Datum Feature Symbol placement

[GreenMindmn]

I have searched through my GD&T books and have come up short on certain situations. All per ASME Y14.5M-1994.

1) Datum Center Plane (see

http://i18.tinypic.com/68j3ode.jpg

)
I know that datum B would be valid in defining the center plane datum from the .325 dimension. But does datum A also work in this same instance?

2) Datum Axis (see

http://i10.tinypic.com/6cyanif.jpg)

For the shown shaft, I know that Datum C is correct. But would datum A,B also be correct?
I know if you are looking at the ends of the shaft, you can define a datum by placing it on the circle of the shaft. Which makes me believe that A would also be valid because you have a center-line defining that the object is cylindrical.

I am still fairly new to GD&T so any help would be appreciated.

 

[sbozy25]

If I am reading the pictures correctly....

1: Your assumptions are correct.

2: Only Datum A & B can be used as a datum axis. Datum C would be interpereted to be the outside surface of the shaft.

I looked this up in a book and referenced a few files of mine. A few years ago my GD&T checker and I had an argument about a situation similar to #2. I thought that datum C could also be a datum axis, however after many wasted hours of arguing I conceeded that he was the expert and it was not.

hope this helps.

 

[ewh]

Your Datum C callout would also be correct... see ASME Y14.5-1994 para 3.3.2(b) and figs. 3-4 (a) thru (c).

As far as refering to the outside surface of the shaft as a datum, this is still correct. The surface would be the datum feature and the centerline would be the datum axis (of that feature). Datum axis' are extensively used but you don't call out the axis (centerline) as a datum.

I know others will feel free to correct me if I'm wrong.

 

[powerhound]

You can also look at fig. 4-8. The B datum is directly attached to the wall of the hole but the datum is understood to be the axis, not the inside surface of the hole.

Powerhound
Production Supervisor
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Mastercam X2
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Taji, Iraq OIF II

 

[vigildesign]

For an axis, datum b would be incorrect. You would have to attach it to leader, which could have a horizontal segment in this case. See Fig. 5-2 of Y14.5. I have never seen a datum attached directly to a number.

Marcelino Vigil
GDTP T-0377

 

[ewh]

I have never seen it attached to a number either, but I felt that the intent was clear.

 

[ctopher]

My 2 cents.
In addition to the GD&T:
Outside machining, arrows on outside; inside mach, arrows on inside; Holes, arrows on inside or point to hole.
Can be the same for GD&T symbols.
This reduces second guessing by machinists as is more clear.
This is my drafting practice for past twenty years and vendors/machinists like it.

Chris
SolidWorks 07 4.0/PDMWorks 07
AutoCAD 06
ctopher's home (updated 04-21-07)

 

[PaulJackson]

There a many ways to identify the datum feature but I don't think that it is proper to attach the symbol to the dimension itself but rather the leader line. Also the symbols should not be attached to center-lines but rather the features that create the centerlines.

see axis_as_datum

http://tinypic.com/625sq6s.gif

see center_plane_as_datum

http://tinypic.com/6g9if7l.gif

 

[GDTGUY]

ASME Y14.5M-1994, paragraph 3.3.2 describes the application of a datum feature symbol in the following acceptable methiods.

-- placed on the outline of a feature surface, or on an extension line of the feature outline, clearly separated from the dimension line, when the datum feature is the surface itself.

-- placed on an extension of the dimension line of a feature of size when the datum is the axis or center plane.

-- placed on the outline of a cylindrical feature surface or an extension line of the feature outline, separated from the size dimension, when the datum is the axis.

-- placed on a dimension leader line to the feature size dimension where no geometrical tolerance and feature control frame are used.

-- placed on the planese stablishedb by datum targets on complex or irregular datum features ....

-- placed above or below and attached to the feature control frame when the feature (or group of features) controlled is the datum axis or datum center plane.

GDT_GUY