Can i use a point as my datum?

[RedNova1]

Im new to this Gd&T stuff and have only had a couple courses on it. I know that you can't use imaginary points as datums... the drawing attached has a datum "A" we want to use but I am unsure if its allowed?
Any help would be great.
Thanks in advanced,

Red.

 

[pmarc]

Apologies for not giving you a quick and direct answer to your question, but why is this dimension so important functionally that it has been chosen as datum feature A? I am asking because at a glance this seems not to be a best choice.

Additional thing to consider - a feature which has been defined/ selected/agreed as a critical one does not have to be a datum feature straight away.

 

[Belanger]

A datum can indeed be a point, and often it's done with a "datum target." But your datum A seems to be a plane, not a point (since it extends into the depth of that .748 dimension). Maybe clarify if it is to continue across the .206 gap, or did you really want just a point?

There are a couple other things, if you are open to critique
Such as using flatness on the .118 dim. Did you mean for just the top surface of the .118 to be flat?

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

http://www.gdtseminars.com

 

[pmarc]

J-P,
I would say OP's datum A is rather a straight line, though drawing presentation is not clear enough to be 100% sure sbout that.

 

[fsincox]

RedNova1,
Welcome,
I really doubt the part will be stabilized for manufacturing or in it's final assembly, captured solely on those "points". Those “points” are really 2 lines on tapered faces and while they could be defined as datum targets lines, I suspect it may not really function that way, Ask yourself how does the part function?
The interesting thing here is I suspect the radius may come into play in the actual assembly but our current standard does not recommend using a radius as a datum a practice I cannot support. If it functions like a datum, it is one, whether the book allows it or not, It is the standards problem not the parts, INHO
Also, the flatness callout is incorrect, while the 2009 standard now allows flatness on features of size the depth you show cannot correctly be called a feature of size as the surfaces are not directly opposing faces.
Frank

 

[Belanger]

Right, pmarc ... it's a line. Still tricky to actually simulate.

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

http://www.gdtseminars.com

 

[fsincox]

To clarify, when I say "does not function that way" I mean as a primary datum. Those line may be important, only you know that.
Frank

 

[RedNova1]

I was told even though the datum may not be in right location for functionality as long as it was a legal call out that we are not to change the drawing...
thanks for the help.
Red.

 

[fsincox]

I would say what you have shown, as it stands, is not technically legal, guys? Just for your own information.
Frank

 

[RedNova1]

hey one more question, the basic 45° in the section view, would it be wrong to call it out like that, or does it need separate 45° on each angle?

 

[fsincox]

Looks like 8X to me?(8X 45deg)
Frank

 

[Belanger]

Maybe Frank is right -- datum A is not quite kosher. For one thing, it's not really a feature of size (directly opposing points). And even if you try to keep A where it is, l think the intent is to have the datum be taken through the full depth. As it is now, we are to simulate datum A along only two of those angled "wings" -- it stops at the gap of .206.

I'm confused by the 45º ... I see only 4 of those diagonal surfaces. But why is it even basic?

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

http://www.gdtseminars.com

 

[powerhound]

I don't think it's legal since it's not clearly defined. The OP is asking about points...but where? All I see is a dimension to an undefined place on a 90 degree angle. I see a .108 basic dimension but what is that saying. It doesn't seem to go to anything. I see a flatness attached to a non feature of size as if it were a feature of size (Frank's point). Datum A is probably not a feature of size, regardless of whether it's comprised of points or two lines so the datum shift on the other FCF's are probably illegal too. The 45 degree is basic...why? (J-P's point)

If all your guys are concerned with is whether or not something is legal, there's a whole lot of that to go around with this print without even talking about the question about datum A.

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

 

[DeanD3W]

I think you're doing a good thing by questioning this drawing. It needs a complete rework to be made meaningful.

No standard is referenced, about half of the characteristics of most features are left uncontrolled, and datum feature A is unclear.

Dean

http://www.d3w-engineering.com

 

[MechNorth]

ok, dealing just with Datum-A, where are you getting the idea that the datum is something other than an axis? I'm not an '82 expert, but as I recall the meaning of a datum callout applied to a FOS is the same from '82 to '94; specifically, it is the center plane or center axis of that FOS. If a circle of contact is intended, then a datum target line (as mentioned above) is needed. Am I missing something?
BTW, I'm assuming '82 (or earlier) because of the datum callout symbol.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[RedNova1]

Really appreciate the help.
I have questioned this drawing and have proposed it be re-dawin with proper GD&T but this well not happen. We have received parts that we excepted and work does not want any changes to be made to drawing in fear of a product problem...
I know this could become a future problem but the guys above me want nothing to do with change.
again thank all you for the help.

Red

 

[fsincox]

Jim,
I still feel that the datum concept shown, if it is correct to the intent, needs at least an opposing datum to capture the part. I suspect the "shaft dia" would does that nicely in the real function.
Frank

 

[fsincox]

I think we all work for those same guys.
Frank

 

[powerhound]

At the very least, the datum A should be changed to be represented by datum targets. In this case, datum lines. Your company's fear of a "product problem" is real but it's nothing that this drawing will alleviate. You can't even measure most of these dimensions so how do you know when a part meets print? Just because it works doesn't mean it meets print. If your method of inspection is just to put it in the assembly and see if it works, then get rid of the ASME spec on the drawing and change it to ISO 2768.

You can be assured of two things with this drawing:
1. If you have received good parts in the past, it is not because of this print, it is in spite of it.
2. If you don't get this print right, there will come a day when your company is going to have to shell out some bucks on parts that don't work.



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

 

[MechNorth]

Sorry, for some reason I was picturing this part as circular, so pls disregard my previous thoughts. Indeed, two datum target lines should be shown & specified, at the BASIC location, and not referenced at MMC.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com