Please help... I need some input on this drawing

[Mike Johnston]

Hello everyone. I've posted 3 pictures. One of them shows where Datum A and B are located and the other ones show Datum C and Datum D. The question is about the post where the position is called out to Datum D. Some think that the rotation should be from Datum B to Datum D but I don't think so. I believe that what is being asked for is for the origin to be moved to Datum D and then where is the position of the post accordingly.

How would you guys and gals interpret this? First of all, I think that Datum C and Datum B are flipped where Datum B should be the rotational feature and Datum C should be the tertiary (sp?) datum. Anyway, any thoughts would be great. I'm sorry if I left out some information. If you need more info, please let me know. Thanks in advance!

Mike

[Jan d.]

So, what's not clear? You have a plane, a cylinder and a rotational feature.

Your problem is going to be the VC on your datums! Also, as Kevin pointed out to me some time ago, you may have simultaneous requirements (see page 93 of ASME Y14.5M-1994). I don't think 3.7 is going to handle that.

I have no idea how to do this in anything before V4.0. I know I can do this easily in V4.1, using the XactMeasure GD&T. Under Insert/Dimensions, you can actually define Simultaneous requirements. I tried it and I think it works.


Jan.

[Qinsp]

Hello everyone. I've posted 3 pictures. One of them shows where Datum A and B are located and the other ones show Datum C and Datum D. The question is about the post where the position is called out to Datum D. Some think that the rotation should be from Datum B to Datum D but I don't think so. I believe that what is being asked for is for the origin to be moved to Datum D and then where is the position of the post accordingly.

How would you guys and gals interpret this? First of all, I think that Datum C and Datum B are flipped where Datum B should be the rotational feature and Datum C should be the tertiary (sp?) datum. Anyway, any thoughts would be great. I'm sorry if I left out some information. If you need more info, please let me know. Thanks in advance!

Mike

D appears to be used to localize the features surrounding it to a higher local tolerance. But it looks like the engineer made a bo-bo. According to the print, base plane is A, origin XY to B, then use D to clock it. Do not move the origin. Or make a setup sheet and explain how you set origin to D.

[cmmguy]

What is the P with the circle around it mean? err.... in the sectional views right after the tolerance in the FCF.

[T Miller]

Projected tolerance

[Qinsp]

P = Projected. Essentially create a plane at that height, and intersect the hole (as a cylinder) to that plane. That is used to check tilt.

[Mike Johnston]

The "P" is a projected distance...

Wouldn't you just move the origin to Datum D and then ask for the position relevant to Datum D instead of Datum B? I don't understand why I would clock to Datum D. For some reason that just doesn't make sense. The basic dimensions on the print are linear to the main coordinate system. I've attached another diagram with the basics shown. If I were to rotate to Datum D then the basic dimensions would not apply. I appreciate your feedback! Thanks a bunch.

[Qinsp]

To clarify my first point why I think the drawing is in error, if the hole that forms D is already at the radial limit to B, you will squawk the features related to D even if they are nearly perfect.

[cmmguy]

Projected tolerance

Thanks, I had a brain fart on that one...

[bw_bob]

By clocking you are saying this

-A-.......level...z orgin
-B-&-C- or -B-&-D-.....rotate
X,Y orgin to -B-

Is that correct ???

[Qinsp]

The "P" is a projected distance...

Wouldn't you just move the origin to Datum D and then ask for the position relevant to Datum D instead of Datum B? I don't understand why I would clock to Datum D. For some reason that just doesn't make sense. The basic dimensions on the print are linear to the main coordinate system. I've attached another diagram with the basics shown. If I were to rotate to Datum D then the basic dimensions would not apply. I appreciate your feedback! Thanks a bunch.

The callout says to use B as an origin and D as tertiary. B would have no effect in that callout if you move the origin to D.

Make a setup sheet, show that you origined to D, then measure it that way, IMO. Your job is not correct eng'g mistakes, but to see if the part will work.

[cmmguy]

Why would you move your origin to D and not just set your alignment through D? Am I missing the section cuts for the D & E? I dont see what is wrong with the drawing. (Maybe not enough info, cause I cant see how the holes are dimensioned.)

[Qinsp]

There is nothing actually wrong with the drawing, just that if the goal was to move the origin to D, it didn't accomplish it. It would have had to been -A-!-D-!-B-.

Guess I'm looking at it this way. They added D to the callout for a reason. Apparently they care about the relationship of certain features to D. That callout doesn't really do it, except rotationally. The absolute distance between D and the features called out to D is not actually being controlled at all, only the rotational relationship. If all they cared about was rotation alone, I guess it would work.

Just my opinion based on seeing alot of prints.

[cmmguy]

The call out to D doesn't necessarily make it the origin. Without seeing the dimensioning to those features it is difficult to really interpret the call out. I think you are correct about the features called out to D as being a localization of the datums, but it probably is more about alignment instead of origin.

edit: Looking at the second picture posted by Mike - I would say that the alignment is shifted to the basic value of distance to D and then the "Y" origin is shifted to D also.

[#2#]

The way I see it...

Following the way a FCF is layed out, a datum fixes as many degrees of freedom (DOF) as possible then you move to the next datum.

A - plane - two rotational and one translational DOF fixed.
B- cylinder - can't fix any rotational (they're already taken by A) - two translational DOF fixed.
D- A and B fix 5 DOF's - D must fix the remaining rotational DOF.