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Hole location

Hole location

(OP)


Hole is used as a secondary datum feature (B) in the assembly:

I was told that using position to control only the orientation of the hole is a nay-nay in ASME 2009.
The the question becomes how to control its location?
A-primary= a flat surface
B=secondary datum feature. Should be only oriented to the primary, correct?
Then what about its location to A?

Should be a basic dimension shown from A?

RE: Hole location

Since the hole is at a distance from A, you can (common wisdom) use position with a basic dimension. The faces of the part can then be located and oriented to the two datums.

RE: Hole location

(OP)
All,

Again, per my (and others around the company I am with) understanding is, since datum feature B is secondary, it needs only an orientation control to the primary (surface A).
So, someone idea was to control only its parallelism to datum feature A. And to control all the other features / surfaces --including the sides --- with profile to A primary and B secondary. (with basic angle to the horizontal datum feature A).

But the issue was (and still is) how far from A datum feature B needs to be?
Is it appropriate to have a ± dimension from A (along with its parallelism)?

Do you have any good examples of this case shown somewhere? (including some GD&T books)?

RE: Hole location

It's a set of rules. There is no need for examples of every possible rules-compliant solution.

Datum B needs to be 5 mm from A.

RE: Hole location

Quote (gabimot)

Again, per my (and others around the company I am with) understanding is, since datum feature B is secondary, it needs only an orientation control to the primary (surface A).

Though many examples show that strategy, there's nothing forcing you to do that.

If you want published examples of a secondary datum feature being located relative to a primary datum feature with basic dims, see Figures 4-29 through 4-32 in the 2009 standard. However, as 3DDave says, a lack of a specific example in the published literature is not proof of much.

RE: Hole location

gabimot,

Datum A is the bottom face of your part. If datum A were the front face, a positional tolerance on the hole would control perpendicularity. As it is, a positional tolerance on the hole controls elevation and parallelism.

--
JHG

RE: Hole location

(OP)

Quote (Nescius)

If you want published examples of a secondary datum feature being located relative to a primary datum feature with basic dims

I guess I would like some examples of a secondary datum feature being ORIENTED relative to a primary datum feature (not located).
4-29 - 4.32 shows B= secondary datum feature being the surface (or the middle plane) as a secondary and in my case the surface is primary (not secondary).

Therefore, the question still remains: how to ORIENT (not locate) the secondary datum feature (the thru hole) to the primary one (the bottom surface)?
Why I want to orient only (the hole) and not locate it? Because it is secondary datum feature and then only needs ORIENTATION (not location).
Then I will locate everything else to it or to the DRF made from A primary and B secondary.


Quote (drawoh)

If datum A were the front face, a positional tolerance on the hole would control perpendicularity

Well, it is not , hence my dilema/ question.....

Quote (drawoh)

As it is, a positional tolerance on the hole controls elevation and parallelism.
yes, but the positional callout/ tolerance will LOCATE the hole and I don't want that the hole to be located. I want it to be only ORIENTED to the primary datum feature A (the bottom surface)


RE: Hole location

gabimot,

It sounds to me as if your hole should be the primary datum, and the bottom face the secondary datum. On the bottom face, you specify parallel, profile, and possibly flatness.

--
JHG

RE: Hole location

(OP)

Quote (drawoh)

It sounds to me as if your hole should be the primary datum, and the bottom face the secondary datum. On the bottom face, you specify parallel, profile, and possibly flatness.


The physical reality of this assembly is that the part is sitting on datum feature A and then the hole is used with its mating component (not vice-versa). The hole is free to float - up and down within some tolerance from datum feature A.

RE: Hole location

gabimot,

How about you place a composite FCF showing an accurate parallel tolerance, and a sloppy positional tolerance?

--
JHG

RE: Hole location

(OP)

Quote (drawoh)

How about you place a composite FCF showing an accurate parallel tolerance, and a sloppy positional tolerance?

Could you please provide more details? Thank you.
Can I locate the hole in only one direction (only vertical) and the position callout is still valid?


RE: Hole location

gabimot,

If your positional tolerance is from datam A, it can only work from one direction, up and down.

--
JHG

RE: Hole location

(OP)
So, will be a composite with no diametral symbol "Ø" vertical aligned?
What about the lower segment? Will it have "Ø" symbol ?

RE: Hole location

If the shape of the hole's tolerance zone is a cylinder then you need the dia. symbol.

RE: Hole location

(OP)
But the issue is that is controlling the position in only one direction?

RE: Hole location

If the hole is the primary datum feature then you can profile the surfaces w.r.t. the hole.

Yes, functionally the bottom surface is the primary datum but sometimes compromises can be made (if there is a good enough reason - I have had to do this before). How much of a difference is there (tolerance-wise) if the bottom surface is profiled from the hole instead of if the hole is positioned from the surface? If there is a difference can you adjust the tolerances of these feature to compensate?

RE: Hole location

gabimot:

It appears you might need to re-familiarize yourself with the control of motion offered by position vs orientation. There are 3 translation and 3 rotation degrees-of-freedom (DOF) of motion - see 4.4 Orientation only restricts rotational DOF - see para 6.4. Position can control all 6 DOF depending on the number of datums specified.

Certified Sr. GD&T Professional

RE: Hole location

mkcski,

Gabimot's problem is that his hole is datum B (C???). The part is not yet restrained when the hole is measured. At some point, the front or rear face must be selected as a datum. The measurable tolerance zone for the hole is a straight line up and down. He could just as easily apply a ±tolerance to the hole, although I understand this is being disparaged by the standard.

--
JHG

RE: Hole location

Neither the front or rear face is required to be a datum. The bottom face and the hole are sufficient to fix the location and orientation of the part.

RE: Hole location

gabimot,

The drawing should have a basic dimension between datum feature A and the hole axis. This is required to properly construct the datum reference frame you describe based on the two features. Refer to ASME Y14.5-2009 para. 4.5.2 for the specific requirement.

To control the distance between datum A and the hole axis, an ordinary RFS position tolerance with a cylindrical tolerance zone is perfectly appropriate. Why do you feel this is a problem?

If you want to control the orientation of the hole axis relative to datum A to a tighter limit than the position, you could additionally apply a parallelism or angularity tolerance. Again, a cylindrical tolerance zone is fine.

pylfrm

RE: Hole location

(OP)

Quote (pylfrm)

To control the distance between datum A and the hole axis, an ordinary RFS position tolerance with a cylindrical tolerance zone is perfectly appropriate. Why do you feel this is a problem?

Because we want to control only the hole's orientaion to datum feature A (the flat surface) and not its location. Everything else is controlled FROM A primary and B secondary.

The idea is that the secondary datum feature should be oriented to the primary. (if there were two holes then position is the correct way to call it since position will also control the mutual relationship between the holes, but in my case there is only one hole hence the conundrum)

Maybe the idea and the concept that the secondary datum feature must only be oriented to the primary is not always applicable. So, I am looking for some examples (GD&T books/ pages or websites) where I can see other good applications of the method.

RE: Hole location

3DDave,

The bottom face locates in elevation, and two rotation axes. Based on the OPs commments, the hole stops side to side movement, and the other rotation axis if it is datum B. What keeps it from moving fore and aft?

--
JHG

RE: Hole location

Nothing. It isn't required if it's just the prismatic shape shown.

Make the front and back faces profile to [A|B] and viola.

RE: Hole location

I haven't read every response in this thread so forgive me if this has already been stated. The control of the secondary datum feature is determined by its relationship to the primary. If the only relationship the secondary has to the primary is orientation, then all you need is an orientation tolerance. If the relationship is one of location then you need a locational tolerance. In this case the relationship is locational since the hole is a certain distance from A. Yes, the hole is parallel to A but location trumps orientation when it comes to datum referencing. If the hole was actually perpendicular to A then there is no more locational relationship and only orientation remains, thus you could legally use just perpendicularity as the datum feature control.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

RE: Hole location

I remember a thread not too long ago where the question of using position to orient the secondary datum feature (of size) was discussed. The secondary datum feature was perpendicular to the primary datum feature (surface). This was discussed at length and pretty much everyone agreed that orientation should be used for a single feature and position can be used if multiple features (of size) made up the secondary datum feature. We never discussed what to do if the secondary datum feature (of size) was parallel to the primary datum feature. I think this is what was tripping people up (myself included). I think that some of us were assuming that the perpendicular case carried over to the parallel case.

RE: Hole location

Quote (gabimot)

Should be only oriented to the primary, correct?

It does not actually matter if the hole is datum feature or if it is just an ordinary hole controlled relative to some datums. It simply needs a location tolerance to avoid incomplete drawing specification. With parallelism only, the hole can wander up and down without any limitation, which is a serious drawing error.


Quote (3DDave)

Nothing. It isn't required if it's just the prismatic shape shown.

Make the front and back faces profile to [A|B] and viola.

True, but you can also just use A on the drawing and control the hole with position to A and the outline with profile to A. B will not be needed then at all.

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