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GDT Standard
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Posted by: raphalla

12/01/2006, 22:11:27

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Are there any standard GDT way to label the center of a cross (+)in either X or Y direction?
Means: When people see how you label, then they definitely know the datum is taking from the center of the cross in X or Y direction.
Thanks!







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Re: GDT Standard
Re: GDT Standard -- raphalla Post Reply Top of thread Forum
Posted by: Kelly Bramble

12/01/2006, 22:31:46

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The center line cross symbol (+) means that the cylindrical feature is perpendicular to the given drawing view. A center line ( -------- - --------) also indicates that the associated cylindrical feature is perpendicular to the given drawing view.

A fundamental rule in GD&T is that a datum is never associated or attached directly to a center line. Datums are always declared and established from a real physical feature. If you declare a cylindrical feature as a datum, then the datum is derived from the highest physical contact areas of the cylinder by orienting and locating a cylindrical datum simulator against these locations. From these highest contact areas devrived from the datum simulator, the datum center line (two perpendicular datum planes) is established.

See the book "Geometric Boundaries" The Interpretation and Applciation of Geometric Dimensioning and Tolerancing per ASME Y14.5M - -1994 for more information on datums.

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Modified by Kelly Bramble at Fri, Dec 01, 2006, 22:43:19


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Re: Re: GDT Standard
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Posted by: traingdt

12/02/2006, 08:26:09

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Also, If you want the datum to operate in only one direction, you need to indicate that direction by how the datum feature symbol is attached to the physical feature. If it's a cylindrical hole, but you only want the datum in the X (horizontal) direction, then use extension lines coming vertically off of the hole and attach the datum feature symbol to a dimension arrow spanning across those extension lines.







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Re: GDT Standard
Re: Re: Re: GDT Standard -- traingdt Post Reply Top of thread Forum
Posted by: raphalla

12/05/2006, 01:13:28

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As per the picture, am i correct to say that, this is the correct way to label the datum?
Datum H is at the real feature, but all dimensions are actually supposed to take from the midline of the 2 ribs (indicated by the dotted line in between of the ribs.)

Pic1.JPG (20.2 KB)  






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Re: Re: GDT Standard
Re: Re: GDT Standard -- raphalla Post Reply Top of thread Forum
Posted by: Kelly Bramble

12/05/2006, 07:36:47

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In this case, since the datum is associated with the dimension extension line on the left, then the datum feature is the assoociated surface on the left.

If you want the datum to resolve into the center (plane) of the slot feature, then you will nned to move the datum anchor so that it is aligned with the dimension limit of size arrow ---<|<---. It os ok if the datum anchor is on top of the datum arrow.








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Re: GDT Standard
Re: Re: GDT Standard -- raphalla Post Reply Top of thread Forum
Posted by: raphalla

12/05/2006, 02:02:28

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Another question:
With the labelling of Datum B and Datum C as in the drawing, does it correctly defined that for the line in Y direction, a middle line should be formed from the midline of both "cross" (+)?
Thanks!

Pic2.JPG (19.7 KB)  






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Re: Re: GDT Standard
Re: Re: GDT Standard -- raphalla Post Reply Top of thread Forum
Posted by: Kelly Bramble

12/05/2006, 07:43:03

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Not even remotely, I assume these are the drive features of phillips head screws.

You realy, realy, realy need study a book, or take a class and read a book on GD&T ASME Y14.5M - 1994. Applying the principles of GD&T is not difficult, however not trivial.

There is so much about identifying, selecting and applying datums that you need to learn. Normally, I spend about four hours of class room time (three day class) teaching just datums, and still most folks have minor questions afterwards..







Modified by Kelly Bramble at Tue, Dec 05, 2006, 07:49:11


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