# Assumed Alignment and Assumed Datum on a Print

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#### kay-hh

Hello! No doubt somebody has posted something similar on this forum at some time or another, but I sure can't find it. I am hoping that I can get a definitive answer here because I seem to find myself stuck between two opinions that both make sense to me.

I have attached a sketch that depicts an assembly of two tubes attached to each other at the end, each with a bracket. the surface of one of the brackets is datum A and the hole in the same bracket is datum B. There is no other datum shown on the drawing, but there is a dotted line going through the center of the attached tubes. There is a true position callout on the hole of the second bracket and a basic dimension showing the distance between the holes, but not the distance from the centerline of the tube to the holes. The hole that is -B- is referenced, but not the hole we are checking(which is also 70). Size and location of both holes are fully drawn out and controlled on sub-assembly drawings.

So, my dilemma: The guys in the met lab say that the dotted line is an assumed datum, so the part is set up on plane -A-, aligned with the tube center line and true position is calculated using data derived from both the X (200) and Y (70). My design engineer says, nope, the Y(70) is controlled at the sub-assembly level, hence no basic dimension for this on the drawing. This is a straight line true position. Set up the part on datum A, call B zero and measure the shortest distance to the hole in the 2nd bracket.

Based on the function of the part, I agree with the design engineers intent, but I am really not sure if he drew it correctly or I need to request a drawing change. Either way, I am going to have to fully understand it myself in order to explain it to one of them. And if I do need a drawing change, what would best fit the intent?

Thank you so much! (boy, I hope the guys in the met lab are not reading this!)

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#### howste

##### Thaumaturge
Super Moderator
I see no reason to assume that the tube center line has anything to do with the true position callout. If it was supposed to be a datum it should have been called out as a datum. I don't claim to be a GD&T expert, but as it's drawn I would interpret it the way the design engineer did.

#### Stijloor

Staff member
Super Moderator
Hello! No doubt somebody has posted something similar on this forum at some time or another, but I sure can't find it. I am hoping that I can get a definitive answer here because I seem to find myself stuck between two opinions that both make sense to me.

I have attached a sketch that depicts an assembly of two tubes attached to each other at the end, each with a bracket. the surface of one of the brackets is datum A and the hole in the same bracket is datum B. There is no other datum shown on the drawing, but there is a dotted line going through the center of the attached tubes. There is a true position callout on the hole of the second bracket and a basic dimension showing the distance between the holes, but not the distance from the centerline of the tube to the holes. The hole that is -B- is referenced, but not the hole we are checking(which is also 70). Size and location of both holes are fully drawn out and controlled on sub-assembly drawings.

So, my dilemma: The guys in the met lab say that the dotted line is an assumed datum, so the part is set up on plane -A-, aligned with the tube center line and true position is calculated using data derived from both the X (200) and Y (70). My design engineer says, nope, the Y(70) is controlled at the sub-assembly level, hence no basic dimension for this on the drawing. This is a straight line true position. Set up the part on datum A, call B zero and measure the shortest distance to the hole in the 2nd bracket.

Based on the function of the part, I agree with the design engineers intent, but I am really not sure if he drew it correctly or I need to request a drawing change. Either way, I am going to have to fully understand it myself in order to explain it to one of them. And if I do need a drawing change, what would best fit the intent?

Thank you so much! (boy, I hope the guys in the met lab are not reading this!)
In order to inspect the part (hole location) 6 degrees of freedom must be constrained. The way it is drawn, the tube is still permitted to move along its axis (in a plane). The axis of the tube is not specified as a datum feature and can not be used to orient the part for proper inspection.

Imagine that this part is requested to be made by an external provider. They would immediately get on the phone and will ask a lot of questions.

GD&T is a very useful engineering language (which I have been teaching for many years ) but often greatly misunderstood. Engineers will not admit this.

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#### optomist1

##### A Sea of Statistics
Super Moderator
Stijloor,

The intent; the part can only be gaged/setup/measured one way....so as to eliminate any question or confusion....correct?

BR,
Marty

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