Using only 2 datums to locate a hole.

[Madhu454]

Hi All,

This is a very basic question, but just wanted to know all of your opinion on this.

Please see the attached drawing, the hole is located with only two datums. Like other cases it does not consider the bottom surface which is perpendicular to the hole axis.

I know this won't control the perpendicularity of the hole axis wrt to bottom surafce. Also this wont create a DRF.

1) Does this is legal representation in ASME Std??
2) Does it is always required to have 3 mutually perpendicular planes to establish a origion for measurement (Like DRF). if my functionality does not demands for perpendicularity of the hole can I use only 2 datums.?
3) In ISO std it is allowed to mention only 2 datums as shown in the drawing. but not sure wheter it is allowed in ASME, I have not seen new std ASME- 2009.

Could yu please clear this doubt?

 

[Madhu454]

sorry some problem with the attachment.

 

[CheckerHater]

In my opinion this is legal, the other question is WHY? It is hard to imagine the design intent. Specifying location RFS also doesn't help. Just try to imagine fixturing of the part.

 

[Belanger]

Sure, it's fine with two datums. It just means that the hole might not be perpendicular to either face of the block.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[fsincox]

It is certainly legal; the standard itself shows it (ASME Y14.5M-1992, fig, 5-51). The question is more a philosophical issue than a "legal' issue. How much should a drawing define the actual function vs. just make me a part that "looks something like this".
Frank

 

[MechNorth]

2009 - "Section 1.4 Fundamental Rules: Dimensioning and tolerancing shall clearly define engineeringintent and hsall conform to the following.
...
(b) Dimensioning and tolerancing shall be complete so there is full understanding of the characteristics of each feature. ..."

The question then is whether the product definition is complete.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[Belanger]

Jim, why isn't it complete? (other than the fact that the print lacks the dimensions L x W x H). We have the implied 90º rule, with a presumed general tolerance, to cover the angles.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[powerhound]

If the thickness was .060" then it would certainly violate the "datums must be of sufficient size" rule. Therre would not be enough surface to sufficiently stabilize the primary datum plane.


Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[MechNorth]

J-P, I think I'll recant that. It may be complete. My mental bias is to have one of the faces (entry or exit) be a datum, but that's just due to habit. Looking at this particular item, I don't see that's the case. Tks for backstopping me.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com