Hole defined as datum feature

[aenman]

Hi,

I'm sure this is a simple question but I've been struggling to find the answer.

I have a hole in a flat rectangular part that carries a position tolerance of dia .002 relative to Datum B. Datum B is defined beneath the feature control frame of another hole which has a position tolerance of dia .005 relative to three mutually perpendicular datums.

My question is: does datum B move with the position of the hole that defines it, and if so, does this affect the position of the second hole? For example, if the first hole (defined as datum B) has basic dimensions of x and y from two perpendicular edges and a dia .002 position tolerance, this hole can theoretically be located x+.002 from one edge. When we go to position the second hole relative to datum B, will it be located by the basic position of datum B x&y, or by the location where the hole actually is located x+position tolerance & y+position tolerance?

Thanks and sorry to ramble.

 

[PaulJackson]

“Datum B is defined beneath the feature control frame of another hole which has a position tolerance of dia .005 relative to three mutually perpendicular datums.”

Assuming “dia.005” means that there is a diameter symbol preceding value .005... then the axis of diameter B must reside within a .005 diameter cylindrical tolerance zone that is oriented and located at the basic dimensions from the origin of the relative to the three mutually perpendicular (unnamed) datum features. If there was not a diameter symbol preceding value .005... then the axis must reside in a .005 cubical tolerance zone oriented and located from that “unnamed” datum reference.

“I have a hole in a flat rectangular part that carries a position tolerance of dia .002 relative to Datum B.”

If the position callout for the second hole declares only “B” as its datum reference...then its .002 diameter cylindrical tolerance zone must be oriented and located at the basic distance from the axis of the cylindrical datum feature “B”. Since the cylinder “B” is incapable of stopping rotation...the second holes’ .002 diameter cylindrical tolerance zone will only have a basic radial displacement from the axis of “B” (assuming that the axes of the two holes are parallel to one another).

No matter how basic dimensions are depicted on the drawing…you must figure (add, subtract, etc) the basic displacements relative to origin constructed by the callout’s datum reference frame. If the depth of the cylindrical datum feature B is too short to establish a stable and repeatable orientation for the cylindrical axis then you should question the design intent of having B as the primary datum feature for the callout.

 

[tgwow]

One thing I might add is that if Datum B is callout at MMC/LMC then the second hole is positioned relative to the virtual condition of Datum B and not the feature defined by B. Therefore, the second hole would only shift if the second hole's geo tol calls out Datum B at RFS.

 

[powerhound]

Let's start with the hole defined as datum B; I don't see how it can have a positional feature control frame associated with it if it isn't located with basic dimensions relative to datums. Are these "mutually perpendicular datums" actually called out as datums (ie., A, B, or C) or are they just dimensioned from the edge of the part and you are calling the edge a datum? Typically the B datum is your secondary datum so to have it called out from 3 other datums that are mutually perpendicular would tell me that those first 3 datums have restricted all degrees of freedom and your datum B hole should actually be a datum D at the least.
All the technicalities aside; on to your actual question: As long as your datum B hole is within locational tolerance with respect to the datums that locate it, it becomes datum B right where it is and all other features located from it should be located relative to where the datum B feature is and NOT to where it should be.

Powerhound
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[drawoh]

aenman,

Datum B is defined by its hole. If that hole moves WRT the three main datums, Datum_B moves with it.

This is a perfecly reasonable solution to a practical problem. Your hole must be located to the Datum_B hole more accurately than to the three main datums. Perhaps your part must interface to two round, accurately located features. You care much less about where the part's outline goes.

JHG

 

[ringman]

AENMAN,

If you have a rectangular part, which you indicate you do, the hole being related to B most likely will require another datum for orientation. It would be beneficial to know what version of Y14.5 is applicable to your drawing. Also, I dont believe that the plus or minus .002 from the edge is valid.

ringman

 

[aenman]

Thanks all for your help. We're in a very very preliminary stage of design here and the designers are just beginning to come up with coherent tolerancing schemes and asking me to do tolerance stackups. Since this is my first real exposure to GD&T I was getting tripped up with some of the details. This forum has been a great help to me while setting out on this quest.

 

[Tunalover]

aenman-
No offense intended and this is only to help. It looks like you need training in ASME Y14.5M-1994. It's really not a good idea to design products without a passable knowledge of this "language."


Tunalover

 

[palecactus]

if the to holes in question have relationships that tie them together then of course if you chang or re-locate on the other will respond depending on the mating relationship between the two,