Basic Dimensions Required for True Position? 3

[Daekar]

An engineer I'm working with has dimensioned a part in a similar fashion to the example provided, where a Datum face is included in a True Position control frame but the basic dimensions giving the position of the hole are referencing another face which has its location relative to the Datum given with a non-basic dimension. The example makes this much clearer, please ignore the fact that the hole doesn't have both X and Y positions defined since it's immaterial to my question.

http://files.engineering.com/getfil...6d-b1e9-0c4136b47424&file=Example_Drawing.JPG

Is this actually a correct way to dimension a part? Shouldn't the second face be another Datum and be referenced in the control frame instead? If the engineer is concerned with the position of the hole relative to the original datum, shouldn't they give the nominal position for the hole in basic dimensions? And if both matter, wouldn't a composite True Position tolerance be necessary?

 

[ewh]

There should be a clear path of basic dimensions relating the feature to the datums specified. It is incorrect to throw a ± dimension as shown in yuor examples.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[dgallup]

According to my guide, a legal tolerance of position specification must use basic dimensions to position the feature of size relative to the datums referenced.

 

[KENAT]

As ewh says, your example is wrong and TP feature needs to be tied via basic dims (can be a chain of basic dims) to the datum.

A very similar question was asked recently and got quite a few responses as I recall, you may be able to find it.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[Daekar]

You gentlemen don't happen to have a direction reference to ASME standard where this is stated, do you? This particular engineer directly stated that "Basic dimensions do not have to be tied to the datums."

 

[KENAT]

ASME Y14.5M-1994 section 5.2. I'm not sure there's a single concise phrase but it does say things like "Basic dimensions establish the true position from specified datum features"

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[ewh]

You could ask him how to accommodate ± tolerances in addition to a positional tolerance when doing a tolerance study. Maybe he can teach us all something. ;-)

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[TheTick]

Whoever came up with datum C-D and pointed it to a centerline needs to be flogged.

 

[Daekar]

TheTick: Actually, that was my ghetto shortcut of demonstrating there was a centerline defined between two Datum diameters without resorting to drawing another part view in Paintbrush. Yes, you're right, it is horrible.

KENAT: Thank you for the alphabet soup! I hate having to have something like that to "point at" but sometimes it's necessary...

ewh: I think it would make his brain hurt, since it sure does mine. I think he'd explain how he intended it to work and I'd be forced to point out that the second face should then be a Datum.

 

[TheTick]

You are granted clemency. Flogging is stayed.

 

[Belanger]

You can also quote from the standard where it defines "true position" (para. 5.2 in 1994 or para. 7.2 in 2009). In black and white, it says that a position tolerance defines the allowable variation "from a true (theoretically exact) position."
It continues: "Basic dimensions establish a true position from specified datums..."
There's your ticket


John-Paul Belanger
Certified Sr. GD&T Professional

http://www.gdtseminars.com

 

[TheTick]

You could have two dimensions for the .15 thickness, one size and one basic. The basic dimensions add without tolerance.

I remember seeing this was allowed the firs time I went through training. Technically, it is not "double-dimensioned", as there is no conflict for measuring and reporting.

 

[dgallup]

While you could do that, what would it accomplish compared to just giving a single basic dim from the datum?

 

[TheTick]

Dunno, does it matter? Just supplying a possible solution. All the easy obvious stuff was taken already.

 

[PeterStock]

If datum A is the mating surface then the dimension should be moved to come from datum A. If not then datum A needs to be moved to the other (mating) side.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

http://www.infotechpr.net

 

[drawoh]

My interpretation of the OP's drawing is that the 1.25 basic dimension varies depending on the actual value of the 0.15 dimension. A 1.40 basic dimension from datum[ ]A would be better drafting practise.

I have not got ASME Y14.5M-1982 in front of me. The datum box is from 1982. Is there any significance to this? This is a general question for everyone.

JHG

 

[ModulusCT]

Isn't Datum B the axis of the cylindrical feature? I don't understand the point of C-D. If B were the outer diameter of the part than the datum should be attached to a extension line of the diameter dimension which is also missing.

Sorry if this is a dumb question. I understand the picture wasn't created to spec.

 

[Daekar]

ModulusCT: The part in question is actually rectangular with a cylindrical bore. Datums C and D would therefore be circular features of (differing) size at each end of the part.

 

[ModulusCT]

Oh, I see... OK thanks for that clarification.