True Position? 1

[metaldork]

So I'm drafting up some process prints for a run of new tombstones we have in our shop. Aside from the surfaces being machined we are putting a dowel hole pattern on the faces. We are looking to control the location of these holes to the bottom and to the center of the tombstone, which I believe will be our primary and secondary datums. My question is what would be ideal for our third datum?

 

[fsincox]

If it is (2) pins I would say you do no need any other. ASME Y14.5M-1994, Fig 4-29.
Frank

 

[CheckerHater]

What you describe most likely will be your secondary and tertiary datums.
Your primary will be face or back.
Hard to tell without seeing some details - how many sides, etc.

 

[metaldork]

Checker
It's a four-sided tombstone ..also I thought I read somewhere that the primary datum should be perpendicular to the feature which is what I think you're saying ...is this true?

 

[KENAT]

Checkerhater probably has it, yes generally the primary is perpendicular to the feature.

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[pmarc]

Is the tombstone rectangular block? Are these holes located on the side faces of the block? A simple sketch of geometry in Paint would help.

Though it's the most common case to have primary datum perpendicular to the hole, it doesn't always have to be what is really needed. For sure it's not a hard rule.

 

[CheckerHater]

Datums are HUGE area of GD&T; and you may need to check some additional sources.
For now, let say primary datum is what is the best to locate your part for machining / inspection.
In case of simple rectangular plate with holes in it, the "flat" would be your primary datum, then longer side the secondary, and the short - tertiary. Then you locate the holes to sides and orient them being perpendicular to "flat".
As far as I can imagine four-sided tombstone, things are little more complicated. The bottom actually can be your primary.
The 2 center planes derived from "width" and "depth" could be secondary and tertiary.
I will try to produce picture after lunchtime

 

[metaldork]

The tombstone is square and the holes are on the faces. I'm looking to control the position to the bottom plane and the center axis of the tombstone most critically, but need a third datum to keep it from spinning ....maybe a second axis off center?

 

[pmarc]

You may not need a tertiary datum. It seems your primary and secondary datum references constrain location of positional tolerance zone in a satisfactory way. Look at attached picture, just imagine there is only one (1) hole not four (4) and forget about two bottom segments of composite position tolerance callout. Tertiary datum feature is not needed in this case.

http://files.engineering.com/getfil...879d-2e8fd756cec9&file=2_datum_references.JPG

Do you see analogy?

 

[fsincox]

Pmarcs, example is a good one for a part not needing the third datum, it is not an absolute requirement. The functional requirement is key for me, which we do not know.
Frank

 

[CheckerHater]

This is how I think the part looks like.
Sorry, not always have time posting from work

 

[metaldork]

Here is a cross section of the tombstone. The bottom of the tombstone and the center of rotation which is the center bore in the bottom are critical. I want to control the tru position of the dowel hole pattern on the sides to these features. As it is I have my primary datum as A (the bottom plane)and secondary as B (center axis). Inspection requires that I have a third datum to stop rotation around datum axis B? I hope this clarifies and appreciate your assistance.

 

[pmarc]

Technically, it is up to you to choose the tertiary datum feature. You are the one who knows which features in the part play the most important functional role and can be considered as canditates for datum feature. From what is shown, this can be one of the holes in the bottom of part, but also width or height of bottom plate or widht or height of the column. It is really hard to tell without knowing how this part functions.

However from theoretical point of view your inspection is wrong. Tertiary datum feature is not needed as long as literally all features here (dowel holes, widths and heights of rectangular elements, holes in the bottom) are located with relation to the same datum reference frame |A|B|. In such case rule of simultaneous requirement is in charge, so everything can indeed rotate around datum axis B, but this is simultaneous rotation, meaning that all those features are in fact tied to each other.

 

[CheckerHater]

Your tertiary datum could be one of the holes in your base. Possibly dowel hole, or some other feature made with precision.

 

[CheckerHater]

pmarc,
From theoretical point your datums will constrain 5 out of 6 degrees of freedom, so your datum reference frame will be incomplete.

 

[pmarc]

CH,
That is right, but it does not have to be fully contrained, because the importance of last rotational degree of freedom around datun axis B will be somehow neglected by simultaneous requirement rule which will tie all features together in terms of rotation. The necessary condition is that all holes, widhts and heights are controlled relative to the same datum reference frame |A|B|.

 

[fsincox]

pmarc's solution may be you simplest one, it is similar to what evan proposed in another thread, just let the simultaneous requirement do it. I am uncomfortable with it, our shops may not like it, the ISO doesn't like it, but it is elegant for situations like these and most likely why ASME likes it.
Frank
thread1103-318114

 

[CheckerHater]

Guys,
You cannot have 3-dimensional part desribed using only 2 axis.
Where exactly ASME "likes" it, and where does it say datum reference frame does not have to be constrained?

 

[KENAT]

Minor point, but what drafting standard are you working to? Your sketch shows -A- which I believe implies an older version of ASME Y14.5.

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[fsincox]

CH,
It is just part of the rules of the ASME game. If they didn't like it they would change it, The ISO doesn't like it. You judge by people's actions, it has been ASME law since at least 1982. To me it is just like the envelope principle and AME's, these assumpltion of perfection ASME likes to make. Sometimes, like here, It makes the engineer's job easier.
Frank