Datum interpretation 4

[SeasonLee]

Attached print is a fiber glass reinforced resin molded part, my question is:

1.The pivot hole is datum B, section B-B defined the datum, but I can’t image how to set up the datum, it’s a datum point, datum line or datum area?

2.There is a position callout on the pivot hole, can we use the pinot hole itself as a 2nd reference datum in the DRF?

Thanks in advance for the replies.

SaesonLee

 

[SeasonLee]

One datum reference is quite often used in positional control of a hole, can I make a change on the pivot hole position callout by deleting the datum reference B, that means only one datum reference will be remained in the FCF.

|POS|Ø.020|A|B| >>> |POS|Ø.020|A|

If this is legal, may I ask the difference between them?

Thanks

 

[dtmbiz]

I’d like to know “if” datum B is two internal target points / lines, and there is some physical gage
pin touching those points/lines; how does one get a gage pin or CMM probe into the full length
of the hole to inspect its location? Datum selection requires “accessibility”.

If Datum A is the only referenced datum, then how do you set this part up to inspect the pivot hole?
It would only be mounted on flat datum feature A. It should be perpendicular to datum center plane C.
It also must have a datum reference to the location from the inserts.
A position control requires basic dimensions

5.2 (1994)
....Basic dimensions establish the true position from
specified datum features and between interrelated
features....

4.2.2.1 Mutually Perpendicular Planes. The
planes of the datum reference frame are simulated
in a mutually perpendicular relationship to provide
direction as well as the origin for related dimensions
and measurements...

Not lecturing you SeasonLee... just supporting my position....

Why not use the insert pattern as datum B and remove the confusion from the target points and datum B being hung off of the hole callout?

It appears that the pivot hole will be the hinge to close the cover and relative to the inserts and fasten the cover.

 

[SeasonLee]

dtmbiz

Thanks for the input, as we discussed earlier that we all accepted the datum B should be a datum target line, and your concern is absolutely right on the datum setup, we can simulate the theoretical datum by CMM but I know some of the inspector will use physical pin gage instead (for easy work) to establish the DRF.

The pivot hole acts as the door hinge, I think the designer will not consider other features as datum B. There is only one basic dimension to define the pivot hole location and it will not help you on the position tolerance calculation with the referenced datum B.

Thanks

SeasonLee

 

[GDTcoach]

SeasonLee, there is no problem dropping the datum feature B reference in the position tolerance on the pivot hole. Since it is the secondary datum feature, other features will be located from its axis. It is the origin for other features in the "vertical" direction. Also, not certain why you think the datum targets are lines. They look like points to me.

 

[SeasonLee]

Thanks for your comments, Don. The question arisen is when should we use one datum reference on the pivot hole position callout and when should we use two referenced datum, and what is the difference between them?

Indeed, the datum target on the print is point, but there is only one point on both sides, how can they establish the datum axis, as what I know it should be at least three points on both sides of the pivot hole, just like Fig. 4-50 on 2009 standard, the three datum target points are equally spaced. Please let me know if I missed anything.

Thanks

SeasonLee

 

[powerhound]

I don't think it's a good idea to automatically say that the two datum points should be interpreted as a line just because two points don't make sense in the setup scheme. If we are to interpret this print to ASME Y14.5M-1994 then those are points.

Datum B is referenced secondary so only two points of contact are required. This certainly ignores the axis and form of the pivot hole. It may be a bad way to set up the DRF for this part but I don't think it's wrong per the standard.

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

 

[SeasonLee]

“If we are to interpret this print to ASME Y14.5M-1994 then those are points.” Are there any changes if this print interprets by 2009 standard? or I missed something.

A DRF is a set of three mutually perpendicular theoretical datum planes, so we can use the 3-2-1 Rule to say only two contact points required to setup the secondary datum plane, but this secondary datum (pivot hole) is not a plane and it is not perpendicular to the primary datum plane, how can we use only two points to establish the secondary datum axis? I really want to learn how to build the datum axis.

Thanks

SeasonLee

 

[dtmbiz]

SeasonLee,

WOW... this thing just doesn’t let you sleep or are you located in Asia? (ref: 1:58)

I am not getting the goal here. The part is obviously not fully defined from what is shown
on the print I see. Could this possibly be a purchased part that is being modified?
I ask because it appears there are some modifications, possibly mount points for a PC board.
It really might be helpful to understand the objective. For instance which features are present
before the modifications; that is if this is a modified / altered part. Point being, is the pivot hole there already and only needs to be identified as a datum for reference; or does it need a positional control to locate it because it is a feature being created?

GDT ... says no problem for dropping datum B. It is a problem as I see it if the pivot hole is only
referenced by datum A. It has a positional tolerance control! Only 2 degrees of freedom restricted in one of the axis plus/minus direction.
Part can still rotate around that axis, while the other degrees of freedom are not addressed by defining
a complete DRF. This part needs a complete DRF IMO. There are not enough basic dimensions locating
the target points for datum B either as has been pointed out.

I believe you mentioned earlier that the designer wouldn’t change the DRF. This drawing is obviously confusing. It appears the pivot hole is positioned to itself. Some have said that’s okay, citing the example of a single axis from 2 coaxial cylindrical features for a runout control. I would take the position that the drawing you have doesn’t meet that criteria. Different animal all together, having only one feature involved... the pivot hole.

Any way, if you can’t appeal to the designer with the common sense, to define a DRF that makes sense then it appears to defeat the entire concept and purpose of ASME Y14.5 of defining the part functionality, clearly showing the relationships, and having one interpretation. (I know I’ll get comments on that one

You might just try using three points 120 degree’s apart; .060 in from the outside surface; however even that is bogus because those basic dimensions are supposed to originate from the DRF origin and that does not appear to be the case at all. They should come from the center plane of datum C.
Even so, that doesn’t clear up all that is lacking as the drawing currently stands.

If it were me, I would propose a DRF and dimensioning scheme that would work for the objective and then try to reason with the powers to be.

Their dog just ain't goina' hunt in this case.

 

[powerhound]

The 2009 interpretation is the same. The points are shown in a direct view of the surface. I think this means that the datum points are on the surface of the hole because that's the only place they could exist anyway. I agree that the feature itself is not correctly toleranced but setting up a DRF according to the standard doesn't appear to be a difficult thing. Datum A is easy and arrests 3 degrees of freedom, datum B be should be two fingers extending into each end of the slot with small, spherically tipped, nubs that come into contact with the cylindrical surface in the places shown, this arrests 2 more degrees of freedom. Datum C is then the center plane of the width of datum feature C.

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

 

[SeasonLee]

dtmbiz

Thanks for your inputs, I am staying in California, the Pacific Time Zone is three hours late from East coast, just try to send the post before sleeping, this thing really make me not sleep well.

Sorry for I can’t provide any functional information regarding this part since we are just an OEM, but glad to know your view point.

Thanks

SeasonLee

 

[Belanger]

Just to interject -- the notion of a datum reference frame having 3-2-1 contact points is not always true. That's helpful to explain simple datum structures such as the good-ole rectangular block examples. But there are many examples where the secondary datum feature may only contact one point.
Think of a shaft where we lock onto the OD as primary, and the bump the end of the shaft over to a flat plate for (potentially) one point of contact only.

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

http://www.gdtseminars.com

 

[dtmbiz]

John-Paul,

It appears to me that the drawing is attempting to establish an axis as a datum with target points. If that conclusion is true, then I believe that it would require 3 points at each of location to accomplish this.

What do you say in regard to establishing an axis with target points? Wouldnt you need at least 3 equally spaced points in each location?

dtm

 

[powerhound]

Thanks for the input JP. I don't know if your post was in response to what I said or what SeasonLee said. If it was in response to my post, I did not mean to imply that the 3-2-1 concept is the only way things work. I was simply offering a possible solution on how to arrest the degrees of freedom on this print given the datum structure. Having two datum target points labeled B1 and B2, along with calling datum B out as secondary, pretty much assigns two points of contact by which those degrees of freedom are arrested...in this configuration.

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

 

[Belanger]

My comment was in reply to SeasonLee's post at 1:48 this morning. I must admit that I haven't really been following this thread... I just jumped in to clarify what seemed to be a blanket statement.

But now that I've roped myself into the discussion, I would simply say that datum B is a line created by the 2 target points B1 and B2. Although referencing B doesn't seem to add any value to the position tolerance, it's not wrong to have it up there.
Dtmbiz ... as it is now I would say that the 2 points simply form a line at the top edge of the hole, not an axis. If the goal was to establish an axis (which might make more sense?) then I would agree that another point is need to encircle the hole.

It's been said that the datum targets don't have enough basic dims to locate them -- I think the .025 in the right view serves to locate the targets, in addition to the TP of the hole's axis. So IMO that is OK.



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

http://www.gdtseminars.com

 

[powerhound]

Using datum target points is definitely not the best way to dimension this but I see no other way to interpret datum B. I guess I just don't see how this could be interpreted as a line.

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

 

[Belanger]

Wait... I guess I was being too casual with terminology. Datum targets B1 and B2 are points. Datum B, however, will be a plane -- you're right about that!

I was using the word line to get away from the notion of B being an axis, but I should have said that B is a plane.

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

http://www.gdtseminars.com

 

[SeasonLee]

Very interesting, if I were a inspector, I will ask how to use these datum targets to establish the DRF, how to create the datum axis since the basic dimension is from the axis of the hole. As I mentioned earlier that some inspector will simply insert a best fit gage pin to stimulate the datum axis, is it a right way to setup the DRF?

Thanks

SeasonLee

 

[powerhound]

Point taken JP. I thought you and others were saying that a datum target line was being specified. I have a feeling I'm the only one who thought that. Everyone else seems to have been on board.

SeasonLee,
If this is a simple hinge hole for a lid and there is nothing really critical about how they interface, then it seems that getting rid of section B-B, and dropping the reference to datum B in the FCF but leaving it specified as datum B, would make this a much simpler part. This has been the direction that most of those here have already recommended and I'm certainly on board with it as well.

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

 

[GDTcoach]

Using the right terminology will maybe help here. Datum feature A establishes the first plane of the datum reference frame. From there I can locate the hole relative to this first datum plane. I then use two points on the surface of the hole to establish the second datum plane. This plane passes through the two datum points (not the axis)and must be at right angles to the first datum plane. You are not establishing a datum axis. Datum feature C should be positioned relative to A and B. So, lastly, the third plane of the datum reference frame is a center plane through datum feature C. This plane is mutually perpendicular to the first two datum planes.
The letters identify the datum features on the part. When you try to label the datum planes of the datum reference frame using these letters, once you get beyond a simple block, things fall apart. Think of a hole as the primary datum feature, A, on some other part. The axis of the hole establishes two datum planes that are perpendicular to each other and intersect at the axis of the hole. So, which one is datum A? There is no datum A, only datum feature A. The planes are the first and second planes of the datum reference frame. Sorry for the lecture. Here is a Tip I did on the subject.

http://www.tec-ease.com/gdt-tips-view.php?q=257

and the video of this Tip:

http://www.tec-ease.com/video/Aug_11.wmv

 

[Belanger]

Good stuff, Don. If I could offer my 2 cents...
One cent: when mentioning the axis of datum feature A, it is true that there are 2 planes formed from there. But I wouldn't say there is no datum A. Datum A is the axis derived from the datum feature simulator.
This comes from paragraph 1.3.13, which says that a datum is a point, axis, line, plane, etc. (I know what you're saying, in that the datum reference frame always consists of planes, but we don't want people to be left with the impression that the term "datum A" can never be used in conjunction with a theoretical axis.)

Second cent: Wouldn't it be better to say that datums exist only in a theoretical setting, and not "the datums exist in our processing equipment"? I think the standard would refer to the planes, axes, etc. on the processing equipment as "simulated datums."

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

http://www.gdtseminars.com