

#4
So, if I'm reading you correctly, if the feature control frame is on the BORE and references as its primary datum, the FACE, then that would tell you that you have to establish the FACE datum then check the BORE.

I agree that using two features, like 1) the center of the bore at the top and 2) the center of the bore at the bottom, so close together to set up an alignment, is setting us up for problems, but its not specific (to the level of CMM feature selection order) in Y14.5. That conclusion is reached by interpretation.

So when looking at a print, when the FCF is on the bore centerline, it means establish the face first. Are there any words in Y14.5 that speak to this more clearly? I can't find any.

Thanks for the replies.
 

Stijloor

Staff member
Super Moderator
#5
So, if I'm reading you correctly, if the feature control frame is on the BORE and references as its primary datum, the FACE, then that would tell you that you have to establish the FACE datum then check the BORE.

I agree that using two features, like 1) the center of the bore at the top and 2) the center of the bore at the bottom, so close together to set up an alignment, is setting us up for problems, but its not specific (to the level of CMM feature selection order) in Y14.5. That conclusion is reached by interpretation.

So when looking at a print, when the FCF is on the bore centerline, it means establish the face first. Are there any words in Y14.5 that speak to this more clearly? I can't find any.

Thanks for the replies.
First of all, do you have ASME Y14.5 available?

Secondly, without an actual sketch or drawing, it remains guesswork for me. ;)

When the Feature Control Frame is applied/attached to the bore, the bore is the controlled feature. The feature control frame should show the datum (sequence). So, if the large surface is identified with a datum feature symbol, let's say "X", and the feature control frame shows "X" as well, than the large surface is your (primary) datum.

Show us a picture!! :agree1:
 
T

True Position

#6
A drawing would help, but if the FCF has the small bore as the datum then you need to measure it that way unless the design responsible engineer can update the print.
 
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