True Position with only 1 Datum - Referenced datum is the plane the hole is on

J

JasonFdz

#1
True position with only one datum? I normaly do not have a problem with true position call outs, but this is confusing. True position on a hole and the referenced datum is the plane the hole is on. Can some one please help??
 
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Cari Spears

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Super Moderator
#3
True position with only one datum? ... True position on a hole and the referenced datum is the plane the hole is on.
Hi, Jason :bigwave:
So it only references one datum in the feature control frame? Are there other datums called out on the drawing? What feature(s) do your basic dimensions come off of?
 
J

JasonFdz

#4
Hi, thanks for replying, basic dimensions are referenced but it seem i cant use them since only the one datum is specified. It seems that it would be the exact same callout twice then. please see the attached pdf file which i posted after you replied.:eek:
 

Cari Spears

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#5
please see the attached pdf file which i posted after you replied.:eek:
I saw we posted simultaneously.:D

You can calculate the true position with only one datum in the feature control frame - you need the deviation from the basic dimension from that basic dimension's origin. In this case the basic dimension 30 from the center of the large (are those gear teeth?) diameter in the middle and the basic dimension 17.3 from the hole labeled "B".

Are you setting this up on a CMM - or using surface plate / height gage set-up?
 
Last edited:
J

JasonFdz

#6
Im setting this up on a cmm, I'm using calypso so values are being generated, Im just not really sure what is being asked, it seems more like perpendicularity or concentricity should be used, how would you set up on a height gage?
 

Cari Spears

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#7
... how would you set up on a height gage?
I wouldn't - given a choice. I just wanted to make sure I didn't go into CMM set up detail if you weren't using one.

Since the feature control frame only calls out A, I would default to alphabetical order and set the part up to datums A/B/C. Then I would rotate the setup to the degrees specified and tell the program the origin of the basic dimension is "Hole XX". (I've never used Calypso.)
 
J

JasonFdz

#8
Ok, thanks, One last question, what then would be the difference of No 130 and 131 since they would basically have the same set up and origin. (nominals)
 
D

David DeLong

#9
The positional that I saw reflected 3 holes marked as "A" relative to datum A to become a datum B. Sounds confusing but that is what I read.

This means there is a hole to hole position of a diametrical tolerance zone of 0.1 mm beyond the MMC (maximum material condition) of the 3 holes to themselves and perpendicular to datum A. If they meet this requirement, the 3 hole pattern now becomes datum B.

To confirm the positional of the three holes, it is best to use a checking fixture since one cannot confirm all three holes at once on a CMM. If you are forced to use a CMM, most CMM Operators would create a datum on one of the holes (not quite theory) and then confirm the others to that hole.

Hope this helps.
 
J

JasonFdz

#10
Dave,
That makes perfect sense............let me make sure i understand you correctly, create a circle using the three referenced holes (center point), create an alignment from each hole to the center of the three, and then simply ensure that the distance is within .1 of each other.
 
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